Singlet nitrenium ions

Because singlet nitrenium ions bear a formal nonbonding electron pair, they can be regarded as Lewis bases. In principle, it should be possible to derive a dicationic species through protonation (10, Fig. 13.4) of a nitrenium ion, or by removal of two... 

The formation of the parent amine 23 in these solvolysis reactions was considered to be the most dehnitive evidence for formation of a discrete nitrenium ion. Gassman and Cryberg" postulated the following, (a) Initial Cl—N bond heterolysis would occur adiabatic ally, generating the singlet nitrenium ion 21. (b) The triplet... 

Gonzales and co-workers extended their work on simple nitrenium ions, examining the reactions of several small, monosubstituted nitrenium ions (XNH+, where X = H, F, Cl, CN, and Me) with water. These computational studies (QCISD(T)/6-311-1—1- G ) treated only the singlet states of the nitrenium ion. According to these calculations, each singlet nitrenium ion adds to water to form an O-protonated hydroxylamine intermediate (R2N—OHj). No activation barrier was detected for... 

Figure 13.31. Protonation of singlet nitrenes to give singlet nitrenium ions.

The DFT calculations (BPW91/cc-pVDZ) on the same singlet nitrenium ions give excellent agreement with these measurements. For calculated and experimental C=C stretches, deviation between experiment and calculation is generally <5 cm On the other hand, similar calculations carried out for the triplet states showed significantly greater deviation from the measured values. 

Shortly after Anderson and Falvey reported the first observation of a shortlived nitrenium ion in CH3CN by UV spectroscopy, Novak and McClelland and co-workers demonstrated that the nitrenium ions 75h and 75o could be observed in aqueous solution after LFP of the pivalic acid ester 76h, the sulfuric acid ester 76o, and its N-chloro analogue N-chloro-4-phenylacetanilide. The transients with A ax of ca. 450 nm were identified as singlet nitrenium ions, based on the kinetics of their decomposition in the presence of NJ, the equivalence of kaz/ks determined by the azide clock method and by direct observation, the lack of sensitivity of the transients to O2, product studies that showed similar products from solvolytic and photolytic decomposition of N-chloro-4-phenylacetanilide, and identical transient UV spectra for 75o derived either from 76o or its N-chloro analogue. A comparison of azide/solvent selectivity data obtained by azide clock and direct observation of 7Sh and 75o is presented in Table 1. 

Our understanding of the chemistry of N-arylnitrenium ions is significantly more advanced than it was a decade ago. Nevertheless, this field of research is still considerably less developed than that of carbenium ions, carbenes, or nitrenes. For example, although singlet nitrenium ions behave as one might expect that their 4-imino-2,5-cyclohexadienyl resonance contributors would in their reactions with H2O, NJ, or Cl, their reactions with carbon, nitrogen, and sulfur nucleophiles, particularly d-G, are not so easily rationalized. Except for d-G, these reactions with soft nucleophiles have not been examined systematically and the regiochemistry exhibited by these nucleophiles is incompletely understood. 

A picosecond Raman spectroscopic examination of the reactions of 2-fluorenyl-nitrene and 4-methoxyphenylnitrene with water revealed singlet nitrenium ions.69 Evidence was presented to suggest that the nitrenes were produced during the photolysis of the corresponding azides. The 4-methoxyphenylnitrene decayed much faster than the 2-fluorenylnitrene. 

In a recent example the singlet nitrenium ion 155 derived from isoxazoli-dines undergoes rearrangement via aryl migration with the participation of the pentacoordinate intermediate [Eq. (6.152)] ... 

Photolysis of aryl azides in acidic media represents a special case. At low pH, singlet aryl nitrenes may be N-protonated, rapidly yielding singlet nitrenium ions. These ionic reactive intermediates undergo reactions different from those of singlet nitrenes. 

Photolysis of aryl azides in acidic media leads to rapid protonation of the initially formed singlet nitrenes with formation of singlet nitrenium ions. Nitrenium ions are nitrogen-centered diradicals that are formally isoelectronic with carbenes. They have been in the focus of research for their role as ultimate carcinogen in the metaboHsm of certain aromatic amines." " Nitrenium ions can also be made by photolysis of 2,2-benzisoxazolium (anthranihum) salts" or by irradiation of N-aminopyridinium salts. 

On the basis of ab initio MO studies on the nitrenium ion 23 derived from 1//-azepine, it has been concluded that the singlet nitrene is a planar, fully conjugated C2v system with substantial 67r-aromatic character. 

Gonzalez, C., Restrepo-Cossio, A., Marquez, M., Wiberg, K. B., De Rosa, M., 1998, Ab Initio Smdy of the Solvent Effects on the Singlet-Triplet Gap of Nitrenium Ions and Carbenes , J. Phys. Chem. A, 102, 2732. 

The first experimental determination of a singlet-triplet energy gap (Mst) for an organic nitrenium ion was made for the 1,3-dimethylbenzotriazolium ion . A powerful ligand, 4,5-bis(diphenylphosphinoyl)-l,2,3-triazole, has been synthesized and found to possess two different modes of chelation <00ACIE3321>. 

Calculations of the singlet-triplet energy gaps of a series of nitrenium ions [X(H)N ]+ in the gas phase and in solution indicate that the gap decreases in the order X = H > CN > F as a result of stabilization of the singlet state by r-donation from the substituent. For strong tt-donors (Cl, F), the singlet state is more stable than the triplet. ... 

Calculations on the isoelectronic series Me(Ph)B , Me(Ph)C , and [Me(Ph)N ]+ show that the singlet-state geometries are different, reflecting differences in the orbital interactions between the hypovalent atom and the 7r-system. The high calculated barrier (21.5 kcal moP ) for [1,2]-H shift in the nitrenium ion is the result of migration using the orbital which is conjugated with the tt-system. 

The nitrenium ion +NH2 has been the subject of a detailed, comprehensive calculation. Calculations on (48) with 15 different X substituents reveal a large substituent sensitivity, and also that aqueous solvation preferentially stabilizes the singlet state. This substiment sensitivity agrees with the results of a time-resolved IR study of the diphenylnitrenium ion (49), which shows that resonance contributors such as (50) and (51) are very important to the overall structure. Substituted 4-biphenyl nitrenium ions... 

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. 

It is important to note that for methyl nitrenium ion the singlet state is not predicted at higher levels of theory to show a potential energy minimum. Rather this species is a transition structure that eliminates H2 without a barrier (Fig. 13.15). It seems likely that the difficulties encountered in attempting to study alkyl nitrenium ions might be traced to their propensity to rearrange or eliminate. 

In order to increase the experimental benchmarks available for comparison with theoretical calculation, Mcllroy et al., determined the singlet triplet energy splitting in a diaminosubstituted nitrenium ion, 1,3-dimethylbenzotriazolium ion 37 (Fig. 13.22). This species can be viewed a nitrenium ion by virtue of mesomer 37a, which places a positive charge on the central nitrogen. Because this species... 

Figure 13.22. Singlet-triplet gap in stable nitrenium ion probed by energy transfer experiments.

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