Recent Advances in the Chemistry of Heavier Group 14 Enolates

Abstract Recently heavier Group 14 enolates showed their importance and applicability in a broad range of chemical transformations. They were found to be key intermediates during the synthesis of photoinitiators, as well as during the formation of complex silicon frameworks. This Minireview presents general strategies towards the synthesis of heavier Group 14 enolates (HG 14 enolates). Structural properties, as well as their spectroscopic behavior are outlined. This study may aid future development in this research area.


Introduction
The chemistry of metal enolates is thoroughly investigated and understood to av ery high degree. [1] Moreover,t he classical aldol reaction is one of the most important biosynthetic tools for life on earth. [2,3] Although the first report on the synthesis of heavier Group 14 enolates by Bravo-Zhivotovskii and coworkers was in 1989, [4] the synthesis and characterizations of these derivatives is still ac hallenging endeavor.I n2 003 Ottosson succeeded in isolatingasilenolate, which had ah igh thermal stability,i no rder to perform ac omplete structural analysis. [5] Historically speaking the preparation of HG 14 enolates was mainlyt riggered by the need of substrates for spectroscopic studies on thesecompounds. Quite recently the Stueger group successfully isolated the first tetraacyl substituted germanes and stannanes,a nd showedt heir ability to serve as long-wavelength photoinitiatorsw ith superior potential. [6,7] During these reactions the key intermediates are HG 14 enolates, which allows as traightforward access to these highly desirable compounds. Another milestonei nt he chemistry of HG 14 enolates was the reporto ft he first sila-aldol reaction, which emphasizes the tight connection between silicon and carbon chemistry. [8] This new synthetic strategy must be considered as ap owerful alternative to standard coupling techniques, such as the Wurtz reaction, [9] hydrosilylation, [10] as wella st ransitionmetal-catalyzed silicon-carbon coupling reactions. [11] Moreover, this novel synthetic methodp rovides as traightforward access to structurallyc omplex silicon frameworks, in quantitative yield. With these findings, HG 14 enolates demonstrate their importance and applicability in ab road range of chemical transformations.
HG 14 enolates can exist in two possible isomeric structures (Scheme 1). 1-HG 14 enolates are still undiscovered, due to the low stability of am etal-carbon double bond. [12] With regard to 2-HG 14 enolates, numerousr eports on this compound class exist. As for metal enolates, two resonances tructures for 2-HG 14 enolates are possible:i nt he enol form (I), the negative charge is primarily located on the oxygen atom, whilst in the keto form (II) the negative charge resides predominantly on the silicon atom (Scheme 1). [13,14] The dominant structure of metal enolates is generally the enol form and preferably occursi ns olid state, asw ell as in solution. [14] HG 14 enolates show as ignificantly different resonance behavior. The position of the equilibrium is strongly influenced by the chosen alkali metal, the solvent system, as well as the substituent at the carbonyl-moiety.
In this Minireview,w ef irst present the most important strategies reported towards the synthesis of heavierG roup 14 enolates with ap articular emphasis on structural assignments and spectroscopic behavior.T hen we focus on the recenta dvances in this field and give ab rief outlook.

