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-hexadienethe isomer corresponding to ring opening by the thermally allowed
(conrotatory) electrocyclic pathwayin 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.