Hydroboration. XXXV. Cyclic hydroboration of 2,4-dimethyl-1,4-pentadiene and 2,5-dimethyl-1,5-hexadiene. Simple synthesis of bis(3,5-dimethyl)borinane and bis(3,6-dimethyl)borepane and their application to the 1,4-addition reaction of organoboranes

Abstract

The hydroboration of 2,4-dimethyl-l ,4-pentadiene (2,4-DMP) or 2,5-dimethyl-l,5-hexadiene (2,5-DMH) with borane in THF at 0°in the molar ratio of 1:1 with the diene-to-borane mode of addition is rapid and complete immediately following the addition of diene. The oxidation product derived from 2,4-DMP consists of a 98:2 mixture of 2,4-dimethyl-l ,5-pentanediol and its isomer, while that from 2,5-DMH is a 90:10 mixture of the 1,6-diol and its isomer. Little or no position isomerization occurs at 25°, but at 70°a slow isomerization is evident in the product from 2,5-DMH. The initial hydroboration mixture contains only ca. 50% of either bis(3,5-dimethyl)borinane (3,5-DMBN) or bis(3,6-dimethyl)borepane (3,6-DMBP). The presence of fully alkylated polymeric species, presumably the 3:2 dumbbell-shaped products, and unreacted borane in THF is indicated. The amount of 3,5-DMBN or 3,6-DMBP increases with time, requiring approximately 24 hr at 25°or 1 hr at 70°to attain the maximum yields of 90-94 % (3,5-DMBN) or 75% (3,6-DMBP). Consequently, the hydroboration of 2,4-DMP or 2,5-DMH with borane in THF in the molar ratio of 1:1 followed by a brief heating of the reaction mixture at reflux provides a simple synthesis of 3,5-DMBN or 3,6-DMBP. 3,5-DMBN has been isolated and identified. 3,6-DMBP has not been isolated, but has been fully identified. The dimeric nature of these compounds has been established by the typical bridge absorption in the ir at 1650-1565 cm-1. Treatment of these compounds with alcohols produces the corresponding B-alkoxy derivatives. The hydroboration of olefins with a solution in THF containing a modest excess of 3,5-DMBN or 3,6-DMBP proceeds smoothly, requiring approximately 1 hr with 1-hexene, 1-octene, isobutylene, 2-butene, cyclopentene, and norbornene, 2 hr with 2-methyl-2-butene, 4-8 hr with cyclohexene, and 16-24 hr with 2,3-dimethyl-2-butene. The hydroboration products containing B-alkyl-3,5-DMBN or B-alkyl-3,6-DMBP can in most cases be used for further operations following quenching of residual hydride. Distillation provides B-alkyl-3,5-DMBN (98-99 % pure) or B-alkyl-3,6-DMBP (~90% pure) in excellent yields. Alternatively, the reaction of B-methoxy-3,5-DMBN with alkyllithium produces the corresponding B-alkyl-3,5-DMBP in excellent yields. These B-alkylboracyclanes undergo a facile reaction with methyl vinyl ketone and similar ,ß-unsaturated carbonyl derivatives to transfer the B-alkyl group from boron to carbon, especially readily when the B-alkyl group is secondary or tertiary. This extension of the 1,4addition reaction via organoboranes to highly branched secondary and tertiary alkyl groups makes this reaction broadly applicable to the formation of carbon-carbon bonds utilizing alkyl and cycloalkyl groups with a wide range of structures.