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?

Abstract

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.

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].…”

“…The Leigh group observed that cycloreversion is generally increased, if the Rydberg instead of the pp* state is excited [115,117]. They also found that at least part of the ring cleavage takes place via intermediate formation of cyclopropyl carbenes (that also deliver methylene cyclopropanes as end products) by ring contraction (alkyl 1,2-migration, Scheme 13) [118] (it is also conceivable that carbene formation and ring splitting result from the same conical intersection).…”

The effect of central bond torsional mobility on the Rydberg state ring opening of alkylcyclobutenes

“…The direct irradiation of simple alkylcyclobutenes in the gas phase or solution results in electrocyclic ring opening and stereospecific [2 + 2]-cycloreversion to yield the corre-sponding alkyne and alkene (1)(2)(3)(4). It has been known for several years that contrary to early indications (5), the ring opening reaction proceeds nonstereospecifically (6,7), in apparent violation of simple orbital symmetry selection rules for photoelectrocyclic reactions (8) as well as the results of high level ab initio calculations (9,10), all of which predict that the process should proceed with preferred disrotatory stereochemistry when initiated by population of the lowest π,π* excited singlet state of the molecule.…”

“…It has been known for several years that contrary to early indications (5), the ring opening reaction proceeds nonstereospecifically (6,7), in apparent violation of simple orbital symmetry selection rules for photoelectrocyclic reactions (8) as well as the results of high level ab initio calculations (9,10), all of which predict that the process should proceed with preferred disrotatory stereochemistry when initiated by population of the lowest π,π* excited singlet state of the molecule. Assuming that orbital symmetry selection rules are, in fact, obeyed in the π,π* reaction, there are at least three possible explanations for this behaviour (6): (i) ring opening proceeds adiabatically, yielding the photochemically allowed (disrotatory) diene isomer(s) in the lowest π,π* excited singlet state, from which the forbidden (thermally allowed, conrotatory) isomer(s) are formed via E,Z-isomerization; (ii) the conrotatory isomers are formed via reaction from upper vibrational levels of the ground state of the cyclobutene, populated by internal conversion in competition with disrotatory π,π* state ring opening; and (iii) the conrotatory isomers are formed via competing reaction of the radical-cation-like π,R(3s) Rydberg excited singlet state, which is known to be of similar energy to the π,π* (valence) singlet state in alkylcyclobutenes (4,11,12), just as it is in other aliphatic alkenes (13)(14)(15). 2 Much of our work in this area over the past 15 years has been directed at attempting to distinguish between these possibilities, through studies of the effects of substituents, excitation wavelength, and various structural constraints on the stereochemistry of the reaction (4,6,12,(16)(17)(18)(19)(20)(21)(22)(23).…”

“…Assuming that orbital symmetry selection rules are, in fact, obeyed in the π,π* reaction, there are at least three possible explanations for this behaviour (6): (i) ring opening proceeds adiabatically, yielding the photochemically allowed (disrotatory) diene isomer(s) in the lowest π,π* excited singlet state, from which the forbidden (thermally allowed, conrotatory) isomer(s) are formed via E,Z-isomerization; (ii) the conrotatory isomers are formed via reaction from upper vibrational levels of the ground state of the cyclobutene, populated by internal conversion in competition with disrotatory π,π* state ring opening; and (iii) the conrotatory isomers are formed via competing reaction of the radical-cation-like π,R(3s) Rydberg excited singlet state, which is known to be of similar energy to the π,π* (valence) singlet state in alkylcyclobutenes (4,11,12), just as it is in other aliphatic alkenes (13)(14)(15). 2 Much of our work in this area over the past 15 years has been directed at attempting to distinguish between these possibilities, through studies of the effects of substituents, excitation wavelength, and various structural constraints on the stereochemistry of the reaction (4,6,12,(16)(17)(18)(19)(20)(21)(22)(23). By the mid-1990s, we had concluded: (i) that the π,R(3s) state is responsible for the [2 + 2]-cycloreversion reaction but is not involved in ring opening (12,24); and (ii) that the adiabatic, π,π* state ring opening mechanism most adequately accounts for the nonstereospecificity of the reaction (19,21,22).…”