Journal of Organic Chemistry, 64, 5256-5263 (1999).

1999 American Chemical Society

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Stereospecific (Conrotatory) Photochemical Ring Opening of Alkylcyclobutenes in the Gas Phase and Solution. Ring Opening from the Rydberg Excited State or by Hot Ground State Reaction?

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

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

Abstract: The photochemistry of 1,2-dimethylcyclobutene and cis- and trans-1,2,3,4-tetramethylcyclobutene has been studied in the gas phase (1 atm; SF6 buffer) and in hydrocarbon solvents with 193-, 214- and 228-nm light sources. The major products are the isomeric dienes from electrocyclic ring opening and 2-butyne + alkene (ethylene or E-/Z- 2-butene) due to formal [2+2]-cycloreversion. The total yields of dienes relative to 2-butyne are generally higher in the gas phase than in solution, but decrease with increasing excitation wavelength under both sets of conditions. In the case of cis-1,2,3,4-tetramethylcyclobutene, 228-nm photolysis results in the stereospecific formation of E,Z-3,4-dimethyl-2,4-hexadiene - the isomer corresponding to ring opening by the thermally-allowed (conrotatory) electrocyclic pathway - in both the gas phase and solution. All three diene isomers are obtained upon 228-nm photolysis of trans-1,2,3,4-tetramethylcyclobutene, but control experiments suggest that the thermally-allowed isomers (E,E- and Z,Z-3,4-dimethyl-2,3-hexadiene) are probably the primary products in this case as well. The results are consistent with cycloreversion resulting (at least in part) from excitation of the low-lying p,R(3s) singlet state, and with ring opening proceeding by at least two different mechanisms depending on excitation wavelength. The first, which dominates at short wavelengths, is thought to involve direct reaction of the second excited singlet (p,p*) state of the cyclobutene. The second mechanism, which dominates at long wavelengths, is proposed to ensue either directly from the lowest energy (Rydberg) state, or from upper vibrational levels of the ground state, populated by internal conversion from this excited state.

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