Kinetic Studies on the Formation of Sulfonyl Radicals and Their

Article pubs.acs.org/JPCA

Kinetic Studies on the Formation of Sulfonyl Radicals and Their Addition to Carbon−Carbon Multiple Bonds Chryssostomos Chatgilialoglu,*,† Olivier Mozziconacci,†,∥ Maurizio Tamba,† Krzysztof Bobrowski,‡ Gabriel Kciuk,‡ Michèle P. Bertrand,§ Stéphane Gastaldi,§ and Vitaliy I. Timokhin§,⊥ †

ISOF, Consiglio Nazionale delle Ricerche, Via P. Gobetti 101, 40129 Bologna, Italy Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland § Aix-Marseille Université, CNRS, Institut de Chimie Radicalaire UMR 7273, Equipe CMO, 13397 Marseille Cedex 20, France ‡

S Supporting Information *

ABSTRACT: The reactions of α-hydroxyl and α-alkoxyl alkyl radicals with methanesulfonyl chloride (MeSO2Cl) have been studied by pulse radiolysis at room temperature. The alkyl radicals were produced by ionizing radiation of N2Osaturated aqueous solution containing methanol, ethanol, isopropanol, or tetrahydrofuran. The transient optical absorption spectrum consisted of a broad band in the region 280−380 nm with a maximum at 320 nm typical of the MeSO2• radical. The rate constants in the interval of 1.7 × 107−2.2 × 108 M−1 s−1 were assigned to an electron-transfer process that leads to MeSO2Cl•−, subsequently decaying into MeSO2• radical and Cl−. The rate constants for the addition of CH3SO2• to acrolein and propiolic acid were found to be 4.9 × 109 M−1 s−1 and 5.9 × 107 M−1 s−1, respectively, in aqueous solutions and reversible. The reactivity of tosyl radical (p-CH3C6H4SO2•) toward a series of alkenes bearing various functional groups was also determined by competition kinetics in benzene. The rate constants for the addition of tosyl radical to alkenes vary in a much narrower range than the rate constants for the reverse reaction. The stabilization of the adduct radical substantially contributes to the increase of the rate constant for the addition of tosyl radical to alkenes and, conversely, retards the β-elimination of tosyl radical.

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INTRODUCTION The main synthetic applications of sulfonyl radicals (RSO2•) are based on their ability to add reversibly to carbon−carbon double and triple bonds (eq 1/−1).1,2 Recent examples are the iron-catalyzed RSO2• radical formation from sulfonylhydrazides and oxidative addition to alkenes affording β-hydroxysulfone derivatives,3 and iron-catalyzed regio- and stereoselective chlorosulfonylation of terminal alkynes.4 Taking advantage from the equilibrium (1/−1), Landais and co-workers have developed synthetically useful radical chain reactions using allylsilanes in oximation, alkenylation, and allylation of alkyl halides.5 The same group finalized three-component carboalkynylation and -alkenylation radical chain processes of olefins.6 Regioselectivity of tosyl radical (Ts• = p-CH3C6H4SO2•) mediated 5-exo-trig cyclization of 3-silyl-1,6-dienes have also been developed.7 In polymer chemistry, it is worth mentioning the recent highly controlled regiospecific free radical copolymerization of 1,3-diene monomers with sulfur dioxide.8,9 Alabugin and co-workers reported the first efficient experimental 5-endo-dig ring closure of a carbon-centered radical, when radical cyclizations of 1,3-diynes are triggered with tosyl radicals.10 The 5-endo-dig product was formed in 51−72% yield upon photolytic generation of Ts• radical from TsBr at room temperature.

Despite the practical importance of sulfonyl radicals, kinetic data of their formation and involvement in equilibrium (1/−1) are scarce.11 Some of us already reported on the rate constants for the β-elimination of Ts• radical from carbon-centered radicals (eq −1).12 Depending on the substituents X and Y, the rate constants vary by more than 2 orders of magnitude in the range of 103−106 s−1 at 293 K. On the other hand, rate constants for the addition of sulfonyl radicals to C−C multiple bonds are limited to an estimated value close to diffusioncontrol for the addition of MeSO2• to 1-hexene in CH3CN (k = 1 × 109 M−1 s−1 at 273 K).11 Rate constants of the same magnitude were also measured for the trapping of MeSO2• radical by a variety of carotenoid antioxidants containing multiconjugated double bonds in tert-butanol/water (60:40 v/ v).13 Spectroscopic data of sulfonyl radicals in solution indicated that they are σ-type species with a pyramidal center at sulfur.14−17 The spin distributions for the unpaired electron in MeSO2• and PhSO2• are similar, as well as the percentage (∼42%) on sulfur and oxygen atoms. 16,17 Moreover, ΔH298(RSO2Cl) values were measured for R = Me and Ph by photoacoustic calorimetry and found to be equal (295 kJ/ Received: May 19, 2012 Revised: June 21, 2012 Published: June 22, 2012

