Stoichiometric Decarbonylation of Acid Chlorides

A considerable amount of work has been performed over the last 15 years to determine the mechanism of acid chloride decarbonylation with RhCl(PPh3)3/ Although the discovery of aldehyde decarbonylation preceded that of acid chlorides/ much more time has been spent on the acid chloride system because it is more easily studied. Many intermediates have been isolated and characterized (see Table 1). Even though the mechanism of the catalytic reaction is not well understood, the mechanism for the stoichiometric decarbonylation of acid chlorides has been proposed. However, the generally accepted mechanism has recently been challenged/ In this section, we will first review the stoichiometric decarbonylation mechanism for acid chlorides, followed by the stoichiometric decarbonylation of aldehydes. Finally, the mechanism of catalytic decarbonlyation of acid chlorides and aldehydes will be discussed. 

The general mechanism proposed for acid chloride stoichiometric decarbonylationunder mild conditions is given in Equations 7a-7e. 

When j3-hydrogens are present, i.e., R = R CH2CH2, olefins and HCl are predominantly formed, as shown in Equation 7f  

The only known exceptions vide infra) to the above mechanism. Equations 7a-7f, occur when R is an aryl group or when R is an a, /8-unsaturated species.In the former case, decarbonylation is not observed under mild conditions, and in the latter case, phosphonium salts are produced. Recent work has, however, substantiated the proposed mechanism for aliphatic and substituted aliphatic acid chlorides. 

Acid chloride Aroyl (or acyl) complex Aryl (or alkyl) complex Reference 

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There is general agreement on the mechanism for the stoichiometric decarbonylation of acid chlorides (9,14,15,16). The overall mechanism is shown by Equation set 2 where X = Cl. The stoichiometric decarbonylation reaction results from initial oxidative addition of the acid chloride to RhCl(PPh3)2 (Equation 2b, X = Cl). RhCl(PPh3)2 is a very reactive, low-concentration intermediate which is likely to be solvated (see Equation 2a) (17). 

Table 1. Isolated or Observed Intermediates during the Stoichiometric Decarbonylation of Acid Chlorides Using RhCliPPhsIs... 

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