Hydroalkylation of Alkynes Catalyzed by Transition Metals

Abstract

Alkynes are a fundamental class of compounds in chemistry. One such use for alkynes is the formation of alkenes, another class of compounds that have been a subject of interest due to their significance in biologically active molecules and highly variable structures. Most alkenes exist in either a Z or E conformation and can have up to four unique substituents branching from them. Formation of alkenes via the reduction of alkynes controlled by transition metal catalysts to favor certain stereo- and regioisomers is an important goal in organic synthesis, which has led to the development of novel methods. Here, we describe two such methods for the synthesis of alkenes from alkynes. The first method allows for the formation of isomerically enriched Z-alkenes from the reductive cross coupling of terminal alkynes with bulky alkylboranes. This silver-catalyzed hydroalkylation method utilizes a 1,2-metalate shift to accomplish its stereoselectivity. Significantly, this method improves upon prior work by allowing for the formation of increasingly complex products by expanding the scope of this type of reaction to include secondary alkylboranes. The second method, whose development is incomplete, harnesses the steric and electronic biases of silyl-protected alkynes to regioselectively synthesize alkenes via nickel-catalyzed hydroalkylation with alkyl iodides. Regioselectivity in this method is substrate dependent, as alkylation can occur at either position on the alkyne depending upon the silyl protecting group used.