SEARCH

SEARCH BY CITATION

Keywords:

  • Carbonylation to ketones;
  • Double bond isomerization;
  • Hydroboration–oxidation;
  • Unsaturated FAME

Abstract

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References
  8. Supporting Information

Unsaturated FAMEs can be modified to higher value compounds by hydroboration of the double bond. The added boron atom can be replaced in a subsequent oxidation by a hydroxy group. This way methyl 10-undecenoate (1b), methyl oleate (3a), methyl ricinoleate ((9Z,12R)-6a), and methyl linoleate (9) were converted into methyl 11-hydroxyundecanoate and methyl hydroxyoctadecanoates in 82–92% yield. Organoboranes isomerize at higher temperatures. Hydroboration of oleic acid and subsequent heating to 220°C led after oxidative work-up to 70% of 1,4-octadecanediol. Presumably a 1,4-oxaborinane intermediate is involved. Alkyl groups of organoboranes can be attached to the carbonyl-equivalent: dichloromethyl methyl ether (DCME). With this reaction the boranes of the esters 1b and 3a were converted in 55–80% yield to dimers with an inserted ketocarbonyl group. Ester 9 afforded in the same reaction methyl nonadecanoates with an integrated cyclopentene ring.

Practical applications: Hydroboration–oxidation allows the partially regioselective conversion of unsaturated FAMEs into hydroxy- and dihydroxyesters. Thermal isomerization of the boranes from methyl oleate, oleic acid, and oleyl alcohol leads after oxidative work-up to the 1,4-diol: 1,4-octadecanediol as main product. These products are useful as components for polyesters, polyurethanes, and precursors for heterocycles. Reaction of organoboranes from unsaturated FAMEs with the carbonyl equivalent: DCME provides a one-step access to 1,ω-diesters with an internal keto group, which can be applied for coupling, cross-linking, and further conversion. The borane · THF complex is commercially available in amounts of 0.8 mol and more to do exploratory research toward such applications.

1 Introduction

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References
  8. Supporting Information

Hydroboration is the addition of hydrogen and boron from BH3 (borane) to a double bond. The reaction is regioselective: boron adds to the less substituted carbon of the double bond, and stereoselective leading to a cis-adduct. The carbon–boron bond can be converted by oxidation into a carbon–oxygen bond with retention of configuration to afford alcohols [1-3].

Organoboranes can isomerize at higher temperatures by migration of the boron atom to the terminal carbon atom of the alkyl chain [1-5]. Mechanistically the isomerization involves a sequence of reversible boron–hydrogen eliminations and additions. The reaction is catalyzed by a small borane excess.

Organoboranes can react with carbon monoxide and different work-up of the intermediate leads to tertiary-, secondary-, and primary alcohols [6]. For bulky substituents the dichloromethyl methyl ether (DCME) method, which uses deprotonated DCME as CO-equivalent, proves to be a better choice [7, 8]. The ether anion reacts with the alkylborane and subsequent alkyl migrations from boron to carbon form a tertiary carbon atom, whose oxidative work-up leads to a tertiary alcohol.

In hydroboration mainly alkenes with a short chain length and the double bond often in the terminal position are used. In this paper, we want to explore the hydroboration and conversion of the boranes from unsaturated FAMEs. There the alkyl chain is longer and contains mostly internal double bonds, which are sterically less accessible and for that reason less reactive. As conversion for the organoboranes we chose the oxidation with hydrogen peroxide, the thermal isomerization and the carbonylation with lithiated dichloromethyl methyl ether (DCME)-reaction. These reactions possibly provide an access to higher value oleochemicals and give insight into the influence of longer alkyl chains on the reactivity and applicability of organoboranes.

2 Materials and methods

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References
  8. Supporting Information

2.1 Analytical equipment

FT-IR-spectra were recorded with a 5-DXC-FT-IR-spectrometer (Nicolet). 1H NMR and 13C NMR-spectra were measured with a WM 300 spectrometer (Bruker) at 300 and 75.5 MHz. Mass spectra (EI, 70 eV) were obtained with a CH-7A instrument (Varian-MAT) and a MAT 312 instrument (Finnigan-MAT). For GC–MS the gas chromatographs 1400, 3400 (Varian, Finnigan), and GC-8A (Shimadzu) were combined with mass spectrometers MAT CH-7a and MAT 8230 (Varian). GC with chemical ionization (GC/CI), direct chemical ionization (DCI), and HR-MS was performed with MAT-instrument 8230. For operation conditions of GC and GC–MS see Supporting Information: Section 'Analytical equipment'. As internal standards in GC methyl palmitate and methyl icosanoate both in 99.5% purity from Janssen were used.

TLC was performed with silicagel 60 F254 (Merck) and for flash chromatography silicagel 60 [70–230 mesh] (Merck) was used. Elemental analyses were done by the analytical laboratory of the Institute of Organic Chemistry at the University of Münster and the microanalytical laboratory M. Beller (Göttingen). Melting points are uncorrected. Yields refer to the substance used in a smaller molar amount.

2.2 General

The applied chemicals were purchased from Aldrich, Fluka, Janssen, Merck, Roth, and Sigma and if not stated otherwise were used without further purification. The fatty acids and their methyl esters had the following purities: 10-undecenoic acid (98%, the acid was converted into the methyl ester with dimethoxypropane [9], the ester was further purified by flash-chromatography to 99%), methyl oleate (99%), methyl (9Z,12R)-12-hydroxy-9-octadecenoate (purified by flash chromatography to 99.5%). Linoleic acid (99.5% pure) was donated by Fa. Henkel KGaA, Düsseldorf and converted into the methyl ester with dimethoxypropane [9]. All solvents were purified by distillation and if necessary residual water was removed. The composition of solvents and eluents are given in volume ratios of the components. All reactions, where oxygen and water had to be excluded, were done under argon in oven-dried vessels being sealed with a septum. Reagents and solvents were added with microliter syringes.

2.3 Hydroboration–oxidation of unsaturated FAMEs 1b, 3a, 6a, and 9

2.3.1 Methyl 11-hydroxyundecanoate (2)

Methyl 10-undecenoate (1b) (1.0 g, 5.04 mmol) was reacted in abs. THF (30 mL) with the BH3 · THF complex (1.85 mL, 1.8 m, 3.33 mmol BH3). The preparation of the BH3 · THF complex and the determination of the borane content are described in [10-13]. In all cases, we used a self-prepared BH3 · THF complex. The hydroboration was performed in a round-bottom two-neck flask (100 mL), closed with a septum and a fermentation tube filled with glycerol or paraffin oil. The ester 1b, THF and the BH3 · THF-complex solution were added with glass syringes being rinsed before with abs. THF (three times). After stirring over night at RT excess borane is destroyed with water (1 mL). Subsequently, the organoborane solution is cooled to 0°C and then dropwise a 3 N NaOH-solution (2 mL) and a 30% H2O2-solution (2 mL) are added. After stirring for 1 h at RT, water (10 mL) is added. After saturation with NaCl-salt and separation of the THF-phase, the aqueous phase is extracted with diethyl ether (3 × 15 mL). The combined extracts are dried (MgSO4), the solvent is removed at a rotary evaporator and the product is isolated by flash chromatography to afford hydroxyester 2 (999 mg, 4.62 mmol, 92%). Rf-value: 0.88 (diethyl ether). IR (film): ν = 3369 cm−1 (s, br), 2928 (s), 2856 (s), 1741 (s, C[DOUBLE BOND]O), 1458 (m), 1438 (s), 1364 (m), 1255 (m), 1200 (s), 1174 (s), 1107 (m), 1057 (m), 723 (m). 1H NMR (CDCl3): δ = 1.26 (s, br, 12H, 6CH2), 1.5–1.7 (m, 4H, CH2CH2OH and CH2CH2CO2CH3), 2.28 (t, J = 7.6 Hz, 2 H, CH2CO2CH3), 3.61 (t, J = 5.6 Hz, 2 H, CH2OH), 3.65 (s, 3H, OCH3), 4.7 (s, br, 1H, OH). 13C NMR (CDCl3): δ = 24.9 (t, C-3), 25.6 (t, C-9), 29.04 (t), 29.12 (t), 29.25 (t), 29.29 (t), 29.41 (t), 32.7 (t, C-10), 34.0 (t, C-2), 51.4 (q, OCH3), 63.0 (t, CH2OH), 174.3 (s, C[DOUBLE BOND]O). MS (GC/MS-coupling, 70 eV) as TMS-ether: m/z (%) = 288 (2) [M+], 273 (63) [M+−CH3], 257 (7) [M+−OCH3], 241 (100) [M+−CH3OH[BOND]CH3], 173 (3), 159 (5) [CH2CHC(OTMS)OCH3+], 149 (5), 131 (4), 129 (5) [C3H4OTMS+], 117 (27), 107 (28), 103 (32), 97 (12), 89 (33), 83 (31), 75 (71), 73 (78), 69 (39), 59 (19), 55 (74), 43 (14), 41 (26). C12H24O3 (216.32): calcd. C 66.63, H 11.18; found C 66.71, H 11.20.

