Borabenzene derivatives. 22. Synthesis of boratabenzene salts from 2,4-pentadienylboranes. Structure of [NMe3Ph][C5H5BMe]

Article abstract of DOI:10.1021/om00001a064

Borylation of potassium pentadienide with BCl(NR₂)₂ and subsequent alcoholysis produce 2,4-pentadienylboranes C₅H₇B(NR₂)₂ (4a-c: R = Me, Et, Prⁱ), C₅H₇B(OR)₂ (4e,f: R = Et, Bu^t), and C₅H₇B(OCR₂)₂ with 1,3,2-dioxaborolane rings (4g,h: R = H, Me) as mixtures of E and Z isomers. Metalation of 4a,b in THF by lithium dialkylamides LiNR′₂ (R′ = Me, Et, Prⁱ) in the presence of TMEDA results (i) in a nonproductive deborylation via primary attack at the boron center, (ii) in ring closure to produce boratabenzene salts [Li(TMEDA)][C₅H₅-BNR₂] (10a,b: R = Me, Et), and (iii) in substitution of B-dialkylamino groups when NR₂ is bulkier than NR′₂. 10a is obtained in 26.5% yield. Metalation of 4h in THF by the sterically demanding lithium dialkylamides LDA and LiTMP affords a precipitate of the spiroborate [Li(THF)][C₅H₆B(OCMe₂)₂] (13) in 63% yield in equilibrium with the presumed isomer Li-[C₅H₅BO(CMe₂)₂OH] (Li(3g)) in the THF solution. 13 can be transformed into boratabenzenes by several methods. (i) Treatment with Me₃SiCl produces a 1:1 mixture of isomeric boracyclohexadienes C₅H₆BOCMe₂CMe₂OSiMe₃ (15a, 2,4-isomer; 15b, 2,5-isomer) which, on subsequent low-temperature metalation, give Li[C₅H₅BO(CMe₂)OSiMe₃] (Li(3h)). (ii) Metalation of 13 at -78°C affords Li₂[C₅H₅BO(CMe₂)O] (Li₂(3i)) and, after treatment with Me₃SiCl, the bora-2,4-cyclohexadiene (5-Me₃Si)C₅H₅BOCMe₂CMe ₂OSiMe₃ (16). (iii) Treatment of 13 with Al2Me₆ in toluene at -78°C affords Li[C₅H₅BMe] (Li(3b)) in 79% yield as a white solid. This salt is also obtained from Li(3h) and Al₂Me₆. (iv) Addition of 15 to LiBu^t in pentane/hexane produces Li[C₅H₅BBu^t] (Li(3c)) in 53% yield. Combining strongly alkaline aqueous solutions of [NMe₃Ph]I and Li(3b) affords [NMe₃Ph][C₅H₅BMe] (18). 18 crystallizes in the monoclinic space group P2₁/n with a = 935.4(1) pm, b = 1557.4(4) pm, c = 988.8(1) pm, β = 95.47(1)°, and Z = 4. The 1-methylboratabenzene ion of 18 displays nearly perfect C_2v symmetry with intra-ring bond distances of 150.1, 138.8, and 139.2 pm (average) for B-C1 (B-C5), C1-C2 (C4-C5), and C2-C3 (C3-C4), respectively.