Triplet nitrenium ions

The triplet nitrenium ion, however, lacks symmetry but, unlike the excited triplet states of other azaaromatic systems such as pyridine, is surprisingly planar.283 Theoretical dipole moments for cyclopent[c]- and cyclopent[rf]azepine, and the molecular geometry of cyclopent[rf]-azepine, have been calculated using semiempirical ab initio methods.3... 

Photolysis in polar media (e.g., H20-MeCN mixtures) results in the same products observed from thermal generation. In addition, however, the parent amine, which is not observed in the thermal reactions, is formed photochemicaUy. This finding suggests that there may be a competition between heterolysis and homolysis in the photochemical reaction. It has also been suggested that the amine might result from formation of the triplet nitrenium ion. In any case this competing process along with the instability of the precursors has hmited interest in this photochemical route. 

Figure 13.59. Hydrogen atom transfer to triplet nitrenium ion.

The para-nitro ester 71d generated only 4-nitroaniline (70%) and 4,4 -dinitroazoxybenzene (10%) when it underwent decomposition (Scheme 29). These products could have been derived from either a triplet nitrene or a triplet nitrenium ion precursor. Homolysis of the N—O bond to generate radical intermediates was ruled out because of the nearly quantitative yield of pivalic acid derived from 71d. The pivaloxy radical would have undergone rapid decarboxylation to generate CO2 and the rert-butyl radical under these conditions. Since no rearrangement product was observed, it was tentatively concluded that this ester underwent direct decomposition to 4-nitrophenyl-nitrene without the intermediacy of a nitrenium ion. ... 

The role of singlet and triplet nitrenium ions in these reactions has been evaluated.228,229 Convincing evidence for the participation of singlet ground-state 2-acetylphenyl nitrenium ions during the photodecomposition of 3-methylanthranils in concentrated sulfuric acid has been obtained.229 For example, in the presence of arenes or anisole, in addition to the usual products, substantial amounts of diphenylamines and 3- and 5-(p-methoxy-phenyl)-2-aminoacetophenones, respectively, are formed by electrophilic attack of the nitrenium ion on the added substrate. 

Scribner. J. D., and Naimy, N. K. Destruction of triplet nitrenium ion by ascorbic acid. Experientia,37 470-471.1975. 

Photolysis of 3-phenyl-2,l-benzisoxazole in 48% HBr produced reduction and substitution products via a proposed triplet state nitrenium ion intermediate (71HCA2111). Photolytic decomposition of 5-bromo-3-phenyl-2,l-benzisoxazole in 48% HBr gave 2-amino-5-bromoacetophenone and 2-amino-3,5-dibromoacetophenone (Scheme 18). A nitrenium ion intermediate was also proposed for the photolytic decomposition of 3-phenyl-2,l-benzisoxazole in concentrated HCl (Scheme 19) (7IHCA2111). 

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. ... 

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. 

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... 

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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