Carbene from carboxamides

Table 2.6. Heteroatom-substituted carbene complexes prepared from carboxamides.

Dilute solutions in aqueous buffers exposed to diffused or direct sunlight gave dimethylamine and 5-diazoimidazole-4-carboxamide (215). At pH 1.0 or pH 7.4 and above, this cyclized to 2-azahypoxanthine (216). In the intermediate pH range, a different product was obtained (217) which happens to be the aglycone of the antibiotic bredinin. Compound (217) was not formed by irradiation of (216). It was suggested that the betaine arose from a carbene (218) which had been quenched by water. However, (217) is colourless. Formulated injection solutions which had been found to lose activity contained 10 mg/ml... 

Experimental Procedure 2.1.2. Preparation of a Chromium Carbene Complex from a Carboxamide [(4)-2,2-Dimethyl-4-phenyloxazolidin-3-yl]methylene pentacarbonylchromium [126]... 

Dinuclear Rh(II) compounds are another class of effective catalysts (227). Electrophilic carbenes formed from diazo ketones and dimeric Rh(II) carboxylates undergo olefin cyclopropanation. Chiral Rh(II) carboxamides also serve as catalysts for enantioselective cyclopropanation (Scheme 95) (228). The catalysts have four bridging amide ligands, and... 

Support from the National Science Foundation and the National Institutes of Health (GM-46503) for investigations of the design and development of chiral dirhodium(II) carboxamidate catalysts for asymmetric metal carbene transformations is gratefully acknowledged. 

Heimgartner and co-workers treated a-diazoketones and a-diazoamides 64 with thiones, with and without a catalyst such as Rh(OAc)2 present (1998HCA285). The products were substituted thiiranes 65 and/or substituted 1,3-oxathioles. In all cases, a thiocarbonyl ylide intermediate, which could undergo either a 1,3- or a 1,5-electro-cyclization, was held responsible. The ylide could arise either from addition of a carbene or a carbenoid to S of the thiocarbonyl compound or by loss of N2 from a primary cycloadduct between the diazo and the thiocarbonyl compounds. In one case, such a primary adduct was isolated. The thiirane carboxamides could be desulfurized with (Me2N)3P in tetrahydrofuran (THF) at 60 °C to afford acrylamides 66 (Scheme 11). 

ChiralcarboxamMatecomplexes. After exchanging the ligands of dirhodium tetraacetate to chiral pyrrolidinones (as well as their heteroatom analogs) bearing a methyl ester at C-5 new carboxamidate complexes are formed. These are catalysts of choice for enantioselective intramolecular metal carbene transformations. One such complex is particularly effective for the formation of P-benzyl-y-butyrolactones from hydrocinnamyl diazoacetates. The lactones are useful for the synthesis of some lignans. 

Cyclopropanation of olefins is currently performed by direct transition metal-catalyzed carbene transfer from a diazo compound to the olefin. Dirhodium(II) carboxylates and carboxamidates have proved to be the catalysts of choice. Other rhodium compounds, such as Rh (CO),6, Rh2(BF4)4, and rhodium(III) porphyrins, have been also investigated, but did not show better reactivity, while rhodium(I) compounds have never been successful [66]. Other complexes containing copper or ruthenium have been tested in cyclopropanation reactions, but have never shown better reactivity or selectivity than rhodinm(II) compounds [67]. 

Exploratory experiments have shown that glycosylidene carbenes also insert into the N-H bond of sulfonamides, whereas carboxamides yield esters, as illustrated by the product obtained from an A-Boc-asparagine benzyl ester (Figure 7c) [32]. Presumably, such esters are formed by hydrolysis of the initially formed imino ether 0-glycosides, suggesting that glycosylidene diazirines may be used for (selective ) cleavage of peptidic bonds. 

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