Nitrenes pentafluorophenyl

Like carbene insertions into carbon-hydrogen bonds, metal nitrene insertions occur in both intermolecular and intramolecular reactions.For intermole-cular reactions, a manganese(III) meio-tetrakis(pentafluorophenyl)porphyrm complex gives high product yields and turnovers up to 2600 amidations could be effected directly with amides using PhI(OAc)2 (Eq. 51). The most exciting development in intramolecular C—H reactions thus far has been the oxidative cychzation of sulfamate esters (e.g., Eq. 52), as well as carbamates (to oxazolidin-2-ones), ° and one can expect further developments that are of synthetic... 

Studies of the photodecomposition of azido derivatives of PCB have attracted interest because of their potential use as photolabels in intracellular distribution studies.Polyfluorinated aryl azides are also of current interest as new reagents for photoaffinity labelling. The major products of photolysis of methyl 4-azidotetrafluorobenzoate in cyclohexane or diethylamine, for example, arise by insertion, a property which enhances its use in labelling. Singlet pentafluorophenyl nitrene, formed in an analogous fashion on irradiation of pentafluorophenyl azide, can be trapped by toluene as the insertion products (79) and (80). °... 

Figure 6. The absorption, fluorescence, and fluorescence excitation spectra of triplet pentafluorophenyl nitrene in EPA glass at 77 K.

Pentafluorophenyl azide was first synthesized by Haszeldine et al. in 1962 [92]. Ten years later, Banks and Sparkes [93] began to study the chemistry of polyfiuorinated aryl nitrenes generated by thermolysis of fluorinated aryl azides. 

The structure of 11 was proven by X-ray crystallography. This demonstrated that ring expsnsion to a dehydroazepine is possible in the pentafluorophenyl nitrene system at elevated temperatures despite the fact that this process cannot be driven photochemically in a matrix or glass at low temperature. 

Abramovitch et al. [97] demonstrated that the pentafluorophenyl nitrene intermediate generated from treatment of nitroso compound 13 with triethyl-phosphite reacted with electron rich aromatics. Adducts were formed with benzene, toluene, anisole, and mesitylene in modest yield. 

Abramovitch et al. [99] studied the reaction of thermally and photochemi-cally generated pentafluorophenyl nitrene with alkenes. 

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 singlet pentafluorophenyl nitrene the ratio of kx/kEXP is much more favorable for nitrene trapping than with singlet phenyl nitrene at ambient temperature. Polyfluorination has probably both increased the rate constant for toluene capture of the singlet nitrene and decreased the rate constant of ring expansion. 

Figure 12. The transient absorption spectrum of triplet pentafluorophenyl nitrene produced by laser flash photolysis of pentafluorophenyl azide in methanol.

Thus the triplet EPR signal of triplet pentafluorophenyl nitrene is rapidly bleached upon extended photolysis in 2-MTHF at 77 K but is stable to prolonged exposure to UV-VIS light in perfluoro-2-n-butyl THF. Furthermore extended photolysis of azide 4 in polycrystalline toluene gives good yields of benzylic CH insertion. 

One of the more interesting products obtained from the capture of singlet pentafluorophenyl nitrene is ylide 19 obtained by photolysis of C6F5N3 in... 

Figure 14. The transient absorption spectrum of the pentafluorophenyl nitrene pyridine ylide observed in neat pyridine, 400 ns after the laser pulse.

Fignre 15. The yield of pentafluorophenyl nitrene pyridine ylide, measured at 390 nm, as a function of pyridine concentration. 

A reasonable value of kPYR is 109M-1s-1 which indicates that kEXP x 10 M 1 s 1 in CH2C12 at ambient temperature, and that the lifetime of singlet pentafluorophenyl nitrene is 10 ns under these conditions [108]. 

Figure 16. The transient absorption spectra of pentafluorophenyl nitrene and pentafluorodehydroazepine derived pyridine ylides observed in cyclohexane containing dilute pyridine, recorded 400 ns after the laser flash.

Poe et al. [ 109] have measured the rate of formation of ketenimine ylide 20 in the presence of traps of singlet C6F5N such as dimethylsulfide. Laser flash photolysis of pentafluorophenyl azide in the presence of various amounts of dimethylsulfide (a scavenger of singlet pentafluoro phenyl nitrene), at a fixed concentration of pyridine, lowers the yield of ylide 20, but does not increase the rate of formation of the ylide. This demonstrates that dimethylsulfide reacts with a precursor of ylide 20 (singlet pentafluorophenyl nitrene) but not the immediate precursor of the ylid (perfluorodehydroazepine). This result... 

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. ... 

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. 

Dunkin, l.R. and Thomson, P.C.P., Pentafluorophenyl nitrene a matrix isolated aryl nitrene that does not undergo ring expansion,/. Chem. Soc., Chem. Commun., 1192, 1982. 

Young, M. J. T. and Platz, M. S., Mechanistic analysis of the reactions of (pentafluorophenyl)nitrene... 

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