Ketenes palladium catalysis

Diazoalkane carbonylation using palladium catalysis gives unobserved acylketenes, which are captured in situ with nucleophiles (Scheme 7.43). The reactions are suggested to involve palladium complexed ketenes and are carried out with various substrates and nucleophiles. 

At the time the chemistry of main group enolates flourished already for a while, that of late transition metals had a shadowy existence in synthetic organic chemistry. Their stoichiometric preparation and the sluggish reactivity - tungsten enolates, for example, required irradiation to undergo an aldol addition [24a] - did not seem to predestine them to become versatile tools in asymmetric syntheses [27]. The breakthrough however came when palladium and rhodium enolates were discovered as key intermediates in enantioselective catalyses. After aldol reactions of silyl enol ethers or silyl ketene acetals under rhodium catalysis were shown to occur via enolates of the transition metal [8] and after the first steps toward enantioselective variants were attempted [28], palladium catalysis enabled indeed aldol additions with substantial enantioselectivity... 

The [2+2] cycloaddition of a ketene and an imine leading to P-lactams can be accomplished using palladium catalysis. Starting from an allyl phosphate or allyl chloride (see also 1), carbonylation with carbon monoxide lead to an intermediate ketene that cycloadds to imines to form the desired P-lactams (Scheme 5-176). ... 

While wanning the catalysis mixture to 55 C (Step D, Scheme 1) leads to no other observable reaction intermediates, the generation of intermediate 8 would allow the series of steps shown in Scheme 1. Insertion of the coordinated CO into the palladium-carbon bond would lead to the overall coupling of acid chloride, imine and carbon monoxide in conq>lex 10. The subsequent loss of HCl from 10, either via direct deprotonation or P-H elimination, would form the a-amide substituted ketene 11. The latter is known to be in rapid equilibrium with its cyclic mesoionic l,3-oxazolium-5-oxide tautomeric 12 (14). These steps would lead to the liberation of the Pd(0) catalyst, which can return to the catalytic cycle. 

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