The reaction scheme below shows the starting materials for a special hydroboration-oxidation procedure. Specifically, chiral iminoalkene A was hydroborated with dicyclohexylborane to provide a mixture of two diastereomeric products in which one diastereomer predominated over the other in a ~6:1 ratio. Steric hindrance proved to be the primary factor governing both the regioselectivity and diastereoselectivity of this hydroboration step. Next, the diastereomeric mixture of organoborane intermediates was converted to the corresponding alcohols by treatment with aqueous sodium hydroxide and hydrogen peroxide. Surprisingly, the rate of the oxidation step was noticeably faster for the minor diastereomer.
a) Suggest the structures of the major and minor diastereomers after the hydroboration step. Be sure to clearly indicate stereochemistry with wedges and dashes and indicate which diastereomer is the major product. Provide a brief explanation for what specific steric factors influenced this diastereoselectivity.
b) Provide a mechanistic explanation for why the oxidation of the major diastereomer would be slower than the minor diastereomer. It would be extremely helpful to review the necessary steps in the mechanism for a standard organoborane oxidation procedure, first.