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 ph otochemical reaction of the corresponding adduct [Rh2(mu-O2CMe)3{(C6H4 )PPh2}{P(C6H4X)3}(HO2CMe)] 1. These compounds contain one equatorial p hosphine which undergoes a facile cyclometallation reaction, catalysed in the presence of protic acids, to give doubly metallated compounds u-O2CMe)2{(C6H4)PPh2}{(XC6H4)-P(C6H4X)2}(HO2CMe)2] 3. The kinetics and mechanism of this cyclometallation have been studied in chloroform an d toluene solutions. A mechanism in which protons facilitate loss of o ne of the acetate groups in the starting compounds is proposed. Prelim inary 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 entro pies. The values obtained for the deuterium kinetic isotopic effect in dicate that, for the acid-catalysed path, the transition state lies in a more advanced position on the reaction coordinate than for the ther mal process. The gap between the isokinetic plots for the acid-catalys ed and thermal reactions is ca. 20 kJ mol-1.