ChemPhysChem 8, 592-598 (2007).

© 2007 WILEY-VCH Verlag GmbH & Co.

Forward and Backward Pericyclic Photochemical Reactions Have Intermediates in Common, yet Cyclobutenes Break the Rules

Werner Fuß*, Wolfram E. Schmid, Sergei A. Trushin, Paul S. Billone, and William J. Leigh

Contribution from the Max-Planck-Institut für Quantenoptik, D-85741 Garching, Germany and the Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, ON Canada L8S 4M1.

Photochemical pericyclic reactions are believed to proceed via a so-called pericyclic minimum on the lowest excited potential surface (S₁), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3-butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward-Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but two common minima, p* and ct*: Starting from the side of the dienes (cyclohepta-1,3-diene and cycloocta-1,3-diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis-trans isomerization. Starting from the side of the corresponding cyclobutenes (bicyclo[3.2.0]hept-6-ene and bicyclo[4.2.0]oct-7-ene), the forbidden isomer is formed from ct*. The path branches at the first (S₂/S₁) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamical effect: the momentum in the C=C twist direction, acquired - such as in other olefins - in the Franck-Condon region of the cyclobutenes.

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