Ketocarbenoids carbenoid reactions

Whereas pyrrole was reported not to give N/H insertion by ketocarbenoids, such a reaction mode does occur with imidazole Copper-catalyzed decomposition of ethyl diazoacetate at 80 °C in the presence of imidazole gives ethyl imidazol- 1-ylacetate exclusively (93 %) small amounts of a C-alkylated imidazole were obtained additionally under purely thermal conditions 244). N/H insertion also takes place at benzimidazole 245 a). The reaction is thought to begin with formation of an N3-ylide, followed by N1 - C proton transfer leading to the formal N/H insertion product. Diazomalonic raters behave analogously however, they suffer complete or partial dealkoxycarbonylation under the reaction conditions 244) (Scheme 34). N-alkylation of imidazole and benzimidazole by the carbenoids derived from co-diazoacetophenone and 2-(diazoacetyl)naphthalene has also been reported 245 b>. 

The use of rhodium(II) acetate in carbenoid chemistry has also been extended to promoting intramolecular C/H insertion reactions of ketocarbenoids 277,280,280 ,). From the a-diazo-P-ketoester 305, highly functionalized cyclopentane 306 could thus be constructed in acceptable yields by regiospecific insertion into an unactivated... 

The use of ketocarbenoids with chiral auxiliaries has not been terribly effective at chiral induction. Menthol and bomeol esters of diazoacetates resulted in very low enantioselectivity.38 Some improvements were obtained by using the chiral amide (29 equation 13), but low overall yields were obtained due to competing intramolecular side reactions.39 Related studies with other types of carbenoids, however, have resulted in high enantioselectivity.60... 

The regioselectivity of the formal [3 + 2] cycloaddition reactions in Scheme 13 corresponds to the polarization of the olefinic bond and to partial charges in the ketocarbenoid. The only exception occurred with benzofuran, in agreement with the behavior of the carbenoid derived from ethyl 2-diazo-3-oxobutyrate As several examples in Scheme 13 show, the cycloaddition is sterospecific, i.e. the configuration about the enol ether double bond is retained in the dihydrofuran. The observation that in the case of 1-methoxy-1,3-butadiene highly selective cycloaddition of the unsubstituted double bond occurs, whereas for 2-methoxy-1,3-butadiene cycloaddition takes place at the more electron-rich double bond, finds a close analogy in the cyclopropanation of these same dienes with ethyl diazoacetate (see Table 9). 

The competition of a triple bond and a double bond for the ketocarbenoid is poorly selective similar to the situation in copper-catalyzed reactions . Alternatively, the carbenoid derived from 2-bromoethyl diazoacetate and Rh2 OAc)4 (25 °C) or CU/CUSO4 (100 °C) reacted with 2-methyl-l,5-hexadien-3-yne at the double bonds exclusively The same holds true for the synthesis of bis(l-methylcyclopropyl)acetylene from 2,5-dimethy -l,5-hexadien-3-yne and diazomethane copper(II) bis (N-a-phenylethylsahcylaldiminate) has been recommended there as an efficient catalyst... 

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