A binuclear palladium(I) hydride. Formation, reactions, and catalysis

Article abstract of DOI:10.1021/om00014a035

(dippp)Pd(Ph)Cl (3) reacts with methanol to yield the novel hydrido Pd(I) dimer {[(dippp)-Pd]₂(μ-H)(μ-CO)}⁺Cl^- (1), (dippp)PdCl₂ (4), H₂, benzene, and formaldehyde. In the presence of NEt₃, HNEt₃⁺Cl^- is formed instead of 4. 1 can also be formed in a reaction of Pd(dippp)₂, HCl, and CO. Labeling studies and modeling reactions indicate that the novel transformation of 3 into 1 involves methanolysis of 3 followed by a β-H elimination from a methoxo intermediate to yield formaldehyde, benzene, and the 14e transient (dippp)Pd (7). Formaldehyde decarbonylation, coupling of the palladium carbonyl complex with 7, and protonation lead to 1. Alternatively, 1 can be formed by electrophilic attack of protonated 7, on the carbonyl complex (dippp)Pd(CO). A number of reactivity modes have been identified for 1. Reaction with acetylenes results in bridge-splitting to form (dippp)Pd(η²-acetylene) and in hydropalladation of the acetylene to form a vinyl complex. The hydropalladation process exhibits high regio- and stereoselectivity, resulting in cis addition and attachment of the Pd atom to the more hindered carbon, indicating electronic control. 1 undergoes exchange of the hydride for deuteride in CD₃COCD₃, most likely via an enol insertion into Pd-H. In the presence of an olefin, such as cyclooctene or ethyl vinyl ether, catalytic transfer deuteration takes place. α-Deuteration of the latter is preferred, indicating anti-Markovnikov Pd-H addition. The integrity of 1 is maintained during this process. With norbornene, bridge-splitting to form (dippp)Pd(norbornyl) (17) and its CO-insertion product 18 takes place. No H/D exchange catalysis is observed in this case with acetone-d₆. 1 behaves as a Pd(O) complex and exhibits oxidative addition reactivity with chlorobenzene or benzyl chloride, yielding (dippp)Pd(R)Cl. The relevance of this reactivity to Pd-catalyzed reactions is discussed.