Lithium-silenolates
The first synthesized silenolates were lithium-silenolates by the group of Bravo-Zhivotovskii. [4] They reported on the synthesis of silenolates and introduced the generals trategy of reacting a germyl-lithium reagent with an acylsilane in order to generate silenolates (Scheme 2). These silenolates werefound to be relatively unstable, with ah alf-life time of approximately 12 h. However,t he decomposition products resulting from 1 were not identified.
In af ollow up paper,A peloig and Bravo-Zhivotovskii succeeded in the identification of ap ossible degradation process of 1. [15] They found that an excess of the used base (e.g.,t wofold excess) leads to the formation of a1:2 mixtureo ft he trisilacyclobutane 2 ando f( adamantoyl)adamantylcarbinol 3. (Scheme 3. Note:S tirring for 48 ha tr oom temperature followed by aqueous work-up). The mechanism is rather complex and involves three silenolate moieties, as well as aP eterson elimination in order to obtain compound 2 and 3.F or the complete mechanism the readerisreferred to the originalpublication. [15] J. Ohshitaa nd M. Ishikawa expanded this strategy andi ntroduced more precursor molecules, as well as the use of different lithium reagents. [16,17] Additionally,t hey extensively explored the chemistry of their synthesized lithium-silenolates. During the course of their studies concerning the chemicalr eactivity of acylpolysilanes with organolithium reagents, they found that the reaction of acylpolysilanes with silyllithium reagents resulted in the formation of lithium-silenolates 4a-d in solution( Scheme 4). These lithium silenolates are thermally instable. 4a is moderately stable at room temperature. 4b undergoes af ast degradation even at temperature below À80 8C. 4c,d are more stable than 4b,b ut undergo uncharacterized degradation processesa tr oom temperature. Therefore, all chemicalmanipulations were performedi ns itu.
Metal enolates are known to react with chlorosilanesu nder the formationo fasilyl enol ether. [1,18] Ohshita and Ishikawa furthers tudied the reactivity of 4a-d towards the reaction with chlorosilanes. Interestingly,t he reactions of lithium-silenolates 4a-d with chlorosilanesu nderwent two different pathways. The chosen pathwayi sd ependent on the substituent at the carbonyl group. 4a,b,w hich bear an aryl substituent at the carbonyl group, form the Brook-types ilenes 9a,b.O nt he other hand 4c,d with alkyl groups at the carbonyl group form the acylsilanes 10 a,b (Scheme6). The cause for the different reactivity wasd etermined by NMR spectroscopy. The negative charge in 4c,d is moderately localized on the central silicon atoms, whereas in 4a,b the negative charge is effectively delocalized over the silicon atoms and carbonyl groups.T hisr eactivity wasa lso found by other groups, which will be discussed in more detail later in this review.
Furthermore, they also demonstrated that oxidative coupling of lithium-silenolates with palladium(II) chloride leads to the formation of bis(acyl)polysilanes 11 a,c,d. [19] This was the first example of polysilanes with two silicon-acyl bonds on the adjacent silicon atoms( Scheme 7). Moreover,t hey reacted their lithium-silenolates with variousa cid chlorides and obtained the first examples of di-and tetraacylsilanes 12 a-e and 13 a,b (Scheme7). [20] Recently Apeloig and co-workersu sed ad ifferent approach towardst he generation of lithium-silenolates.T hey synthesized them by metal-halogen exchange between silyl-lithium reagents (in excess) and bromo-acylsilanes in hexane. With this methodology they were able to isolate their silenolates and to obtain X-ray moleculars tructures of thef irst enol-form silenolates 14 a,b (Scheme 8). [21] The X-ray structure is depicted in Figure 1a nd will be discussed Section 5.
In af ollow up paper this group also investigated the reactivity of their isolateds ilenolates 14 a,b.U pon addition of ap olar solvents uch as THF an interesting rearrangement occurred and lithium-silenides were formed. [22] For the complete mechanism the readeri sr eferred to the original publication.

