Photochemistry of 1-(2- and 3-Thienyl)diazoethanes: Spectroscopy and Tunneling Reaction of Triplet 1-(3-Thienyl)ethylidene.

Photolysis (λ > 613 nm) of 1-(3-thienyl)diazoethane ( 21 ) yields the s-E rotamer of triplet 1-(3-thienyl)ethylidene ( 3 ), as characterized by UV/vis and EPR spectroscopy. The s-Z rotamer of 3 was not observed. EPR and UV/vis signals attributed to carbene 3 decrease by approximately 50% upon standing in the dark for 68 h at 10 K. Although formally spin-forbidden, an intramolecular [1,2]-hydrogen shift in triplet carbene 3 to afford singlet s-E 3-vinylthiophene ( 8 ) is presumed to occur via quantum mechanical tunneling. The behavior of the CD 3 analogue supports this interpretation. Photolysis (λ > 613 nm) of 1-(3-thienyl)diazoethane- d 3 ( 21 - d 3 ) yields triplet 1-(3-thienyl)ethylidene- d 3 ( 3 - d 3 ), as characterized by IR, UV/vis, and EPR spectroscopy. No change in the signal intensity of EPR and UV/vis signals of triplet 3 - d 3 is observed upon standing in the dark for 68 h at 10 K. In a series of 2-substituted thienyl derivatives, irradiation of 1-(2-thienyl)diazoethane ( 22 ), 1-(2-thienyl)diazoethane- d 3 ( 22 - d 3 ), or (3-methyl-2-thienyl)diazomethane ( 23 ) does not yield triplet carbene intermediates. Positioning and labeling of the methyl group proved to have a large effect on products observed for these species. 1-(2-Thienyl)diazoethane ( 22 ) yields the products of [1,2]-hydrogen migration, s-Z and s-E 2-vinylthiophene ( 7 ), while 22 - d 3 and 23 give products derived from opening of the thiophene ring.