Syntheses and reactivity of exocyclic unsaturated heterodiborolanes and their diborylhexadiene precursors - Formation of 2-aza-4,5-dicarba-nido-hexaboranes(6)

The aryl-substituted 3,4-diboryl-2,4-hexadienes 4c-e are generated when 3,4 -bis(dimethoxyboryl)-2,5-dimethyl-2,4 hexadiene (4a) and 3,4-bis(dimenthoxy boryl)-2,5-dimethyl-2,4-hexadiene (4b) react with two equivalents of xylyll ithium or duryllithium. Cyclization reactions of 4b with heptamethyldisilaz ane and of 4c with MeN(SiMe3)(2), S(SiMe3)(2), and O(SiMe3)(2) lead to the corresponding 2,5-diaryl-3,4-diisopropylidene-1-hetero-2,5-diborolanes 5a,b , 6, and 7 in excellent yields. l-Aza-2,5-di-tert-butyl-3,4-diisopropyliden e-l-methyl-2,5-diborolane (5c) is obtained from the reaction of 5a with two equivalents of tert-butyllithium. When the azadichlorodi-borolane 5a is re acted with Li[BH4] or Li[DurBH(3)] the Cl atoms are replaced by hydrogen or duryl and the liberated BH3 hydroborates the isopropylidene groups to form the 2-aza-4,5-dicarba-4,5-diisopropyl-2-methyl-nido-hexaboranes 10a,b. The new compounds are characterized by MS and H-1, B-11 and C-13 NMR spectrosc opy. X-ray structure analyses of 4c and 4e show that the two halves of the hexadienes are rotated by 80 to 90 degrees. The solid state structures of t he three duryl-dubstituted heterodiborolanes 5b, 6, and 7 reveal twist conf ormations of the C2B2X rings.