© 2012 American Chemical Society

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dx.doi.org/10.1021/jp304863y | J. Phys. Chem. A 2012, 116, 7623−7628

The Journal of Physical Chemistry A

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mol), indicating that the radical chemistry of RSO2• should be independent of the R group.18 The one-electron reduction of MeSO2Cl in aqueous solutions leads, in the first instance, to an electron adduct MeSO2Cl•− which lives long enough for direct detection and decays into sulfonyl radicals MeSO2• and Cl−, with k = 1.5 × 106 s−1 (eqs 2 and 3).19 The electrochemical reduction of a series of para substituted arenesulfonyl chlorides in CH3CN has also been investigated.20 A “sticky” dissociative electron transfer (ET) mechanism takes place with 4-CN and 4-NO2 derivatives, whereas with other substituents (MeO, Me, H, Cl, and F) a “classical” dissociative ET is followed. Polymerization processes initiated with sulfonyl chlorides and copper catalysts are thought to proceed through an analogous one-electron reduction step.21 eaq − + MeSO2 Cl → MeSO2 Cl•− •−

MeSO2 Cl

•

−

→ MeSO2 + Cl

sample from at least five points, and the average value is given for each pair of alkenes. The errors in the relative rate constants correspond to one standard deviation.

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RESULTS AND DISCUSSION Pulse Radiolysis Studies. Radiolysis of neutral water leads to the species eaq−, HO•, and H• as shown in eq 4 where the values in parentheses represent the chemical radiation yields (G) expressed in μmol J−1. In N2O-saturated solution (∼0.02 M), eaq− are scavenged efficiently by nitrous oxide. Indeed N2O transforms eaq− into the O•− species (eq 5, k5 = 9.1 × 109 M−1 s−1). The HO• radical [pKa(HO•) = 11.9] is in equilibrium with its conjugated base O•− (eq 6/−6, k6 = 1.2 × 1010 M−1 s−1 and k−6 = 1 × 108 s−1). Under these conditions, H• and HO• radicals accounted for 10 and 90%, respectively, of the reactive species.25,26

(2) (3)

In the present work we extended our studies on (i) the reaction of α-hydroxyl and α-alkoxyl alkyl radicals with MeSO2Cl and the reactivity of MeSO2• radical toward acrolein and propiolic acid in aqueous medium using time-resolved spectroscopy (pulse radiolysis) and (ii) the relative reactivity of Ts• radical toward a series of alkenes in benzene by kinetic and product studies.

H 2O ⇝ eaq −(0.27), HO•(0.28), H•(0.062)

(4)

eaq − + N2O → O•− + N2

(5)

HO• + HO− ⇌ O•− + H 2O

(6/−6)

Reactions 4 and 5 were used to generate α-hydroxyalkyl radicals. The H• and HO• radicals are scavenged efficiently by CH3OH (eq 7, k7 = 9.7 × 108 M−1 s−1, and eq 8, k8 = 2.6 × 106 M−1 s−1), CH3CH2OH (eq 7, k7 = 1.9 × 109 M−1 s−1, and eq 8, k8 = 1.7 × 107 M−1 s−1) and (CH3)2CHOH (eq 7, k7 = 1.9 × 109 M−1 s−1, and eq 8, k8 = 7.4 × 107 M−1 s−1). The yields of the α-hydroxyalkyl radicals from the reaction with HO• are 93% for methanol and ca. 85% for the other two alcohols.27,28 The reactions of H• and HO• radicals with THF were used to generate α-alkoxyalkyl radicals. The reactions occur with rate constants of k = 5 × 107 M−1 s−1 and k = 4.0 × 109 M−1 s−1, respectively.

