Transition-metal-promoted reactions of boron hydrides. 12. Syntheses, polymerizations, and ceramic conversion reactions of B-alkenylborazines

Abstract

High yield synthetic routes for the production of new B-alkenylborazines and poly(B-alkenylborazine) polymers and oligomers as well as the use of the poly(B-alkenylborazines) as polymeric precursors to boron nitride ceramics are reported. The syntheses of the B-alkenylborazines are based on the use of transition-metal reagents to activate the B-H bonds of borazine for either alkyne-addition or olefin-dehydrocoupling reactions and are the first examples of transition-metal-catalyzed reactions of borazine to be reported. Thus, the complexes RhH(CO)(PPh{sub 3}){sub 3} and IrCl(CO)(PPh{sub 3}){sub 2} were found to activate borazine/alkyne-addition reactions with acetylene, propyne, 1-butyne, and 2-butyne to give the previously unknown B-substituted alkenylborazines, B-(RHC{double bond}CR{prime})-B{sub 3}N{sub 3}H{sub 5} (R,R{prime} = H, Me, or Et). For reactions employing RhH(CO)(PPh{sub 3}){sub 3}, typical yields of B-alkenylborazines range from 70 to 85% with the catalyst undergoing 25-50 turnovers an hour. Palladium bromide was found to catalyze the dehydrocoupling reactions of either borazine or N-trimethylborazine with olefins, such as ethylene and propene, to give the corresponding B-alkenyl- and B-alkylborazine products. The B-alkenylborazines were found to readily undergo thermally induced polymerizations to produce insoluble poly(B-alkenylborazines). Soluble, low molecular weight (< 1,500) oligomers were formed when the thermal polymerization reactions were carried out in the presence of additional borazine.more » It was found that pyrolysis of these polymers yielded ceramic materials with B/N ratios of {approximately} 1.0 which, depending upon the polymer and pyrolysis conditions, ranged from black, high carbon materials to white, crystalline h-boron nitride. In all cases, the polymer/ceramic conversions were found to take place with both high ceramic and chemical yields.« less