Model Studies of Oxidative Addition in the Rh system

The original work by Monsanto identified [Rh(CO)2l2] as the major Rh spedes under their process conditions and reaction of Mel with this complex as rate controlling for the process [3]. However, the proposed primary product of oxidative addition of Mel to [Rh(CO)2l2r, [RhMe(CO)2l3] , was not observed in early work. Forster [10] studied the reaction of Mel with [Rh(CO)2l2] by IR and found that it gave the acyl complex [Rh(C(0)Me)(C0)l3] , which was also isolated and characterized in the solid state as a dimer. The reaction could be followed quantitatively but the observed rate constant would be antidpated to be a composite of the rate constants for the formation of the intermediate [RhMe(CO)2l3r and its further reaction to [Rh(C(0)Me)(C0)l3] (Eq. (15)) and (Eq. (16)). 

The first quantitative studies of the reaction of [Rh(CO)2l2] with Mel were reported by Maitlis and Hickey [30] who used IR to follow the formation of [Rh(C(0)Me)-(CO)l3j in aprotic media and in the presence of iodide salts. They proposed that the promotion of the overall reaction rate was due to the formation of a species such as [Rh(CO)2l3] , which would be a more potent nudeophile towards Mel than [Rh(CO)2l2r, (Eq. (17-18)). 

Similar observations on the promotion of the reaction of [Rh(CO)2l2] with Mel in protic media were published by workers at Celanese [14, 15]. They investigated by IR the promotional effect of both iodide and acetate salts on the reaction, since for metals such as Li, a preferred promoter for carbonylation, substantial amounts of LiOAc, formed in situ (Eq. (19)), might be present in the working carbonylation media. 

Lil and LiOAc both promoted the reaction of [Rh(CO)2l2] with Mel in protic media and again the formation of a more nucleophilic [Rh(CO)2l2X] (X = F or OAc ) 

It was reasoned that the likelihood of observing the [RhMe(CO)2l3] species would be favored by working at as high a concentration of Mel as possible, because this would enhance the formation of [RhMe(CO)2l3] from [Rh(CO)2l2] and, being a relatively nonpolar medium, would retard its subsequent reaction to [Rh(C(0)Me)(C0)l3]-. 

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