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EXPERIMENTAL SECTION Materials. Methanesulfonyl chloride (Aldrich) was distilled before use under nitrogen. Alcohols (HPLC grade) and THF from Sigma-Aldrich Co. were used without further purification. Solutions were freshly prepared using water purified with a Millipore (Milli-Q) system. Alkenes were commercially available from Aldrich and used without additional purification. Amides (entries 1, 4, and 6 in Table 2) were prepared according to standard procedures from the corresponding amines.22 TsBr was prepared according to known procedures23 and was dried under vacuum before use. Pulse Radiolysis. All time-resolved investigations were conducted by means of the radiation chemistry technique of pulse radiolysis, using the 12 MeV LINAC (linear accelerator) facility at the CNR-ISOF in Bologna. Details of the optical detection system, methods, and computer treatment of data are described elsewhere.24 Pulses from 50 to 200 ns were used according to the dose (10−40 Gy) to be delivered to the samples. For a radiation chemical yield of G = 0.27 per J absorbed energy, as it is known to be for eaq− and •OH in irradiated water, this corresponds to a concentration of (0.25− 1.0) × 10−5 M for either of these species. Besides eaq− and •OH also H• atoms are generated upon the radiolysis of water. Their radiation chemical yield amounts to G = 0.06 J−1. The pulse irradiation was performed at room temperature (295 ± 2 K) on samples contained in Spectrosil quartz cells of 5 cm optical path length. Experimental error limits for individual radiation chemical experiments are typically 10%. Competitive Kinetic Studies. A benzene solution, containing weighed quantities of all the reagents and the internal standard (pentamethylbenzene), was stirred under argon, in a pyrex cell (volume 1−2 mL). This cell was irradiated with a medium pressure mercury lamp for 35 min. The reactions were carried out at 293 K and monitored by 1H NMR analysis of a ∼0.1 mL sample diluted in ∼0.6 mL of CDCl3, each 5 min. The rate constant ratios were determined for each

N2O-saturated aqueous solutions containing 0.5 M of an alcohol or THF and CH3SO2Cl at various quantities (1−20 mM) were used for these studies. In all cases, the same optical absorption spectrum is obtained 2 μs after the pulse, which consisted of a broad band in the region 280−380 nm with a maximum at 320 nm typical of the CH3SO2• radical.19 The build-up of sulfonyl radical was monitored at 350 nm in order to avoid a residual absorption of the α-hydroxyalkyl radical at 320 nm. Tracing the MeSO2• radical at 350 nm, two distinct processes were revealed as expected. Indeed, eaq− is partitioned between the reaction with N2O (eq 5) and the reaction with MeSO2Cl (eq 2, k2 = 3.3 × 1010 M−1 s−1) followed by dissociation of the resulting radical anion (eq 3, k3 = 1.5 × 106 s−1). This can be realized by inspection of the insets in Figure 1A,B, which specifically pertain to a N2O-saturated aqueous solution containing 10 mM MeSO2Cl and 0.5 M alcohol. Under these conditions, MeSO2Cl traps two-thirds of eaq− based on the above-mentioned rate constants. The second, considerably slower step, made up for the remaining third. The time profile of the slow formation of CH3SO2• radical leads to a pseudo-first-order rate constant, kobs, and was measured for a number of different alcohols and THF concentrations. Figure 1 7624



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associated with the reaction of secondary alkyl radicals with CH3SO2Cl, which is less than 2 orders of magnitude compared to the analogous reactions in Table 1.30 However, it was suggested that the reactivity of alkyl radicals toward sulfonyl chlorides is a result of a fairly “loose” transition state with important polar contributions like [CH3SO2−Cl•R+] based on the Arrhenius parameters.31 The present results in aqueous solution with the attacking radical being an α-hydroxyl or α-alkoxyl alkyl radicals suggest that the reaction probably occurs by electron transfer (eq 9) followed by dissociation of the resulting radical anion (eq 3). Indeed, the rate constant for reaction 9 increases more than 1 order of magnitude for the series of α-hydroxyl alkyl radicals following the order of reduction potential. CH3SO2Cl also follows the same trend of reactivity of Fe3+ (Table 1), suggesting an analogous electron-transfer process. Moreover, CH3SO2Cl•− has been shown to decay by reaction 3 with k = 1.5 × 106 s−1 at 295 K.19

As reported in the introduction, the main synthetic applications of RSO2• radicals are based on their ability to add reversibly to carbon−carbon double and triple bonds (eq 1/−1). Therefore, we also investigated the reactivity of the CH3SO2• radical with acrolein and propiolic acid as examples of water-soluble compounds with carbon−carbon double and triple bonds (eqs 10/−10 and 11/−11).

