Synthesis and properties of (η³-1-methylallyl)palladium(II) formates as models of intermediates in the palladium-catalyzed reductive cleavage of allylic carboxylates and carbonates with formic acid

In order to gain insight into the mechanism of the regioselective formation of alkenes in the palladium-catalyzed reductive cleavage of allylic carboxylates or carbonates with formic acid, two types of (η³-1- methylallyl)palladium formates (P1 and P2 types) have been prepared as simple models of catalytic intermediates. The P1 type is a neutral complex coordinated with one tertiary phosphine ligand and a formato ligand: [Pd(η³-MeCHCHCH₂)(O₂CH)(L)] (L = PMePh₂, PMe₂Ph, PMe₃, P(o-tolyl)₃); the P2 type is a cationic complex bearing two tertiary phosphine ligands and a formate counter anion: [Pd(η³-MeCHCHCH₂)L₂]⁺CO₂H^- (L = PMePh₂, PMe₂Ph, PMe₃). The structures and dynamic behavior of the complexes in solution have been examined in detail by NMR spectroscopy. Studies on the thermolysis of the P1 and P2- type complexes have clearly provided the following mechanistic viewpoints: (1) 1-butene and 2-butene are formed from the PI species; (2) butadiene is liberated from the P2 species; (3) the ratio of 1-butene to 2-butene increases as the bulkiness of phosphine ligand increases. A mechanism involving two geometrical isomers of [Pd(η³- MeCHCHCH₂)(H)(L)], which are formed by decarboxylation of the P1-type complexes, has been proposed for the formation of butenes.