The photochemistry of alkylcyclobutenes at 228-nm: An investigation of the Rydberg excited state reactivity

Abstract

<p>In apparent contradiction with the Woodward-Hoffmann rules of pericyclic reactions, the photochemical ring-opening of alkylcyclobutenes to 1,3-dienes has been shown to proceed nonstereospecifically. There is now substantial evidence to support a mechanism in which the reaction occurs adiabatically to produce the symmetry-allowed diene in the excited singlet state. Relaxation to the ground state provides the observed mixture of diene isomers. Although the ring-opening is presumed to occur from the π,π* excited state alone, the potential involvement of the low-lying Rydberg π,R(3s) has never been completely ruled out. In order to assess the contribution of the Rydberg state to the photochemistry of cyclobutenes, a series of 1,2-dialkyl substituted derivatives were irradiated at 228-nm where the π,R(3s) state can be excited selectively. Under these conditions, it was found that these cyclobutenes continue to give rise to ring-opening products in substantial yields. Furthermore, with cis - and trans -1,2,3,4-tetramethylcyclobutene, the reaction proceeds with conrotatory (thermal) stereospecificity. The possibility of reaction from upper vibrational levels of the ground state was investigated. The quantum yields for ring-opening at 228-nm for a series of cyclobutenes were determined, and compared to values calculated with the assumption that reaction proceeds entirely via a hot ground state mechanism. The calculated quantum yields were based on rate constants from RRKM theory and the rate of relaxation from upper vibrational levels of the ground state. Hot ground state reactivity was shown to be insignificant by the lack of any correlation between the experimental and calculated quantum yields. The ring-opening reaction from the Rydberg state was shown to be dependent on the incipient torsional mobility about the central C1-C2 bond of the cyclobutene. Derivatives with small ancillary rings attached to the central bond displayed reactivity similar to the π,π* state, where those with large ancillary rings gave products enriched in the conrotatory diene isomers. Both the π,π* (adiabatic) and Rydberg state ring-opening reactions can be rationalized in terms of these results. The pathway to ring-opening on the Rydberg state potential energy surface likely encounters a barrier associated with twisting about the central bond. If the barrier is sufficiently large, the state can cross with the π,π* surface and products of adiabatic ring-opening are formed. When there is free rotation about the central bond, conrotatory ring-opening from the Rydberg state occurs.</p>