Nitrenes triplet

In contrast to 6-azidobenzo[/)]thiophene, which yields only benzo[i]thiophen-6-amine (9 %) and JVh,Ar(1-diethylbenzo[/)]thiopheiie-6,7-diamine (25 % bp 175-177 C/0.7 Torr), 6-azido-2,3-dibromobcnzojhjthiophene (1 a, R = R2 = Br) on irradiation in diethylamine in the presence of pyrene, a triplet nitrene quenching agent, yields a mixture of 2,3-dibromo-./V6,./V6-diethyl-benzo[5]thiophene-6,7-diamine (2a, R1 = R2 = Br 13%) and the 8W-thieno[2,3-r]azepine 3a.14<1 Likewise, methyl 6-azidobenzo[6]thiophene-2-carboxylate (lb, R1 = C02Me R2 = H) yields the thienoazepine ester 3b.147... 

These results clearly showed that biradical 83 was not formed from ground state triplet nitrene 82, either classically or via tunneling. The anthors specnlated that the biradical might have been prodnced from a higher energy electronically excited nitrene than was available from irradiation of 82. Alternatively, it is possible that the biradical might have formed directly from excited azide. 

More recent work 8> shows that the S—N bond can be cleaved by hydroperoxides and that aromatic sulphonyl azides only undergo free radical thermal decomposition if a source of radicals is provided. Some light on the nature of the radical transfer agent has recently been shed by the observation 14> that dodecyl azides are formed (2.3%) in the thermolysis of mesitylene-2-sulphonyl azide (3) at 150 °C in w-dodecane under nitrogen. It seems likely that a dodecyl radical is produced by hydrogen abstraction by the triplet nitrene (5) [mesitylene-2-sulphonamide was also formed (1.1%)] which then attacks undecomposed sulphonyl azide... 

Photolysis of ferrocenylsulphonyl azide in cyclohexene gave the corresponding aziridine derivative (9%), but thermolysis did not 17>. This could be an addition of the triplet nitrene to the olefin and studies on the stereospecificity of this reaction are under way. 

In contrast, the much lower enthalpy computed for 3 lb, compared to 3lc, means that the N-H BDE of the anilinyl radical 8b is much lower than the C-H BDE of the 3-pyridylmethyl radical 8c. The results in Table 5 show that this is indeed the case, not only for R=Ph and R =3-pyridyl, but also for R=R =Ph and R=R —H.77 The data in Table 5 indicate that, not just for lb and lc but in general, triplet nitrenes are ca. 20 kcal/mol more thermodynamically stable than comparably substituted triplet carbenes. 

Matrix isolation photolytic studies on tetrazolo[l,5- ]pyridazine 25 have been reported by Hill and Platz <2003PCP1051> (Scheme 5) and formation of the l-cyano-3-diazopropene 27, triazacycloheptatetraene 28, and cyano-cyclopropene 29 was detected. Upon the absence of electron spin resonance absorptions at 7 K, the authors concluded that triplet nitrene was not formed but, instead, the resulting singlet nitrene rapidly underwent further ring openings. 

Irradiation of tetrazolo[l,5- ]pyrimidine 1 in benzene in the presence of trifluoroacetic acid was investigated by Takeuchi and Watanabe <1998JP0478> (Scheme 10). Formation of 2-anilinopyrimidine 57, 2-aminopyrimidine 58, and biphenyl was observed. The authors assumed that 57 was formed via a singlet nitrene, whereas formation of the other products may result from the triplet nitrene intermediate. In contrast to this early assumption, Hill and Platz <2003PCP1051> exclude the formation of a triplet nitrene upon spectroscopic evidence as discussed in Section 11.18.5.1. 

A photolytic study with a 5-azidotetrazole derivative also led to the formation of a tetrazolo[l,5-/z]quinazoline compound, although the yield was fairly low. Araki et al. published findings that irradiation of 110 in aqueous medium resulted in formation of the mesoionic enolate 112 in 8% yield <2000JHC1129> (Scheme 20). The authors concluded that the formation of the tricyclic structure can be rationalized by an intramolecular insertion of the triplet nitrene formed from 110 to a C-H bond of an ortho-methyl group to give at first intermediate 111, which was converted under the applied reaction conditions to produce 112. 

Azido-4,6-dichloro-133-triazine 35, matrix isolated in argon at 10 K, yields triplet nitrene 36 and the strained cyclic carbodiimide 37 upon photolysis <99CC2113>. 

The imidogen anion (NH ) can ionize to form either the singlet or triplet nitrene, thus, the difference in the kinetic energies of the photoelectrons leading to NH and NH is just the singlet-triplet splitting of NH. 

In fact, rearrangements of singlet alkylnitrenes generated in frozen matrices cannot even be suppressed by developing strain in imine products such as 1-3, although some triplet nitrene is detected by EPR and UV-vis spectroscopy upon low-temperature photolysis of 1-azidonorbornane. ... 

Each of these azides contains an internal sensitizing group. Upon LFP the triplet states of all three azides are observed. However, in the case of 17 and 18, only singlet nitrene derived products are formed. Photolysis of 17 and 18 at 8 K in fluorolube fails to produce persistent triplet EPR spectra. It seems clear that these aroyl nitrenes also have singlet ground states. As mentioned previously, photolysis of 14 does produce a triplet nitrene EPR spectrum. 

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 presence of oxygen had no effect, outside of experimental error, on the transient hfetime in ethanol. On the basis of the solution and glassy matrix work the transient absorbing at 345 nm was assigned to the triplet nitrene. LFP of 25 in acetonitrile produced a longer hved transient (r > 150 ps) also absorbing near 345 nm, which may be the result of formulation of a nitrene-solvent ylide. 

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