Phenylnitrene Ring expansion, energies

Figure 7. Comparison of the energetics of the ring expansions of phenylcarbene ( A -la) and phenylnitrene (1A2-lb), calculated at the CASPT2(8,8)/6-31 G //CASSCF(8,8)/6-31 G level.57-61 The numbers in parentheses represent corrections for the known deficiencies of CASPT2/6-31G in computing the energies of singlet phenylnitrene61 and singlet phenylcarbene.55 The small differences in the energies in Fig. 5 are a consequence of the difference between the basis sets used in the two sets of calculations.

Figure 11.6. Relative energies (in kcal/mol) of species involved in the ring expansions of singlet fluoro-substituted phenylnitrenes calculated at the CASPT2/cc-pVDZ//CASSCF(8,8)/ 6-3IG level, (a) Difluorinated phenylnitrenes. (b) Monofluorinated phenylnitrenes. [Reproduced with permission from N. R Gritsan, A. D. Gudmundsdottir, D. Tigelaar, Z. Zhu, W. L. Kamey, C. M. Hadad, and M. S. Platz, J. Am. Chem. Soc. 2001, J23, 1951. Copyright 2001 American Chemical Society.]...

Fig. 8 CASPT2N/6-311G(2d,p) relative energies of species involved in the ring expansion of phenylnitrene (PN). Energies are for CASSCF/6-31G geometries and include ZPE corrections." ...

Table 9 (8/8)CASSCF/6-31G, CASPT2/6-31G and CASPT2/cc-PVDZ relative energies (kcal/mol) for species involved in the first step of the ring expansion of fluoro-substituted phenylnitrenes (Scheme 5). ...

Fig. 14 Relative energies (in kcal/mol) of species involved in the ring expansions of singlet fluoro-substituted phenylnitrenes calculated at the CASPT2/cc-pVDZ// CASSCF(8,8)/6-31G level, (a) Difluorinated phenylnitrenes. (b) Monofluorinated phenylnitrenes.

Johnson, W. T. G. Sullivan, M. B. Cramer, C. J. Meta and para substitution effects on the electronic state energies and ring-expansion reactivities of phenylnitrenes, Int. J. Quantum Chem. 2001, 85,492-508. 

Quantum-chemical (Extended Hiickel, CNDO/2, and INDO) calculations fully corroborate the contention tht arylcarbenes emd -nitrenes can fnnction as nucleophiles during ring expansion. The argument is independent of the question of formation of bicyclic intermediates. Calculations of the phenylnitrene energy surface show that the nitrene loses electron density, and C 1 gains electron density until the transition state for ring expansion is reached 5 ),... 

CASSCF and CASPT2 calculations both overestimate the stability of the open-shell electronic structure of singlet nitrenes 62a-d by about 3 kcal/mol, as in the case of parent 149 101 pjj g opening is computed to require passage over a 2 to 3 kcal/mol lower energy barrier than reversion of the intermediates to the reactants. Therefore, cyclization is the rate-determining step in the ring-expansion reactions of cyano-substituted phenylnitrenes 62a-d to derivatives of 64 and 64 (Scheme 5.3). 

As with phenylcarbene (35) (c/. Scheme 5), the ring expansion of phenylnitrene (75) could occur directly on the singlet potential energy surface or via a bicycHc intermediate, in this case azirine 92. Such azirines were first detected in the matrix photolysis of 1- and 2-azidonaphthalene. For example, photolysis of 93 generated a photoproduct with an IR absorption at 1730 cm, which in turn gave a cyclic ketenimine [v(C=C=N) at 1926 cm ] on further photolysis. It was proposed that the intermediate with the 1730 cm IRband was the tricyclic azirine 94, which subsequently rearranged to 95. At the time of these experiments, however, reliable computations of IR transitions were not available, so a definitive identification of the azirines was scarcely possible. Nevertheless, a clear parallel was found here with the behavior of aryl carbenes, which give detectable cyclopropenes in the naphthalene series (c/. 44 and 45) but not with the monocyclic carbenes (see the brief discussion of this point in Section 14.4). 

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