2.3.2
2.3.3
2.3.4 Mixture of methyl 9,12-dihydroxyoctadecanoate (7a) and methyl 10,13-dihydroxyoctadecanoate (10), methyl 10,12-dihydroxyoctadecanoate (8a) and mixture of isomeric methyl hydroxyoctadecanoates (11a11d)

Methyl linoleate (9) (1.0 g, 3.40 mmol) was hydroborated in abs. THF (30 mL) with the BH3 · THF complex (2.52 mL, 1.8 m, 4.54 mmol BH3) and subsequently oxidized as described in Section 'Methyl 11-hydroxyundecanoate (2)' above. The product mixture was separated by chromatography (petroleum ether:diethyl ether = 5:1) into a fraction consisting of 7a, 10, and 8a and a fraction of 11a11d. The mixture of 7a, 10, and 8a was then separated by chromatography (diethyl ether) into a mixture of 7a and 10 and pure 8a. The separation afforded 7a and 10 (736 mg, 2.23 mmol, 66%, GC-ratio 7a: 10 = 1:1), 8a (184 mg, 0.56 mmol, 16%) and 11a11d (106 mg, 0.34 mmol, 10%). Rf-value (diethyl ether): 0.8 for 11a11d, 0.6 for 8a, 0.4 for 7a and 10.

The 1,4-diols 7a and 10 were characterized as mixture. Their IR, 1H, and 13C NMR-spectra are nearly identical with these of pure 8a. Elemental analysis of the mixture of 7a and 10: C19H38O4 (330.51): calcd. C 69.05, H 11.59; found C 69.26, H 11.89.

The mass spectra of the trimethylsilyl (TMS)-ethers of 7a, 8a, and 10 are given below. For the preparation of the TMS-ether, the substance was dissolved in diethyl ether and a 10- to 50-fold excess of N-methyl-N-(trimethylsilyl)-trifluoroacetamide (MSTFA) is added. After one night at RT, the reaction is quantitative.

Methyl 9,12-bis(trimethylsilyloxy)octadecanoate, TMS-derivative of 7a: MS (GC/MS-coupling, 70 eV): m/z (%) = 459 (0.1) [M+−CH3], 443 (0.6) [M+−OCH3], 389 (2) [M+−CH3(CH2)5, α-cleavage], 360 (4) [(CH2)2CH(OTMS)(CH2)7C(OTMS)OCH3+], 317 (7) [M+−(CH2)7-CO2CH3, α-cleavage], 299 (52) [389-HOTMS], 259 (49) [TMSOCH(CH2)7CO2CH3+, α-cleavage], 227 (62) [317-HOTMS], 187 (59) [CH3(CH2)5CHOTMS+, α-cleavage] (For further fragments see Supporting Information Section 'Mixture of methyl 9,12-dihydroxyoctadecanoate (7a) and methyl 10,13-dihydroxyoctadecanoate (10), methyl 10,12-dihydroxyoctadecanoate (8a) and mixture of isomeric methyl hydroxyoctadecanoates (11a11d)').

Methyl 10,13-bis(trimethylsilyloxy)octadecanoate, TMS-derivative of 10: MS (GC/MS-coupling, 70 eV): m/z (%) = 443 (0.3) [M+−OCH3], 403 (1) [M+−CH3(CH2)4, α-cleavage], 384 (1) [M+−HOTMS], 374 (3) [(CH2)2CH(OTMS)(CH2)8C(OTMS)OCH3+], 313 (32) [403-HOTMS], 303 (8) [M+−(CH2)8CO2CH3, α-cleavage], 273 (41) [TMSOCH(CH2)8CO2CH3+, α-cleavage], 213 (52) [303-HOTMS], 173 (45) [CH3(CH2)4CHOTMS+, α-cleavage], 169 (19) [CHO(CH2)8CO+], 147 (16) [(CH3)2SiOTMS+], 129 (38) [C3H4OTMS+], 73 (100) [TMS+] (For further fragments see Supporting Information Section 'Mixture of methyl 9,12-dihydroxyoctadecanoate (7a) and methyl 10,13-dihydroxyoctadecanoate (10), methyl 10,12-dihydroxyoctadecanoate (8a) and mixture of isomeric methyl hydroxyoctadecanoates (11a11d)').

Methyl 10,12-bis(trimethylsilyloxy)octadecanoate, TMS-derivative of 8a: MS (GC/MS-coupling, 70 eV): m/z (%) = 459 (0.3) [M+−CH3], 443 (0.4) [M+−OCH3], 389 (0.1) [M+−CH3(CH2)5, α-cleavage], 384 (3) [M+−HOTMS], 360 (1) [CH2CH(OTMS)(CH2)8C(OTMS)OCH3+], 303 (0.1) [M+−(CH2)8CO2CH3, α-cleavage], 299 (6) [389-HOTMS], 273 (67) [TMSOCH-(CH2)8CO2CH3+, α-cleavage], 213 (9) [303-HOTMS], 187 (100) [CH3(CH2)5CHOTMS+, α-cleavage] (For further fragments see Supporting Information Section 'Mixture of methyl 9,12-dihydroxyoctadecanoate (7a) and methyl 10,13-dihydroxyoctadecanoate (10), methyl 10,12-dihydroxyoctadecanoate (8a) and mixture of isomeric methyl hydroxyoctadecanoates (11a11d)').

Methyl 10,12-dihydroxyoctadecanoate (8a): m.p. = 40–41°C. IR (KBr): ν = 3408 cm−1 (m, br), 2928 (s), 2853 (s), 1742 (s), 1467 (m), 1438 (m), 1380 (w), 1209 (m), 1176 (m), 1090 (w), 725 (w). 1H NMR (CD3OD): δ = 0.90 (t, 3H), 1.25–1.65 (m, 26H), 2.32 (t, J = 7.4 Hz, 2H), 3.66 (s, 3H), 3.7–3.85 (m, 2H), 4.56 (s, br, 2H). 13C NMR (CD3OD): δ = 14.4 (q), 23.7 (t), 26.0 (t), 26.4 (t), 26.8 (t), 30.17 (t), 30.32 (t), 30.53 (t), 30.72 (t), 33.0 (t), 34.8 (t), 38.7 (t), 39.2 (t), 44.9 (t), 45.6 (t), 51.9 (q), 69.3 (d), 71.8 (d), 176.0 (s). Mass spectrum as TMS-ether see above. C19H38O4 (330.51): calcd. C 69.05, H 11.59; found C 69.05, H 11.53.

