Investigation of cyclization reactions of dicyclohexyl-6-iodo- and -6-tosylhexenylborane. A facile radical cyclization diverted to a rearrangement-cyclization with base.
Dicyclohexyl-6-iodohexenylborane efficiently undergoes radical cyclization at room temperature using tri-n-butyltin hydride as a hydrogen donor and without the aide of a radical initiator. Efforts to develop environmentally friendly reagents using hypophosphites as substitutes for tin hydrides in this reaction provided a 60% yield of cyclopentanemethanol when tetrabutylammonium hypophosphite was used. Air was necessary as an initiator when hypophosphites were used. During investigations of the radical cyclization reactions, it was discovered that excess tetrabutylammonium hydroxide provided the rearrangement-cyclization product in excellent yield. Since this product was chiral, efforts were focused on achieving enantioselectivity. Oxazaborolidines made from chiral amino alcohols and 6-tosyl-1-hexenylboronic acid were treated with methyllithium to give 1-cyclopentylethanol after oxidation in 60% ee, demonstrating that oxazaborolidines were effective chiral directors for this reaction.
Department of Chemistry, University of California, Riverside, California 92521, USA.
Diane M
DM
Hinkens
M Mark
MM
Midland