Nitrenes phenylnitrene

Wentrup s experimental work on carbene-to-nitrene rearrangements10 suggested that calculations on phenylnitrene (lb) and the isomeric pyridylcarbenes might provide some useful relative energies. Kemnitz et al. carried out calculations on lb and 3-pyridylcarbene (lc).77 The latter molecule was used to provide a link between the energies of not only phenylcarbene (la) and phenylnitrene (lb), but also between CH2 and NH. 

Our calculations showed that the first step, cyclization of the nitrene to an azabicyclo[4.1.0]heptatriene, is rate-determining. Our calculated barriers for cyclization of four fluorinated derivatives of lb are given in Table 6.87 The CASPT2/cc-pVDZ barrier of 13.4 kcal/mol for cyclization of 2,6-difluoro-phenylnitrene (10b) is 4.1 kcal/mol higher than the barrier computed for lb -+ 2b. In contrast, the calculated barriers to rearrangement of 3,5-difluoro-phenylnitrene (lOe) and 4-fluorophenylnitrene (10c) are very similar to that computed for unsubstituted phenylnitrene (lb). These computational results are consistent with the observed reluctance of pentafluorophenylnitrene (10a) and 2,6-difluorophenylnitrene (10b) to rearrange,48 1 81 83 and with the relative ease... 

With the purpose of understanding the crosslinking mechanism of 1,2-polybutadiene with aromatic nitrene, we studied the reaction of phenylnitrene with unsaturated olefine monomers such as 3-methyl-1-butene and cyclohexene. These monomers are structually similar to a unit segment of 1,2-polybutadiene. 

The triplet state of NH is 36 kcal/mol lower in energy than the singlet state. The triplet state is favored because, on the average, electrons with parallel spin spend less time in proximity with each other than electrons with antiparallel spin. Consequently, the Coulombic electron-electron repulsion in the triplet state is less severe than in the singlet state (cf. Borden, Chapter 22 in this volume.) As we will see in vinyl- and phenylnitrene, delocalization of an unpaired electron by conjugation dramatically stabihzes the singlet relative to the triplet states of nitrenes. 

To add to the confusion, various groups reported that gas-phase photolysis of phenyl azide produced the absorption and emission spectra of triplet phenylni-trene. " These observations were reconciled by the work of Leyva et al. who discovered that the photochemistry of phenyl azide in the presence of diethylamine was very sensitive to temperature. Above 200 K, azepine 30 is formed, but <160 K, azobenzene, the product of triplet nitrene dimerization, is produced. The ketenimine can react with itself or with phenyl azide to produce a polymer, which can be converted into an electrically conducting material. Gritsan and Pritchina pointed out that at high-dilution ketenimine 30 can interconvert with singlet phenylnitrene which eventually relaxes to the lower energy triplet that subsequently dimerizes to form azobenzene. 

The formation of the products [cyclic ketemimine (30) and/or triplet nitrene (33t)] was monitored at 380 nm. The decay of singlet phenylnitrene and the growth of the products are exponential and can be analyzed to yield an observed rate constant An Arrhenius treatment of these data (open circles), is presented in... 

For most aryl azides, the rate constants of singlet nitrene decay and product formation (triplet nitrene and/or ketenimine) are the same. Thus, in all these phenyl-nitrenes cyclization to substituted benzazirines is the rate-limiting step of the process of isomerization to ketenimine, as is the case for the parent phenylnitrene. The only known exception, o-fluorophenylnitrene, will be discussed in the next section. 

Two para-substituents, phenyl and cyano depress and retard the rate of cyclization significantly (Table 11.2)." p-Phenyl and p-cyano are both radical stabilizing substituents. These conjugative substituents reduce the spin density on the carbon ortho to the nitrene nitrogen. The reduced spin density at carbons ortho to the nitrogen lowers the rate at which the 1,3-biradical cychzes. The effect with p-cyano and p-biphenyl singlet phenylnitrene is quite dramatic. The lifetimes of these singlet nitrenes at ambient temperature are 8 and 15 ns, respectively, and the activation barriers to cychzation are 7.2 and 6.8 kcal/mol, respectively. 

As illustrated by the methyl derivative, the pyridylcarbene gives high yields of aryl nitrene derived products indicating that phenylnitrene is substantially lower in energy than the isomeric pyridylcarbenes, as first suggested by Wentrup and co-workers. ... 

Phenylnitrene is intrinsically more stable than the isomeric pyridylcarbene. Kamey and Borden explained that the reason why the N—H BDEs in RNH radicals are smaller than the C—H BDEs of RCH radicals, that is, rehybridization, is also responsible for the fact that nitrenes are thermodynamically more stable than carbenes.The lone pair of electrons of a nitrene reside in a low-energy sp hybrid orbital. This effect dramatically stabilizes nitrenes relative to carbenes in which the nonbonding electrons reside in either pure p or pseudo sp orbitals. 

Analysis of the data of Table 1 verifies the same finding that the shift of the near-UV absorption band of singlet arylnitrenes correlates with the shift of the intense near-UV absorption band of triplet nitrenes. Furthermore, the ort/io-substituents influence the absorption spectra of singlet and triplet phenylnitrenes more significantly than do para-substituents. 

Values of obs of substituted singlet phenylnitrenes were measured by following the decay of singlet nitrene absorption at the wavelength of their maxima as a function of temperature. In all cases the magnitude of A obs decreases as the temperature decreases until a limiting value is reached... 

Fig. 7 Arrhenius treatment of Atobs values for para-methyl (1), ortho-methyl (2) singlet phenylnitrene in pentane and singlet ortho,ortho-dimethylphenyl nitrene in hexane (3).

However, from elegant labelling studies by Wentrup and coworkers on the gas-phase thermal interconversion of phenylnitrene and 2-pyridylcarbene, it became evident that an alternative and equally viable intermediate in these nitrene ring expansions is the azacycloheptatrienylidene (216) or its tautomer, the cumulated system (217) (81AHC(28)23l). 

Relative reactivity studies with primary, secondary, and tertiary C-H bonds have revealed the following order of selectivity phenylnitrene > carbethoxynitrene > sulfonyl-nitrene > car bene.88 ... 

---