2.4 Thermal isomerization of hydroborated oleyl alcohol, oleic acid, and methyl oleate

2.4.1 General procedure for the thermal isomerization

According to the procedure in Section 'Methyl 11-hydroxyundecanoate (2)' oleyl alcohol, methyl oleate, and oleic acid were hydroborated with the named amount of BH3 · THF complex in the given solvent (see Table 1). After the completed hydroboration the reaction apparatus (see Section 'Methyl 11-hydroxyundecanoate (2)') was placed into an autoclave, which is then flushed with nitrogen, filled with nitrogen at a pressure of 60 atm and heated with an electrical oven to the given temperature (see Table 1 for temperature and time). For the reaction in bis-2-methoxy-ethyl ether, the reaction vessel is equipped with a reflux condenser and a heating jacket. After cooling to RT, water (1 mL) is added and then a 3 N NaOH-solution (1.5 mL) and a 30% H2O2-solution (1.5 mL) are added. The solution is stirred for 1 h at RT and then diluted with water (10 mL); for salting out NaCl-salt is added and thereafter the solution is extracted with diethyl ether (3 × 10 mL) and dried (MgSO4). The solvent is removed at the rotary evaporator. A sample is silylated with MSTFA (see silyl ethers in Section 'Mixture of methyl 9,12-dihydroxyoctadecanoate (7a) and methyl 10,13-dihydroxyoctadecanoate (10), methyl 10,12-dihydroxyoctadecanoate (8a) and mixture of isomeric methyl hydroxyoctadecanoates (11a11d)') and analyzed by GC. The diols 14, 15, and 16 and 1-octadecanol (17) from the thermal isomerizations were identified with authentic references and quantified by GC with methyl palmitate and methyl icosanoate as internal standard. For the preparation of 15 and 16, see Sections 2.4.2 and 2.4.3, for the preparation of 12, 14, and 17 see Sections 2.4.4–2.4.6 in the Supporting Information. The products, their yields and further data are compiled in Table 1.

Table 1. Thermal isomerization of hydroborated oleyl alcohol (12), methyl oleate (3a) and oleic acid (13); reaction conditions and products
Nr.Starting compounds (mmol)Solvent (mL)Reaction temperature (°C), time (h)Products and yields (%, isol., GC)a)Refs.
  • a)

    Calibrated GC.

  • b)

    External generation from 150 mmol NaBH4, 240 mmol BF3 · Et2O in bis-2-ethoxyethyl ether.

  • c)

    Milliliters of BH3 · THF solution in THF.

  • d)
1.12 (75), B2H6b)Bis-2-ethoxy-ethyl ether (150)c)160, 415 (34 isol.)d), 16 (10 isol.)Logan [20]
2.12 (1.58), BH3 · THF (1.58)Bis-2-methoxy-ethyl-ether (5) [Kb.p. 162°C], THF (0.88)c)Reflux, 1214 (>90 GC)This paper, [12]
3.12 (3.16), BH3 · THF (3.16)THF (1.76)c)220, 14, Autoclave15 (70 GC; 66 isol.), 16 (16, GC), 17 (7, GC)This paper, [12]
4.12 (3.16), BH3 · THF (3.16)THF (1.76)c)240, 8, Autoclave15 (56, GC), 16 (4, GC), 17 (28, GC)This paper, [12]
5.3a (1.48), BH3 · THF (1.8)THF (1.0)c)240, 12, Autoclave15 (55, GC), 16 (2 GC), 17 (30 GC)This paper, [12]
6.13 (1.58), BH3 · THF (2.52)THF (1.4) and THF as solvent (5.0 mL)220, 30, Autoclave15 (70 GC)This paper, [12]
2.4.2 Preparation of 1,4-octadecanediol (15)

Diol 15 was prepared by using the procedure in Section 'General procedure for the thermal isomerization'. Oleyl alcohol (12) (0.85 g, 3.16 mmol) and BH3 · THF complex (3.16 mmol BH3) were reacted in an autoclave (14 h, 220 °C) to yield after flash chromatography (acetone:ethyl acetate:dichloromethane = 1:1:1) 1,4-diol 15 (0.60 g, 2.09 mmol, 66%). M.p. = 67°C. Rf-value: 0.3 (acetone:ethyl acetate:dichloromethane = 1:1:1). IR (KBr): ν = 3241 cm−1 (m, br), 2923 (s,), 2849 (s), 1465 (m), 1440 (w), 1380 (w), 1120 (m), 1050 (m), 724 (m). 1H NMR (CD3OD): δ = 0.90 (t, J = 6.7 Hz, 3H), 1.29 (s, br, 24H), 1.4–1.5 (m, 4H), 1.5–1.7 (m, 2H), 3.51 (s, br, 1H), 3.56 (t, J = 6.4 Hz, 2H). 13C NMR (CD3OD): δ = 14.7 (q), 23.7 (t), 26.7 (t), 29.8 (t), 30.37 (t), 30.68 (t), 30.79 (t), 33.0 (t), 34.6 (t), 38.4 (t), 63.1 (t), 72.3 (d). MS (GC/MS-coupling, 70 eV) as TMS-ether: m/z (%) = 430 (0.2), 415 (1), 373 (5), 299 (82) [M+−(CH2)3OTMS, α-cleavage), 285 (6), 233 (42) [M+-CH3(CH2)13, α-cleavage], 149 (11), 147 (23), 143 (100), 129 (9), 116 (12), 103 (14), 97 (16), 85 (19), 83 (18), 75 (18), 73 (62), 71 (45), 69 (15), 57 (15), 55 (14), 43 (13). C18H38O2 (286.50): calcd. C 75.46, H 13.37; found C 75.49, H 13.23.

2.4.3 1,18-Octadecanediol (16)

18-Hydroxystearic acid (1 g, 3.33 mmol) [14] dissolved in abs. THF (10 mL) is added dropwise to a suspension of LiAlH4 (190 mg, 5.00 mmol) in abs. THF (5 mL) and stirred over night at RT. After addition of ice water and dissolution of precipitated Al(OH)3 with 2 N H2SO4 the solution was extracted with diethyl ether (3 × 15 mL) and the ether phase was dried (MgSO4) and the ether evaporated. Flash chromatography afforded the 1,18-diol 16 (620 mg, 2.16 mmol, 65%). M.p. = 89–91°C. Rf-value: 0.3 (petroleum ether:acetone = 3:1). IR (KBr): ν = 3408 cm−1 (m, OH), 2918 (s), 2850 (s), 1473 (m), 1463 (m), 1062 (m, C[BOND]O), 720 (w, CH2). 1H NMR (CD3OD): δ = 1.29 (s, br, 28H, 14CH2), 1.52 (m, 4H, 2CH2CH2OH), 3.53 (t, J = 6.6 Hz, 4H, 2CH2OH). 13C NMR (CD3OD): δ = 27.0 (t), 30.61 (t), 30.76 (t), 33.7 (t), 63.0 (t). MS (GC/MS-coupling, 70 eV as TMS-ether): m/z (%) = 430 (0.5) [M+], 415 (7) [M+−CH3], 399 (8) [M+−OCH3], 340 (10) [M+-HOTMS], 325 (13) [340-CH3], 250 (5), 177 (7), 165 (15), 149 (100), 147 (46), 125 (8) [C9H17+], 111 (25) [C8H15+], 103 (30), 97 (50) [C7H13+], 83 (52), 75 (51), 73 (44), 69 (47), 57 (39), 55 (32), 43 (17). C18H38O2 (286.50): calcd. C 75.46, H 13.37; found: C 75.46, H 13.32.

