A Laser Flash Photolysis and Quantum Chemical Study of the Fluorinated Derivatives of Singlet Phenylnitrene

Abstract

Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ or KrF excimer laser, 10 ns, 249 nm, 50 mJ) of 2-fluoro, 4-fluoro, 3,5-difluoro, 2,6-difluoro, and 2,3,4,5,6-pentafluorophenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected by transient absorption spectroscopy, and their spectra are characterized by sharp absorption bands with maxima in the range of 300-365 nm. The kinetics of their decay were analyzed as a function of temperature to yield observed decay rate constants, k(OBS). The observed rate constant in inert solvents is the sum of k(R) + k(ISC) where k(R) is the absolute rate constant of rearrangement of singlet nitrene to an azirine and k(ISC) is the absolute rate constant of nitrene intersystem crossing (ISC). Values of k(R) and k(ISC) were deduced after assuming that k(ISC) is independent of temperature. Barriers to cyclization of 4-fluoro-, 3,5-difluoro-, 2-fluoro-, 2,6-difluoro-, and 2,3,4,5,6-pentafluorophenylnitrene in inert solvents are 5.3 +/- 0.3, 5.5 +/- 0.3, 6.7 +/- 0.3, 8.0 +/- 1.5, and 8.8 +/- 0.4 kcal/mol, respectively. The barrier to cyclization of parent singlet phenylnitrene is 5.6 +/- 0.3 kcal/mol. All of these values are in good quantitative agreement with CASPT2 calculations of the relative barrier heights for the conversion of fluoro-substituted singlet aryl nitrenes to benzazirines (Karney, W. L. and Borden, W. T. J. Am. Chem. Soc. 1997, 119, 3347). A single ortho-fluorine substituent exerts a small but significant bystander effect on remote cyclization that is not steric in origin. The influence of two ortho-fluorine substituents on the cyclization is pronounced. In the case of the singlet 2-fluorophenylnitrene system, evidence is presented that the benzazirine is an intermediate and that the corresponding singlet nitrene and benzazirine interconvert. Ab initio calculations at different levels of theory on a series of benzazirines, their isomeric ketenimines, and the transition states converting the benzazirines to ketenimines were performed. The computational results are in good qualitative and quantitative agreement with the experimental findings.

A computational study of the chemistry of substituted 3-nitrenopyridine 1-oxides

“…It has been noted that CASSCF and CASPT2(N) methods tend to overestimate the stability of the nitrene S 1 state, leading to a systematic overestimation of the barrier to cyclization in arylnitrenes [6,7,9,16]. Our current interest lies in the qualitative description of the relative effects of substitution on cyclization rather than quantitative energy values, so this is not of immediate concern but should be taken into account when comparing against experimental data.…”

“…Previous workers have shown that the presence of substituents ortho-to the nitreno-group have significant impacts on the rate and selectivity of the nitrene cyclization reaction [6][7][8][9][10]. Thus, judicious substitution of the 3-nitrenopyridine 1-oxide system may lead to selective nitrene cyclization toward and away from the N-oxide group.…”

3,4,5,6‐Tetrafluorophenylnitren‐2‐yl: A Ground‐State Quartet Triradical

“…[6] Fluorine substituents in the ortho positions of 1 decrease the tendency of these rearrangements, [7] and therefore ortho-fluorinated phenyl nitrenes are obtained in higher yields under the same conditions. [5][6]8] The only nitreno radicals 4 that could be matrix-isolated and spectroscopically characterized are therefore those bearAbstract: The photochemistry of 2-iodo-3,4,5,6-tetrafluorophenyl azide (7 d) has been investigated in argon and neon matrices at 4 K, and the products characterized by IR and EPR spectroscopy. The primary photochemical step is loss of a nitrogen molecule and formation of phenyl nitrene 1 d.…”

Photochemistry of Azides: The Azide/Nitrene Interface

“…The lifetimes of these singlet nitrenes and Arrhenius parameters for their rearrangement are summarized in Table 11 However for a few ortho -substituted phenylnitrenes (namely, ortho -fl uorophenyl -, ortho -biphenyl -and 2,4,6 -tri -tert -butylphenylnitrene), the ring -opening reaction was found to be the rate -limiting step (Scheme 11.31 ). 154,198,213,214 In the case of these nitrenes, the Arrhenius parameters for the ring -opening reaction ( k RO , A RO , E RO ) could be obtained (Table 11.3 ).…”