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J . Am. Chem. SOC.1990, 112, 9648-9649
248-nm Laser Flash Photoprotonation of Mesitylene, Hexamethylbenzene, and 1,3,5-Trimethoxybenzene in 1,1,1,3,3,3-HexafluoroisopropylAlcohol. Formation and Electrophilic Reactivities of the Cyclohexadienyl Cations Steen Steenken*.la and Robert A. McClelland*qib Max- Planck-Institut fur Strahlenchemie 0-4330 Mulheim, Germany Department of Chemistry, University of Toronto Toronto, Canada MSS I A I Received August 14, 1990 Revised Manuscript Received October I I , 1990
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Proton exchange upon photolysis of aromatic compounds in acidic solutions suggests that the excited aromatic species is protonated to form an intermediate cyclohexadienyl cation2 Quenching of the fluorescence of aromatic compounds by acids" and ipso photosubstitutionk have been interpreted in terms of the same mechanism. Photoreactions proceeding by way of carbocations are well establi~hed,~ and there have been a number of recent reports using flash photolysis to detect the intermediates and to study directly the kinetics of their reactions6 This, however, has not been done for photoprotonation of aromatics (eq 1). In fact, although cyclohexadienyl cations are well established
in strong acids,' there is no information regarding their lifetime under solvolytic conditions (in the presence of nucleophiles), except for some highly stabilized examples.* We have recently found that the solvent 1 ? I , I ,3,3,3-hexafluoroisopropylalcohol (HFIP) has a dramatic kinetic stabilizing effect for the 9-fluorenyl cation.6' In this paper we report that this solvent has the additional property of being sufficiently acidic to effect protonation of electronically excited aromatic compounds without the need for added stronger acids. It is thus possible to produce, characterize spectroscopically, and kinetically study cyclohexadienyl cations. As examples, data ( I ) (a) Max-Planck-Institut. (b) University of Toronto. (2)(a) Smith, B. E. J. Chem. Soc. A 1969,2673.(b) Lodder, G.; Havinga, E. Tetrahedron 1972,28, 5583. (c) Stevens, C. G.; Strickler, S. J. J. Am. Chem. Soc. 1973, 95,3922. (d) Shikuza, H.; Tobita, S.Ibid. 1982,104, 6919. (e) Shizuka, H.Acc. Chem. Res. 1985, 18, 141. (3)(a) Brandon, W.; Pincock, A. L.; Pincock, J. A.; Redden, P.; Sehmbey, C. J. Am. Chem. Soc. 1987,109,2181.(b) Pinaxk, J. A,; Redden, P. R. Can. J. Chem. 1989,67,2181. (c) Wan, P.; Wu, P. J. Chem. Soc.. Chem. Commun. 1990,822. (4)For an alternative interpretation for compounds with reactive side chains, see: (a) Wan, P.; Culshaw, S.; Yates, K. J . Am. Chem. SOC.1982, 104,2509. (b) Wan, P.; Yates, K. J . Org. Chem. 1983,48, 869. (c) Turro, N. J.; Wan, P. J. Photochem. 1985.28.93. (d) Wan, P.; Chak, B. J. Chem. SOC.,Perkin Trans. 2 1986, 1751. (5)(a) Cristol, S.J.; Bindel, T. H. In Organic Photochemistry; Padwa, A,, Ed.; Marcel Dekker: New York, 1983;Vol. 6,pp 327-415. (b) Kropp, P. J. Acc. Chem. Res. 1984,17, 131-137. (c) Wan, P.; Yates, K. Reu. Chem. Intermed. 1984,5, 157. (6)(a) McClelland, R. A.; Banait, N.; Steenken, S.J . Am. Chem. SOC. 1986,108,7023;(b) 1989,111, 2935. (c) McClelland, R. A,; Kanagasabapathy, V. M.; Stemken, S.Ibid. 1988, 110, 6913. (d) McClelland, R. A,; Kanagasabapathy, V. M.; Banait, N.; Steenken, S. Ibid. 1989,111,3966.(e) Steenken, S.;McClelland, R. A. Ibid. 1989,1 1 1 , 4967. (f) McClelland, R. A,: Mathivanan. N.: Steenken. S.Ibid. 1990,112,4857.(8) Schnabel, W.; Naito, 1.; Kitamura, T.; Kobayashi, S.; Taniguchi, H. Tetrahedron 1980,36, 3229-3231. (h) Kobayashi, S.;Kitamura, T.; Taniguchi, H.; Schnabel, W. Chem. Left. 1983,1 117-1 120. (i) Van Ginkel, F. 1. M.; Visser, R. J.; Varma, Lodder, G. J. Photochem. 1985,30,453-473. (j) Kobayashi, C. A. G. 0.; S.;Zhu, Q. Q.; Schnabel, W. Z . Naturforsch. 1988,436,825-829. (k) Minto, R. E.; Das, P. K. J . Am. Chem. SOC.1989,I l l , 8858. (I) Johnston, L. J.; Lobaugh, J.; Wintgens, V. J . Phys. Chem. 1989,93,7370. (m) Alonso, E. 0.;Johnston, L. J.; Scaiano, J. C.; Toscano, V. G. J. Am. Chem. SOC.1990, 112, 1270.
(7) Brouwer, D. M.; Mackor, E. L.; MacLean, C. In Carbonium Ions; Olah, G . A,, Schleyer, P. v. R., Eds.; Wiley-Interscience: New York, 1970; Vol. 11, Chapter 20. (8) Effenberger, F. Acc. Chem. Res. 1989,22, 27.
