Phenyl nitrenium ion

As shown in Figure 7.10c, because of the lower energy of the nitrenium ion orbitals, interaction of the 2p orbital with the n bonding orbitals of C substituents will be stronger than the interaction with the n orbitals. Substantial stabilization ensues. The interaction raises the separation of the 2p and spn orbitals. Thus, the ground state for C -substituted nitrenium ions is expected to be So, unlike the situation in C -substituted carbenes. This expectation has been supported by ab initio calculations on phenyl nitrenium ion [183, 184]. Because the LUMO remains very low, this type of nitrenium ion will be very electrophilic. 

SJ-l-Phenylethanol, preparation, 12 Phenyl carbene ( CHC6H5), 116, 275 Phenyl nitrene (CeHsN), 118 Phenyl nitrenium ion, 120 [4]Phenylene, 150... 

Robbins, R. J., Yang, L. L.-N., Anderson, G. B., and Falvey, D. E., Photogenerated arylnitrenium ions reactions of N-tert-hutyl (2-acetyl-4-substituted)phenyl nitrenium ions with alcohols and water studied by laser flash photolysis, J. Am. Chem. Soc., 117, 6544, 1995. 

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). 

Ford and Thompson used semiempirical methods to evaluate the relative stabilities of various nitrenium ion guanine adducts where the exocyclic 2-amino group of guanine is coupled to nitrogen and various ring positions of phenyl or... 

Phenyl azide was also found to react with alkenes in TFA (Scheme 11). The reaction was assumed to go via an aziridinium ion generated from attack of the alkene on the nitrenium ion because of the overall /rans-addition to the alkene noted in product 29. An alternative Sn2 mechanism was disfavored because phenyl azide decomposes at 21 °C in 50 vol% cyclohexene/TFA with an almost identical first-order rate constant as in 50vol% benzene/TFA even though cyclohexene is a considerably stronger nucleophile than benzene. ... 

The substituent effects calculated for A show that aromatic and vinylic jT-donors in the para-position have a stabilizing effect on the nitrenium ions that is much larger than is seen in Op. For example, Op for Me, MeO, and Ph are —0.31, —0.78, and —0.18, respectively, while A for 75y (Ar = 4-tolyl), 75cc (Ar = 4-MeOphenyl) and 75n (Ar = 4-biphenylyl) are 8,1 kcal/mol, 22.7 kcal/mol, and 19.3 kcal/mol, respectively. The calculations and experimental data show that a para-phenyl substituent is about as stabilizing for a nitrenium ion as is a para-methoxy substituent. This unusual stabilization is the major reason that correlations of logS vs. cr are so scattered for nitrenium ions. Substituent effects at N are relatively small. Replacement of NH by NAc destabilizes the ion toward hydration by 4.5 l.Okcal/mol. Based on the correlation line, at 20°C this amounts to a predicted increase in by a factor of 4 to 11 when NH is replaced by NAc. The experimentally observed range of 1.5 to 9.0 (Table 1) is very close to this prediction. These calculated substituent effects on the thermodynamics of hydration and the calculated geometries of nitrenium ions (discussed in another section) indicate that for most nitrenium ions the canonical structure II of Scheme 38 is dominant. ... 

Aziridine formadon from arylnitrenes, rather than via tiiazolines, is known for highly fluorinated arenes. Phenyl azide with trifluotoacetic acid generates a nitrenium ion which adds stereospecifically to alkenes to give aziridines. Yields are rather low, pardy due to concurrent ring opening of the aziridine by addidon of trifluoroacedc acid. Similar reacdons can be adiieved with Lewis acids such as AlCb. Enamines with aryl azides can yield either 2-aminoaziridines or amidines. ... 

Nitrenium ions have been observed following flash photolysis of 4-ethoxy- and 4-methoxyphenyl azides in aqueous solution and a series of polyfluorinated phenyl azides in acidic media, such as acetonitrile containing sulfuric acid. Measured lifetimes of the phenylnitrenium ions varied from 1 ps up to milliseconds. 

The only known way to synthesize 3-substituted 6-phenyl-l,4,2-oxathiazines 8 is by ring expansion of 1,3-oxathiolinm salts 7 (see Houben-Weyl, Vol. E8a, p lOff) with azide ions (Method A), iodine/ammonia (Method B) or sulfenamides 11,12 (Method C).47 48 As intermediates, sextet structures of the nitrenium ion or nitrene type are under discussion. It is remarkable that the ring expansion obviously only incorporates the nitrogen function into the C—O bond and therefore only produces 8. 1,4,3-Oxathiazine derivatives, which could equally be formed, were not discovered.48 All three methods A C produce 1,4,2-oxathiazines 8 in yields which are moderate at best. The reaction of 2-dialkylamino-5-phenyl-l,3-oxathiolium perchlorates 7b, 7c with N, V-dimethylthiocarbamoylsulfenamide 11 yields, sometimes even as the main product, also bis(4-pheny -2-dialkylamino-5-thiazolyl) sulfides 9.48... 

---