Can. J. Chem., 81, 680-688 (2003).

2003 National Research Council of Canada

The Effect of Central Bond Torsional Mobility on the Rydberg State Ring Opening of Alkylcyclobutenes

Bruce H. O. Cook1 and William J. Leigh*

Contribution from the Department of Chemistry, McMaster University, Hamilton, ON  Canada  L8S 4M1

Abstract: The stereochemistry of the p,R(3s) excited state ring opening of a series of bicyclic alkylcyclobutenes has been studied in hydrocarbon solution with 228 nm excitation. In these compounds, the C=C bond is shared between the cyclobutene ring and a 5-, 6-, or 7-membered ancillary ring, which has the effect of restricting the torsional mobility about the central C-C bond in the isomeric diene products. It has previously been shown that monocyclic alkylcyclobutenes undergo stereospecific conrotatory ring opening upon excitation at the long wavelength edge of the p,R(3s) absorption band (228 nm), and non-stereospecific ring opening upon irradiation at shorter wavelengths (within the p,p* absorption band). Different behavior is observed for the bicyclic systems studied in the present work. The bicyclo[3.2.0]hept-1-ene, bicyclo[4.2.0]oct-1-ene, and one of the bicyclo[5.2.0]non-1-ene derivatives yield nearly the same mixtures of E,E-, and E,Z-diene isomers upon irradiation at 214- and 228-nm, with the product mixtures being heavily weighted in favor of the isomer(s) corresponding to disrotatory ring opening. The results suggest that the stereochemical characteristics of the Rydberg-derived ring opening of alkylcyclobutenes depends on the ability of the molecule to twist about the “central” bond (i.e., the C=C bond in the cyclobutene) as ring opening proceeds. It is proposed that restricting the torsional mobility about the central bond activates internal conversion from the p,R(3s) to the p,p* potential energy surface, from which predominant disrotatory ring opening ensues.

1 Present address:  3M Canada Company, 1840 Oxford St. E., London, ON Canada N6A 4T1


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