Potassium-silenolates
Av ery importantm ilestone in the chemistry of HG 14 enolates was the introduction of potassium as counterion by Ottosson and co-workers. They reacted tris(trimethylsilyl)acylsilane with potassium tert-butoxide (KOtBu) and observed the quantitative formation of ap otassium-silenolate (Scheme 9). [5] In contrastt o the more difficult preparation of lithium-silenolates,t hese silenolatesa re found to be thermodynamically more stable, and thus could be stored under an inert atmosphere and ambient temperature over af ew months without degradation. Thiss tability allowed Ottosson to obtain the first X-ray structure of a silenolate reported in the literature. The molecular structure is depicted in Figure 1and will be discussed in Section5.
They also briefly investigated the reactivity of 15.T he trapping of 15 with MeI leads to the formation of the silicon-methylatedp roduct 16.T he reaction of 15 with 2,3-dimethyl-1,3butadiene yields exclusively to the formation of the [4+ +2] adduct 17 (Scheme 10). Ohshitaa nd Ishikawa found the same reactivity for their lithium-silenolates. [16] Later on, the Stueger group demonstrated the possibility of synthesizing and characterizing cyclic silenolates 18 a-c. [23] 18 a-c wereo btainedw ith remarkable selectivity by the addition of 1.05 equiv of KOtBu to the corresponding acylcyclohexasilanes either in THF or in toluene solution in the presence of 1.05 equivo f [ 18]-crown-6 ([18]-cr-6)a tÀ50 8C( Scheme 11). In the absence of air these cyclic potassium-silenolates have the same stability as the acyclic derivatives. However, 18 a-c decompose immediatelyt ou ncharacterized material upon exposure to the atmosphere, or the attempted removal of the solvent and other volatile components in vacuum. Nevertheless, they obtained crystal structures for 18 b and 18 c.
Furthermore, they studied the reactivity of 18 a-c towards the reaction with chlorosilanesa nd MeI. They found the same reactivity as for acyclic lithium silenolates (see Scheme 12). The reaction of 18 a,b with chlorosilanes allowed the formation of exocyclics ilenes 19 a,b.T he reaction of 18 c with chlorosilanes give rise to the formation of acylcyclohexasilane 19 c.I nt he reaction of 18 a,c with MeI, same reactivities in terms of reaction sites weref ound. In both cases, alkylation of the silicon atom was observed in nearly quantitative yields.
In afollow up paper they also examined the thermal stability of 18 a,c. [24] As elective rearrangement cascade was found when 18 a was stirred for 5hat 60 8Cl eading to the formation of highly interesting carbanion 2-oxahexasilabicyclo[3.2.1]octan-8-ide 21.T his observation indicates that 18 a was only the kinetic product, which thermodynamically rearranged through am ild, selective silyl-migration cascade to the bicyclic carban-Scheme7.Reactivity of 4a,c,d with acid chlorides.
Scheme8.Synthesis of lithium-silenolates. Chem www.chemeurj.org ion 21 (Scheme13). Thisc ascade represents the first example of an intramolecular Sila-Petersonr eaction, where the formed silene is trapped by the present oxygen nucleophile intramolecularly( For the complete mechanism the readeri sr eferred to the originalp ublication). [24] Upon the addition of MeI to a freshly prepared toluene solution of 21,the corresponding methylated bicyclic adduct 22 was formed in the diastereomeric ratio of endo:exo = 2:1.
Interestingly the same reaction set-up for 18 c led to complete degradation of 18 c to uncharacterized material. They assumed that, in the case of alkyl-substituted systems, the negative charge could not be distributed in the same way as in compound 21 and the primarily formed carbanion reacted further under the applied reaction conditions.

Lithium-germenolates
Again the first synthesized germenolate was al ithium-germenolate by the group of Bravo-Zhivotovskii. [4] They reported briefly on the formation of this lithium-germenolate 23 from the reactiono fa na cylgermane with Et 3 GeLi (Scheme14). Spectroscopic and structurald ata of 23 were not given.

Potassium-germenolates
Potassium-germenolates were found to be crucial intermediates for the synthesis of tetraacylgermanes. [6] To verify this assumption tris(trimethylsilyl)acylgermane 24 was reactedw ith 1.05 equiv. of KOtBu (see Scheme 15). Aq uantitative formation of the corresponding germenolate was observed. The molecular structure of 24,a sd etermined by single-crystal X-ray crystallography,a nd the complete set of consistentN MR data can be found in Section 5 Our group also succeeded in the isolation and characterization of the first cyclic germenolates.T herefore, the corresponding acyl-1,4-digermacyclohexasilanes were reacted with 1.05 equiv. of KOtBu at À70 8C( see Scheme 16). [25] The stability of thesec yclic germenolates is comparable to their silicon homologs. After addition of [18]-cr-6 in toluene, we were able to grow crystalso ft he 1:1[ 18]-cr-6 adducts of 25 b and 25 c, which were suitable for single-crystal X-ray crystallography (see Section5).
The reactivity of 25 a-c versus chlorosilanes parallels that observed earlierf or silenolates. Thus, 25 c,w ith an alkyl group attached to the carbonyl moiety,r eactedw ith Me 3 SiCl at 0 8C under formation of the correspondingc yclic acylgermane 26 c, while the aryl-substituted compounds 25 a,b,u nder the same conditions, exclusively afforded the O-silylated germenes 26 a,b (Scheme 17).
Furthermore our group demonstrated the possibility to generate the first examples of dianionic germenolates 27 a,b, which were synthesized by the reaction of the corresponding cyclic acylgermanes with 2.1 equiv.o fK O tBu (see Scheme 18). After addition of [18]-cr-6 in toluene we were able to grow crystalso ft he 1:2[ 18]-cr-6 adducts of 27 a and 27 b,w hich were suitable for single-crystal X-ray crystallography (see Section 5). [26] Scheme12. Reactivity of 18 a-c with chlorosilanes and MeI.

