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Addicott C, Wong MW, Wentrup C: 2-pyrazinylnitrene and 4-pyrimidylnitrene. J Org Chem. 2002 Nov 29;67(24):8538-46.
Ring expansion to 1,3,5-triazacyclohepta-1,2,4,6-tetraene and ring opening to (2-Isocyanovinyl) carbodiimide.. Tetrazolo [1,5-a] pyrazine/2-azidopyrazine 9T/9A undergo photolysis in Ar matrix at cryogenic temperatures to yield 1,3,5-triazacyclohepta-1,2,4,6-tetraene 21 as the first observable intermediate, and 1-cyanoimidazole 11 and (2-isocyanovinyl) carbodiimide 22 as the final products. The latter tautomerizes to 2-(isocyanovinyl) cyanamide 23 on warming to 40 K. The same intermediate 21 and the same final products are obtained on matrix photolysis of the isomeric tetrazolo [1,5-c] pyrimidine/4-azidopyrimidine 24T/24A. These photolysis results as well as those of the previously reported thermal ring contraction of (15) N-labeled 2-pyrazinyl- and 4-pyrimidylnitrenes to 1-cyanoimidazoles can all be rationalized in terms of selective ring opening of 21 or nitrine 10 to a nitrile ylide zwitterion 28 prior to formation of the final products, 11 and 22. The results are supported by high-level ab initio and DFT calculations (CASPT2-CASSCF (6,6), G3 (MP2), and B3LYP/6-31+G) of the energies and IR spectra of the intermediates and products. |
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