Stereospecific (conrotatory) photochemical ring opening of alkylcyclobutenes in the gas phase and in solution. Ring opening from the Rydberg excited state or by hot ground state reaction?

The photochemistry of 1,2-dimethylcyclobutene and cis- and trans-1,2,3,4-te tramethylcyclobutene 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]-cyclore version. The total yields of dienes relative to 2-butyne are generally high er in the gas phase than in solution but decrease with increasing excitatio n wavelength under both sets of conditions. In the case of cis-1,2,3,4-tetr amethylcyclobutene, 228-nm photolysis results in the stereospecific formati on of E,Z-3,4-dimethyl-2,4-hexadiene- the isomer corresponding to ring open ing by the thermally allowed (conrotatory) electrocyclic pathway-in both th e 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 resu lts are consistent with cycloreversion resulting from excitation of the low -lying pi,R(3s) singlet state and with ring opening proceeding by at;least two different mechanisms depending on excitation wavelength. The first, whi ch dominates at short wavelengths, is thought to involve direct reaction of the second excited singlet (pi,pi*) state of the cyclobutene. The second m echanism, which dominates at long wavelengths, is proposed to ensue either directly from the lowest energy (Rydberg) state or from upper vibrational l evels of the ground state, populated by internal conversion from this excit ed state.