MECHANISM OF THE ACID-CATALYZED CYCLOMETALATION REACTION OF DIRHODIUM(II) COMPOUNDS WITH GENERAL FORMULA [RH2(O2CME)(MU-O2CME)2((C6H4)PPH2)(P(C6H4X)3)(OH2)]

Compounds of general formula [Rh2(O2CMe)(mu-O2CMe)2{(C6H4)PPh2}{P(C6H4X)3}(OH2)] 2 (X = H, p-Me, p-Cl, m-Me or m-Cl) have been prepared by photochemical reaction of the corresponding adduct [Rh2(mu-O2CMe)3{(C6H4)PPh2}{P(C6H4X)3}(HO2CMe)] 1. These compounds contain one equatorial phosphine which undergoes a facile cyclometallation reaction, catalysed in the presence of protic acids, to give doubly metallated compounds [Rh2(mu-O2CMe)2{(C6H4)PPh2}{(XC6H4)-P(C6H4X)2}(HO2CMe)2] 3. The kinetics and mechanism of this cyclometallation have been studied in chloroform and toluene solutions. A mechanism in which protons facilitate loss of one of the acetate groups in the starting compounds is proposed. Preliminary experiments showed that the addition of phosphines enhances the cyclometallation rate. The mechanism is fully concerted with a highly ordered transition state as seen by the very negative activation entropies. The values obtained for the deuterium kinetic isotopic effect indicate that, for the acid-catalysed path, the transition state lies in a more advanced position on the reaction coordinate than for the thermal process. The gap between the isokinetic plots for the acid-catalysed and thermal reactions is ca. 20 kJ mol-1.