Substituent effects on the photolysis of methyl 2-carboxylate substituted aliphatic 2H-azirines

In this study, the UV induced photochemical reactions of two 2H-azirines – methyl 2-chloro-3-methyl-2H-azirine-2-carboxylate (MCMAC) and methyl 3-methyl-2H-azirine-2-carboxylate (MMAC) – isolated in argon matrices are compared. For both compounds, irradiation with λ > 235 nm led to observation of two primary photoprocesses: (a) CC bond cleavage, with production of nitrile ylides (P1-type products), and (b) CN bond cleavage, with production of methylated ketene imines (P2-type products). However, subsequent photoprocesses were found to be different in the two cases. In MCMAC, both primary photoproducts were shown to undergo further reactions: P1-type products decarboxylate, giving [(1-chloroethylidene)imino]ethanide, which bears a CN+C− group (P3-type product); P2-type products decarbonylate, yielding a substituted ylidene methanamine (P4-type product). In MMAC, only P2-type primary photoproducts appeared to react, undergoing decarbonylation or decarboxylation (both reactions leading to P4-type products), whereas P1-type products were found to be non-reactive. The non-observation of any secondary photoproduct resulting from photolysis of P1-MMAC revealed the higher photostability of this species when compared with the corresponding photoproduct obtained from MCMAC.The CN photochemical cleavage is an unusual process in aliphatic 2H-azirines. In the studied compounds, its preference over the commonly observed CC azirine-ring bond photocleavage is attributed to the presence of electron withdrawing substituents (methylcarboxy group in both azirines and also the chlorine atom in MCMAC), which accelerates intersystem crossing towards the triplet state from where the cleavage of the CN bond takes place. The lack of the chlorine atom in MMAC may be partially compensated by the significantly higher stabilization of the P2-type photoproduct derived from this molecule (ca. −52 kJ mol−1) relatively to the reactant, when compared to that obtained from MCMAC (ca. −26 kJ mol−1). Nevertheless, the obtained results indicate that the methylcarboxy substituent plays the most important role in determining the photochemical behavior of these aliphatic 2H-azirines, in particular the preference they exhibit for the unusual CN bond cleavage over the “classic” CC bond cleavage.