Nitrenes intersystem crossing rates

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. 

Intersystem crossing rate constants of ortho- and meto-substituted singlet phenyl nitrenes are presented in Table 3. Mono- and di-ort/io-fluorine substituents have no influence on ISC rate constants. No effect with meta, metfl-difluoro substitution is observed either. Pentafluoro substitution has no efffect on kisc in pentane although a modest acceleration is observed in the more polar solvent methylene chloride. 

Table 3 Intersystem crossing rate constants of ortho- and meM-substituted phenyl-nitrenes.

From the above general discussion, it is obvious that the proposed mechanisms of the Curtius rearrangement must be regarded as tentative. In particular, measurements of lifetimes and rates of intersystem crossing of intermediate nitrenes, coupled with spectroscopic data on the available excited states of acyl azides, should resolve the problem. 

First we assume that intersystem crossing and ring expansion of singlet phenyl nitrene are both irreversible processes in the presence of 1.0 M diethylamine. The ratio of 3f/-azepine 2 to azobenzene 3 formed on photolysis of phenyl azide is then related to the ratio of rate constants of ring expansion (fcEXP) to intersystem crossing (fclsc). 

Recently, " the LFP of 47 was studied at 77 K where singlet nitrene 52 cleanly relaxes to the triplet state 52. The rate constant of intersystem crossing at 77 K was found to be 3.8 0.3 X 10 s. Thus kisc for 52 is indeed temperature independent. The spectrum of 52 at 77 K is similar to that detected in solution (Figure 11.7, spectrum i). ... 

The effect of substituents placed para to the nitrene center was investigated in a series of matrix isolation, laser flash photolysis, and product studies. Table 44.1 Ksts a number of rate constants (k,sc for intersystem crossing to the triplet ground-state and for rearrangement to the didehydroazepine). For comparison, the kinetic parameters of several other singlet aryl nitrenes are also given. 

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