The Mechanism for Transition-Metal-Catalyzed Hydrochalcogenation of Unsaturated Organic Molecules

Abstract

In this chapter, discussions are focused on two types of mechanisms of transition-metal-catalyzed hydrochalcogenation, Type I and Type II, which are classified by the initial behavior of precatalysts. In Type I mechanism, precatalyst M–X (M = Pd, Ni, Zr, Ln, and An) first undergoes protonolysis with REH (E = O, S, and Se) to generate active catalyst M–ER, which then undergoes insertion of alkyne into the M–ER bond (chalcogenometalation) to give 2-chalcogenovinyl complex, followed by protonolysis of M–Cvinyl with REH to produce the product and to regenerate active catalyst M–ER. Type II mechanism starts from oxidative addition of REH (E = S and Se) to complex [M] (M = Pd, Pt, Rh, and Ir) to give chalcogenolato–hydrido complex, [M]H(ER). In the next alkyne insertion, [M]–H insertion (hydrometalation) to give [M](ER)(vinyl) or [M]–E insertion (chalcogenometalation) to give [M]H(2-RE-vinyl) occurs and then reductive elimination of the resulting vinyl [M] complexes yields the product and [M]. Reactions where transition metal catalysts exert as Lewis acid to activate unsaturated bonds and those proceeding through vinylidene intermediates are mentioned only shortly.