Cyclopropene Photochemistry. Mechanistic and Exploratory Organic Photochemistry

Article abstract of DOI:10.1021/jo00139a002

Four new cyclopropenes have been synthesized and their photochemistry has been investigated.Thus, 3-methyl-1,2-diphenyl-3-(2-phenylallyl)cyclopropene was found on direct photolysis to afford four photoproducts, the two major products of which derived from novel photochemistry. 2-Methylene-4-methyl-5,6-diphenyltetracyclo<5.4.0.0.^1,50^4,6>undeca-8,10-diene derived from <2 + 2> cycloaddition of the excited cyclopropenyl ? bond to the C-1,C-2 ? bond of the phenyl of the 2-phenylallyl side chain.The quantum yield for this novel transformation was 0.023.A second photoproduct was 2-methyl-1,4,6-triphenyltricyclo<2.2.0.0^2,6>hexane.This product results from a formal <2 + 2> cycloaddition of the excited cyclopropenyl ? bond to the allyl double bond.A diradical mechanism is structurally equivalent.Formation of such a tricyclic <2.2.0.0^2,6> system is normally not found in direct irradiations.The quantum yield for this product was 0.088.Also formed was 3-methyl-1,2,5-triphenylbicyclo<3.1.0>hex-2-ene with an efficiency of 0.019.This product is understood as deriving either (i) from cyclopropene opening to a carbene which then adds to the allyl ? bond or (ii) from a bicyclic diradical arising from vinyl-vinyl bonding.The fourth photoproduct is 2-methyl-1,3-diphenyl-3-(2-phenylallyl)cyclopropene.The efficiency was 0.018.The bicyclic diradical above provides a common species leading to the last three products.Sensitization led only to the first two products: the tetracyclic diene (φ=0.26) and the tricyclo<2.2.0.0^2,6>hexane (φ=0.15).Product structures were established by X-ray, degradation, independent synthesis, and spectral analysis.In addition, the photochemistry of cis- and trans-2,6-dimethyl-1,6-diphenylspiro<2.5>oct-1-ene was investigated.Stereoisomerization resulted from the singlet but not the triplet.Mechanisms for the above transformations are considered.Finally, corresponding thermal chemistry exhibited by the above compounds was investigated.