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(dichloroboryl)‐2,5‐dimethyl‐2,4‐hexadiene ( 4b ) react with two equivalents of xylyllithium or duryllithium. Cyclization reactions of 4b with heptamethyldisilazane and of 4c with MeN(SiMe 3 ) 2 , S(SiMe 3 ) 2 , and O(SiMe 3 ) 2 lead to the corresponding 2,5‐diaryl‐3,4‐diisopropylidene‐1‐hetero‐2,5‐diborolanes 5a,b , 6 , and 7 in excellent yields. 1‐Aza‐2,5‐di‐ tert ‐butyl‐3,4‐diisopropylidene‐1‐methyl‐2,5‐diborolane ( 5c ) is obtained from the reaction of 5a with two equivalents of tert ‐butyllithium. When the azadichlorodiborolane 5a is reacted with Li[BH 4 ] or Li[DurBH 3 ] the Cl atoms are replaced by hydrogen or duryl and the liberated BH 3 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 1 H, 11 B and 13 C NMR spectroscopy. X‐ray structure analyses of 4c and 4e show that the two halves of the hexadienes are rotated by 80 to 90°. The solid state structures of the three duryl‐substituted heterodiborolanes 5b , 6 , and 7 reveal twist conformations of the C 2 B 2 X rings.