Cross triplet nitrene

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 irradiation of azides mainly results in the formation of the singlet nitrene and the reactions of the singlet nitrene have to compete with the intersystem crossing process leading to the production of the triplet nitrene. Therefore intermolecular reactions with singlet state nitrenes may be possible at high nitrene quencher concentrations. 

Sensitized decomposition by singlet or triplet energy transfer, yielding singlet or triplet nitrenes that may undergo intersystem crossing. 

In addition, the singlet nitrene can undergo intersystem crossing to the triplet nitrene. The Ti-azide state can thus be populated by two routes (a) and (b). These possibilities are indicated in the following scheme ... 

Dilution Experiments Effecting Intersystem Crossing to Triplet Nitrenes... 

Sensitized photolysis is a much more efficient method to produce triplet nitrenes than the collisional deactivation method. This technique, which populates the triplet state of a nitrene exclusively by energy transfer, employs triplet sensitizers such as aromatic ketones with a high intersystem-crossing efficiency. Energy is... 

Aziridine yields in intermolecular additions are good addition of ethoxycarbonylnitrene to cyclohexene gave the aziridine in 56% yield at room temperature, and in 75% yield at Dry Ice temperature (azide photolysis) . At a 0-2 mole% concentration of cyclohexene (in dichloromethane) a 35% yield of aziridine was obtained . Other olefins give similar yields. The good yields at low olefin concentrations are possible because both the singlet and the triplet nitrenes will add readily, and the intersystem crossing of the nitrene does not interfere with the reaction (except for its stereospecificity, see above ). With isoprene , the singlet ethoxycarbonylnitrene does not measurably discriminate between the two double bonds, while the triplet species prefers the more substituted double bond by a factor of two. 

Unsubstituted organic azides absorb appreciably in the near-UV region (<380 nm). Their direct irradiation leads to the extrusion of molecular nitrogen to form singlet nitrene (Section 5.4.2), which can intersystem cross to triplet nitrene (Scheme 6.175).1112,1114,1162 Triplet nitrene can also be obtained by photosensitization. In general, nitrenes are reactive intermediates that can undergo various reactions. 

However, the azepine yield was low when a 5-OMe substituent was present, and no azepine was formed from the 5-N02 compound. This is possibly due to a rapid singlet-triplet crossing in the nitro compound the triplet nitrene then gives the corresponding azo compound.258 With the acyl group in meta or para position with respect to the nitrene, the azepine yields were lower. It was concluded that a reasonably electrophilic singlet nitrene is essential for azepine formation.258... 

It is clear that the ortho and para substituted diarylamines 16 and 17 are derived from capture of the singlet nitrene and the product of benzylic CH insertion 15 can be formed from either the triplet or singlet state of the nitrene. Decafluoroazobenzene is derived from a dimerization reaction of the triplet nitrene and pentafluoroaniline is formed by hydrogen atom abstraction reactions of triplet pentafluorophenyl nitrene, and possibly by some photoreduction of an excited state of the azide. It is clear from this data that the singlet and triplet nitrene are not rapidly interconverting, and there is no evidence for uphill intersystem crossing from the triplet to the singlet nitrene. 

Scheme 7.9 Cross-linking of polymers through the reaction of triplet nitrene.

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