Imidoylnitrenes R′C(═NR)–N, Nitrile Imines, 1H-Diazirines, and Carbodiimides: Interconversions and Rearrangements, Structures, and Energies at DFT and CASPT2 Levels of Theory

Abstract

The structures, energies, and rearrangements of imidoylnitrenes H-C(═NH)-N, HN-C(═NH)-N, Ph-C(═NH)-N, H-C(═NPh)-N, and MeO-C(═NCN)-N (10a-e) are investigated at DFT and CASPT2 levels of theory. Imidoylnitrenes are potentially formed by pyrolysis or photolysis of azides, tetrazoles (6, 6'), or sydnones. Unlike most acylnitrenes, the imidoylnitrenes 10 have triplet ground states. The first excited states are the open-shell singlets (OSSs), lying between ca. 4 and 20 kcal mol above the triplets at the CASPT2 level. The second excited states are the closed-shell singlets (CSSs), lying >50 kcal mol higher in energy. The OSS imidoylnitrenes can ring-close to 1H-diazirines 9 with very low activation energies (2-12 kcal mol), and the 1H-diazirines can then rearrange to nitrile imines 8 with activation energies of 37-48 kcal mol. Conversely, nitrile imines generated directly by pyrolysis or photolysis of 2,5-substituted tetrazoles 6 can rearrange to 1H-diazirines 9 and imidoylnitrenes 10 with activation energies of 37-60 kcal mol. Finally, the imidoylnitrenes 10 can rearrange to carbodiimides 11 with modest activation barriers of 12-20 kcal mol. Calculated vibrational data, UV-vis spectra, and spin densities in the triplet states are also reported, and zero-field splitting parameters |D/hc| in the range 0.9-1 cm and nonzero |E/hc| values are predicted.

Photolysis and Pyrolysis of Phenyltetrazoles: Formation of Phenylcarbodiimide, N‐Phenylnitrile Imine, Phenylnitrene, Indazole, and Fulvenallene

“…The Z ‐isomer of imidoylnitrene PhN=CH–N 5b could be expected to cyclize to benzimidazole 12 (Scheme ) . Putative imidoylnitrenes formed from 1,5‐disubstituted tetrazoles have been reported to undergo such cyclizations, sometimes thermally, sometimes photochemically, but usually in yields inferior to those of the carbodiimides.…”

“…It is noteworthy that indazole is not formed on FVP of 1‐phenyltetrazole 2b in agreement with the computational finding that the activation barrier for rearrangement of the putative N ‐phenylimidoylnitrene 5b to phenylcarbodiimide 6 is 15.7 kcal/mol, whereas it is about 50 kcal/mol for rearrangement to phenylnitrile imine 3b …”

“…The formation of phenylcarbodiimide and absence of benzimidazole are in agreement with the calculated activation barriers: 15.7 kcal/mol for the rearrangement to the carbodiimide and 21.8 kcal/mol for the cyclization to benzimidazole at the B3LYP/6‐311G(d,p) level . Other things being equal, this would correspond to a rate ratio of ca.…”

“…The photolysis of 2b is expected to lead initially to the open‐shell singlet imidoylnitrene 5b , which rearranges rapidly to phenylcarbodiimide 6 , i.e. the same product as obtained from 1a , .…”

“…In the FVP of 2‐phenyltetrazole 1b , weak‐to‐medium intensity bands assigned to PhN=C=NH 6 and PhNHCN 7 are formed as expected (Figure S9). But in this case, the cyclization of the N‐ phenylnitrile imine 3b to indazole 15 has a lower calculated barrier (27 kcal/mol) than the rearrangement to phenyldiazirine 4b (40 kcal/mol) and hence N‐ phenylimidoylnitrene 5b and phenylcarbodiimide 6 . Therefore, indazole 15 is, in fact, the main product (Scheme ) as clearly seen in the IR spectrum of the pyrolysate (Figure ).…”