The Thermally and Photochemically Induced Ring Opening ofcis-3,4-Dichlorocyclobutene andtrans-3,4-Dichlorocyclobutene: New Insights from a Matrix-Spectroscopic Study

Abstract

The thermally induced ring opening of 1 and 5 proceeds with high stereospecifity in a conrotatory fashion in agreement with the Woodward-Hoffmann rules. Upon flash pyrolysis of 1 the matrix-isolated (Xe, 12 K) products 2a and 2b were found, whereas the corresponding reaction of 5 yielded 3a and 3b. The s-cis conformers 2b and 3b were identified for the first time by comparison of the experimental and calculated (BLYP-6-311G*) IR absorptions. A non-stereospecific photochemical reaction could be observed if matrix-isolated (Ar or Xe, 12 K) 1 was excited directly with λ = 193 nm. Products of the allowed disrotatory pathway as well as those of the forbidden conrotatory reaction were found, a behaviour that corresponds with the known photochemistry of cyclobutenes. A surprising reaction was found if 1 and 5 were irradiated in a xenon matrix with λ > 270 nm. The precursors, which were photostable in an argon matrix, opened the ring in a conrotatory fashion, the expected pathway for a thermal reaction. In this case the light is absorbed by the solid xenon in a cooperative process with the precursor molecules. A detailed discussion of the possible energy transfer mechanisms is given. Most probably, a hot ground-state reaction of vibrationally excited 1 and 5 occurs, although the reaction of the radical cations cannot be excluded.