Arylnitrenes singlet

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

Assuming that singlet nitrene reacts with alkanes at near diffusion controlled rates allowed deduction of a rate constant of singlet-to-triplet nitrene intersystem crossing (ISC) of 2-8 X 10 s . This ISC rate is slower than in carbenes, but significantly faster than with arylnitrenes, which are discussed in a subsequent section. 

The strongest chemical evidence for the existence of singlet arylnitrenes comes not from the parent system, but from intermolecular capture of ortho-substituted aryl azides,and intermolecular capture of certain highly electron-deficient aryhii-... 

The electronic absorption spectra of 33s and 33t are very similar (Figs. 2 and 4), but all of the calculated and experimental bands of 33s exhibit a red shift compared to those of 33t. This result is very reasonable because both these species have very similar open-shell electronic configurations ( 2 and 2). The absorption maxima of several singlet arylnitrenes are given in Table 11.1. 

An o-cyano group has little influence on kisc, but two o-cyano groups slightly accelerate intersystem crossing. ° ° Singlet arylnitrenes with electron-withdrawing groups in the para-position have little influence on the rate constant of ISC. 

In the case of insertion toward fluorine (41), there is an even greater amount of positive-positive charge repulsion between the ortho and ipso carbons than in the transition state and this effect is responsible, in part, for a higher activation barrier for insertion toward F to form 41 than away from fluorine to form 40. Therefore, the origin of the pronounced influence of ortho,ortho-difluoro substitution on the lifetime of singlet arylnitrene and the increased activation energy of its cyclization is the result of combination of the steric effect and the extraordinary electronegativity of fluorine atom. 

The assignment of the transient absorption spectrum of Fig. 2 to PN is supported by the similarity of its spectrum to that of the longer-lived per-fluorinated singlet arylnitrenes. " The decay of this transient absorption is accompanied by the formation of cyclic ketenimine K. Furthermore, the electronic absorption spectrum of PN in the A2 state calculated at the CASPT2 level is in good agreement with the transient spectrum (Fig. 2). 

Direct observations of singlet arylnitrenes before 1997 were exceedingly rare. Before the detection of the spectrum of and its 2,4,6-tribromo-... 

Fig. 4 Electronic absorption spectra of selected singlet arylnitrenes phenylnitrene (1), 2-fluorophenylnitrene (2) and 2-cyanophenylnitrene (3) in pentane, and 2,6-dicyanophenylnitrene (4) in CH2Q2.

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. 

Figs 6 and 7) similar to the trend observed with parent singlet phenyl nitrene (Fig. 3). The temperature-independent rate constant observed at low temperature was associated with kisc- In the case of 4-bromo-, 4-iodophenyl-nitrene (Fig. 6) and 2,2-dimethylphenylnitrene (Fig. 7) the values of koBs re independent of temperature over a very large temperature range ( 120-200 K). This proves that indeed the rate constants of ISC in arylnitrenes are temperature independent in solution over typical temperature ranges. 

As shown in the previous seetion (Figs 6 and 7), the magnitude of koBs decreases as the temperature decreases until a limiting value is reached. The pattern is similar to that observed with parent singlet phenylnitrene (Fig. 3). As before, the temperature-independent observed rate constants are associated with kisc- Plots of In (koBs —kjsc) were linear (Figs 10, 11) and these plots were used to deduce the Arrhenius parameters to cyclization of the substituted singlet arylnitrenes (Tables 2, 4 and 6). 

One of the most interesting products obtained from the capture of singlet nitrene 16e is ylide 22e produced by photolysis of 15e in pyridine (Scheme 4). Ylide 22e has a very intense absorption band with maximum absorption at 390 nm. The pyridine-ylide method was successfully used by our group to probe the dynamics of the fluoro-substituted singlet arylnitrenes - " -" ... 

In 1997 the electronic absorption spectra of phenylnitrene" and its perfluorosubstituted analogues " were detected. Recently the kinetics of bimolecular reactions of the singlet fluoro-substituted arylnitrenes were studied using direct spectroscopic methods. The absolute rate constants of reaction of singlet perfluoroarylnitrene 16f and 16g with amines, pyridine and dimethylsulfoxide are presented in Table 11. 

In subsequent work, the protonation of fluoro-substituted singlet arylnitrenes 16f and 16g was studied in 1 4 water-acetonitrile mixture on the nanosecond timescale. LFP studies of aryl azides 15f and 15g in the presence of HCl demonstrated that the decay of the arylnitrene absorption was accompanied by the formation of transient absorption with maximum at... 

---