Unusual Photochemical C−N Bond Cleavage in the Novel Methyl 2-Chloro-3-methyl-2H-azirine-2-carboxylate

Abstract

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 matrixes and in the neat solid amorphous state by infrared spectroscopy and quantum-chemical calculations. Two conformers of the compound were observed in argon, krypton, and xenon matrixes, 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 matrixes, whereas the more polar Cc conformer became the most stable form in the xenon matrix and in the neat solid amorphous phase. In situ broadband UV (lambda > 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 the C-N bond had never been previously observed in the case of aliphatic 2H-azirines. Two electron withdrawing substituents (methoxycarbonyl group and chlorine atom) are connected to the azirine ring in the novel MCMAC azirine. The simultaneous presence of these two groups accelerates intersystem crossing toward the triplet state 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.

Methyl 3-Methyl-2H-azirine-2-carboxylate Photochemistry Studied by Matrix-isolation FTIR and DFT Calculations

“…For example, the set of vibrations in the 1280-1260 cm -1 range can doubtlessly be attributed to the Cc conformer, while the groups of absorptions in the 1370-1330 and 1205-1185 cm -1 regions may be assigned predominantly to the Ct conformer, since only very low intensity bands of the Cc form are predicted by calculations to lie near these spectral regions (see Table 2). Similar to what was found for MCMAC, 35 considerable band splitting is observed for both conformers of MMAC, due to trapping of the solute molecules in different matrix sites.…”

“…[48][49][50][51] The trans-cis (C-O) energy differences in MMAC are similar to those found previously for MCMAC, where the T conformers were disfavored relative to the most stable conformer by more than 30 kJ mol -1 . 35 For MMAC, the energies of the Tt and Tg conformers (relative to the most stable conformer (Ct)) were predicted by the calculations to be 35.5 and 46.4 kJ mol -1 . Taking into account the high relative energies of the two T forms, they can be assumed to be of no practical importance and will not be further considered in this study.…”

“…Note that the reactivity of the primary photoproducts was found to be different for MMAC and the previously studied analogue chloro-substituted compound, MCMAC. 35 In MCMAC, both primary photoproducts (equivalent to P1 and P2) were found to take part in secondary photoreactions, with P1 undergoing decarboxylation and P2 decarbonylation. For MMAC, as described above, only P2 undergoes subsequent photoreactions.…”

“…These conformers can be correlated with the two most stable conformers of MCMAC. 35 It must, however, be pointed out that in MCMAC the dipole moments of the two conformers differ considerably (Ct, 2.6 D; Cc, 4.9 D), while in MMAC the replacement of the electronegative chlorine atom by the hydrogen atom leads to a significant decrease in the dipolar moment of the Cc conformer and the dipole moments become nearly similar in the two forms (Ct, 3.1 D; Cc, 3.7 D; see Table 1). Therefore, contrary to what was found for MCMAC, in the case of MMAC, no additional stabilization of the higher-energy Cc conformer is expected to take place in the matrixes relative to the gas phase.…”

“…Comparison between the photochemical reactivity of the two compounds can then be used to shed light on the relative importance of the chlorine and methoxycarbonyl substituents in determining the preference for the C-N bond photocleavage previously observed for MC-MAC. 35 The low-temperature matrix environment previously applied in the study of MCMAC is also used in the present investigation to allow for stabilization of the obtained photoproducts and reduction of the number of undesired cross reactions. The interpretation of the experimental results now obtained is also supported by extensive B3LYP/6-311++G(d,p) calculations, which enabled undertaking a detailed structural characterization of both the reactants and products of the studied photoreactions.…”