2.5 Carbonylation of hydroborated fatty compounds

2.5.1 General procedure for the DCME-reaction

Hydroboration was performed according to procedure in Section 'Methyl 11-hydroxyundecanoate (2)' in the given solvent (see Sections 'Dimethyl 12-oxotricosanedioate (18)''Preparation of a mixture of 1-hexyl-3-(methyl 8-octanoate)-cyclopentene, 3-hexyl-1-(methyl 8-octanoate)-cyclopentene (20a), 1-(methyl 9-nonanoate)-3-pentyl-cyclopentene and 3-(methyl 9-nonanoate)-1-pentyl-cyclopentene (20b), 2-hexyl-5-(methyl 8-octanoate)-cyclopentanone (21a), and 2-(methyl 9-nonanoate)-5-pentyl-cyclopentanone (21b) (see Supporting Information Section 2.5.4)') with 0.5 equivalents of borane per double bond. The solution was stirred over night at RT and then after cooling to −78°C DCME was added. At −78°C a lithium 3-ethyl-3-pentanolate solution in hexane was added within 10 min. After 45 min, the reaction mixture was slowly warmed up to RT and stirring was continued for 8 h. With unsaturated ester 9 stirring was limited to 2 h at RT. After adding a 3 N NaOH-solution (2 mL) and a 30% H2O2-solution (2 mL) at 0°C, the mixture remained for 1 h at RT and then was diluted with water (10 mL) and diethyl ether (10 mL), saturated with NaCl-salt and extracted with diethyl ether (3 × 15 mL). After drying (MgSO4) and evaporation of the solvent, the esters were isolated by flash chromatography.

The lithium 3-ethyl-3-pentanolate solution (1.96 M) was prepared by adding a butyl lithium solution in hexane (2.67 M, 27 mL, 72.1 mmol) at −10°C under inert gas to 3-ethyl-3-pentanol (10 mL, 8.42 g, 72.5 mmol). The commercially available DCME-reagents DCME and 3-ethyl-3-pentanol were distilled prior to use. Best results were obtained with a freshly prepared base.

2.5.2 Dimethyl 12-oxotricosanedioate (18)

Methyl 10-undecenoate (1b, 1.0 mL, 0.90 g, 4.53 mmol) in abs. diglyme (20 mL) was hydroborated with BH3 · THF complex (2.34 mmol BH3). The solution was stirred over night at RT and then at −78°C DCME (0.5 mL, 0.64 g, 5.55 mmol) was added. Then within 10 min a lithium 3-ethyl-3-pentanolate solution in hexane (3.7 mL 1.96 M, 7.25 mmol) was added and further proceeded as described in Section 'General procedure for the DCME-reaction' to afford after flash chromatography (petroleum ether:acetone = 8:1) the diester 18 (530 mg, 1.24 mmol, 55%). M.p. = 73°C. Rf-value: 0.6 (petroleum ether:acetone = 4:1). IR (KBr): ν = 2917 cm−1 (s), 2850 (s), 1736 (s, estercarbonyl), 1712 (m, ketocarbonyl), 1473 (m), 1464 (m), 1438 (m), 1379 (m), 1336 (m), 1272 (m), 1241 (m), 1209 (m), 1181 (m), 1118 (m), 720 (w). 1H NMR (CD3OD): δ = 1.30 (s, br, 24 H), 1.45–1.65 (m, 8H), 2.31 (t, J = 7.4 Hz, 4H), 2.44 (t, J = 7.3 Hz, 4H,), 3.65 (s, 6H). 13C NMR (CD3OD): δ = 24.9 (t), 26.0 (t), 30.08 (t), 30.17 (t), 30.22 (t), 30.35 (t), 34.9 (t), 43.5 (t), 51.8 (q), 176.0 (s), 214.2 (s). MS (GC/MS-coupling, 70 eV): m/z (%) = 426 (1) [M+], 395 (16) [M+−OCH3], 363 (21) [M+−OCH3[BOND]CH3OH], 353 (7) [M+−CH2CO2CH3], 321 (11) [353-CH3OH], 242 (100) [M+−CH2CH(CH2)7CO2CH3, McLaff.], 227 (42) [CO(CH2)10CO2CH3, α-cleavage], 210 (32) [242-CH3OH], 195 (17), 185 (26) [C9H18CO2CH3+], 184 (35) [CH2CH(CH2)7CO2CH3+, McLaff.], 55 (100) [C4H7+]. For further fragments see Supporting Information Section 'Dimethyl 12-oxotricosanedioate (18)'. C25H46O5 (426.64): calcd. C 70.38, H 10.87; found: C 70.63, H 10.87.

2.5.3 Preparation of a mixture of dimethyl 9,11-dinonyl-10-oxononadecanedioate, dimethyl 10,12-dioctyl-11-oxoheneicosanedioate and dimethyl 9-nonyl-11-octyl-10-oxoeicosanedioate (19)

Methyl oleate (3a, 1.0 mL, 0.88 g, 2.96 mmol) in abs. THF (20 mL) was converted as described in Section 'General procedure for the DCME-reaction' with BH3 · THF complex (1.53 mmol BH3), DCME (0.3 mL, 0.38 g, 3.32 mmol) and a lithium-3-ethyl-3-pentanolate solution in hexane (2.3 mL 1.96 M, 4.51 mmol) to afford 19 as mixture (740 mg, 1.19 mmol, 80%). Rf-value: 0.5 (petroleum ether:diethyl ether = 5:2). The regioisomers could not be separated by flash chromatography and were further characterized as mixture. IR (film): ν = 2927 cm−1 (s), 2855 (s), 1743 (s, estercarbonyl), 1708 (s, ketocarbonyl), 1465 (m), 1436 (m), 1363 (m), 1248 (m), 1197 (m), 1172 (m), 1118 (w), 1020 (w), 723 (w). 1H NMR (CDCl3): δ = 0.88 (t, J = 6.7 Hz, 6H), 1.25 (s, br, 48H), 1.50–1.65 (m, 8H), 2.30 (t, J = 7.5 Hz, 4H), 2.51 (m, 2H), 3.66 (s, 6H). 13C NMR (CDCl3): δ = 14.1 (q), 22.6 (t), 24.9 (t), 27.4 (t), 29.10 (t), 29.20 (t), 29.29 (t), 29.45 (t), 29.49 (t), 92.56 (t), 29.65 (t), 29.83 (t), 30.6 (t), 31.9 (t), 34.1 (t), 51.1 (d), 51.4 (q), 174.2 (s), 216.7 (s). MS (direct inlet, 70 eV): m/z (%) = 622 (8) [M+], 591 (6) [M+−OCH3], 573 (7) [591-H2O], 559 (4) [M+−OCH3[BOND]CH3OH], 510 (18) [M+−CH3(CH2)5CHCH2, McLaff.], 496 (19) [M+−CH3(CH2)6-CHCH2, McLaff.], 466 (19) [M+−CH2CH(CH2)5CO2CH3, McLaff.], 452 (18) [M+-CH2CH(CH2)6CO2CH3, McLaff.], 398 (6) [M+−2CH3(CH2)5CHCH2, 2 McLaff.], 384 (14) [M+−CH3(CH2)5CHCH2[BOND]CH3(CH2)6CHCH2, 2 McLaff.], 370 (8) [M+−2 CH3(CH2)6CHCH2, 2 McLaff.], 354 (12) [M+−CH3(CH2)5CHCH2[BOND]CH2CH(CH2)5-CO2CH3, 2 McLaff.], 340 (32) [M+−CH3(CH2)5CHCH2[BOND]CH2CH(CH2)6CO2CH3, and M+−CH3(CH2)6CHCH2[BOND]CH2CH(CH2)5CO2CH3, 2 McLaff.], 326 (18) [M+−CH3(CH2)6-CHCH2[BOND]CH2CH(CH2)6[BOND]CO2CH3, 2 McLaff.], 325 (21) [M+−C17H34CO2CH3, α-cleavage], 57 (100) [C4H9+]. For further fragments see Supporting Information Section 'Preparation of a mixture of dimethyl 9,11-dinonyl-10-oxononadecanedioate, dimethyl 10,12-dioctyl-11-oxoheneicosanedioate and dimethyl 9-nonyl-11-octyl-10-oxoeicosanedioate (19)'. C39H74O5 (623.01): calcd. C 75.19, H 11.97; found: C 75.22, H 11.98.

