Reactivities of tertiary phosphines towards allenic, acetylenic, and vinylic Michael acceptors

The addition of phosphines (PR₃) to Michael acceptors is a key step in many Lewis-base catalysed reactions. The kinetics of the reactions of ten phosphines with ethyl acrylate, ethyl allenoate, ethyl propiolate, ethenesulfonyl fluoride, and ethyl 2-butynoate in dichloromethane at 20 °C was followed by photometric and NMR spectroscopic methods. The experimentally determined second-order rate constants k ₂ show that electronic effects in sterically unencumbered phosphines affect their nucleophilicity towards different classes of Michael acceptors in the same ordering. Michael acceptors with sp-hybridised electrophilic centres, however, are less susceptible to changes in the PR₃ nucleophilicity than those with sp²-hybridised reactive sites. Linear correlations of lg k ₂ from this work with published rate constants for S_N2 and S_N1 reactions as well as with Brønsted basicities and fugalities for PR₃ demonstrate the generality of the detected reactivity trends. Computed reaction barriers (ΔG ^‡ _calc) as well as reaction energies (ΔG _add) for Michael adduct formations show excellent correlations with experimentally obtained reaction barriers (ΔG ^‡ _exp) corroborating the interpretation of the kinetic data and revealing the philicity/fugality features of the reactants in phospha-Michael additions.