Palladium complexes carbomethoxy species

When carbon monoxide is bubbled through a methanol solution of (dppp)Pd(triflate)2 a carbomethoxy-palladium species is formed, which can undergo insertion of alkenes and hence this is a feasible alternative initiation route to chain-growth polymerisation (Figure 12.4) [13], To ensure a clean formation of the carbomethoxy species, however, exclusion of water is a prerequisite. If during the preparation water was present the formation of a palladium hydride complex (dppp)PdFT was observed (reaction (1), Figure 12.2). 

Palladium(O) forms a complex with quinone that is now electron rich and can be protonated to give hydroquinone and palladium(II). The latter can start a new cycle via a carbomethoxy species after reaction with methanol and CO (c.f. reaction (6), Figure 12.4). Thus we have formally switched from a hydride initiator to a carbomethoxy initiator species. Addition of quinone to a nonactive or moderately active palladium system is a diagnostic tool that tells us whether zerovalent palladium is involved as an inactive state. Likewise, one might add dihydrogen to a system to see whether palladium(II) salts need to be converted to a hydride to reactivate our dormant catalyst. 

Pd complexed with 2-pyridyldiphenylphosphine (or its 6-methylpyridyl derivative) and weakly coordinating counter ions, such as methylsulfonate, are employed as catalyst [84, 85]. After the coordination of methanol and CO the migration of the methoxy group takes place to form the palladium carbomethoxy species. Then the coordination of propyne takes place. The steric bulk of the pyridyl group, and in particular of the 6-methylpyridyl group, induces the con-... 

Nitroso compounds as a reaction intermediate for nitro compound reductions were investigated as substrates for mechanism understanding by Cenini and his colleagues [18]. The isolation of a paramagnetic complex suggested that the first step of the interaction between Pd(0) and the nitroso species is an electron transfer reaction. The palladium carbomethoxy complex from the reaction of a palla-dium(O) complex with PhNO under CO pressure in methanol was also isolated, which fits with the published reaction mechanism [19]. 

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