2.5.4 Preparation of a mixture of 1-hexyl-3-(methyl 8-octanoate)-cyclopentene, 3-hexyl-1-(methyl 8-octanoate)-cyclopentene (20a), 1-(methyl 9-nonanoate)-3-pentyl-cyclopentene and 3-(methyl 9-nonanoate)-1-pentyl-cyclopentene (20b), 2-hexyl-5-(methyl 8-octanoate)-cyclopentanone (21a), and 2-(methyl 9-nonanoate)-5-pentyl-cyclopentanone (21b) (see Supporting Information Section 2.5.4)

3 Results and discussion

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References
  8. Supporting Information

3.1 Conversion of unsaturated FAMEs to alcohols by hydroboration–oxidation

In the hydroboration of ethyl 10-undecenoate (1a) with the BH3 · THF complex and the oxidation of the organoborane with hydrogen peroxide Brown et al. obtained 88% of ethyl hydroxyundecanoate. The ratio of ethyl 11-hydroxyundecanoate (2a) to ethyl 10-hydroxyundecanoate was 91: 9 (Scheme 1a) [15]. In the same reaction of 1a with disiamylborane, only the primary alcohol was isolated in 81% yield [15]. With methyl ester 1b we found with the BH3 · THF complex 92% of methyl 11-hydroxyundecanoate (2b) with less than 2% of the (ω-1)-regioisomer (Scheme 1a) [12]. With the reagent: NaBH4/J2 the ester 1b has been converted to 72% of 2b [16].

image

Scheme 1. (a) Preparation of ethyl and methyl 11-hydroxyundecanoate (2a and 2b) from ethyl and methyl 10-undecenoate (1a and 1b); (b) preparation of a mixture of methyl 9-hydroxyoctadecanoate (4a) and methyl 10-hydroxyoctadecanoate (5a) and a mixture of 9-hydroxyoctadecanoic acid (4b) and 10-hydroxyoctadecanoic acid (5b) from methyl oleate (3a).

Download figure to PowerPoint

With methyl oleate (3a) we found in 90% yield a 1:1 – mixture of methyl 9-hydroxyoctadecanoate (4a) and methyl 10-hydroxyoctadecanoate (5a) [12] (Scheme 1b). Applying the same conditions but a different work-up Fore and Bickford found in 85% yield a 1:1 – mixture of 9-hydroxyoctadecanoic acid (4b) and 10-hydroxyoctadecanoic acid (5b) [13] (Scheme 1b). We avoided the use of thexylborane, disiamylborane, and 9-borabicyclo[3.3.1]nonane for the hydroboration–oxidation of the esters 3a, 6a, and 9a, because the yields were poor with these reagents. Possibly with these organoboranes the addition of the sterically less accessible boron atoms to the disubstituted internal double bonds is hindered. We could unequivocally assign the structures of the esters 4a and 5a from the mass spectra of their silyl ethers by the fragments of the α-cleavage at the CHOTMS group (see Supporting Information Section '').

Ester (9Z,12R)-6a afforded under the same conditions as in our conversion of ester 3a the two regioisomers: methyl (12R)-9,12-dihydroxyoctadecanoate ((12R)-7a) and methyl (12R)-10,12-dihydroxyoctadecanoate ((12R)-8a) in together 90% yield and a ratio of (12R)-7a:(12R)-8a = 49:51 (Scheme 2) [12]. The position of the hydroxy groups in the products has been assigned by means of characteristic fragments in the mass spectra of the silyl ethers (see below).

image

Scheme 2. Hydroboration–oxidation of methyl (9Z,12R)-12-hydroxy-9-octadecenoate ((9Z,12R)-6a) with the borane · THF complex and hydrogen peroxide to methyl (12R)-9,12-dihydroxyoctadecanoate ((12R)-7a) and methyl (12R)-10,12-dihydroxyoctadecanoate ((12R)-8a) and with BF3 · Et2O/NaBH4 and hydrogen peroxide to the triols (12R)-7b and (12R)-8b.

Download figure to PowerPoint

image

Scheme 3. Hydroboration–oxidation of methyl linoleate (9) to the diols 7a, 10, and 8a and the isomeric monoalcohols 11a11d.

Download figure to PowerPoint

Recently, a Russian group isolated from the hydroboration–oxidation of ester (9Z,12R)-6a two triols (12R)-7b and (12R)-8b in 98% yield [17]. Different from our hydroboration with a pure borane · THF complex, the reagent in [17] was generated in situ by adding BF3 · Et2O in anhydrous THF to a suspension of ester (9Z,12R)-6a and NaBH4 in anhydrous THF. Under this condition also the ester group was reduced to a primary alcohol and the (12R)-1,9,12-octadecanetriol ((12R)-7b) and (12R)-1,10,12-octadecanetriol ((12R)-8b) were obtained in a regioselectivity of 87:13. The Russian group separated the diastereomers of (12R)-7b and (12R)-8b and assigned their structures. The new asymmetric centers at C-9 and C-10 had preferentially a (S)-configuration; up to 87% of (10S,12R)-8b and up to 100% of (9S,12R)-7b were found.

In the hydroboration–oxidation of methyl linoleate (9), we [12] obtained two 1,4-diols 7a and 10 in together 66% yield, 16% of the 1,3-diol 8a, and additionally 10% of four unsaturated monoalcohols 11ad. Surprisingly, no 1,5-diol was detected. The position of the hydroxy groups in the products has been deduced from the characteristic fragments in the mass spectra of the silyl ethers. The fragments are for 7a: m/z (%) = 259 (49), 187 (59) [α-cleavage], for 10: m/z (%) = 273 (41), 173 (45) [α-cleavage], for 8a: m/z (%) = 273 (67), 187 (100) [α-cleavage], for 11a: m/z (%) = 259 (91), 227 (10) [α-cleavage], for 11b: m/z (%) = 273 (71), 213 (5) [α-cleavage], for 11c: m/z (%) = 270 (100), 187 (100) [α-cleavage] and for 11d: m/z (%) = 313 (4), 173 (100) [α-cleavage] (see assignments in Section 'Mixture of methyl 9,12-dihydroxyoctadecanoate (7a) and methyl 10,13-dihydroxyoctadecanoate (10), methyl 10,12-dihydroxyoctadecanoate (8a) and mixture of isomeric methyl hydroxyoctadecanoates (11a11d)').

For the product formation, the following reaction sequence is proposed (Scheme 4). In the first step, the monounsaturated organoboranes AD are formed. They can cyclize intramolecularly to the five membered borolanes E and F and to the more strained four membered boretane G. On oxidative work-up, E and F react to 10 and 7a, and G to 8a. AD can in principle also react intermolecularly with the borane · THF complex and afford after oxidation 10, 7a, and 8a. The unreacted AD are converted to the unsaturated monoalcohols 11a11d. In the products, the expected 1,5-diol is missing. This could result from a preferred conformation of 9, A, and D. The 1,4-diene structure of ester 9 can be simplified to the structure of (2Z,5Z)-2,5-heptadiene (9′). Conformations of 9′ and extended structures have been studied by Hartree–Fock and DFT methods [18]. The hydroboration of 9′ in its preferred conformation leads to A′, which is a simplified structure of A. In the concerted intramolecular addition of the organoborane A′, a borolane corresponding to E is formed, where the boron atom binds to C-5 of A′ and the hydrogen atom to C-6. The formation of a borinane corresponding to H would be sterically not possible in this conformation. Other conformations of A and A′ should be less populated because of steric interactions between the methyl groups in A′ or the CH3(CH2)4-group and the CH3O2C(CH2)7-group in A. As the intermediate D should react similar as A, the formation of the borinane H and of the 1,5-diol is suppressed.

image

Scheme 4. Proposed mechanism for the hydroboration–oxidation of methyl linoleate (9) to the esters 7a, 10, 8a, and 11a11d (see also the text).

