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Spectral Properties and Reactivity of Diarylmethanol Radical Cations in Aqueous Solution. Evidence for Intramolecular Charge Resonance1 Massimo Bietti*,2a and Osvaldo Lanzalunga*,2b Dipartimento di Scienze e Tecnologie Chimiche, Universita` Tor Vergata, Via della Ricerca Scientifica, I-00133 Rome, Italy, and Dipartimento di Chimica, Universita` La Sapienza, P.le A. Moro, 5 I-00185 Rome, Italy
[email protected] Received November 16, 2001
Spectral properties and reactivities of ring-methoxylated diarylmethane and diarylmethanol radical cations, generated in aqueous solution by pulse and γ-radiolysis and by the one-electron chemical oxidant potassium 12-tungstocobalt(III)ate, have been studied. The radical cations display three bands in the UV, visible, and vis-NIR regions of the spectrum. The vis-NIR band is assigned to an intramolecular charge resonance interaction (CR) between the neutral donor and charged acceptor rings, as indicated by the observation that the relative intensity of the vis-NIR band compared to that of the UV and visible bands does not increase with increasing substrate concentration and that the position and intensity of this band is influenced by the ring-substitution pattern. In acidic solution (pH ) 4), monomethoxylated diarylmethanol radical cations 1a•+-1e•+ decay by CR-H deprotonation [k ) (1.7-1.9) × 104 s-1] through the intermediacy of a ketyl radical, which is further oxidized in the reaction medium to give the corresponding benzophenones, as evidenced by both time-resolved spectroscopic and product studies. With the dimethoxylated radical cation 2•+, CR-H deprotonation is instead significantly slower (k ) 6.7 × 102 s-1). In basic solution, 1a•+-1e•+ undergo -OH-induced deprotonation from the R-OH group with kOH- ≈ 1.4 × 1010 M-1 s-1, leading to a ketyl radical anion, which is oxidized in the reaction medium to the corresponding benzophenone. Side-chain deprotonation of alkylaromatic radical cations represents one of the main reaction pathways of these important reactive intermediates, and the reactions of this class of carbon acids attract continuous attention.3 In relation to our interest on this topic, we have recently shown the existence of a pH-dependent mechanistic change for the deprotonation reactions of 1-(4-methoxyphenyl)alkanol radical cations in aqueous solution. Accordingly, while in neutral and acidic solution 4-methoxybenzyl alcohol radical cation undergoes CR-H deprotonation to give the corresponding benzylic radical, in alkaline solution it reacts with -OH at a diffusioncontrolled rate undergoing O-H deprotonation.4,5 To obtain a better understanding of this mechanistic picture, we felt that useful information could be provided through the study of the reactivity of mono- and dimethoxylated diarylmethanol radical cations (compounds 1a-1e and (1) This paper is dedicated to Professor Enrico Baciocchi on the occasion of his 70th birthday. (2) (a) Universita´ Tor Vergata. (b) Universita´ La Sapienza. E-mail:
[email protected]. (3) See, for example: (a) Electron Transfer in Chemistry (Organic, Organometallic, and Inorganic Molecules; Part 1: Organic Molecules); Balzani, V., Ed., Wiley-VCH: Weinheim, 2001; Vol. 2. (b) Baciocchi, E.; Bietti, M.; Lanzalunga, O. Acc. Chem. Res. 2000, 33, 243-251. (c) Parker, V. D.; Zhao, Y.; Lu, Y.; Zheng, G. J. Am. Chem. Soc. 1998, 120, 12720-12727. (d) Bockman, T. M.; Hubig, S. M.; Kochi, J. K. J. Am. Chem. Soc. 1998, 120, 2826-2830. (e) Freccero, M.; Pratt, A.; Albini, A.; Long, C. J. Am. Chem. Soc. 1998, 120, 284-297. (4) Baciocchi, E.; Bietti, M.; Steenken, S. Chem. Eur. J. 1999, 5, 1785-1793. (5) Baciocchi, E.; Bietti, M.; Gerini, M. F.; Manduchi, L.; Salamone, M.; Steenken, S. Chem. Eur. J. 2001, 7, 1408-1416
2 in Chart 1), i.e., substrates containing an additional aromatic ring on the benzylic carbon, both in acidic and basic solution. For comparison, the radical cations of diarylmethanes 3a and 3c have also been investigated. The results of these studies are reported herein. Results Generation of the Radical Cations. Radical cations of substrates 1-3 were generated in aqueous solution by means of radiation chemical techniques (pulse radiolysis and steady-state 60Co γ-radiolysis) employing sulfate radical anion (SO4•-) as the oxidant (method 1: eqs 1-4, X ) H, OH): hν, γ-ray
H2O 98 H+, •OH, e-aq
(1)
•
OH + CH3C(CH3)2OH f H2O + •CH2C(CH3)2OH (2) e-aq + S2O82- f SO42- + SO4•-
(3)
SO4•- + ArCH(X)Ar′ f SO42- + Ar•+CH(X)Ar′ (4) The hydroxyl radical (•OH) is scavenged by 2-methyl2-propanol (eq 2) with k ) 6 × 108 M-1 s-1,6 while the (6) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513-886.
