Singlet phenylnitrene

Singlet phenylnitrene, and hence /V,A -diethyl-3//-azcpin-2-amines, e. g. 102, can be generated by the thermolysis of A,-phenyl-Af,<9-bis(trimcthylsi]yl)hydroxylamine (100) in the presence of dialkylamines the reaction fails, however, with arylamines.210 Photofragmentation of the spiro oxaziridine 101 in diethylamine solution also produces the 3//-azepine 102,2,1 and an oxaziridine intermediate is probably involved in the formation, in low yield (1 %), of azepine 102 by the photolysis of A/,A( -diarylbenzoquinonc diimine A/,A/ -dioxides in benzene/die-thylamine solution.212... 

A. Electronic Structures of the Lowest Singlet States of Phenylcarbene and Phenylnitrene... 

Figure 2. Orbital occupancies for the nonbonding electrons and CASSCF(8,8)/6-31G optimized geometries of the lowest triplet and singlet states of phenylnitrene (1 b).61 Bond lengths in angstroms, and bond angles in degrees.

Our calculations on the ring expansion of the lowest singlet state of phenylnitrene ( A2-lb) to azacycloheptatetraene (3b) predict a two-step mechanism that is analogous to that for the rearrangement of la to 3a and which involves the bicyclic azirine intermediate 2b.61 The CASPT2 energetics are depicted in Fig. 5, and the CASSCF optimized geometries of the stationary points are shown in Fig. 6. 

Figure 5. Energetics of the ring exponsion of singlet phenylnitrene (1A2-lb), colculated at the CASPT2(8,8)/6-31 lG(2d,p)//CASSCF(8,8)/6-31G level.61...

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.

As discussed in Section HI, calculations on the ring expansions of phenylcar-bene (la) and phenylnitrene (lb) suggest that singlet lb is thermodynamically... 

Table 6. CASPT2/cc-pVDZ Barrier Heights (kcal/mol) for Ring Expansions of Fluorinated Singlet Phenylnitrenes °...

The apparent absence of large electronic effects on the cyclization reactions of derivatives of phenylnitrene has been attributed to the nature of the wave function for the lowest singlet state, which, as discussed earlier, resembles a... 

Galbraith, J. M. Caspar, P. P Borden, W. T. What Accounts for the Difference between Singlet Phenylphosphinidene and Singlet Phenylnitrene in Reactivity toward Ring Expansion J. Am. Chem. Soc. 2002, 124, 11669-11674. 

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. 

Photolysis of phenyl azide (32) produces singlet phenylnitrene (33s), but what happens next depends on temperature and phase. In the gas-phase 33s isomerizes to cyanocyclopentadiene 31, in solution at ambient temperature, it isomerizes to ke-tenimine 30 and in cryogenic matrices, singlet phenylnitrene isomerizes to triplet phenylnitrene (33t). 

Although AEst of phenylnitrene is small in comparison to that of imidogen, it is still very large compared to phenylcarbene. Thus, while singlet and triplet phenylcarbene interconvert rapidly, singlet to triplet intersystem crossing of phen-... 

Singlet phenylcarbene is a closed-shell species, but singlet phenylnitrene has an open-shell electron configuration. This observation explains why singlet... 

TABLE 11.1. Maxima (in nm) of the Most Intense Absorption Bands in the Electronic Absorption Spectra of Snbstituted Singlet and Triplet Phenylnitrenes (near UV and vis)... 

According to calculations, there is essentially no barrier to cyclization of singlet vinylnitrene, but at the same level of theory, the corresponding barrier in singlet phenylnitrene is predicted to be 9 kcal/mol. Thus, the latter species should be more easily detected in solution. Taking into account that this level of theory overestimates the barrier to cyclization of singlet vinylnitrene by 3.4 kcal/mol, one can extrapolate that the best prediction of the barrier to cyclization of singlet phenylnitrene is close to 6 kcal/mol. 

In 1997, two groups simultaneously reported that LFP of phenyl azide 32 or compounds 34 and 35 produces a previously undetected transient with Lmax = 350 nm and a lifetime of 1 ns at ambient temperature.The transient decays at the same rate that cyclic ketenimine 30 is formed, implying that the newly detected transient is singlet phenylnitrene 33s. 

Laser flash photolysis (266 nm) of phenyl azide in pentane at 233 K produces a transient absorption spectrum with two sharp bands with maxima at 335 and 352 nm (Fig. 11.4). Spectrum 1 was measured, point by point, 2 ns after the laser pulse. In later work, the spectrum of 33s was reinvestigated and an additional very weak, long wavelength absorption band at 540 nm was observed (Spectrum 2). The transient spectrum of Figure 11.4 was assigned to singlet phenylnitrene in its lowest open-shell electronic configuration ( 2). 

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