Theoretical Aspects, Calculations, and Physical Data

Theoretical Aspects, Calculations, and Physical Data.—The basis of orbital-corre-pondence analysis in maximum symmetry has been described in terms of the familiar A and S notation, and the relationship to the Woodward-Hoffmann and Longuet-Higgins-Abrahamson analyses of pericyclic reactions discussed. In a useful and 

Kniezo, and J. Hapala, Coll. Czech. Chem. Comm., 1975,40, 3862. T. Sasaki, K. Kanematsu, and A. Kondo, Chem. Letters, 1976, 783. 

Complete lineshape analysis of the ABCj DEF2 exchange process observed in the n.m.r. spectra of 2,6-disubstituted homotropylidenes has been employed in the evaluation of the thermodynamic parameters for Cope rearrangement, The phenyl groups at the 2,6-positions of the barbaralane (155) destabilize the transition state relative to the unsubstituted hydrocarbon (154) as evidenced by the AGj g values, respectively 9.3 and 7.6 ( 0.1) kcalmol Kinetic data for the Diels-Alder addition of acrolein to cydohexa-1,3-diene, and its reverse reaction, in the gas phase have been interpreted in favour of a biradical mechanism. Both the heat and entropy of formation of the endo-adduct have been estimated. Structural modification of 2,3-dimethylenenorbornane (156) or of 2,3-dimethylenenorbornene (158) by the introduction of oxygen in place of C-7, as in (157) and (159) respectively, leads to a decrease in rate of Diels-Alder reaction of the dienes with dimethyl acetylenedi-carboxylate. The double bond in (158) likewise resulted in deactivation relative to (156), and epoxidation of the endocyclic double bond in (158) also afforded an s-cis-diene of decreased reactivity. 

An ab initio MO computational study of the Diels-Alder addition of ethylene to butadiene reveals a symmetrical transition state and a concerted reaction profile. Extended Htickel MO calculations on the endo.exo product distribution in [4 + 2] reactions of cyclopentadiene indicate, within the range of intermolecular distances investigated, that the ewdo-orientation is energetically favoured. A study of the relationship between isomer distribution and solvent polarity in the reactions of cyclopentadiene with acrylonitrile and with methyl acrylate reveals that the endo-adduct. dicyclopentadiene and exo-adduct rdicyclopentadiene ratios are increased linearly with the empirical solvent polarity factor The favoured endo- 

Seguchi, A. Sera, and K. Maruyama, Bull. Chem. Soc. Japan, 1976,49, 3558. 

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