Diaryl-imidazolylidene

To conclude the details on zero-valent nickel carboxylation catalysts, some recent synthetic approaches worthy of note showed that this area of research still has a rich chemistry. For example, Louie and coworkers reported on the use of N-heterocyclic carbenes (diaryl-imidazolylidene) as new efficient ligands in the Ni-catalyzed coupling of various symmetrical di-ynes with C02 (Scheme 5.19) [60a]. 

Having synthesized enol ester 51b and 51c, examination of the NHC-catalyzed rearrangement was undertaken. Previously, NHCs such as diaryl imidazolylidene 13 had proven optimal for the preparation of simple pyra-nones. Unfortunately, subjection of enol ester 51b to carbene 13, generated in situ from precatalyst 13 HC1, failed to bring about any reaction, even when the reaction was heated to reflux (Table 2, entry 1). Similar results were achieved when the reaction was conducted in tetrahydrofuran (THF), with only trace amounts of the desired pyranone observed by NMR analysis of the crude reaction mixture (Table 2, entry 2). Thus, the challenges of total synthesis had found our methodology wanting. 

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See also in sourсe #XX -- [ ]

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