Chlorotris(triphenylphosphine)rhodium has been shown to catalyze the selective rearrangement of many vicinal disubstituted epoxides to ketones between 150 and 210 °C. Kinetic measurements for various trans-1,2-diarylethylene oxides and RhCl(PAr3)3 catalysts were carried out. The reaction rate was shown to increase by introduction of electron-donating substituents into either the catalyst ligands or the substrate. The catalysis is inferred to proceed in the following order: (a) dissociation of RhCl(PAr3)3, (b) reversible nucleophilic cis addition of the epoxide to the activated catalyst to give a Rh(III) hydride, (c) intramolecular hydrogen transfer from the rhodium atom to the noncoordinated oxirane carbon, (d) reductive elimination to form the ketone and activated catalyst. The data are compatible with the expression rate = k1 k2[S][C]0/k-1 + k2 + k1 [S]) where [S] and [C]0 are substrate and initial catalyst concentration, respectively. Step c is considered rate determining on the basis of kinetic isotope effect measurements. Complexes RhCl(PAr3)3 have been shown to catalyze also an unusual carbon‒carbon bond cleavage in stilbene oxides having potent electron-attracting substituents to yield benzaldehydes and polymers. Epoxides in which one aromatic ring is more electron attracting than the other form aldehydes with the least electronegative groups.
ASJC Scopus subject areas
- Organic Chemistry