ORGANOBORANES FOR SYNTHESIS. 1. PROTONOLYSIS OF TRIALKYLBORANES. A CONVENIENT NON-CATALYTIC CONVERSION OF ALKENES INTO SATURATED COMPOUNDS via HYDROBORATION-PROTONOLYSIS

Article abstract of DOI:10.1016/S0040-4020(01)88151-5

The protonolysis of trialkylboranes with carboxylic acids proceeds very rapidly for the first group, somewhat slower for the second, but requires elevated temperatures for removal of the third.A close examination of the protonolysis of representative symmetrical, as well as mixed trialkylboranes, reveals that the steric requirements of the alkyl groups attached to boron play an important role in the rate.Secondary alkyl groups protonolyze less readily than primary alkyl groups.More hindered alkyl groups are still more resistant.Since the hydroboration-protonolysis sequence involves relatively mild conditions, this procedure can be applied for hydrogenating olefinic derivatives containing labile groups, such as active sulfur, halogen and nitrogen functionalities.The stereochemistry of protonolysis has been established via deuteroboration of norbornene and deuterolysis of the product.It is evident that protonolysis proceeds with retention of configuration at the migrating carbon.Partially alkylated boranes can be used for hydroboration-protonolysis with improved regioselectivity.Subsequent protonolysis of the resulting mixed trialkylboranes provides the desired alkanes in good yields.Products sensitive to the action of hot acetic acid offer difficulties.For example, when this procedure was applied to the preparation of 1-menthene from d-limonene, the product was racemic.