Homotropylidenes, addition

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. 

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