Synthesis, Characterization, and Functionalization of 1‐Boraphenalenes

Abstract 1‐Boraphenalenes have been synthesized by reaction of BBr3 with 1‐(aryl‐ethynyl)naphthalenes, 1‐ethynylnaphthalene, and 1‐(pent‐1‐yn‐1‐yl)naphthalene and they can be selectively functionalized at boron or carbon to form bench‐stable products. All of these 1‐boraphenalenes have LUMOs localized on the planar C12B core that are closely comparable in character to isoelectronic phenalenyl cations. In contrast to the comparable LUMOs, the aromatic stabilization of the C5B ring in 1‐boraphenalenes is dramatically lower than the C6 rings in phenalenyl cations. This is due to the occupied orbitals of π symmetry being less delocalised in the 1‐boraphenalenes.

Rachel J. Kahan, Daniel L. Crossley,J essica Cid, James E. Radcliffe,and Michael J. Ingleson* Abstract: 1-Boraphenalenes have been synthesized by reaction of BBr 3 with 1-(aryl-ethynyl)naphthalenes,1 -ethynylnaphthalene,a nd 1-(pent-1-yn-1-yl)naphthalene and they can be selectively functionalizeda tb oron or carbon to form benchstable products.A ll of these 1-boraphenalenes have LUMOs localized on the planar C 12 Bcore that are closely comparable in character to isoelectronic phenalenyl cations.I nc ontrast to the comparable LUMOs,the aromatic stabilization of the C 5 B ring in 1-boraphenalenes is dramatically lower than the C 6 rings in phenalenyl cations.This is due to the occupied orbitals of p symmetry being less delocalised in the 1-boraphenalenes.
Phenalenyl (1)i sa no pen-shell polyaromatic hydrocarbon (PAH) containing 13 carbon atoms and 13 p electrons. [1] Since Haddonss eminal report in 1975, [2] 1,a nd derivatives,h ave been of considerable interest for studying fundamental bonding phenomena (multi-centre bonding/s Vs. p dimerisation), [2,3] and for ar ange of applications (organic semiconductors,s pin memory,e lectrode materials). [4] Then onbonding SOMO of phenalenyl is key to its unique properties (Figure 1), and phenalenyls display amphoteric redox behaviour,w ith oxidation furnishing a1 2p electron cation and reduction a14p electron anion. [1] Thekey properties of 1 can be modulated by functionalisation of the periphery or by incorporation of heteroatoms. [5,6] While the incorporation of N, O, and Si nto phenalenyls is well-documented, [1,5,7] there are only two reports incorporating boron to the best of our knowledge,and in both boron is co-doped with nitrogen (for example, 2 and 3;F igure 1). [8][9][10] However,c omputational studies on boraphenalenes have indicated potentially interesting molecular and bulk properties. [11] While notable work on di-and tetrabenzophenalenes containing boron (with and without co-doping with N/O/S) has been reported, [12] these more extended compounds have distinct electronic structures and thus are not directly comparable to the phenalenyls.E ven 3 which has at ricyclic core isoelectronic to 1 + has some LUMO character located on the exocyclica romatic groups and is not completely planar (Supporting Information, Figure S8), and therefore is distinct to 1 + .T og enerate ab oron-doped PA Hm ore comparable to the phenalenyl cation, the analogue should be planar, be isoelectronic to 1 + ,a nd have aL UMO that is closely comparable in character to 1 + .N otably our calculations indicate this is the case for the 1-boraphenalenes (for example,F igure 1, bottom right).