Download figure to PowerPoint

In the hydroboration–oxidation of 1,4-pentadiene the 1,4- and 1,5-pentane diol are formed in a ratio of 62:38. A borolane corresponding to E or F (Scheme 4) is proposed as intermediate for the 1,4-diol [19]. Regarding the observed formation of a 1,5-diol with 1,4-pentadiene one has to keep in mind that the boron atom is adding for steric and electronic reasons preferentially to the unsubstituted carbon atom of the double bond, which is present in 1,4-pentadiene but not in ester 9. Additionally, in the 1,4-pentadiene a conformation for the cyclization to a six-membered borinane is populated because of the low steric repulsion of the hydrogen atoms at C1 and C5.

In the intermolecular additions of the borane · THF complex, the rates for the boron-addition to C9 in A and to C13 in D should be higher than these to C9 in B and C13 in C, because of the smaller steric repulsion between the two boron atoms in the first case. As no 1,5-diol is found the intermolecular addition appears to be unimportant.

In summary, the hydroboration–oxidation of the esters 1a and b, 3a, 6a, and 9 affords with the BH3 · THF complex in yields between 82 and 92% the corresponding hydroxy- and dihydroxyesters. As shown by us and others, the ester 1 with a terminal double bond leads to a highly regioselective hydroxylation at the less substituted carbon atom. The esters 2a and 2b, 4a and 5a that are accessible from esters 1a, 1b, and 3a are useful precursors for polyesters. The esters 6a and 9 afford in one step 1,3-diols and 1,4-diols. The 1,4-diols could serve as 1,4-butanediol unit for the synthesis of polyesters. In addition, the 1,4-diols may be oxidized to 1,4-dicarbonyl compounds, which can be converted into heterocycles that are incorporated into a octadecanoic acid chain. The borane · THF complex is commercially available in amounts of 0.8 mol and more to do exploratory research in small-scale syntheses toward such applications.

3.2 Double bond isomerization of hydroborated oleyl alcohol, methyl oleate, and oleic acid

Logan [20] found that hydroborated oleyl alcohol (12) affords after 4 h heating at 160°C in bis-2-ethoxyethyl ether and subsequent oxidation with hydrogen peroxide at RT 10% of 1,18-octadecanediol (16), 34% of 1,4-octadecanediol (15), and 32% of unreacted oleyl alcohol (the numbers are calculated on the isolated amount of 15, 16, and converted 12 given in the experimental part of [20]). We wanted to see, whether this synthetically interesting reaction could be transferred to methyl oleate and oleic acid that are technically produced in larger scale from plant oils than oleyl alcohol. The results are summarized in Table 1 and in Scheme 5.

image

Scheme 5. Thermal isomerization of hydroborated oleyl alcohol (12), methyl oleate (3a), and oleic acid (13); products and proposed mechanism (see also text).

Download figure to PowerPoint

We found with oleyl alcohol and the BH3 · THF complex in refluxing diglyme (bp. 162°C) after 12 h and oxidative work-up in 90% yield (by calibrated GC) a 1:1-mixture of 1,9- and 1,10-octadecanediol (14 (R = CH2OH), Table 1, Nr. 2) but no isomerization products. At 220 and 240°C, oleyl alcohol (12) gave in an autoclave under nitrogen at 60 atm pressure and with residual THF from the BH3 · THF solution the products shown in Table 1, Nr. 3 and 4. With methyl oleate (3a, Table 1, Nr. 5) the same product mixture as with 12 under the same conditions (Table 1, Nr. 4) was obtained. With oleic acid (13, Table 1, Nr. 6) the diol 15 was found in 70% yield at the same temperature as applied to 12 (Table 1, Nr. 3).

The products were identified and quantified by calibrated GC. C18-diol 15 was isolated and characterized from a larger scale isomerization of 12. The structure of 15 is additionally assigned from the TMS-ether by fragments in the mass spectrum. These originate from α-cleavages and have m/z (%) = 299 (82) and 233 (42) (see Section 'Preparation of 1,4-octadecanediol (15)'). C18-diol 16 was prepared independently from 18-hydroxyoctadecanoic acid and C18-diol 14 was obtained by hydroboration–oxidation of 12.

In the thermal isomerization, the boron atom can migrate by elimination and addition toward the terminal methyl group at C-18 as also proposed by Logan [20]. The primary organoborane at C-18 is more stable than the intermediate secondary organoboranes as shown by Brown [4, 5]. In the opposite migration toward the boronic ester, which is formed at C1 from the hydroxy group or the reduced ester or acid group, the migrating boron atom is trapped at C-4 as borinane as also proposed by Logan [20]. Oxidative work-up of the borinane leads to the 1,4-octadecanediol. A further migration of the boron atom to C-2 can induce a 1,2-elimination with the terminal borate. This elimination is also observed in the thermal isomerization of organoboranes prepared from unsaturated alcohols [21]. The produced 1-octadecene is hydroborated and the oxidative work-up leads to 1-octadecanol (17). The portion of alcohol 17 increases at higher temperatures, the portion of the diols 15 and 16 decreases (Table 1).

In summary, Logan [20] and we found that the thermal isomerization of the boranes from methyl oleate, oleic acid, and oleyl alcohol leads after oxidative work-up to 1,4-octadecanediol (15) as main product. Compared to the earlier results in [20] the scope has been broadened to include methyl oleate and oleic acid, the yield of 15 has been increased and the amount of solvent was decreased to the small quantity of THF in the BH3 · THF solution (Nr. 3. to 5. in Table 1).

A related product with a carboxyl group and a hydroxy group at C-1 and C-4 of fatty acids can be obtained from unsaturated fatty acid derivatives that are converted at lower temperature with catalytic amounts of silver triflate to γ-lactones [22]. Furthermore, the catalyst [Ir(COE)2Cl]2/dppe allows at RT the isomerization of the double bond in methyl oleate to form in 45% yield the 18-pinacolylboronate of methyl octadecanoate [23]. With another efficient and robust system consisting of a nano-iridium (0) species and a carefully designed ligand methyl oleate has been converted at RT in 78% yield into a 18-boronate ester of methyl octadecanoate [24]. Further catalytic isomerizations of internal double bonds in unsaturated fatty acids to terminal positions have been reported with other transition metal complexes [25, 26].

3.3 Carbonylation of methyl 10-undecenoate (1b), methyl oleate (3a), and methyl linoleate (9)

With lithium DCME we could transform the hydroborated ester 1b into the symmetrical ketone 18. By variation of the reaction conditions, the yield could be optimized to 55% of 18 (Scheme 6a). For that purpose, the solvent THF was replaced by diglyme, furthermore freshly prepared lithium alkoxide was used for the metalation and the reaction time was extended to 8 h at RT.

image

Scheme 6. (a) Reaction of lithium dichloromethyl methyl ether with hydroborated 1b to the ketone 18; (b) reaction of lithium dichloromethyl methyl ether with hydroborated 3a to the isomeric ketones 19ac.