Stannenolates
No stable stannenolates are reported so far.R ecently our group publishedapaper on previously unknown tetraacylstannanes. During their formation,s tannenolates were found to be crucial intermediates. [7]

Characterization and Bonding in Group 14 Enolates
In the following section, the spectroscopicb ehavior,a sw ell as important structural features of HG 14 enolates will be discussed. Moreover,ashort summary of theoretical studies concerning HG 14 enolates will be given.

NMR spectroscopy
The chemical shift of the centralm etal atom for HG 14 enolates is strongly influenced by the dominant resonance structure, the solvent as well as the used counter ion. The 29 Si NMR chemicals hifts of the central silicon atom of silenolates, which adopt the keto resonance structure, weref ound in the region from d = À59 to d = À93 ppm. The measured 29 Si NMR shifts are in the typical range for silyl anions. [27] Theu se of crown ethers leads to an even stronger high field shifto ft he central silicon atoms ( 29 Si chemical shifts of the Si atom of 15 are d = À78.7 ppm in THF and d = À93.8 ppm when [18]-cr-6 is present). The measured 13 CNMR shifts of the carbonyl Ca toms of HG 14 enolates adopting the keto resonance structure were found in the regiono fd = 262 to d = 274 ppm, which are typi-cal for sp 2 hybridization,a nd close to the ones measured for the corresponding acyl-derivatives. The measured 29 Si NMR shifts for the central silicon atom of the silenolate, which adopts the enol form is d = 8.4 ppm. This is significantly downfield shifted compared to keto derivatives, and in the region for 29 Si shifts of silicon atoms of aS i =Cd ouble bond. No 13 CNMR spectrum for this compound is reported.T he exact valuesf or the reportedH G1 4e nolates are depicted in Table 1.

UV/Vis spectroscopy
To gain more insighti nto the electronic nature of silenolates, as well as of germenolates, our group recorded the absorption spectra of 18 a,c and 25 a-c and assigned the longest wavelengtha bsorption through time-dependent density functional in combinationw ith the polarizable continuumm odel (TDDFT-PCM) calculationsa tt he B3LYP/6-31 + G(d,p) level. All HG 14 enolates exhibit an intense absorption maximum between 400 and 500 nm. According to calculations, these bands are unequivocally assigned to aH OMO!LUMO or aH OMO! LUMO + 1t ransition. The HOMOs mainly correspond to the p z orbitalo ft he metal atom with little variation in shape and energy.U pon excitation, electron density is displaced into the p*-orbital of the carbonyl moiety (LUMO or LUMO + 1). The LUMOs of the aryl-substituted speciess howeda dditionally conjugation of the carbonyl and the aromatic p-systems, which results in the observed bathochromic shifts of the corresponding absorption bands.T he obtained experimental and computational data are summarized in Table 2. Therefore, silenolates 18 a,c and 25 a-c are best described as acyl metal anions (keto form in Scheme 1) in solution, irrespective of the nature of the Rg roup attached to the carbonyl moiety.I nc ontrast to that, Apeloig also calculated the HOMO orbitals for their silenolates with enol character and found that they have pronounced p-character. [21]

Structural assignments
All silenolates adoptingt he keto resonance structure, which were characterized by single-crystal X-ray crystallography,h ave Scheme17. Reactivity of 25 a-c with chlorosilanes.
Scheme18. Synthesis of cyclic dianionic potassium-germenolates.  All silenolates adopting the enol resonance structure, which were characterized by single-crystal X-ray crystallography,h ave lithium as counter ion. Generally,they have aplanar central silicon atom and the central SiÀCb ond shows double bond character.One example is depictedi nF igure 1.
All germenolates, whichw ere characterizedb ys ingle-crystal X-ray crystallography, have potassium as counter ion and adopt the keto resonance structure. Generally,t hey have a strong pyramidal central germanium atom with an elongated Ge-C singleb ond. One example of these compoundsi sd epicted in Figure 1. Selected bond lengths d []a nd selected sum of valence angles SaGe1 and SaC1 [deg] for the germenolates are depicted in Ta ble 4.

