Rosenmund reduction mechanism

From these facts, a mechanism of the Rosenmund reduction has been proposed, in which the formation of the acylpalladium species 893 is the first step of the aldehyde formation and also the decarbonylation, although the Rosenmund reduction proceeds under heterogeneous conditions[744]. 

The mechanism of the Rosenmund reduction has attracted occasional attention. The discovery of Tsuji et al that palladium metal can catalyze the conversion of acyl chlorides (11) into alkenes (13), carbon monoxide and HCl, presumably via intermediates of the form (12 Scheme 5), suggests that (12) may also be an intermediate in the Rosenmund reaction. Consistent with this suggestion is the observation that the unsaturated acyl chloride (14) is converted to phenol by a palladium catalyst.22... 

The mechanism (Scheme 48) ° is expected to proceed through the acylpalladium species much as in the Rosenmund reduction. Indeed, the acyl complex 56 from oxidative addition of vinyl chloride with Pd(CO)(Ph3P)3 was isolated (Scheme 49). " The reaction of acid chlorides with the same catalyst provides aldehydes. However, aliphatic acid chlorides do not reduce effectively. The phosphine ligands present in the Heck acylpalladium intermediate are thought to be the cause, allowing decarbonylation and elimination to occur. Interestingly, the formylation will not occur with the Rosenmund catalyst. 

D. MECHANISM OF PALLADIUM-CATALYZED DECARBONYLATION OF ALDEHYDES AND ACYL HALIDES, AND ITS RELATION WITH ROSENMUND REDUCTION... 

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