Nitrenes lifetimes

The LFP of p-biphenyl azide produces singlet-p-biphenylnitrene. The phenyl group has little influence on the electronic spectra of either singlet or triplet p-biphenylnitrene (Table 11.1), but it extends the singlet nitrene lifetime from 1 to 17 ns at ambient temperature by diluting the spin density ortho to the nitrene nitrogen. 

Dilution of toluene with the inert solvent methylene chloride was attempted in an effort to extend the singlet nitrene lifetime and enhance the yield of triplet nitrene [104]. Dilution, however, did not change the ratio of aryl C-H to benzyl C-H insertion products formed, instead the yield of all volatile products decreased at the expense of tar formation. Dilution with CH2C12 did not increase the yield of triplet nitrene derived products such as C6F5NH2 and decafluoroazobenzene, thus the yield of triplet phenyl nitrene is negligible (Table 7) in methylene chloride. The results can be understood with the aid of Scheme 10, which is identical to the mechanistic hypotheses written for parent phenyl azide (Scheme 7). 

Placement of fluorine substituents at both ortho positions (70d) raises the barrier to cyclization by about 3 kcal/mol relative to the unsubstituted system (Table 5.13). The lifetimes of singlet phenylnitrene ( 49) and 4-fluorophenylnitrene (70b) are about 1 ns or less at 298 K. The lifetime of 3,5-difluorophenylnitrene (70c) is about 3 ns at 298 K, but that of 2,6-difluorophenylnitrene (70d) is 260 ns, in CCI4. Because a para-fluoro group fails to exert an electronic influence on the cyclization process, it is tempting to attribute the effect of two or /io-fluorine substituents on the singlet nitrene lifetime to a simple steric effect. 

With the exception of thermodynamically stabilized [64] or sterically protected [65] carbenes, these species and their hetero-analogs, nitrenes, are very reactive and therefore special conditions are required for their direct observation. Fast spectroscopic techniques capable of characterizing species with lifetimes of a few picoseconds have been used [1-3]. More recently, time-resolved IR (TRIR) experiments have been used to characterize species with lifetimes of microseconds and even nanoseconds [4-6]. 

Polyfluorination does seem to suppress rearrangements of alkyl azides and to extend the lifetimes of the corresponding singlet nitrenes. Photolysis of 5 in cyclohexane produces insertion adduct 6." ... 

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. 

Two para-substituents, phenyl and cyano depress and retard the rate of cyclization significantly (Table 11.2)." p-Phenyl and p-cyano are both radical stabilizing substituents. These conjugative substituents reduce the spin density on the carbon ortho to the nitrene nitrogen. The reduced spin density at carbons ortho to the nitrogen lowers the rate at which the 1,3-biradical cychzes. The effect with p-cyano and p-biphenyl singlet phenylnitrene is quite dramatic. The lifetimes of these singlet nitrenes at ambient temperature are 8 and 15 ns, respectively, and the activation barriers to cychzation are 7.2 and 6.8 kcal/mol, respectively. 

The putative benzyloxynitrene can be intercepted with tetramethylethylene to form the expected aziridine. Time-resolved IR spectroscopy was unable to detect the 0-nitrene, but detected the presence of PhCH2N=0 formed with a time constant of 250 ns after the laser pulse. Thus, the lifetime of benzyloxynitrene is also 250 ns. The TRIR spectroscopic studies indicated that benzyloxynitrene reacts with oxygen a rate constant of 10 M s. This value strongly suggests that the 6>-nitrene, in contrast to the Al-nitrenes has a triplet ground state. 

Nitrenium ions (or imidonium ions in the contemporaneous nomenclature) were described in a 1964 review of nitrene chemistry by Abramovitch and Davis. A later review by Lansbury in 1970 focused primarily on vinylidine nitrenium ions. Gassmann s ° 1970 review was particularly influential in that it described the application of detailed mechanistic methods to the question of the formation of nitrenium ions as discrete intermediates. McClelland" reviewed kinetic and lifetime properties of nitrenium ions, with a particular emphasis on those studied by laser flash photolysis (LFP). The role of singlet and triplet states in the reactions of nitrenium ions was reviewed in 1999. Photochemical routes to nitrenium ions were discussed in a 2000 review. Finally, a noteworthy review of arylnitrenium ion chemistry by Novak and Rajagopal " has recently appeared. 

Nitrenes have a short lifetime (only several microseconds)86- 8 and undergo stabilization by the following reactions isomerization to imines, dimerization to azo compounds, hydrogen abstraction followed by ring closure to heterocyclic compounds, bimolecular insertion into C-H bonds to secondary amines, addition to solvent yielding ylids, and addition to unsaturated systems yielding heterocyclic compounds. Table 117-106 includes the reaction products and references for the different classes of nitrenes. 

These reaction products would result if an intermediate nitrene were formed (see Table I). The values of the entropy of activation (Table III) are also in good agreement with a synchronous mechanism. Some values, however, are rather high for such a reaction. If, nevertheless, in these cases the loss of nitrogen would precede the rearrangement, the lifetime of the intermediate nitrene should be so short that it would not have time to react with the solvent. 

Carbenes are highly reactive, have short lifetimes, and undergo characteristic chemical changes, the most important of which are listed with examples in Table 5.7. Monovalent nitrogen intermediates (57), called nitrenes, are also known their chemistry is in many ways similar to that of carbenes.143... 

The lifetime of the nitrene is governed by its stability this in turn governs its reactivity with respect to added substrates. The most reactive nitrenes, i.e., those having electron acceptor substituents such as sulfonyl and acyl nitrenes are extremely short-lived [6b], In contrast, amino nitrenes that are stabilized by resonance can be stored for several hours at —78 °C [11]. 

The photolysis of aryl azides in low-temperature matrices yields triplet (ground) state nitrenes which have been identified by and absorption spectroscopy. Dinitrenes and trinitrenes have also been reported in the solid-state photolysis of di- and triazides. Quantum yields of photolysis of some aromatic azides are listed in Table 21 and it appears that nitrenes are produced in solution, at room temperature, as well. The lifetimes of some aromatic nitrenes and the absolute rates of some of their reactions have been measured . Some interesting features of photolytic azide decompositions will now be briefly described. 

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. 

That the absence of hydrogens at the a-carbon atom in azides (105) and (112) does increase the lifetime of the nitrene sufficiently to make possible non-rearrangement processes may be seen from the... 

The singlet lifetime has been experimentally established only for cyanonitrene which in the gas phase converts to the ground state triplet at the rate of about lO sec . An estimate of singlet lifetimes in solution can be obtained from the work of McConaghy and Lwowski . They find that in the thermolysis of ethyl azidoformate in the presence of cfj-4-methylpent-2-ene the rate of spin reversal of the nitrene is about 10 times slower than the rate of addition to the... 

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