Photocycloaddition and ortho-hydrogen abstraction reactions of methyl arylglyoxylates: structure dependent reactivities

In photocycloaddition reactions between methyl arylglyoxylates and certain cyclo-1,3-dienes, the diastereochemical outcome of the photoproducts is sho wn to depend on the steric demand of the aryl group in the glyoxylate. Excl usive endo-aryloxetanes were produced with bulky arylglyoxylates while no s ignificant stereoselectivity was observed with glyoxylates containing less demanding aryl groups. This observation is rationalized by considering the stability of possible conformers of the intermediate 1,4-biradical at the i nstance of intersystem crossing. When these ortho-substituted phenylglyoxyl ates were irradiated in benzene, different reaction patterns were observed with different substituents on the aryl rings. The rates of ortho-hydrogen abstractions vary significantly among the individual compounds. Photoproduc ts thus resulting are dependent on the substrate structures.