Modern valence-bond description of the mechanisms of six-electron pericyclic reactions

TY - JOUR

T1 - Modern valence-bond description of the mechanisms of six-electron pericyclic reactions

AU - Karadakov, P B

AU - Cooper, D L

AU - Thorsteinsson, T

AU - Gerratt, J

PY - 2000

Y1 - 2000

N2 - The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ring-opening of cyclohexadiene.The SC descriptions of the electronic mechanisms of these three reactions are shown to substantiate the use of the long-established reaction schemes with full- and half-arrows, well-known from organic chemistry textbooks, in a context which is very meaningful, albeit slightly different from the classical interpretation. The half-arrows now indicate changes in the shapes of individual orbitals, accompanying the breaking of the bonds in which they participate in the reactant(s), and their re-engagement in new bonds within the product(s), rather than the movements of individual electrons. The full-arrows correspond to relocations of orbital, rather than electron pairs.The SC results strongly suggest that the Diels-Alder reaction between butadiene and ethene and the ring-opening of cyclohexadiene pass through aromatic conformations, while in the case of the 1,3-dipolar cycloaddition of fulminic acid to ethyne, the reacting system remains distinctly nonaromatic throughout the course of the reaction.

AB - The combination of modem valence bond theory, in its spin-coupled (SC) form, and intrinsic reaction coordinate calculations utilizing a complete-active-space self-consistent field (CASSCF) wavefunction, is demonstrated to provide quantitative and yet very easy-to-visualize models for the electronic mechanisms of three gas-phase six-electron pericyclic reactions, namely the Diels-Alder reaction between butadiene and ethene, the 1,3-dipolar cycloaddition of fulminic acid to ethyne, and the disrotatory electrocyclic ring-opening of cyclohexadiene.The SC descriptions of the electronic mechanisms of these three reactions are shown to substantiate the use of the long-established reaction schemes with full- and half-arrows, well-known from organic chemistry textbooks, in a context which is very meaningful, albeit slightly different from the classical interpretation. The half-arrows now indicate changes in the shapes of individual orbitals, accompanying the breaking of the bonds in which they participate in the reactant(s), and their re-engagement in new bonds within the product(s), rather than the movements of individual electrons. The full-arrows correspond to relocations of orbital, rather than electron pairs.The SC results strongly suggest that the Diels-Alder reaction between butadiene and ethene and the ring-opening of cyclohexadiene pass through aromatic conformations, while in the case of the 1,3-dipolar cycloaddition of fulminic acid to ethyne, the reacting system remains distinctly nonaromatic throughout the course of the reaction.

KW - CHEMICAL-REACTION MECHANISMS

KW - CASSCF WAVE-FUNCTIONS

KW - STEPWISE MECHANISMS

KW - REPRESENTATIONS

KW - OPTIMIZATION

KW - 1,3-DIPOLES

KW - MOLECULES

KW - BUTADIENE

KW - BENZENE

M3 - Article

VL - 2

SP - 327

EP - 344

JO - QUANTUM SYSTEMS IN CHEMISTRY AND PHYSICS, VOL 1

JF - QUANTUM SYSTEMS IN CHEMISTRY AND PHYSICS, VOL 1

ER -