CHROMIUM AND TUNGSTEN PENTACARBONYL GROUPS AS REACTIVITY AND SELECTIVITY AUXILIARIES IN [3+2] CYCLOADDITION OF ALKYNYL FISCHER CARBENE COMPLEXES WITH N-ALKYL NITRONES

Alkynyl Fischer carbene complexes were found to undergo chemoselective, regioselective, and rate-enhanced 1,3-dipolar cycloaddition with nitrones to give 2,3-dihydroisoxazole carbene complexes in excellent yields. These alkynyl complexes can serve as synthons for substituted propiolate esters since the metal pentacarbonyl group of the cycloadducts can be easily oxidatively removed with DMSO. The tungsten carbene complex 4b reacted with 3 different series of N-alkyl nitrones: N-tert-butyl 2a-f, N-methyl 1a-h and N-benzyl 3 to give cycloadducts 6a-f, 7a-h, and 8-9, respectively, while the chromium carbene complex 4a reacted only with N-tert-butyl nitrones 2a-g to give isolable cycloadducts 5a-g. The structure of N-methyl cycloadduct 7d has been established by a single-crystal X-ray analysis. The reactivity of N-alkyl nitrones toward carbene complexes was in the order: N-Me > N-Bn > N-(t)Bu. In addition, our results showed that the rate of cycloaddition reaction increased as the electron donating ability of para-substituent in N-tert-butyl nitrone 2a-f increased.