ELECTROSTATIC ACCELERATION OF ELECTROCYCLIC REACTIONS BY METAL CATION COMPLEXATION - THE CYCLIZATION OF 1,3-CIS-5-HEXATRIENE INTO 1,3-CYCLOHEXADIENE AND THE 1,5-HYDROGEN SHIFT IN CYCLOPENTADIENE - THE AROMATICITY OF THE TRANSITION STRUCTURES

Appreciable electrostatic acceleration of the cyclization of 1,3-cis-5-hexatriene to 1,3-cyclohexadiene by Li+ complexation is predicted at theoretical levels where the computed kinetic parameters of the uncatalyzed reaction (E(a) = 30.4-30.6 kcal/mol, A = 6.1 x 10(11)) agree with experiment (E(a) = 29.9 +/- 0.5 kcal/mol, A = (7.2 +/- 5.2) x 10(11)). The calculated Li+ electrostatic acceleration effect is very large, 9.2 kcal/mol (RMP4SDTQ) and 11.5 kcal/mol (Becke3LYP), even in this hydrocarbon system. A similar accelerating effect (10.9 kcal/mol, Becke3LYP) is found for the complexation by Li(H2O)(+), a simple solvation model. These accelerations are due to the greater electrostatic stabilization of the transition structure than the ground state. The aromaticity of the 6-electron cyclization transition structure is characterized by its exalted magnetic susceptibility (Lambda, -17.9 ppm cgs) and by upfield lithium chemical shift (-7.8) ill the Li+-complexed transition structure, computed by the IGLO method. The accelerations of various metal cations have been compared with the 8.0 kcal/mol effect for Li+. The reductions of the 1,5-hydrogen shift barrier ill cyclopentadiene are 4.7 kcal/mol for Na+, 12.3 kcal/mol for Mg2+, 13.7 kcal/mol for HBe+, and 19.9 kcal/mol for Be2+, with 8.0 kcal/mol for Li+, respectively. The computed order of the cation acceleration is Be2+ > HBe+ greater than or equal to Mg2+ > Li+ > Na+, comparable with the measured Mg2+ > Li+ > Na+.