Dimerization of terminal epoxides by homogeneous transition-metal complexes. A novel synthesis of carboxylic esters

Terminal epoxides have been shown to dimerize to carboxylic esters at 180°C in the presence of some Rh(I) and Ru(II) catalysts. Mixing of two epoxides of different electronic nature gives as the main product "crossed" esters, in which the electron-donating substituents are attached to the carboxylic part and the electronegative groups to the alcoholic residue. Kinetic measurements were carried out using styrene oxide as substrate and RuCl₂(PPh₃)₃ as catalyst. The reaction rate proved to increase either by increasing the electron density of the metal or by introduction of electron-donating groups in the substrate. The proposed reaction mechanism includes (a) dissociation of RuCl₂(PPh₃)₃; (b) oxidative addition of the epoxide (through the less-substituted carbon atom) to the activated catalyst; (c) slow hydrogen transfer from the metal to the β-oxirane carbon, followed by ring cleavage of the epoxide; (d) α-hydrogen transfer and formation of a ruthenium-acyl complex; (e) addition of the second epoxide molecule to the acyl carbonyl carbon; (f) reductive elimination of ester RCH₂CH₂OCOCH₂R from the rearranged ruthenium complex.