BORABENZENE DERIVATIVES .22. SYNTHESIS OF BORATABENZENE SALTS FROM 2,4-PENTADIENYLBORANES - STRUCTURE OF [NME(3)PH][C(5)H(5)BME]

Borylation of potassium pentadienide with BCl(NR(2))(2) and subsequent alcoholysis produce 2,4-pentadienylboranes C5H7B(NR(2))(2) (4a-c: R = Me, Et, Pr-i), C5H7B(OR)(2) (4e,f: R = Et, Bu(t)), and C5H7B(OCR(2))(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, Pr-i) in the presence of TMEDA. results fi) in a nonproductive deborylation via primary attack at the boron center, (ii) in ring closure to produce boratabenzene salts [Li(TMEDA)][C5H5-BNR(2)] (10a,b: R = Me, Et), and (iii) in substitution of B-dialkylamino groups when NR(2) 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(OCMe(2))(2)] (13) in 63% yield in equilibrium with the presumed isomer Li-[C5H5BO(CMe(2))(2)OH] (Li(Sg)) in the THF solution. 13 can be transformed into boratabenzenes by several methods. (i) Treatment with Me(3)SiCl produces a 1:1 mixture of isomeric boracyclohexadienes C(5)H(6)BOCMe(2)CMe(2)OSiMe(3) (15a, 2,4-isomer; 15b, 2,5-isomer) which, on subsequent low-temperature metalation, give Li[C5H5BO(CMe(2))OSiMe(3)] (Li(3h)). (ii) Metalation of 13 at -78 degrees C affords Li-2[C5H5BO(CMe(2))O] (Li-2(3i)) and, after treatment with Me(3)SiCl, the bora-2,4-cyclohexadiene (5-Me(3)Si)C(5)H(5)BOCMe(2)CMe(2)OSiMe(3) (16). (iii) Treatment of 13 with Al(2)Me(6) in toluene at -78 degrees C affords Li[C(5)H(5)BMe] (Li(3b)) in 79% yield as a white solid. This salt is also obtained from Li(3h) and Al(2)Me(6). (iv) Addition of 15 to LiBu(t) in pentane/hexane produces Li[C(5)H(5)BBu(t)] (Li(3c)) in 53% yield. Combining strongly alkaline aqueous solutions of[NMe(3)Ph]I and Li(3b) affords [NMe(3)Ph][C(5)H(5)BMe] (18). 18 crystallizes in the monoclinic space group P2(1)/n with a = 935.4(1) pm, b = 1557.4(4) pm, c = 988.8(1) pm, beta = 95.47(1)degrees, and Z = 4. The 1-methylboratabenzene ion of 18 displays nearly perfect C-2 nu 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.