10.1021/jo016287f CCC: $22.00 © 2002 American Chemical Society Published on Web 03/16/2002
Diarylmethanol Radical Cations in Aqueous Solution
J. Org. Chem., Vol. 67, No. 8, 2002 2633
Chart 1
hydrated electron (e-aq) reacts with the peroxydisulfate anion, leading to the formation of SO4•- (eq 3), with k ) 1.2 × 1010 M-1 s-1.6 SO4•- is known to react with anisoletype derivatives by electron transfer to give the corresponding radical cations (eq 4) with a rate constant k ) 5 × 109 M-1 s-1.4,7,8 In the case of 2, Tl2+ was also used as the oxidant, produced by irradiating N2O-saturated aqueous solutions (method 2, eqs 1, 5-7):
e-aq + N2O + H2O f N2 + •OH + -OH •
OH + Tl+ + H+ f Tl2+ + H2O
(5) (6)
Tl2+ + ArCH(OH)Ar′ f Tl+ + Ar•+CH(OH)Ar′ (7) -
The function of N2O is to scavenge e aq, leading to the formation of an additional hydroxyl radical (eq 5), with k ) 9.1 × 109 M-1 s-1.9 Tl2+ is then produced by oxidation of Tl+ by •OH (eq 6) with k ) 1.2 × 1010 M-1 s-1.10 Also Tl2+ reacts with anisole-type derivatives by one-electron transfer to give the corresponding radical cations (eq 7) with k ≈ 5 × 108 M-1 s-1.7 Product Analysis. Product analysis was carried out under acidic conditions (pH ) 4.0) for substrates 1a-e and 2 using potassium 12-tungstocobalt(III)ate, K5[Co(III)W12O40] (from now on indicated as Co(III)W), a genuine one-electron chemical oxidant,11,12 to generate the radical cations. 4-Methoxy-4′-X-benzophenones (5ae) and 3,4-dimethoxy-4′-methylbenzophenone were the exclusive products observed. Product analysis of 1a was also studied under both acidic (pH ) 4.0) and basic (pH ) 10.0) conditions, generating the oxidant by means of steady-state 60Co γ-radiolysis. Argon-saturated aqueous solutions containing 1a (1 mM), K2S2O8 (0.5 mM), and 2-methyl-2-propanol (0.2 M) were irradiated at room temperature with a 60Co γ-source at dose rates of 0.5 Gy s-1 for the time necessary to obtain a 40% conversion with respect to peroxydisulfate. 4,4′-Dimethoxybenzophenone (5a) was the exclusive product observed under both conditions. Spectral Properties. Spectral information about radical cations 1•+-3•+ was obtained using the pulse radiolysis technique, generating the radical cations by reaction of 1-3 with SO4•-, as described in eqs 1-4. As an example, in Figure 1 are displayed the time-resolved (7) O’Neill, P.; Steenken, S.; Schulte-Frohlinde, D. J. Phys. Chem. 1975, 79, 2773-2777. (8) Neta, P.; Madhavan, V.; Zemel, H.; Fessenden, R. W. J. Am. Chem. Soc. 1977, 99, 163-164. (9) Janata, E.; Schuler, R. H. J. Phys. Chem. 1982, 86, 2078-2084. (10) Schwarz, H. A.; Dodson, R. W. J. Phys. Chem. 1984, 88, 36433647. (11) Weinstock, I. A. Chem. Rev. 1998, 98, 113-170. (12) Eberson, L. J. Am. Chem. Soc. 1983, 105, 3192-3199.
Figure 1. Time-resolved absorption spectra observed on reaction of SO4•- with 1b (0.2 mM) at T ) 25 °C, recorded after pulse radiolysis of an Ar-saturated aqueous solution (pH ) 3.9), containing 0.1 M 2-methyl-2-propanol and 5 mM K2S2O8, at 4 µs (filled circles), 32 µs (empty circles), 80 µs (filled squares) and 280 µs (empty squares) after the 50 ns, 10 MeV electron pulse. Inset: Time-resolved absorption spectra observed on reaction of SO4•- with 1b (0.2 mM) at T ) 25 °C, recorded after pulse radiolysis of an O2-saturated aqueous solution (pH ) 3.9), containing 0.1 M 2-methyl-2-propanol and 5 mM K2S2O8, at 4 µs (filled circles), 32 µs (empty circles), and 280 µs (empty squares) after the 50 ns, 10 MeV electron pulse. Table 1. Spectral Data for the Radical Cations of Diarylmethanols 1 and 2 and Diarylmethanes 3, Generated by Pulse Radiolysis in Aqueous Solution (pH ≈ 4.0) λmax/nm radical cation
A
B
C
∆ODC/∆ODB
1a•+
290 290 290 290 290 300 300 300
440 450 450 450 450 420 440 440
980 800 680 690 560a 620 960 680
0.81 0.61 0.49 0.47 9, 1 mM sodium tetraborate was added to avoid undesired pH variations upon irradiation. A flow system was employed in all the experiments. The observed rates (kobs) were obtained by averaging 6-12 values, each consisting of an average of 5-15 shots and were reproducible to within 5%. The second-order rate constants for reaction of the radical cations with -OH (kOH-) were obtained from the slopes of the plots of kobs vs the concentration of NaOH.
Bietti and Lanzalunga The ketyl radical and radical anion, (4-MeOC6H4)2C(•)OH (4a•) and (4-MeOC6H4)2C(•)O- (4a•-), were generated after pulse radiolysis of an argon-saturated aqueous solution (pH ) 5.0 and 11.0, respectively) containing 0.05 mM (4-MeOC6H4)2CO (5a), 1 M 2-methyl-2-propanol, 1 mM Na2B4O7, and 20% MeCN (to solubilize 5a).
Acknowledgment. Thanks are due to the Ministero dell’Istruzione, dell’Universita` e della Ricerca (MIUR) and the Consiglio Nazionale delle Ricerche (CNR) for financial support. Pulse and γ-radiolysis experiments were performed at the Paterson Institute for Cancer Research Free Radical Research Facility, Manchester, UK, with the support of the European Commission through the Access to Large-Scale Facilities activity of the TMR Program. JO016287F
