Unusual photochemical C-N Bond Cleavage in the novel Methyl 2-Chloro-3-Methyl-2H-Azirine-2-Carboxylate

A. GÓMEZ-ZAVAGLIA, A. KACZOR, A. L. CARDOSO T. M. V. D. PINHO E MELO AND R. FAUSTO

Estambul, Turquía

Congreso; European Congress on Molecular Spectroscopy EUCMOS 2006; 2006

EUCMOS

The structure, preferred conformers, vibrational spectrum and photochemical behavior of the novel azirine, methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate (MCMAC) were investigated in low-temperature matrices and neat solid amorphous state by infrared spectroscopy and quantum-chemical calculations. Two conformers of the compound were observed in the argon, krypton and xenon matrices, in agreement with the DFT(B3LYP)/6-311++G(d,p) and MP2/6-311++G(d,p) theoretical calculations. Both conformers were found to exhibit the carboxylic ester group in the Cis conformation, differing in the arrangement defined by the O=C-C-Cl dihedral angle (cis and trans, for Ct and Cc forms, respectively). The Ct conformer was found to be the most stable conformer in the gaseous phase as well as in both argon and krypton matrices, whereas the more polar Cc conformer became the most stable form in the xenon matrix and in the solid amorphous phase. In situ broadband UV (l> 235 nm) excitation of matrix-isolated MCMAC led to azirine ring C-C and C-N bond cleavages, the latter process corresponding to the most efficient reaction channel. The photochemical cleavage of C-N bond had never been observed in case of aliphatic 2H-azirines. Its observation in MCMAC is attributed to the simultaneous presence in this novel azirine of two electron withdrawing substituents (methylcarboxy group and chlorine atom) connected to the azirine ring, which accelerates intersystem crossing towards triplet state from where cleavage of the C-N bond takes place. The primary photoproducts resulting from the C-N and C-C ring-opening reactions were also found to undergo further photochemical decarbonylation or decarboxylation reactions. Acknowledgements: This work was funded by Fundação para a Ciência e a Tecnologia, Portugal (Grant #SFRH/BPD/17081/2004 and projects POCTI/QUI/59019/2004, POCTI/QUI/58937/2004, SeCyT-GRICES: PO/PA04-EVI/001 and PO/PA04-EIX/018, FEDER, and Agencia Nacional de Promoción Científica y Tecnológica (PICT 13080)