The Initiation Reaction of Hoveyda-Grubbs Complexes with Ethene

The initiation reactions of the three Hoveyda-Grubbs complexes (GH(H), GH(4-NO2), and GH(5-NO2)) with ethene were studied via UV-vis spectroscopy. The unsubstituted complex GH(H) initiates approximately 5 times slower with ethene than with 1-hexene, and the electron-deficient GH(4-NO2) and GH(5-NO2) initiates 1.2 and 1.7 times slower with ethene than with 1-hexene. A detailed multiwavelength analysis of the absorbance-time traces reveals the multistep nature of the reaction of the Hoveyda-Grubbs complexes with ethene. In addition to the established rate constant k(1) (being first order in ethene), two additional, c(olefin) independent reactions k(2) and k(3) were identified: k(2) depends on the nature of the GH precatalyst, while k(3) is nearly independent of the precatalyst. The reaction associated with k(1) precedes the faster k(2) leading to low stationary concentration of an intermediate formed from the respective Hoveyda-Grubbs complex and ethene, corresponding to a postinitiation species.