An ab initio molecular orbital study of the conformational energies of 2-alkyltetrahydro-2H-pyrans (tetrahydropyrans, oxacyclohexanes, oxanes)

Ab initio Hartree-Fock and Density Functional Theory calculations were used to obtain the geometries and relative energies of the rotamers in the chair conformations of 2-alkyltetrahydro-2H-pyrans and 2- (trimethylsilyl)tetrahydro-2H-pyran. The MP2/6-31G*//6-31G* conformational energies (-ΔG°or A values, kcal/mol) of the 2-alkyltetrahydro-2H-pyrans (Me = 3.18; Et = 3.04; i-Pr = 3.03; t-Bu = 7.56; neo Pent = 2.84) and 2- (trimethylsilyl)tetrahydro-2H-pyran (SiMe₃ = 4.77) are larger than those calculated for the corresponding alkylcyclohexanes and 2-alkyltetrahydro-2H- thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes). Plots of the calculated conformational energies for the 2-substituted tetrahydro-2H-pyrans versus the calculated -ΔG°values for the corresponding alkylcyclohexanes (slope = 1.34 and r = 0.983) and for the corresponding 2-substituted tetrahydro-2H-thiopyrans (slope = 2.01 and r = 0.986) are linear. (C) 2000 Elsevier Science B.V.