Cycloaddition Reactions of a Nitrogen-Substituted Oxyallyl Cation with Cyclopentadiene and Substituted Furans. Reaction Conditions, Diastereoselectivity, Regioselectivity, and Transition State Modeling
journal contributionposted on 23.02.1996, 00:00 by Michael A. Walters, Helene R. Arcand
An investigation of the cycloaddition reactions of a nitrogen-substituted oxyallyl cation is presented. The nitrogen-substituted oxyallyl cation, M+-1 (M+ = H+ or Li+), can be generated from the dibromide 2 using either CF3CH2OH/Et3N or LiClO4/CH3CN/Et3N. These oxyallyl cations were found to undergo [4 + 2] cycloaddition reactions with furan, cyclopentadiene, 2-methylfuran, and 2-methoxyfuran. The diastereo- and regioselectivities in these reactions were found to be positively influenced by the presence of Li+. Reaction of M+-1 with the 2-substituted furans led to primarily those adducts arising from endo addition (as was observed with the unsubstituted dienes) and which had the 2-substituent syn to the bromine. Consideration of the frontier molecular orbitals of the reacting species (Li+-1 and 2-substituted furans calculated at the PM3 semiempirical level of theory) led to the conclusion that FMO theory does not explain the regiochemistry observed in this process, although the relative electrophilicity of M+-1 (M+ = H+ or Li+) could be rationalized. Transition structure modeling was consistent with empirical observations in that it predicted an endo addition of furan to Li+-1 via a stepwise reaction. Calculation of the reaction coordinate for this nonconcerted process predicted a ΔHrxn ∼ −50 kcals/mol and ΔHact ∼ +11 kcal/mol, with the second bond-forming process having a ΔHact ∼ +1.7 kcal/mol.