From pentafluorophenyl azide

Some studies have shown that the generation of the nitrene from pentafluorophenyl azide or from 2,6-dimethylphenyl azide from irradiation in a low-temperature matrix, permits its trapping by carbon monoxide and the formation of the corresponding isocyanate. No evidence for the ring-expansion process was seen in these experiments. Other low-temperature studies in the absence of CO trap and using N-labelled phenyl azide have shown that even at 12 K there is evidence for the ring expansion to the didehydroazepine. ... 

The first nickel tetrazadiene complex, Ni(C6FjNNNNC6F5XC8H,2), obtained from Ni(CgHj2)2 and pentafluorophenyl azide, reacts with... 

A different transient is produced upon flash photolysis of pentafluorophenyl azide in the presence of dilute pyridine (Figure 16) [109]. The transient (xmax = 520 nm) observed upon LFP of pentafluorophenyl azide in dilute pyridine is very similar to the transient observed upon LFP of phenyl azide in neat pyridine (Figure 17). The transient derived from photolysis of pentafluorophenyl azide in dilute pyridine is formed in a process which is first order in pyridine concentration and is formed with bimolecular rate constant fc = 3.1 x 109 M 1 s. The transient derived from phenyl azide reacts with pyridine with k = 4.8 x 105 M -1 s. The lifetime of the intermediate which reacts to form the transient of Figure 17, in the absence of pyridine is found... 

Photolysis of 2-, 3- and 4-fluorophenylazide (39a,b,c) in aniline gives the products (40), (41) and (42) in the yields shown in Scheme 8. " With 2,6-difluoro- and 2,3,4,5,6-pentafluorophenyl azide, however, ring expansion to azepine products analogous to (42) is suppressed, and the corresponding unsymmetrical azobenzenes cf. 41) are formed in 50% and 55% yield, respectively. The azobenzenes are presumed to arise from insertion of a singlet nitrene into an N-H bond of aniline, followed by oxidation of the resulting hydrazine. 

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. 

In line with results of solid-phase experiments (Scheme 28), the first solution-phase cyclodimerization attempts were conducted with 142 by conversion to a pentafluorophenyl ester and subsequent reduction of the azide. This strategy proved to be unsatisfactory, leading to a 1 1 mixture of cyclodimer 136 and cyclomonomer 137, each in a low 5 % chromatographed yield from 141 (Scheme 30). The poor efficiency of this step might have been due, at least in part, to the sensitivity of the pentafluorophenyl ester to the water required in the Staudinger reaction. 

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