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| - Methyl Substitutent Effects in Radical Cation Diels−AlderReactions
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| Abstract
| - The effect of methyl substitution on the Diels−Alder radical cation reaction was studied using B3LYPwith a 6-31G* basis set. Five separate pathways, one concerted and four stepwise, were examined foreach possible position of a methyl substituent. None of the concerted transition structures could be locatedwithout symmetry constraints, and all but one of the structures obtained under these conditions weredestabilized by a second-order Jahn−Teller distortion. A concerted pathway with simultaneous bondformation at C1 and C4 is therefore excluded. Stepwise pathways that had the methyl group either on acarbon involved in the initial bond formation or in a position where it could not stabilize the radical/cation were a few kcal/mol above alternate pathways. High transition state energies for the formation ofvinylcyclobutane derivatives cause it to be a minor product in general. The pathway that proceeds throughan anti-intermediate is the most favored, while the pathways forming the gauche-out intermediate thatconverts to the anti-intermediate is also strongly represented. Both of the major pathways lead directlyto the formation of the methylcyclohexene product.
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