TRANSFER HYDROGENATION OF DIARYLACETYLENES BY POLYMETHYLHYDROSILOXANE IN THE PRESENCE OF THE RHCL3-ALIQUAT-336 CATALYST

The ion pair [(C8H17)3NCH3]+ [RhCl4(H2O)2]-, formed from aqueous rhodium chloride and Aliquat(R) 336, has been shown to catalyze transfer hydrogenation of diarylacetylenes by polymethylhydrosiloxane (PMHS) to give Z- and E-stilbenes as the major and the minor products, respectively. The reduction of diphenylacetylene was found to follow first-order kinetics in the substrate between 0.025 and 2.0 M. Both the steric and the electronic nature of the substrate proved to affect the rate. The electronic effect could be expressed in terms of two separate Hammett plots, one with rho = 0.64 for electron-withdrawing substituents and one with rho = -1.10 for the alkynes with electron-releasing groups. Application of either a deuterium-labelled organic phase or D2O led to mono- and dideuterated products. The observed activation energy E(a) = 11.47 kcal mol-1 suggests that the process is both chemically and diffusion controlled.