Making Photochemically Generated Phenyl Cations Visible by Addition to Aromatics: Production of Phenylcyclohexadienyl Cations and Their Reactions with Bases/Nucleophiles S. Steenken,* ,1 M. Ashokkumar, 1,2 P. Maruthamuthu, 1,2 and R. A. McClelland* ,3 Contribution from the Max-Planck-Institut fu ¨ r Strahlenchemie, D-45413 Mu ¨ lheim, Germany, and Department of Chemistry, UniVersity of Toronto, Canada M5S 1A1 ReceiVed March 3, 1998 Abstract: The benzenediazonium cation and its 4-fluoro, 4-chloro, 4-bromo, 4-methyl, and 4-methoxy derivatives were photolyzed in 1,1,1,3,3,3-hexafluoroisopropyl alcohol (HFIP) with 20-ns pulses of 308-nm light from a XeCl excimer laser. This leads to a pronounced and permanent depletion of the parent compounds (quantum yields 0.89-0.96), but no signal from a transient is seen. However, on addition of aromatics such as, e.g., mesitylene, strong signals due to species with λ max at 250-260 and 350-390 nm are detected. In the absence of bases/nucleophiles other than the aromatics, these species, which do not react with oxygen, have a lifetime in the microsecond to millisecond range. On the basis of their absorption spectra and their reactivity with typical bases/nucleophiles such as halides, alcohols, and ethers, the transients are identified as cyclohexadienyl cations formed from the photoproduced “invisible“ phenyl cations by addition to the ring of the added aromatics. On the basis of competition data for reaction with HFIP and aromatic, the substituents Me, MeO, and Cl lead to a weak increase in selectivity of the phenyl cation, whereas F and Br do not influence the selectivity. Introduction The reactions of arenediazonium ions have attracted consider- able interest, both from a synthetic and from a mechanistic point of view. 4 The diazonium ions are able to undergo homolytic decomposition, as in the Gomberg-Bachmann reaction, 5 or heterolysis, as in, e.g., solvolysis reactions. A particularly fascinating aspect is the gradual changeover from heterolytic to homolytic dediazoniations, which leads to the question of a common intermediate for these two types of reactions. 6 A point of interest for the heterolytic reactions has been whether aryl cations are real intermediates. These cations are expected to be highly reactive due to the large degree of localization of charge at C R in an orbital of significant s character. 7,8 This question has been positively answered on the basis of carefully performed product analysis and kinetic studies on the thermal decomposition of PhN 2 + in H 2 O and in 2,2,2- trifluoroethanol. 9 It was, however, concluded that the phenyl cation is extremely short-lived. This result has been qualita- tively supported by 337-nm laser flash photolysis studies of monosubstituted arenediazonium cations in aqueous solutions, 10 where it was concluded that the lifetime of the aryl cations is probably e0.5 ns. In the case of p-amino-substituted phenyl cations, which exist in the triplet state, their lifetime in liquid solution has recently been measured to be <15 ps. 11 Phenyl cations stabilized by seVeral methoxy groups appear to have long lifetimes in LiCl matrices at 77 K. 12 We have recently found that the solvent 1,1,1,3,3,3-hexafluoro- 2-propanol (HFIP) 13 is sufficiently weakly nucleophilic to allow the direct, time-resolved detection of highly reactive carboca- tions such as the (antiaromatic) 9-fluorenyl cation 14 or benzyl cations. 15 The solvent HFIP is also moderately polar (ǫ ) 16.6), 16 so it should support ionic processes. Diazonium salts are obviously good precursors for carbocations because N 2 is such an excellent leaving group. Finally, photochemical excita- tion should speed up bond breakage as found in many analogous cases. 17,18 Therefore, the combination of benzenediazonium cations as precursors, HFIP as a solvent, and photolysis to accelerate bond rupture should provide conditions suited for the (1) Max-Planck-Institut. (2) Department of Energy, University of Madras, India. (3) University of Toronto. (4) For reviews, see e.g.: Zollinger, H. Acc. Chem. Res. 1973, 6, 335. Zollinger, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 141. (5) See: Ru ¨chardt, C.; Freudenberg, B.; Merz, E. Chem. Soc., Spec. Publ. 1965, 19, 154. (6) Broxton, T. J.; Bunnett, J. F.; Paik, C. H. J. Chem. Soc., Chem. Commun. 1970, 1363. (7) Gleiter, R.; Hoffmann, R.; Stohrer, W.-D. Chem Ber. 1972, 105, 8. (8) Dill, J. D.; Schleyer, P. v. R.; Pople, J. A. J. Am. Chem. Soc. 1977, 99, 1. (9) (a) Swain, C. G.; Sheats, J. E.; Harbison, K. G. J. Am. Chem. Soc. 1975, 97, 783. (b) Bergstrom, R. G.; Landells, R. G. M.; Wahl, G. H.; Zollinger, H. J. Am. Chem. Soc. 1976, 98, 3301. (10) Scaiano, J. C.; Kim-Thuan, N. J. Photochem. 1983, 23, 269. (11) Gasper, S. M.; Devadoss, C.; Schuster, G. B. J. Am. Chem. Soc. 1995, 117, 5206. (12) Ambroz, H. B.; Przybytniak, G. K.; Stradowski, C.; Wolszczak, M. J. Photochem. Photobiol. A 1990, 52, 369. (13) For quantitative data on the solvolyzing power and the nucleophi- licity of HFIP, see: Schadt, F. L.; Schleyer, P. v. R.; Bentley, T. W. Tetrahedron Lett. 1974, 2335. (14) (a) McClelland, R. A.; Mathivanan, N.; Steenken, S. J. Am. Chem. Soc. 1990, 112, 4857. (b) Cozens, F. L.; Mathivanan, N.; McClelland, R. A.; Steenken, S. J. Chem. Soc., Perkin Trans. 2 1992, 2083. (c) Cozens, F. L.; Li, J.; McClelland, R. A.; Steenken, S. Angew. Chem., Int. Ed. Engl. 1992, 31, 743. (d) McClelland, R. A.; Cozens, F. L.; Li, J.; Steenken, S. J. Chem. Soc., Perkin Trans. 2 1996, 1531. (15) McClelland, R. A.; Chan, C.; Cozens, F.; Modro, A.; Steenken, S. Angew. Chem. 1991, 103, 1389; Angew. Chem., Int. Ed. Engl. 1991, 30, 1337. (16) Murto, J.; Kivinen, A.; Lindell, E. Suomen Kemistilehti B 1970, 43, 28. These authors also determined the viscosity of HFIP to be 1.579 cP at 25 °C. (17) Bartl, J.; Steenken, S.; Mayr, H.; McClelland, R. A. J. Am. Chem. Soc. 1990, 112, 6918. 11925 J. Am. Chem. Soc. 1998, 120, 11925-11931 10.1021/ja980712d CCC: $15.00 © 1998 American Chemical Society Published on Web 11/06/1998