Fulmonitrile oxide

The controversy between Huisgen and Firestone concerning the mechanism for 1,3-dipolar cycloaddition is longstanding.9,11 For nitrile oxide cycloadditions, experimental data have been interpreted either as supportive of a concerted mechanism9 or in favor of a stepwise mechanism with diradical intermediates.11 Theory has compounded, rather than resolved, this problem. Ab initio calculations on the reaction of fulmonitrile oxide with acetylene predict a concerted mechanism at the molecular otbital level,12,13 but a stepwise mechanism after inclusion of extensive electron correlation.14 MNDO predicts a stepwise mechanism with a diradical intermediate.13 The existence of an extended diradical intermediate such as (4 Scheme 2) has been postulated by Firestone in order to account for the occasional formation of 1,4-addition products such as the oxime (5).11 Of course, the intermediates (4) and (5) for the Firestone mechanism do not correspond to the initial transition states in Firestone s theory. These are attained prior to the formation of, and at higher energy than, the intermediates. 

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