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Communication | Special issue | Vol 25, No. 1, 1987, pp.61-64
Published online, 1st January, 1970
DOI: 10.3987/S-1987-01-0061
Stereospecificity in Diels-Alder and 1,3-Dipolar Cycloadditions Does Not Prove the Concerted Mechanism

Raymond A. Firestone

*Medical Chemistry, Immunology and Inflammation Dept., Merck Sharp and Dohme, Research Laboratories, P.O.Box 2000, Rahway, NJ 07065, U.S.A.


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 cyclizations t o 5- and 6-membered rings generally enjoy large rate advantages over their inrermolecular counterparts.