Novel mixed organoboranes for the reductive alkylation of p-benzoquinone
- Author(s): Hincapié, Gloria
- et al.
In the early 1960's, Hawthorne discovered the quantitative synthesis of alkylhydroquinones by reacting 1,4- benzoquinone with trialkylboranes formed from hydroboration. Later work revealed limitations to the reductive alkylation of benzoquinones due to the poor regioselectivity of the hydroboration and the migratory aptitudes of the alkyl groups, namely transferring the more substituted group impacting product purity. During this process, only one alkyl substituent transfers while the others are lost, thus resulting in formation of a borinic ester that is difficult to remove, air-sensitive, radical generating, and ultimately degrades product purity. Mixed organoboranes can overcome these difficulties using auxiliary groups with relatively low migratory aptitudes, such as phenyl groups or methyl groups. Our group has developed the first general preparation of alkyldiphenylboranes. They readily react with p-benzoquinone successfully demonstrating the applicability of using mixed organoboranes. In this study, we are investigating using methyl groups as our auxiliary. Our initial studies used Matteson's hydroboration procedure using triethylsilane and trichloroborane to form the unsolvated dichloroborane, HBCl₂, a highly regioselective reagent for a variety of olefins and avoids limitations of ligand complexed boranes. Once the alkyldichloroboranes were synthesized, they were methylated using the Grignard reagent or dimethylzirconocene. The reaction conditions were somewhat dependant on the alkyl group. The purity of the alkyldimethylborane was analyzed by boron NMR after complexation to a ligand. Addition of the alkyldimethylboranes to p-benzoquinone quickly resulted in ca. 90% isolated yields of the alkylhydroquinone. The resulting dimethylborinic acid is easily removed preventing any radical side reactions. Overall, we have developed a general route for the reductive alkylation of quinones, with good selectivity and in high yields, demonstrating the applicability of methyl auxiliary groups. As such, attempts were made to synthesize vinyldimethylboranes using the developed synthetic routes