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 from excitation of the low-lying π,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 (π,π*) 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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Quantum yields for photochemical ring opening of six alkylcyclobutenes have been measured in hexane solution using 228-nm excitation, which selectively populates the lowest pi,R(3s) excited singlet states of these molecules and has been shown previously to lead to ring opening with clean conrotatory stereochemistry. The compounds studied in this work-1,2-dimethylcyclobutene (1), cis- and trans-1,2,3,4-tetramethylcyclobutene (cis- and trans-5), hexamethylcyclobutene (8), and cis- and trans-tricyclo[6.4.0.0(2,7)]dodec-1(2)-ene (cis- and trans-9)-were selected so as to span a broad range in molecular weight and as broad a range as possible in Arrhenius parameters for thermal (ground-state) ring opening. RRKM calculations have been carried out to provide estimates of the rate constants for ground-state ring opening of each of the compounds over a range of thermal energies from 20 000 to 49 000 cm(-1). These have been used to estimate upper limits for the quantum yields of ring opening via a hot ground-state mechanism, assuming a value of k(deact) = 10(11) s(-1) for the rate constant for collisional deactivation by the solvent, that internal conversion to the ground state from the lowest Rydberg state occurs with close to unit efficiency, and that ergodic behavior is followed. The calculated quantum yields are significantly lower than the experimental values in all cases but one (1). This suggests that the Rydberg-derived ring opening of alkylcyclobutenes is a true excited-state process and rules out the hot ground-state mechanism for the reaction.

The stereochemistry of the π,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 five-, six-, or seven-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 π,R(3s) absorption band (228 nm), and nonstereospecific ring opening upon irradiation at shorter wavelengths (within the π,π* absorption band). Different behaviour 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 may indicate 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 π,R(3s) to the π,π* potential energy surface, from which predominant disrotatory ring opening ensues.Key words: cyclobutene, Rydberg, ring opening, photopericyclic, electrocyclic.

Introduction Ground State Conformational Equilibria and E , Z ‐Isomerization The Excited Singlet States of Conjugated Polyenes: UV Absorption and Emission Spectra Singlet State Photochemistry of Conjugated Dienes and Trienes Conclusion