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Figure , Absorption spectrum of MESH' produced by 248-nmphotolysis of 0 mM MES in HFIP at 20 f 1 O C and measured at 100 ns after the laser pulse. The inserts show the decay of MESH' a t (a) 258 nm and (b) 355 nm and (c) the dependence on [MeOH] of kobd for decay monitored a t 355 nm. The arrows show the position of the laser pulse.
on those derived from mesitylene (MES), hexamethylbenzene (HMB), and 1,3,5-trimethoxybenzene (TMB) are reported. The result obtained on photolysis of MES is shown in Figure 1: A transient with two absorption peaks appears within the 204s laser pulse and decays exponentially, with the same rates at the two wavelengths (inserts a and b). There is no difference between oxygen- and argon-saturated solutions while added bases accelerate the decay (insert c). That this transient is the cyclohexadienyl cation MESH+ is seen by comparison with literature spectra obtained for MES in highly acidic solutions where the cation is stable. As shown in Table I, the peaks of the species match closely with respect to both position and relative inten~ity.~This table also shows that HMB and TMB behave in a similar manner upon photolysis in HFIP, giving the corresponding cyclohexadienyl cations HMBH+ and TMBH+. Table 11 lists the quantum yields for the formation of the cyclohexadienyl cations and rate constants for their decay. The former were determined as previously described,6bwith the photoionization of iodide in water as actinometer, and using values reported in highly acidic media for the extinction coefficients of the cation^.^ Quantum yields measured at the two maxima are in good agreement in each case. Rate constants for decay of cation for replicate measurements with the same solution were reproducible to f3%. However, for MESH' and HMBH+, decay rates for different solutions showed larger deviations (f30%), and there was an effect of dose, rate constants being slightly higher at very high laser intensities with a poorer obeyance to first-order decay. Both these observations were made previously with 9-fluorenyl cation6' The former can be attributed to varying amounts of trace water in HFIP (see rate constants in Table III), the latter to reaction with the hexafluoroisopropoxide ion which must also form in the photoprotonation so that its initial concentration increases with laser dose. TMBH+ is extremely long-lived in the solvent HFIP, with little decay in 10 ms. This cation is also observed in trifluoroethanol (TFE) and even water (with 20% acetonitrile (AN)), although the quantum yields for its formation are an order of magnitude lower. The amount formed was increased by adding HCIO,, although even 3 M acid did not give as much cation as in HFIP. There was no cation signal for MES in any of these other solvents, including 5 M HC10, in 20% AN. As expected, added bases/nucleophiles accelerate the decay of the cyclohexadienyl cations. Table I11 lists some preliminary (9)With both MESH+ and TMBH*, the optical density does not decay (see inserts a and b to Figure l ) , due to the presence to zero at either ,A, of other, as yet unidentified, photoproducts. The optical densities employed in the comparisons at the two wavelengths and in the quantum yield measurements were calculated from differences between the initial value and the value after decay of the cation, using the z values at XI and X2 for MESH' and HMBH' and that at A, for TMBH'. Due to the assumptions involved in the c values of the cations in HFIP, the error in the quantum yields for cation formation is estimated as &20%
0 1990 American Chemical Society
J . Am. Chem. SOC.1990, 112, 9649-9651
9649
Table 1. UV Spectra of Cyclohexadienyl Cations e l , M-I, cm-l X,(max) t2, M-I, cm-I OD(Xl)/OD(A2) solvent h(max) HFlP 355 260 1 .276 254 H F/ BF3' 355 1 1 000 8700 1.26 262 HCI/AIC13/lmCld 360 10700 1.05 10200 C F3SOjH' 358 HMBHt HFlP 390 275 1.5Ib 285 HCI/AIC13/lmCld 396 9880 6700 1.47 283 98% H2S0,f 395 ==10000 7240 1.38 TMBH' HFlP 345 250 0.58b 65% HClOAg 346 IO 000 I5000 0.65 25 1 'This work. bSee footnote 9. 'Dallinga, G.; Mackor, E. L.; Verrijn Stuart, A. A. Mol. Phys. 1958, I, 123. dSmith, G. P.; Dworkin, A. S.; Pagni, R. M.; Zingg, S. P. J . Am. Chem. Soc. 1989, 111, 525. lmCl is I-ethyl-3-methylimidazolium chloride. cBokoss, H. J.; Ransom, R. J.; Roberts, R. M. G.; Sadri, A. R. Tetrahedron 1982, 38,623. IDeno, N. C.; Groves, P. J.; Jaruzelski, J. J.; Lugasch, M. N.; J. Am. Chem. Sot. 1960.82.4719. gKresge, A. J.; Chiang, Y.;Kakke, L. E. J . Am. Chem. Soc. 1971, 93, 6167.
cation MESHt
Table 11. Quantum Yields for Formation of Cyclohexadienyl Cations and Rate Constants for Decay at 20 f I OC cation solvent v &(decay),s-I MESHt HFlP 0.08 I x 105 HMBH' 7 x 103 0.04 HFIP < I x 102 TMBHt HFIP 0.06 7 x 102 -0.006 TMBHt TFE 5.8 X IO' -0.003 1:4 AN:H,O TMBHt 0.4 M HCIO4 (20% AN) 0.01 3.2 x 105 TMBH' 0.03 6.3 X 10' TMBHt 3 M HClO, (20% AN) 'See footnote 9. Table 111. Rate Constants, kNu for the Reaction of Bases/Nucleophiles Nu with MESH+, HMBH', and TMBH' in HFlP at 20 f 1 OC kNu: M-' S-I Nu MESHtb HMBHtC TMBHtd 8.8 X IO6 2.7 X IOs
