Model Studies of Reductive Elimination in the Rh System

The reductive elimination has been less studied than the oxidative addition and migratory insertion steps. Forster reported that [Rh(C(0)Me)(CO)l3f decomposed to Acl and [Rh(CO)2l2] in several solvents without any marked dependence of the rate on the solvent. Acl was detected in aprotic media from reaction of Mel with CO in the presence of [Rh(CO)2l2r. However, it was also concluded that oxidative addition of AcX to [Rh(CO)2X2] (X = Cl, Br) was not thermodynamically favorable [34]. 

The reaction of [Rh(C(0)Me)(C0)2l3] with various nucleophiles has been studied by IR [17]. Acetate ion gave immediate formation of AC2O. Amines gave amides, the more nucleophilic dialkylamines reacting more rapidly than methyl anilines. The kinetics of these reactions were interpreted as consistent with two pathways, one being elimination of Acl from [Rh(C(0)Me)(CO)l3] and the other direct nucleophilic attack at the acyl (Eq. (22)). 

Spectroscopic Studies of the Model Reaction Steps of the Ir Carbonylation Cycle 

As with Rh catalysed carbonylation of MeOH, much enduring work on the mechanism of Ir catalysed carbonylation was carried out and reported by Forster at Monsanto [11], Combining data from catalytic reactions, synthesis and reactions of intermediates and IR studies of reaction solutions, three regimes or cycles, designated I, II and III, were identified, compared with the one for Rh. A key reactor composition variable determining which regime is operating is [H2O]. 

Regime II, at intermediate [H2O], referred to as the ionic cycle, parallels the Rh cycle and involves complexes such as [IrMe(CO)2l3] , [Ir(CO)2l4] , Ir(C(0)Me) (CO)2l3] and [Ir(CO)2l2], with the first two of these being the predominant species in solution. 

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