Abstract
The effects of donor/acceptor substitution on the conrotatory cyclobutene/butadiene interconversion have not yet been experimentally studied. MNDO Synchronous Transit/MINIMAX calculations for all possible combinations of NH2/NH3 + 3 groups at the fissile single bond reveal very large substituent effects on the opening reactions and reverse cyclisations. Diamino(1b/2b), diammonio-(1c/2c) and amino-ammonio-substitutions (1d/2d) change the activation energy of the reference ring opening reaction (1a →2a) from 49.9 kcal/mol to 34.7, 29.8 and 26.0 kcal/mol, respectively. This rate enhancement parallels the monotone shift of the transition state pathcoordinate towards the cyclobutenes. Relative to the electrocyclic reactions1a/2a —1c/2c the calculated result for1d/2d is in opposition to the Hammond postulate, but a detailed analysis of the reaction paths suggests a mechanistic deviation in this kind of reaction. The calculated data for the reference process (1a/2a) and the oxetene isomerisation (3/4) when compared with available experimental data indicate that the MNDO calculations accurately reflect the experimental trends.
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Jensen, A., Kunz, H. The electronic substituent influence on the thermally allowed electrocyclic interconversion cyclobutene/butadiene. A MNDO study. Theoret. Chim. Acta 65, 33–47 (1984). https://doi.org/10.1007/BF02427578
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DOI: https://doi.org/10.1007/BF02427578