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

Abstract

The stereochemistry of the π,R(3s) excited state ring opening of a series of bicyclic alkylcyclobutenes has been studied in hydrocarbon solution with 228 nm excitation. In these compounds, the C=C bond is shared between the cyclobutene ring and a five-, six-, or seven-membered ancillary ring, which has the effect of restricting the torsional mobility about the central C—C bond in the isomeric diene products. It has previously been shown that monocyclic alkylcyclobutenes undergo stereospecific conrotatory ring opening upon excitation at the long wavelength edge of the π,R(3s) absorption band (228 nm), and nonstereospecific ring opening upon irradiation at shorter wavelengths (within the π,π* absorption band). Different behaviour is observed for the bicyclic systems studied in the present work. The bicyclo[3.2.0]hept-1-ene, bicyclo[4.2.0]oct-1-ene, and one of the bicyclo[5.2.0]non-1-ene derivatives yield nearly the same mixtures of E,E- and E,Z-diene isomers upon irradiation at 214 and 228 nm, with the product mixtures being heavily weighted in favor of the isomer(s) corresponding to disrotatory ring opening. The results may indicate that the stereochemical characteristics of the Rydberg-derived ring opening of alkylcyclobutenes depends on the ability of the molecule to twist about the "central" bond (i.e., the C=C bond in the cyclobutene) as ring opening proceeds. It is proposed that restricting the torsional mobility about the central bond activates internal conversion from the π,R(3s) to the π,π* potential energy surface, from which predominant disrotatory ring opening ensues.Key words: cyclobutene, Rydberg, ring opening, photopericyclic, electrocyclic.

Predistortion amplified in the excited state

“…So, the momentum model can readily explain also the unexpected anti-WH selectivity on Rydberg excitation. A further confirmation is the finding (also by Cook and Leigh [114]) that the violation of the WH rules on Rydberg excitation is substantially turned down, if the double-bond torsional (conrotatory!) mobility is limited by fusing a four-, five-, or six-membered ring to it.…”

“…This is because the anti-WH momentum stemming from the FC region can partially compensate the less favorable energetics of the conrotatory path in the branching region, so that both paths become competitive [56]. This mechanism also found considerable support in the observation that formation of anti-WH products is substantially reduced in cyclobutenes, in which double-bond torsion is hindered by a small additional ring fused to the C¼C bond [56,114].…”

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

“…This mechanism finds support in the observation that formation of anti-WH products is substantially reduced in cyclobutenes in which double-bond torsion is hindered by the presence of an adjoining ring. [31] This suggests that the mechanism and considerations of this work can be applied generally to diene/cyclobutene systems.…”

Visible-Light-Induced Dehydrohalogenative Coupling for Intramolecular α-Alkenylation: A Way to Build Seven- and Eight-Membered Rings

“…Subsequently, the cyclobutene might undergo a retro-[2 + 2]-cycloaddition to yield the desired α-alkenylated product. As a key step of this strategy, the synthetically useful retro-[2 + 2]-cycloaddition of cyclobutene is normally a thermo-promoted or UV irradiation-promoted reaction. The reported UV irradiation promoted example of a retro-[2 + 2]-cycloaddition leads to a complicated mixture owing to several uncontrollable side reactions or isomerizations.…”