3-Nitrene-2-formylthiophene and 3-Nitrene-2-formylfuran: Matrix Isolation, Conformation, and Rearrangement Reactions.

Two new heteroarylnitrenes, 3-nitrene-2-formylthiophene (/) and 3-nitrene-2-formylfuran (/), in the triplet ground state have been generated in solid Ar (10.0 K) and N (15.0 K) matrices by the 266 nm laser photolysis of 3-azido-2-formylthiophene () and 3-azido-2-formylfuran (), respectively. According to the characterization with matrix-isolation IR spectroscopy and quantum chemical calculations at the B3LYP/6-311++G(3df,3pd) level, both nitrenes exhibit two conformations depending on the orientation of the formyl groups. Upon subsequent green-light irradiation (532 nm), both the nitrenes / and / undergo ring closure to form 3,2-thienoisoxazole () and 3,2-furoisoxazole (), respectively. Traces of 3-imino-4,5-dihydrothiophene-2-ketene (), formally formed through the intramolecular 1,4-H shift in the corresponding nitrenes /, have been also identified among the laser photolysis products of the azide . In sharp contrast to the photochemistry, the high-vacuum flash pyrolysis (HVFP) of the azide at ca. 1000 K mainly yields imino ketene in two conformations / together with traces of isoxazole . In addition to the reversible conformational interconversion in the imino ketene ↔ , the photoisomerization from isoxazole to imino ketene has also been observed. The HVFP of the azide at ca. 1000 K results in complete dissociation to HCN, CH, CO, CO, HO, and N. Unlike the recently disclosed hydrogen-atom tunneling (HAT) in the transformation from the structurally related 2-formyl phenylnitrene () to imino ketene in a cryogenic Ar-matrix, the absence of HAT in nitrenes and can be reasonably explained by the higher barrier heights and also larger barrier widths in the isomerization reactions.