The exceptionally high stereospecificites in some recent cycloaddition cases do not rule out diradical intermediates, because (1) rotation barriers for free radicals >0, ( 2 ) those for diradicals may exceed those for free radicals, and (3) strain-free cycliratians t o 5and 6-membered rings generally enjoy large rate advantages aver their inrermolecular counterparts.A prime f a c t o r in mechanistic thinking D i d -A l d e r and 1.3-dipolai cyclaadditions is stereospecifi~ity, which means that groups disposed cis or trans to each orher in the dienophile or dipalarophile remain so in the cycloadducr.It is required by the concerted mechanism, and permirted by t h e diradical mechanism.Although it is often held to general, in m o s t examples this is 1 based on a mass balance of only 90-95% .Recently, however, several papers have appeared describingexamples of borh reactions that are stereospecific to within very small error limits2'3'4.The authors say that these must be concerted, because if a diradical intervened, some rotation in the dieno/dipolarophile would have been observed.This paper is a reply to that argument.
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Raymond A. Firestone (1987) studied this question.