A Joint Experimental and Theoretical Study of the Palladium-Catalyzed Electrophilic Allylation of Aldehydes

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Abstract
Palladium-catalyzed electrophilic allylation of aldehydes with allylstannanes has been proposed in the literature as a model reaction illustrating the potential of nucleophilic η1-allyl palladium pincer complexes to promote new catalytic processes. This reaction was studied by a joint experimental and theoretical approach. It was shown that pincer palladium complexes featuring a S∼P∼S and a S∼C∼S tridentate ligand are efficient catalysts for this reaction. The full mechanism of this transformation was studied in detail by means of DFT calculations. Two pathways were explored: the commonly proposed mechanism involving η1-allyl palladium intermediates and a Lewis acid promoted mechanism. Both of these mechanisms were compared to the direct transformation that was shown experimentally to occur under mild conditions. The mechanism involving an η1-allyl palladium intermediate has been discarded on energetic grounds, the nucleophilic attack and the transmetalation step being more energetically demanding than the direct reaction between allyltin and the aldehyde. On the other hand, a mechanism where the palladium acts as a Lewis acid proved to be fully consistent with all experimental and theoretical results. This mechanism involves (L∼X∼L)Pd+ species which activate the aldehyde moiety toward nucleophilic attack.