BROMO DERIVATIVES OF AMINE AND PHOSPHINE COMPLEXES OF CYANODIHYDROBORANE - SYNTHESIS AND REACTIVITY

Bromocyanohydroborane complexes [L.BH(Br)CN], containing a chiral boron atom and dibromocyanoborane complexes (L.BBr2CN) have been synthesized from amine- and phosphine-dihydrocyanoboranes [L.BH2CN, where L=trimethylamine (Me3N), tetramethylethylenediamine (TMEDA)/2, quinuclidine (Q), picoline (Pic), tributylphosphine (Bu3P) and triphenylphosphine (Ph3P)] and bromine. Trialkylamine-bromocyanohydroboranes show fairly high hydrolytic stability, while the analogous picoline complexes hydrolyze readily. The leaving group character of bromine in amine-bromocyanohydroboranes was tested in reactions with amines. TMEDA.2BH2CN, on reaction with tetramethylethylenediamine, picoline or on beating, is transformed into a cationic complex [(TMEDA)B(H)CN+]Br- which contains a five-membered ring. This compound is also formed under mild conditions in the reaction of Pic.BH(Br)CN and TMEDA. A chiral cationic complex [Pic(Q)B(H)CN+]Br- was formed from Q.BH(Br)CN and picoline whereas reaction Of Me3N-BH(Br)CN with picoline resulted in the formation of pure [Pic2B(H)CN+]Br- through substitution and transamination. Amine-dibromocyanoboranes, in contrast to amine-bromocyanohydroboranes, did not react with amines even under harsher reaction conditions. These experiments indicate that bromine can be considered as a good leaving atom in amine-bromocyanohydroboranes and these new molecules might be useful as starting materials for the synthesis of novel boron analogs of alpha-amino acids.