Theoretical studies
The keto-enol equilibrium in metal silenolates has also been investigated computationally.A peloig et al. found that in nonsolvating media the enol-form of the silenolate dominates. The effective solvation of the cation, for example, by crowne thers, results in the formation of the keto-form. [21] Additionally,O ttosson et al. revealed in ar elateds tudy that coordinationo fasolvated metal ion to the oxygen atom in silenolates results in shorter SiÀCb ond lengths, as maller degree of pyramidalization around the central silicon atom, andalower charge difference at the carbon and at the silicon atom (Dq(SiC)) compared to the nakeds ilenolate. [28]

Recent Advances
The synthesis and characterization of HG 14 enolates was mainlyt riggered by fundamentali nvestigations in the field of main group chemistry. This changed drastically when the Stueger group found an elegant synthetic protocolt owards the synthesis of the first tetraacyl substituted germanes and stannanes.M oreover,t hese compounds showed their ability to serve as long-wavelength photoinitiators with superior potential. [6,7,29] During these reactions, the key intermediates are HG 14 enolates, which allows as traightforwarda ccess to these highly desirable compounds. Another importantd iscovery in the chemistry of HG 14 enolates was the report of the first silaaldol reaction. [8] In the following sectiono ft his review these recenta dvances will be summarized.

Tetraacylgermanes and -stannanes as photoinitiators
Photoinitiators (PIs) playavery important role in aw ide range of industrial processes (coatings,a dhesives, dental filling materials, and the manufacture of 3D objects). Among the promising PI systems, tetraacylgermanesa nd tetraacylstannanes can act as suitable radicalp recursors generating acyl-a nd metalcentered radicals upon irradiation, which add very rapidly to double bondso fv ariousm onomers. [30] Moreover,t hey offer the advantages of significantly redshifted absorption bands and reduced toxicity compared to the frequently applied phosphorus-based PIs. [31] Furthermore, the synthetic protocoli sv ery robust, one-pot, and outperforms the methods in the synthesis of state-of-the-art photoinitiators. [32] The Stueger group discovered that the reactiono fapotassium germanide and stannide

Sila-aldol chemistry
The classical aldol reaction, with its power in the reversible formation of carbon-carbonb onds, is one of the most important organic reactiont ypes. [2,33] The aldol reaction for silicon-based compounds, were unknown until our group reported on the first sila-aldol reaction. This reactionp rovides as traightforward access to structurally complex silicon frameworks. The starting point for the development of this transformation was the reaction of 1,4-bis-(acyl)cyclo-hexasilane 28 with KOtBu. As expected, the base abstracted one SiMe 3 group to give monosilenolate 29. 29 immediately reacted further in the proposed silaaldol reaction. Interestingly,the silicon-carbon addition product 30 was not directly observed by NMR spectroscopy,a st his reaction selectively led to the formation of the bicyclic carbanion 31 in an ensuing anionic rearrangementc ascade (Scheme 20). This transformation introduces ap ioneering strategy for the formationo fs ilicon-carbon bonds by establishing af urther link between the two relatedf ields of silicon and carbon chemistry. The sila-aldolr eactionp rovides as ignificant addition to the synthetic methods available for the formation of an ew class of silicon-based compounds.

Conclusions and Outlook
Given that the chemistry of HG 14 enolates is relativelyy oung, we have summarized in ac oncise and ac omplete way,t he most important synthetic strategiest owards this compound class. We have also shownt heir reactivity towards selected exampleso fe lectrophiles and trapping agents. Furthermore,w e have summarized the spectroscopic behavior and the structural data for HG 14 enolates. Inspired by the promising potential of tetraacylgermanes and -stannanes acting as long-wavelength photoinitiators, we have highlighted their synthesis, where HG 14 enolates are crucial intermediates during their formation.M ore research towards the chemistry of this new photoinitiator class is likelyt o emerge soon.
The sila-aldolc hemistry has been shown to be highly efficient in the formation of complex silicon framework. This new synthetic strategy can be ap owerful alternative to standard coupling techniques, such as the Wurtz reaction, hydrosilylation, as well as transition-metal-catalyzed silicon-carbon coupling reactions.