Deoxy Breslow intermediate

S. Matsuoka, Y. Tochigi, K. Takagi, M. Suzuki, Tetrahedron 2012, 68, 9836-9841. Sequential one-pot and three-component reactions of an N-heterocychc carbene to form 4-(l,2,4-triazol-5-ylidene)pyrrolidine-2,5-diones a tandem umpolung/ annulation sequence via deoxy-Breslow intermediates. 

A series of azolium enolates (99 Ar = phenyl, mesityl) have been synthesized and characterized. Their ambident reactivities have been measured by studying their reactions with benzhydryl cations, Ar2CH , in r/ -acetonitrile, using known electrophilicity parameters for the latter. NMR shows predominantly 0-attack initially, with a switch to C-product over 1-2 days, with second-order rate constants for the two processes calculable. The azolium enolate reactivities have been compared with those of the corresponding free carbenes, and deoxy-Breslow intermediates. 

Mechanistic studies on the tail-to-tail dimerization of methyl methacrylate (317), catalysed by NHC (316), revealed noticeable differences in the kinetics, reversibility, and stability of the intermediates compared to benzoin condensation (Scheme 16). The initial addition, involving a rapid intermolecular proton transfer, generates the deoxy-Breslow intermediate (318), which has been found to be quite stable. Its addition to the second molecule of (317), followed by the final catalyst elimination, producing (320), is the rate-limiting step. ... 

Aza- and deoxy-Breslow intermediates are regarded as stable and catalytically relevant surrogates of standard Breslow intermediates. Aza-Breslow intermediates (157) and (158) derived from the reaction of chiral triazolium salt (159) and an iminium salt have been isolated and fully characterized.Those stable and unique architectures are... 

Finally, the involvement of deoxy-Breslow species as intermediates and related to the reactivity profile of acrylates under NHC catalysis has been reported twice. The first report dealt with detailed mechanistic studies of the well-known tail-to-tail dimerization of methyl acrylate. By means of complementary and robust experiments (including kinetic isotope effects, deuterium-labelling studies and competitive reactions), the formation of the dimer (148) has been unambiguously rationalized. The second report has described/or the first time the NHC-catalysed cyclotetramerization of acrylates. Using imidazolium chloride (135) as NHC source, various trisubstituted cyclopen-tenones (149), thus resulting from the cyclotetramerization of acrylates, have been obtained in moderate yields. 

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