Recently,anumber of routes have been developed to synthesize B-doped PA Hs. [6,13] In this area the combination of alkyne borylative cyclisation [14] and intramolecular boron-Friedel Crafts enabled the formation of boracycles (Scheme 1, top). [15] Compound B and derivatives (Scheme 1, top right) do contain a1 -boraphenalene (C 12 B) subunit. However,the additional fused rings present in B leads to nonplanarity within the C 12 Bs ubunit and LUMOs that are delocalised beyond the tricyclic subunit. Herein we report the serendipitous synthesis of aplanar 1-boraphenalene containing no additional annulation. This enabled the subsequent  [8,9,12] Bottom:boraphenalene isomers. The LUMOs are calculated at the B3LYP/6-311G(d,p) level (0.05 isovalue). Scheme 1. Top: the combinedb orylative cyclisation/intramolecular S E Ar reaction. Bottom:this work, to form 1,2-and 1,3-dibromo-1boraphenalenes. development of as imple route to 1-boraphenalenes,w hich can be readily functionalized at varying positions.C alculations revealed comparable LUMOs for these 1-boraphenalenes and isoelectronic 1 + ,however, the occupied p orbitals of the 1-boraphenalenes are distinct in character to 1 + ,l eading to lower aromatic stabilization of the C 5 Bring.
Previously,t he cyclisation of 2-(phenylethynyl)-1,1'biphenyl led to the 9-borylated phenanthrene,c ompound A (Scheme 1, top left). [14] However,o na ttempting the borylative cyclisation of 4a the expected compound, C (Scheme 2, left), was not observed. Instead the addition of BBr 3 in orthodichlorobenzene (o-DCB) and heating to 140 8 8Cl ed on workup to the 1-hydroxy-1-boraphenalene, 6a (Scheme 2, right). Along with 6a,t he formation of products from HBr addition to the triple bond of 4a were observed (HBr is the by-product from the S E Ar reaction). Repeating the reaction in the presence of 2,4,6-tri-tert-butylpyridine (TBP) and using excess BBr 3 (as [H-TBP][BBr 4 ]i sn ow the ultimate byproduct from S E Ar) prevents this side reaction and leads to good yields of 6a.A nalogous reactivity is observed when terphenyl is replaced by p-tolyl and 6c also was isolated as ab ench stable solid. In contrast, the formation of 5b was complicated by the 6-endo-dig cyclisation to form the 9borylated phenanthrene (analogous to C)w hich is competitive in this case,with a1:1 ratio of products formed (see the Supporting Information). For 4a,t he 6-endo-dig cyclisation presumably is disfavoured because of the greater steric bulk around the alkyne giving rise exclusively to 5a (while the 6endo-dig cyclisation is not possible with 4c). The 1 HNMR spectrum of 6a has ac haracteristic broad singlet at d = 5.75 ppm for the BÀOH, which is at 5.97 ppm for 6b and 6.11 ppm for 6c.T he 11 BNMR resonances for 6a À6c are typical for boracyclic borinic acids (d 11B = 37-38 ppm).
Thediffering reactivity observed for 4a-c compared with 2-(phenylethynyl)-1,1'-biphenyl (which forms A)presumably arises because the naphthyl moiety can intercept the vinyl carbocation in a5-endo-dig cyclisation (step B, Scheme 3). A plausible mechanism involving tautomerisation and B À C bond cleavage can be proposed (step C) followed by a1 ,2migration of bromide.L astly,a ni ntramolecular S E Ar of the proximal naphthalene moiety can occur (step E) to form the six membered boracycle of the 1,2-dibromo-1-boraphenalenes 5a-c.Arelated trapping of avinyl cation by aproximal naphthalene during the borylative cyclisation of 1,2-bis(1naphthylalkynyl)benzene with B(C 6 F 5 ) 3 has been reported. [16] Theproposed mechanism has an aryl group to stabilise the vinyl cation formed after step A. Replacement of this aryl with an alkyl or hydrogen would disfavour the formation of this vinyl cation. Therefore we anticipated that reaction of the terminal alkyne,1 -ethynylnaphthalene, 7a,w ith more than 1equiv BBr 3 would instead result in trans-haloboration [17] to form 8 (Scheme 4) which positions av inylBBr 2 group for intramolecular S E Ar (akin to step E, Scheme 3) to form 1,3dibromo-1-boraphenalene (9). Thus 7a and excess BBr 3 were combined and NMR spectroscopy indicated the quantitative   6a and 6c with ellipsoids set at 50 % probability;hydrogen atoms (except BÀOH) have been omitted for clarity. [20] Scheme 3. Proposed mechanism for the formation of 6a-c.
formation of the haloborated product 8 within minutes of BBr 3 addition (d 11B = 49.9 ppm). In solution, 8 slowly transforms to 9 over 48 ha t2 08 8C. Compound 9 forms quantitatively (by in situ NMR spectroscopy), and crystallises from the o-DCB solvent during the reaction. Thes olid-state structure of 9 (Scheme 4, bottom right) has positional disorder of B1 and C2, and am irror plane along the C1-C4-C5 axis,precluding detailed discussion of any metrics.
