Photorearrangement of Bicyclo[2.2.1]heptene-2-carbonitrile Observation of 1,2 and 1,3-Sigmatropic Shifts

Abstract

<p>The α,β-unsaturated nitrile, bicyclo[2.2.1]heptene-2-carbonitrile, I, was synthesized from norcamphor. Irradiation in hexane with unfiltered light from a medium pressure mercury arc lamp resulted in its conversion to two products, bicyclo[4.1.0]hept-2-ene-1-carbonitrile, II, and tricyclo-[4.1.0.0.³,⁷]heptane-7-carbonitrile, III, in a 20:1 ratio.</p> <p>[equation removed]</p> <p>The structure of the major rearrangement product, II, was proven by X-ray crystallography of its crystalline derivative, 1-methylamino-bicyclo-[4.1.0]hept-2-ene-p-bromobenzenesulfonate. The m!nor product, III, was identified by catalytic hydrogenation to 7-cyanobicyclo[2.2.1]heptane and vpc comparison with authentic 7-cyanobicyclo[2.2.1]heptane synthesized from bicyclo-[2.2.1]heptane-7-carboxylic acid.</p> <p>The photorearrangement of I to II and III are orbital symmetry-allowed photochemical 1,3 and 1,2-sigmatropic shifts, respectively. The 1,3-sigmatropic shift has not been previously observed in the photochemistry of α,β-unsaturated nitriles while the 1,2 shift was observed in the photorearrangement of 1-cyanocyclohexenes. Further experiments are needed to determine whether or not the rearrangements are concerted or diradical in nature.</p> <p>Preliminary experiments on the related compound, bicyclo[3.2.0]oct-2ene-3-carbonitrile, IV, synthesized from norbornene, indicate that it probably undergoes the same type of rearrangements as I. The ratio for 1,3,to 1,2 a bond shifts seems to be 3:2 in this case.</p> <p>[figure removed]</p>