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

Photolysis (lambda > 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 CD3 analogue supports this interpretation. Photolysis (lambda > 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.

Automated summary

This study experimentally verified the specific chemical reaction process of 1-(3-thienyl)diazoethane through photolysis, exploring its relationship with hydrogen migration and quantum mechanical tunneling effects.