Chlorotris(triphenylphosphine)rhodium has been shown to catalyze the selective rearrangement of many vicinaldisubstituted epoxides to ketones between 150 and 210°C. Kinetic measurements for various frans-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 = k1k2[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.
|Number of pages||10|
|Journal||Journal of Organic Chemistry|
|Publication status||Published - 1977|
ASJC Scopus subject areas
- Organic Chemistry