Why Doeso-Fluorine Substitution Raise the Barrier to Ring Expansion of Phenylnitrene?

Abstract

In an attempt to understand the dramatic difference in reactivity between phenylnitrene (1a) and pentafluorophenylnitrene (1b), the ring expansion reactions of several fluorinated arylnitrenes have been studied computationally at the CASPT2N/cc-pVDZ//CASSCF(8,8)/6-31G* + ZPE level of theory. The nitrenes considered include 2,6-difluorophenylnitrene (1c), 3,5-difluorophenylnitrene (1d), 4-fluorophenylnitrene (1e), and 2-fluorophenylnitrene (1f). In all cases ring expansion of the open-shell singlet nitrenes (1) to the ketenimines (3) was calculated to occur via the same two-step mechanism as that predicted for the parent system (1a), with the first stepcyclization to the azirines (2)being rate-determining. The calculated barrier of 13.4 kcal/mol for cyclization of 1c is 3.5−4.5 kcal/mol higher than the corresponding barriers for nitrenes 1e and 1f, and for 1a. For nitrene 1f, the calculated barrier of 13.0 kcal/mol for cyclization toward fluorine is 3 kcal/mol higher than the barrier for cyclization away f...