Rearrangement transition state analysis

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

The reasons for such retardation effects were disclosed by quantum chemical calculations. Analysis of geometric characteristics for the initial and transition states of the rearrangements obtained by DFT method with the use of PRIRODA program (27, 28) showed that unfavorable steric interactions in transition states of the rearrangements of cations lb,c were responsible for the retardation effects (25). 

The aromaticities of symmetry-allowed and -forbidden transition states for electrocyclic reactions and sigmatropic rearrangements involving two, four, and six r-electrons, and Diels-Alder cycloadditions, have been investigated by ab initio CASSCF calculations and analysis based on an index of deviation from aromaticity. The order of the aromaticity levels was found to correspond to the energy barriers for some of the reactions studied, and also to the allowed or forbidden nature of the transition states.2 The uses of catalytic metal vinylidene complexes in electrocycliza-tion, [l,5]-hydrogen shift reactions, and 2 + 2-cycloadditions, and the mechanisms of these transformations, have been reviewed.3... 

One classic example is an experiment reported by Doering and Roth in 196217 (Scheme 1.XIII). Upon heating, racemic 3,4-dimethylhexa-l,5-diene (13) rearranged to a mixture of (2 ,6 )-octa-2,6-diene (90%), (2Z,6Z)-octa-2,6-diene (9%), and a trace amount of (2 , 6Z)-isomer. The experimental results are explained in terms of a six-membered transition state17 (Scheme 1.XIV). Chairlike transition state A is favored over transition state B based on the conformational analysis of 1,2-dimethylcyclohexane, in which the methyl substituents prefer to be in an equatorial position. The observation that 14 Z was formed in only trace amounts indicates that boatlike transition state C is of significantly higher energy than transition state A or B. 

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