Conrotatory Photochemical Ring Opening of Alkylcyclobutenes in Solution. A Test of the Hot Ground-State Mechanism

Abstract

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.

Predistortion amplified in the excited state

“…In this case, ring opening occurs with perfect conrotatory selectivity, i.e., in anti-WH direction, as found by Leigh and Cook [115]. It was therefore first supposed that the reaction actually takes place in the hot ground state; but this was soon disproved by the same authors [116]. A photochemical anti-WH ring opening can again be understood by momentum effects: the Rydberg state (symmetry B 1 ) has a low-lying CI with the pp* state (symmetry B 2 ) along the C¼C twist coordinate, because the former is practically not displaced along the torsion, whereas the latter is lowered by it.…”

Forward and Backward Pericyclic Photochemical Reactions Have Intermediates in Common, Yet Cyclobutenes Break the Rules

“…A hot ground-state mechanism for formation of the conrotatory diene isomer has been considered in detail, but has been excluded by experimental evidence. [10] The behavior is in conflict with Scheme 2 a, which shows a common funnel (CI p ) for ring opening and closing and a separate cis-trans isomerization path (via CI ct ) for the diene.…”

11 Organic photochemistry

“…Irradiation of 1,2-dialkyl-substituted cyclobutenes results in electrocyclic conrotatory ring opening, in competition with [2ϩ2] cycloreversion. 66 The possibility of a hot ground state mechanism for the ring opening was ruled out due to the lack of correlation between experimental and RRKM-calculated quantum yields for a ground state process. Thus the ring opening of alkylcyclobutenes is a true excited state process.…”