Notably,o ne xposure to wet solvent, the borinic acid derived from 9 is not observed;i nstead protodeboronation occurs.This can be used to transform 9 into 10 a by addition of Hünigs base/pinacol (Scheme 4, top right), with 10 a forming via protodeboronation and an E2 elimination from the haloalkene.C omparable reactivity was observed for 1-(pent-1-yn-1-yl)naphthalene (7b)t of urnish 10 b.T he identification of 10 a was confirmed by X-ray diffraction studies, which revealed distorted C1-C2-C3 angles (174.1(3)8 8), C2-C3-CtA (where CtA = centroid of ring A, 175.60(17)8 8)a nd B1-C11-CtB (168.33(16)8 8). 10 a/b can be synthesised directly from 7a/b with no isolation of intermediates,a nd are the first reported 8-borylated-1-alkynyl naphthalenes to the best of our knowledge.T oconfirm the formation of 9 proceeds via 8, as olution of pinacol was added after 20 minutes to the mixture derived from 7a/BBr 3 to form 8-BPin.T his led to quantitative conversion to 8-BPin but it was isolated in only 30 %yield by crystallisation (8-BPin decomposes under basic conditions to furnish 7a and was unstable on silica).
1-Boraphenalene derivatives that are bench stable and contain exocyclicb oron substituents that do not p donate to boron were next targeted. 6c reacts with MesMgBr to form two species in ac irca 9:1r atio (Scheme 5, top). Them ajor product (11)i sf unctionalised at carbon, leaving the borinic acid moiety intact, as indicated by aresonance at 6.04 ppm in the 1 HNMR spectrum for the B À OH. Theminor product (12) is functionalised at boron, leaving the vinyl bromide group intact. In contrast, functionalisation of the bromo congener 5c with MesMgBr results predominantly in the formation of 12 along with minor unidentified species,w ith the formation of 11 not observed. 12 is bench stable and can be isolated in 55 % yield with a d 11B of 58.0 ppm. Increasing the ratio of MesMgBr: 5c/6c to more than 10:1 did not result in any significant double arylation of either compound at 20 8 8Corat raised temperatures.F urthermore,t he reaction of 12 in as ealed tube with excess MesMgBr at 100 8 8Cr esulted in the formation of the di-arylated compound 13 as only the minor product with 14 the major product (Scheme 5, middle). The formation of 14 presumably occurs by Grignard metathesis generating the Grignard reagent derived from 12 which upon aqueous work up is hydrolysed to 14.A so bserved for 5c, 9 can be readily functionalised at boron, but in this case superior yields were obtained using ZnMes 2 ,w hich afforded bench stable 15 in 88 %yield. Despite repeated attempts,only compound 12 was amenable to crystallisation in our hands. Thes olid-state structure of 12 revealed ap lanar 1-boraphenalene unit and effectively orthogonal aryl groups.T he bond metrics in the boracycle were closely comparable to those found in 6a and 6c,i ncluding as hort C11ÀC12 distance of 1.360 (4) .
Compounds 5c, 9,a nd 12-15 were calculated at the B3LYP/6-311G(d,p) level. All six calculated structures have C = Cd istances for the C1 À C2 unit (numbering as per Scheme 4, bottom right) between 1.35-1.37 comparable to those found in the solid-state structures of 6a,6c ,a nd 12.  oriented effectively orthogonal in each compound. Most notably,the LUMO for each compound is effectively identical (Supporting Information, Figure S7), being located on the C 12 Bc ore and being predominantly non-bonding in nature, with zero orbital coefficients on exocyclic groups in contrast to the LUMO of isoelectronic 3.Thus the replacement of {CÀ H} + in 1 + for {BÀR} has minimal effect on the nature of this frontier orbital. This is notable as the non-bonding character of this frontier orbital is crucial for the unique redoxproperties of phenalenyls. [1] In contrast to the LUMO,t he occupied p orbitals are distinct for the 1-boraphenalenes compared to 1 + .F or 5c and 9 the HOMO is principally located on rings Aand C(Scheme 5; Supporting Information, Figure S7) with some contribution from the exocyclicb romines.For the B-Mes substituted compounds the HOMO and HOMOÀ1are both located on the mesityl group (Supporting Information, Figure S7), but the occupied p orbitals on the C 12 Bc ore are also more localized than in in 1 + where the highest energy occupied p orbitals are delocalized throughout the phenalenyl C 13 core (Supporting Information, Figure S6).
