THEORETICAL-STUDIES OF THE NAZAROV CYCLIZATION .2. THE EFFECT OF BETA-SILYL AND BETA-METHYL GROUPS

Ab initio molecular orbital calculations have been carried out at the RHF/3-21G(*) level on the protonated form of 1,4-pentadien-3-one bearing a methyl, silyl (SiH3), or no (i.e., H) substituent at the beta position. Electron correlation was accounted for by MP3/6-31G** single-point calculations. Substituent-effects on the rate of reaction are steric and inductive, while the trends observed for heats of cyclization are attributed primarily to hyperconjugation, in agreement with earlier work by Jorgensen on the effects of these substituents on carbenium ions. Cyclization of the unsubstituted (-2.4 kcal/mol) and beta-SiH3-substituted (-5.9 kcal/mol) compounds are exothermic, while the beta-CH3-substituted cyclization is endothermic by 4.5 kcal/mol. The transition states for the electrocyclization reactions are all reactant-like and relatively invariant as a function of beta-substitutent; the newly forming C-C bond length varies from 2.09 angstrom with no substituent to 2.07 angstrom for beta-SiH3 to 2.05 angstrom for beta-CH3. The activation energies are 20.4, 21.7, and 21.9 kcal/mol, respectively.