Metalloformyl complexes

The (octaethylporphyrin) rhodium dimer [Rh(OEP)]2 reacts in the presence of H2 and CO to produce the formyl species, Rh(OEP)CHO . The rhodium(II) porphyrins are the only systems to date to react with H2 and CO to produce formyls directly. Alternatively, the metalloformyl complex Rh(OEP)CHO can be prepared as ... 

Metalloformyl complexes are the most probable first organometallic intermediates in metal complex promoted reactions of H2 and CO that produce organic oxygenates. Production of large equilibrium concentrations of T Carbon bonded formyl complexes from reactions of metal hydrides with CO (Equation 6) requires that the M-H bond... 

A neutral metalloformyl complex is formed by the apparent insertion of carbon monoxide between the rhodium hydrogen bond of rhodium(III) hydride, Rh(OEP)H (Eq. 4) ... 

Different mechanisms for production of the rhodium formyls are followed in different reaction media. In benzene solution where [Rh(OEP)]2 and the metalloradical [Rh(OEP)] are present, a radical chain reaction is involved in CO activation and production of the formyl complex . In pyridine solution where [Rh(OEP)] dispropor-tionates to [Rh(OEP)(py)2] and [Rh(OEP)] , interaction of the metalloanion [Rh(OEP)] with CO is believed to be the dominant pathway for CO activation and production of the metalloformyl species by protonation . 

Additional evidence for a concerted reaction of two metalloformyl radical units Rh—CO comes from studies of reactions with ethanol, resulting in predominant formation of the formyl-ethoxy ester. Reaction of ethanol with Rh—CO units from two separate molecules would yield the nonobserved symmetrical products [(H(0)C)Rh (por)-tether-(por)Rh (C(0)H)] and [(OEt(0)C)Rh (por)-tether-(por)Rh -(C(0)OEt)]. In addition, nonconcerted reaction of ethanol with a monocarbonyl complex [Rh (por)-tether-(por)Rh (CO)] would yield nonobserved species containing Rh—H units. Taken together, it seems most likely that the formation of Rh C(0)H and Rh —C(0)Y units proceeds a concerted cleavage of the H Y bonds via a trimolecular transition state, as shown in Fig. 44. 

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