Nucleus-independent chemical shifts (NICS) were determined for the reported compounds,the perprotio 1-boraphenalene (C 12 BH 9 )and the isoelectronic analogue 1 + (Supporting Information, Table S3). Fora ll the 1-boraphenalenes the boracycle (ring C) is effectively non-aromatic (NICS(1) values between À0.3 and À1.6) while the naphthyl unit has significant aromaticity (NICS(1) values for rings Aand Bare between À10.4 and À9.6). This is distinct to the more symmetric aromatic structure of D 3h 1 + (NICS(1) À7.8) and to 3 (which is dominated by as ingle highly aromatic Clar sextet). Therefore while the LUMO of the 1-boraphenalenes and 1 + are closely comparable in character,t he overall p electronic structures are different owing to the lower symmetry on incorporating boron and the higher energy of the Bp p orbital relative to the Cp p orbitals (Supporting Information, Figures S6, S7). To estimate the effect of this on the aromatic stabilization of the C 5 Br ing in 1-boraphenalenes relative to isoelectronic carbocations the isomerization method was used (calculations at the B3LYP/6-311G(d,p) level, Eq. (1) and (2) in Scheme 6). [18] This revealed that this 1-boraphenalene has am uch lower aromatic stabilization energy than the isoelectronic carbocation congener.T he lower aromatic stabilization energy for the 1-boraphenalenes was supported by an isodesmic reaction (Eq. (3) in Scheme 6), [19] which confirmed the greater aromatic stability of phenalenyl cations (> 12 kcal mol À1 ). While it has been demonstrated numerous times that the LUMOs of Bd oped PA Hs and carbocation analogues are often similar in nature, [6] to fully understand the properties of these isoelectronic pairs consideration of the occupied p orbitals is also essential.
With an understanding of the electronic structure of 12-15 in hand, the propensity of these to undergo redox was investigated. Thef irst reduction wave is reversible (Table 1; Supporting Information, Figures S2-S5) and its potential mirrors the trend observed computationally with 12 and 15 containing one inductively withdrawing bromine substituent having aless negative reduction potential than 13 and 14.F or 12-14,t he second reduction event is significantly more negative than the first. This separation and the reversible nature of the first reduction wave indicates that 13 p electron radical anions should be accessible.H owever,a ttempts to date to chemically reduce 13 and 14 have led to either complex diamagnetic mixtures (with 14)o rN MR silent product(s) that have frustrated isolation (with 13).
In conclusion, the first boron-only doped phenalenes are reported, that are available in one step from commercially available precursors (for 9), or in two steps in all other cases. These can be selectively functionalized to provide compounds possessing good bench stability.N otably,t he nature of the LUMO in these 1-boraphenalenes is closely comparable to that in the extensively studied all carbon phenalenyl cation analogues.However, the 1-boraphenalenes have significantly lower aromatic stabilization of the C 5 Bring than observed in each ring in the D 3h phenalenyl cations due to the less delocalized nature of the occupied orbitals of p symmetry in the 1-boraphenalenes.F or the boraphenalenes containing BÀ Mes,areversible reduction wave is observed well separated from the second reduction process,i ndicating that the 13 pelectron radical anion, analogous to the phenalenyl radical, is accessible.F urther studies into generating 1-boraphenalenenes,particularly examples enabling access to isolable 13 pelectron radical anions,are currently ongoing. Scheme 6. Electronic energies (kcal mol À1 )f or arange of isodesmic reactions.