Download figure to PowerPoint

The structures of 18 and 19 are in accord with their IR, 1H NMR, 13C NMR, mass spectra, and their elemental analyses. The compounds 18 and 19 show a keto- as well as an ester-carbonyl group in the IR-spectrum. Additional support for the structure of 18 comes from the mass spectrum by the fragments with m/z (%) = 242 (100) and 184 (35) originating from Mc Lafferty rearrangements and m/z (%) = 227 (42) from a α-cleavage (see assignments in the mass spectrum in Section 'Dimethyl 12-oxotricosanedioate (18)').

The formation of ketone 18 (Scheme 6a) can be explained with a of 1,2-migration of two methyl undecanoates bound at C-11 to the boron atom. For trialkylboranes from short-chain alkenes and without a functional group, even the transfer of three alkyl groups to form a tertiary alcohol is reported [7].

In the DCME-reaction of hydroborated methyl oleate, we also obtained ketones (Scheme 6b). The three regioisomers of 19 could not be separated by flash chromatography or GC. However, the fragments in the mass spectrum arising from a twofold Mc Lafferty rearrangement strongly support the structures of the isomers. The isomer with a twofold C-9-connection of two methyl oleates at the ketocarbonyl group is confirmed by the fragments with m/z = 370 and 340, the connection at C-9 and C-10 by the fragments with m/z = 384, 354, and 326 and the twofold connection at C-10 by the fragments with m/z = 398, 340 (see assignment in the mass spectrum in Section 'Preparation of a mixture of dimethyl 9,11-dinonyl-10-oxononadecanedioate, dimethyl 10,12-dioctyl-11-oxoheneicosanedioate and dimethyl 9-nonyl-11-octyl-10-oxoeicosanedioate (19)').

As borane forms two isomeric adducts at C-9 and C-10 of 3a, the boron-carbon exchange produces three isomeric ketones 19a19c in together 80% yield. The keto-group is connected with the 9,9′-, 9,10′-, and 10,10′-carbon atoms of hydroborated 3a.

Methyl linoleate 9 reacts with the BH3 · THF complex presumably to the borolane E and/or F (Scheme 4) and the base initiated DCME-reaction should lead to cyclopentanones. However, in reproducible reactions we found the cyclopentene derivatives 20a and 20b as main products and the cyclopentanones 21a and 21b are obtained only as side products (Scheme 7).

image

Scheme 7. DCME-reaction of hydroborated methyl linoleate (9) to substituted cyclopentenes 20a and 20b and cyclopentanones 21a and 21b.

Download figure to PowerPoint

Maximal yields are obtained in diglyme, when after 45 min at −78°C the mixture is warmed up to RT and is stirred at this temperature for not longer than 2 h. The oxidation with hydrogen peroxide then leads to 70% of 20a and 20b and to 15% of 21a and 21b. A longer time at RT or a short warm-up for 5 min to 60°C leads even without oxidative work-up to 20a and 20b only.

In the mass spectrum of 20a and 20b, the structures are supported by intensive fragments at m/z = 223 and 237 (M+- CH3(CH2)5/4 and 151, 137 (M+- (CH2)7/8(CO2CH3). An evidence of the cyclopentene-unit is the homologous series CnH2n−3 represented by the ions with m/z = 67, 81, 95, 109. These data are confirmed by the corresponding fragments from the hydrogenated compounds of 20a and 20b (see assignments in Supporting Information Section 'Preparation of a mixture of 1-hexyl-3-(methyl 8-octanoate)-cyclopentene, 3-hexyl-1-(methyl 8-octanoate)-cyclopentene (20a), 1-(methyl 9-nonanoate)-3-pentyl-cyclopentene and 3-(methyl 9-nonanoate)-1-pentyl-cyclopentene (20b), 2-hexyl-5-(methyl 8-octanoate)-cyclopentanone (21a), and 2-(methyl 9-nonanoate)-5-pentyl-cyclopentanone (21b) (see Supporting Information Section 2.5.4)').

In compound 21, the ring strain increases the wave number of the keto-carbonyl group in the IR-spectrum, which coincides with the ester-carbonyl group at 1741 cm−1. Fragments in the mass spectrum at m/z = 240, 168, 84 and at m/z = 254, 154 and 84 due to Mc Lafferty rearrangements give a further support for the structure (see assignments in Supporting Information Section 'Preparation of a mixture of 1-hexyl-3-(methyl 8-octanoate)-cyclopentene, 3-hexyl-1-(methyl 8-octanoate)-cyclopentene (20a), 1-(methyl 9-nonanoate)-3-pentyl-cyclopentene and 3-(methyl 9-nonanoate)-1-pentyl-cyclopentene (20b), 2-hexyl-5-(methyl 8-octanoate)-cyclopentanone (21a), and 2-(methyl 9-nonanoate)-5-pentyl-cyclopentanone (21b) (see Supporting Information Section 2.5.4)').

A mechanism for the olefin formation has been proposed by Brown et al. in [27, 28], which can be applied to explain the formation of the cyclopentenes 20a and 20b (Scheme 8).

image

Scheme 8. Possible mechanism for the reaction of hydroborated methyl linoleate (E and F in Scheme 4) in the DCME reaction (see following text and [27, 28]).

Download figure to PowerPoint

Hydroborated methyl linoleate E, F (Scheme 4) reacts with deprotonated DCME to the cyclopentylborate A (the marking of the intermediates with the characters AD is only valid for Scheme 8). A subsequent 1,2-shift converts A to B, which dissociates to a carbenium ion in the betain C that undergoes a hydride shift to a carbenium ion in the betain D. This is transformed in a β-elimination of the boron group to the cyclopentene 20a and correspondingly to 20b.

In the DCME-reaction, the bulky base lithium 3-ethyl-3-pentanolate is used to prevent the addition of the base to the borane. In the reaction of the esters 1b and 3a, it was possible to replace this base by the cheaper lithium tert-butanolate [7, 29]. Applying otherwise identical reaction conditions, we found that the esters 1b and 3a gave nearly the same yields of 18 and 19, when lithium tert-butanolate was used as base.

In summary, reaction of organoboranes from the esters 1b and 3a with the carbonyl equivalent: DCME provides in one step 1,ω-diesters with an internal keto group. These products are useful for the conversion to polyesters or polyamides, they can be used for the cross-linking of alcohols, for reductive cyclizations to medium sized and large rings and additionally the rich chemistry of the ketocarbonyl group can be applied. Ester 9 affords two isomeric methyl nonadecenoates 20a and 20b in together 70% yield; they have the structure of methyl tetradecanoates with an embedded cyclopentene ring. These compounds have some similarity with naturally occurring cyclopentenyl fatty acids, which exhibit antibacterial activity and are potential precursors for pheromones and perfumes [30, 31].

4 Conclusions

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References
  8. Supporting Information

Hydroboration and conversion of the resulting alkylborane has been applied by us and others (see references) to unsaturated FAMEs. Hydroboration–oxidation affords in good to high yields hydroxylated FAMEs. A high regioselectivity is obtained with terminal double bonds as in esters 1a and 1b and in the formation of 1,4-diols with ester 9 due to an intramolecular hydroboration involving a boracyclopentane. Thermal isomerization of the alkylborane produces with ester 3a, oleic acid, and oleyl alcohol in good regioselectivity 1,4-octadecanediol in up to 70% yield, probably via a borinane as intermediate. With lithiated DCME and the boranes of esters 1b and 3a, we obtained in good yield ketones with two fatty esters connected to the carbonyl group. Ester 9 affords in this reaction two isomeric methyl nonadecenoates in 70% yield, they have the structure of a methyl tetradecanoate with an embedded cyclopentene ring. The compounds have some similarity with naturally occurring cyclopentenyl fatty acids.

Successful conversions with the reactive lithium reagent in the DCME reaction, good yields in hydroboration–oxidation and in thermal isomerizations to 1,4-diols with a very small quantity of solvent indicate useful applications of organoboranes for the synthesis of oleochemicals. The BF3 · THF complex is commercially available to prepare samples for testing, which facilitates further explorations.

