Borabenzene derivatives. 28. Pinene-fused dihydroborinines, boratabenzenes, and a borabenzene-pyridine adduct

The known enantiomerically pure, (1S,5S)-(-)-alpha-pinene-derived tetrahydroborinine (1R,9R)-4-(dimethylamino)-10,10-dimethyl-6-methylene-4-boratricyclo[7.1.1.0(2,7)]undec-2(7)-ene (3) is used as starting material for the synthesis of enantiomerically pure, pinene-fused borabenzene derivatives. In the presence of catalytic amounts of HCl compound 3 can be isomerized to the corresponding dihydroborinine 5a (120 degrees C, 3 days, 100%). Treatment of 5a with BCl3 in CH2Cl2 at -60 degrees C affords the B-chloro derivative 5b (85%). Compound 5b reacts with ZnMe2 in toluene to produce the B-methyl derivative Sc (84%), and with Me3SiOMe in CH2Cl2 to give the B-methoxy compound 5d (91%). The compounds 5a-d are 2,6-dienes and are free of double-bond isomers. Lithiation of 5c with LiN(SiMe3)(2) in hexane and of 5d with LDA in THF affords the corresponding boratabenzenes Li(8) (83%) and Li(9) (similar to 100%). The structure of the crystalline solvate Li(TMEDA)(8) (10) is of the same type as that of Li(TMEDA)(4) with a Li(TMEDA)(+) ion attached to the exo-face of the anion 8(-). The reaction of Li(4) with CuI in toluene gives oxidative coupling products 11 with a C5-C5' bond and some dihydroborinine 5a (up to 25%). The mixture 11 consists essentially of diastereomeric exo,exo- and endo,exo-isomers in a 4:1 ratio, respectively. The major isomer, exo,exo-11, can be isolated as crystalline clathrate exo,exo-11.1/2C(6)H(6) (=12); the structure of 12 (as determined by X-ray single-crystal structure analysis) proves the C-2-symmetric exo,exo-linkage across the 5,5'-C-C bond with the 2,6-position of the double bonds as in 5a-d. Silylation of Li(4) gives a 4:1 mixture 14 of 5-exo- and 5-endo-(trimethylsilyl) derivatives; (similar to 100%). This mixture reacts with BCl3 to give the B-chloro derivatives 15 (again as a 4:1 mixture of stereoisomers, 93%). Treatment of 15 with pyridine affords red crystals of the pyridine adduct 16 (43%) as the first Lewis base adduct of apr enantiomerically pure borabenzene. The structure of 16 is very similar to that of borabenzene-pyridine; the angle between the borabenzene best plane and the pyridine best plane amounts to 33.02(14)degrees.