Triplet aryl cations

The thermal methods available for the formation of aryl cation in solution are limited to the solvolysis of suitable perfluorophenylalkyl esters and aromatic diazo-nium salts [6]. Furthermore, these methods lead to unselective chemistry due to the singlet nature of the aryl cation formed. Under photostimulation, and according to the substituents, aromatic halides give either the singlet or triplet phenyl cation. The mediation of triplet aryl cations is essential for achieving the chemoselectivity desired in these reactions [12]. In order to ensure the formation of such intermediates, certain conditions should be met ... 

Arylation is possible by the addition of triplet aryl cations to neutral nucleophiles such as alkenes, alkynes, enols, and enamines (photo SN1 mechanism). For alkenes, the... 

Figure 6.47 Alternative modes of stabilization in singlet and triplet aryl cations.

Cox, A., Kemp, T. J., Payne, D. R., Symons, M. C. R., and Pinot de Moira, R, Electron spin resonance characterization of ground state triplet aryl cations substituted at the 4-position by dialkylamino group, /. Am. Chem. Soc., 100, 4779, 1978. 

Scaiano and Kim-Thuan (1983) searched without success for the electronic spectrum of the phenyl cation using laser techniques. Ambroz et al. (1980) photolysed solutions of three arenediazonium salts in a glass matrix of 3 M LiCl in 1 1 (v/v) water/acetone at 77 K. With 2,4,5-trimethoxybenzenediazonium hexafluorophos-phate Ambroz et al. observed two relatively weak absorption bands at 415 and 442 nm (no e-values given) and a reduction in the intensity of the 370 nm band of the diazonium ion. The absence of any ESR signals indicates that these new bands are not due to aryl radicals, but to the aryl cation in its triplet ground state. 

Ambroz and Kemp (1979 a, 1982 a, b) photolyzed benzenediazonium salts with donor substituents in the 4-position (2,5-di- -butoxy-4-morpholine-, 2,5-diethoxy-4-H-butylthio-, and 2,4,5-trimethoxy-benzenediazonium hexafluorophosphate in LiCl - H20 - acetone matrices at low temperatures, 77-130 K). Under these conditions the aryl cation formed is sufficiently stable to be identified spectroscopically. It is a triplet, a result that is consistent with earlier ab initio calculations by Dill et al. (1977), and with earlier observations by Lee et al. (1961), also at low temperature (77 K). 

Aryl cations are highly reactive intermediates with two possible electronic configurations, the singlet and triplet states. The former is a localized cation with a vacant a orbital at the dicoordinated carbon atom, whereas the latter has a diradical character with single occupancy of the g orbital and the charge being delocalized in the... 

Accordingly, whilst five-membered heterocycles exclusively give arylation in a-position toward aryl cations, the Imposition is only arylated in 2,5-dimethyl derivatives. Such selectivity for attacking in the a-position is greater than in other electrophilic substitutions, which indicates that triplet phenyl cations are not localized cations and therefore are more selective electrophiles [53],... 

Sensitized) photolysis of diazonium salts as a mild general method for the generation of aryl cations, chemoselectivity of the singlet and triplet 4-substituted phenyl cations. Journal of Organic Chemistry, 70, 603-610. 

The bromide ion apparently has trapped the excimer. One might consider homolysis of the C-Cl bond in the triplet, followed by electron transfer from the aryl radical to chlorine to generate aryl cation which then reacts with bromide ion, using as a model the mechanism put forward to explain the phenol products from the photolysis of 4-chlorobiphenyl in water [25]. However, this mechanism seems unlikely, since the excited chloroarene is in the micellar interior and such a mechanism has not been observed in hydrocarbon solvents. In addition, this... 

As a matter of fact the photochemically generation of the triplet phenyl cation is synthetically equivalent to activation by metal catalysis that likewise produces a sort of phenyl cation complexes to the metal by oxidative addition of the aryl halide and elimination of the halide anion (see above). In fact, a photochemical parallel has been found for most type of metal catalyzed arylations, via reactions that occur from the same or similar reagents. 

Such a mechanism has been hypothesized for the reaction of aryl halides substituted by strong electron-donating groups.838,845 4-Chloroaniline (239), for example, reacts in the triplet state to give a phenyl cation 240, apparently of triplet character (Jt501), which has a selective reactivity toward Jt-, but not n-, nucleophiles (Scheme 6.94), in contrast to the unselective reactions of common singlet aryl cations.847 Interestingly, the cation is added to an alkene to yield 241 even in nucleophilic methanol. 

Laah, K. K., Rasul, G., Prakash, G. K., Olah, G. A. (2002). DFT Study of Substituted and Benzannelated Aryl Cations Substituent Dependency of Singlet/Triplet Ratio. Journal of Organic Chemistry, 67, 2913-2918. 

Ambroz, H. B. and Kemp, T. J., Substituent factors influencing net stabilization of the triplet level in aryl cations, J. Chem. Soc., Perkin Trans. 2,1420, 1979. 

---