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"A binuclear palladium(I) hydride. Formation, reactions, and catalysis"
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- Abstract
- (dippp)Pd(Ph)Cl3 reacts with methanol to yield the novel hydrido Pd(1) dimer ([(dippp)-Pdl2(p-H)(p-CO)]+C1-( l),( dippp)PdClZ (4), Hz, benzene, and formaldehyde. In the presenceof NEt3, HNEt3+C1- is formed instead of 4. 1 can also be formed in a reaction of Pd(dippp)z,HC1, and CO. Labeling studies and modeling reactions indicate that the novel transformationof 3 into 1 involves methanolysis of 3 followed by a 0-H elimination from a methoxo intermediateto 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 withacetylenes results in bridge-splitting to form (dippp)Pd(+acetylene) and in hydropalladationof the acetylene to form a vinyl complex. The hydropalladation process exhibits high regio- andstereoselectivity, resulting in cis addition and attachment of the Pd atom to the more hinderedcarbon, indicating electronic control. 1 undergoes exchange of the hydride for deuteride inCD&OCD3, most likely via an enol insertion into Pd-H. In the presence of an olefin, such ascyclooctene or ethyl vinyl ether, catalytic transfer deuteration takes place. a-Deuteration ofthe latter is preferred, indicating anti-Markovnikov Pd-H addition. The integrity of 1 ismaintained 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 thiscase with acetone-&. 1 behaves as a Pd(0) complex and exhibits oxidative addition reactivitywith chlorobenzene or benzyl chloride, yielding (dippp)Pd(R)Cl. The relevance of this reactivityto Pd-catalyzed reactions is discussed.
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