Mechanistic insights into N- or P-centered nucleophile promoted thiol-vinylsulfone Michael addition

The mechanisms of nitrogen- and phosphorus-centered nucleophile promoted thiol-vinylsulfone Michael addition reactions have been studied by theoretical calculations. It is found that the trimethylamine-promoted thiol-vinylsulfone Michael addition proceeds by the base-catalyzed mechanism, in which the attack of the methanethiolate-trimethylammonium ion pair at divinylsulfone is the rate-limiting step. Comparison of different types of nitrogen-centered nucleophiles suggests that the aliphatic amine-promoted reactions undergo the base-catalyzed mechanism whereas the aza-aromatic compound-promoted reactions undergo the nucleophile-initiated mechanism. The trimethylphosphine-promoted thiol-vinylsulfone Michael addition is found to proceed via the nucleophile-initiated mechanism, in which the attack of trimethylphosphine at divinylsulfone is the rate-limiting step. An amine is superior to an organophosphine for the base-catalyzed mechanism because of its stronger basicity. On the other hand, an organophosphine tends to undergo the nucleophile-initiated mechanism more easily than an amine because phosphorus can stabilize the charged intermediates.