An NMR study of alkyl exchange reactions between boron and aluminum: effect of such exchange reactions on the polymerizability of alkenyldialkylboranes by the Ziegler-Natta process

Abstract

Alkyl exchange reactions between boron and aluminum were studied using ¹H-NMR, to gain a better understanding of the behavior of various ω-alkenyldialkylboranes towards Ziegler-Natta catalysts. The reaction of a model borane, octyldimethylborane, with Et₃Al, Et₂AlCl and EtAlCl₂ was investigated. The transfer of the methyl group from boron to aluminum was monitored as a function of time. The rate of transfer was found to be rapid in the case of Et₃Al and decreased in the order: Et₃Al » Et₂AlCl > EtAlCl₂. The extent of this transfer at equilibrium, followed the same order and a maximum of 92 mol percent of the methyl group was transferred in the case of Et₃Al. Sterically hindered aluminum alkyls such as tri(isobutyl)aluminum was also found to undergo rapid exchange with octyldimethylborane. Sterically crowded boranes such as 1-hexenyl-9-BBN, however, did not undergo any exchange reaction with Et₂AlCl.