Support of this work by the Minister of Research and Technology of FRG (Projekt: Intensivierung der Forschung auf dem Gebiet der Fettchemie), a Fellowship to T. L. by the Fonds der Chemischen Industrie and a gift of methyl linoleate from Henkel KGaA is gratefully acknowledged.

The authors have declared no conflict of interest.

References

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References
  8. Supporting Information
  • 1
    Pelter, A., Smith, K., in: Barton, D. H. R., Ollis, W. D. (Eds.), Comprehensive Organic Chemistry, Vol. 3, Pergamon Press, Oxford 1979, pp. 689940.
  • 2
    Brown, H. C. (Ed.), Hydroboration Benjamin/Cummings, Reading, Massachusetts 1979.
  • 3
    Burkhardt, E., Matos, K., Boron reagents in process chemistry: Excellent tools for selective reductions. Chem. Rev. 2006, 106, 26172650; there 2619–2621.
  • 4
    Brown, H. C., Zweifel, G., Organoboranes. III. Isomerization of organoboranes derived from the hydroboration of acyclic olefins. J. Am. Chem. Soc. 1966, 88, 14331439.
  • 5
    Brown, H. C., Bhatt, M. V., Organoboranes. IV. The displacement reaction with organoboranes derived from the hydroboration of branched-chain olefins. A contrathermodynamic isomerization of olefins. J. Am. Chem. Soc. 1966, 88, 14401447.
  • 6
    Brown, H. C., Organoborane-carbon monoxide reactions. A new versatile approach to the synthesis of carbon structures. Acc. Chem. Res. 1969, 2, 6572.
  • 7
    Brown, H. C., Carlson, B. A., The fast base-induced reaction of α,α-dichloromethyl methyl ether with organoboranes. A new general route from organoboranes to the corresponding carbon structures. J. Org. Chem. 1973, 38, 24222424.
  • 8
    Carlson, B. A., Brown, H. C., A remarkably simple route from 1,5-cyclooctadiene to bicyclo[3.3.1]nonan-9-one. Synthesis 1973, 776777.
  • 9
    Lorette, N. B., Brown, J. H., Jr., Use of acetone dimethyl acetal in preparation of methyl esters. J. Org. Chem. 1959, 24, 261262.
  • 10
    Brown, H. C., Organic Synthesis via Boranes, John Wiley, New York 1975, p. 18 and 241.
  • 11
    Brown, H. C., Rao, B. B. S., Hydroboration of olefins, a remarkably fast RT addition of diborane to olefins. J. Org. Chem. 1957, 22, 11361137.
  • 12
    Lucas, Th., Beiträge zur Veredelung Nachwachsender Rohstoffe durch Additionsreaktionen an Ungesättigte Fettstoffe, Ph.D. Thesis, University of Münster, Germany 1991, and this paper.
  • 13
    Fore, S. P., Bickford, W. G., Hydroboration of fats. I. Positional isomerism in the methyl oleate hydroboration reaction. J. Org. Chem. 1959, 24, 920922.
  • 14
    Jensen-Korte, U., Schäfer, H., J. Kolbe-Synthese von 29-tert-Butyldimethylsilyloxy-3,11-dimethyl-1-nonacosen, einer Schlüsselverbindung zur Darstellung eines optisch aktiven Sexuallockstoffes der Deutschen Hausschabe. Liebigs Ann. Chem. 1982, 15321542.
  • 15
    Brown, H. C., Keblys, K. A., Hydroboration, X. X. I. I., The reaction of unsaturated esters with diborane and disiamylborane. J. Am. Chem. Soc. 1964, 86, 17951801.
  • 16
    Prasad, A. S. B., Kanth, J. V. B., Perisamy, M., Convenient methods for the reduction of amides, nitriles, carboxylic esters, acids and hydroboration of alkenes using NaBH4/J2 system. Tetrahedron 1992, 48, 46234628.
  • 17
    Ishmuratov, G. Yu., Vydrina, V. A., Yakovleva, M. P., Nasibullina, G. V. et al., Hydroboration–oxidation of ricinoleic acid ester derivatives. Russ. J. Org. Chem. 2012, 48, 15091511.
  • 18
    Law, J. M. S., Setiadi, D. H., Chass, G. A., Csizmadia, I. G., Viskolcz, B., Flexibility of polyunsaturated fatty acid chains and peptide backbones: A comparative ab initio study. J. Phys. Chem. A 2005, 109, 520533.
  • 19
    Zweifel, G., Nagase, K., Brown, H. C., Hydroboration. XII. The hydroboration of dienes with diborane. J. Am. Chem. Soc. 1962, 84, 183189.
  • 20
    Logan, T. J., Thermal isomerization of hydroborated olefins. J. Org. Chem. 1961, 26, 36573660.
  • 21
    Sisido, K., Naruse, M., Saito, A., Utimo, K., Hydroboration and thermal isomerization of unsaturated alcohols. J. Org. Chem. 1972, 37, 733738.
  • 22
    Gooßen, L., Ohlmann, D. M., Dierker, M., Silver triflate-catalysed synthesis of γ-lactones from fatty acids. Green Chem. 2010, 12, 197200.
  • 23
    Ghebreyessus, K. Y., Angelici, R. J., Isomerizing-hydroboration of the monounsaturated fatty acid ester. Methyl oleate. Organometallics 2006, 25, 30403044.
  • 24
    Zhu, Y., Jang, S. H. A., Tham, Y. H., Algin, O. B. et al., An efficient and recyclable catalytic system comprising nano-iridium(0) and a pyridinium salt of nido-carboranyldiphosphine for the synthesis of one-dimensional boronate esters via hydroboration reaction. Organometallics 2012, 31, 25892596.
  • 25
    Schwartz, J., Labinger, J. A., Hydrozirconation: A new transition metal reagent for organic synthesis. Angew. Chem. Int. Ed. 1976, 15, 333340.
  • 26
    Biermann, U., Bornscheuer, U., Meier, M. A. R., Metzger, J. O., Schäfer, H. J., Oils and fats as renewable raw materials in chemistry. Angew. Chem. Int. Ed. 2011, 50, 38543871.
  • 27
    Katz, J. J., Carlson, B. A., Brown, H. C., Novel, A., α-Elimination in the mild thermal treatment of α-chloroboronic esters. A new route to olefins. J. Org. Chem. 1974, 39, 28172818.
  • 28
    Brown, H. C., Katz, J. J., Carlson, B. A., A remarkable rearrangement and elimination reaction in the solvolysis of tertiary α-chloroboronates under mild conditions. J. Org. Chem. 1975, 40, 813814.
  • 29
    Brown, H. C., Srebnik, M., Bakshi, R. K., Cole, T. E., Chiral synthesis via organoboranes. 10. Preparation of α-chiral acyclic ketones of exceptionally high enantiomeric excess from optically pure borinic esters. J. Am. Chem. Soc. 1987, 109, 54205426.
  • 30
    Spener, F., Tober, I., Focus on cyclopentenyl fatty acids. Eur. J. Lipid Sci. Technol. 1981, 83, 401402.
  • 31
    Abdel-Moety, E. A., Cyclopentenylfettsäuren als Ausgangsmaterial zur Gewinnung neuer Wirkstoffe. Fette Seifen Anstrichmittel 1981, 83, 6570.

Supporting Information

  1. Top of page
  2. Abstract
  3. 1 Introduction
  4. 2 Materials and methods
  5. 3 Results and discussion
  6. 4 Conclusions
  7. References
  8. Supporting Information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

FilenameFormatSizeDescription
ejlt201300220-sm-0001-SuppData.pdf102K

Supporting Data

Please note: Wiley Blackwell is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.