Figure 1. Plots of kobs vs [CH3SO2Cl] for the buildup of CH3SO2• radical monitored at 350 nm from the pulse radiolysis of N2Osaturated solutions containing 0.5 M methanol (A) or 2-propanol (B) at 295 K. Insets: Time dependence of absorption at 350 nm in the presence of 10 mM CH3SO2Cl.

The decay of the CH3SO2• radical at 350 nm followed second-order kinetics (dose dependent).19,32 However, the decay of the transient at 350 nm followed first-order kinetics in the presence of acrolein and propiolic acid (see insets of Figure 2A,B, respectively). From the slope of kd vs [acrolein] and [propiolic acid], the bimolecular rate constants were found to be k10 = (4.9 ± 0.1) × 109 M−1 s−1 and k11 = (5.9 ± 0.8) × 107 M−1 s−1, respectively (Figure 2). Figure 2A,B also shows intercepts of 4.6 × 106 s−1 and 6.7 × 105 s−1, respectively. These intercepts would also include the rate constants for the possible back reactions. Smaller parts of these values reflect the decay of CH3SO2• radical in the absence of unsaturated compounds (ca. 2.5 × 104 s−1 is the contribution on the half-life of the bimolecular termination, see Supporting Information). Therefore, we suggest that the k−10 and k−11 for the β-elimination of CH3SO2• radical from the acrolein- and propiolic acid-adducts should be close to 4.6 × 106 s−1 and 6.7 × 105 s−1, respectively. The rate constants for the β-elimination of a Ts• radical from a variety of carbon-centered radicals have been measured, for example, kf = 1.5 × 106 s−1 at 293 K for PhCH2•CHCH2Ts.12 It is worth underlining that the rate constant for the addition of MeSO2• to acrolein is a near-diffusion-limited reaction with a K ≈ 1000 M−1 for the equilibrium 10/−10, whereas the rate constant for the addition of MeSO2• to propiolic acid is 2 orders of magnitude slower, with K ≈ 100 M−1 for the equilibrium 11/−11. Relative Reactivity of Alkenes toward the Addition of Tosyl Radical. The reactivity of tosyl radical (Ts•) toward a

shows the plots of kobs vs [CH3SO2Cl] for methanol and 2propanol. From the slope of the linear plots, the bimolecular rate constants were calculated and reported in the second column of Table 1. It is worth mentioning that α-hydroxyl alkyl Table 1. Rate Constants for the Reaction of α-Hydroxyl and α-Alkoxyl Alkyl Radicals with CH3SO2Cl and Fe3+ at 295 K and Reduction Potentials of Radicals (vs NHE) radical •

CH2OH 2-tetrahydrofuranyl CH3•CHOH (CH3)2•COH a

k, M−1 s−1 (1.7 (4.1 (5.4 (2.2

± ± ± ±

0.2) 0.7) 0.6) 0.2)

× × × ×

7

10 107 107 108

k(Fe3+),a M−1 s−1

E0,b,c V

7

8.0 × 10

−1.39

3.8 × 108 5.8 × 108

−1.25 −1.18

From ref 26. bE0(R′RCO, H+/R′RC•OH). cFrom ref 29.

radicals are good reducing agents. In this respect, the rate constants of α-hydroxyl alkyl radicals with Fe3+ in aqueous solutions and their reduction potentials are also collected in Table 1. Rate constants and Arrhenius parameters for the reaction of primary and secondary alkyl radicals with RSO2Cl (where R = CH3 or PhCH2) have been previously reported in organic solvents.30,31 A rate constant of 2.9 × 105 M−1 s−1 at 298 K is 7625



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alkenes M1 and M2, at time t. For eq 12 to hold several conditions must be fulfilled: (i) the radical addition must be the rate-determining step, (ii) it must be the only reaction consuming M1 and M2, and (iii) the addition of tosyl radical to both alkenes must be irreversible. The latter is achieved when the abstraction of bromine atom from tosyl bromide by both carbon-centered radicals is significantly faster than the βelimination of tosyl radical from the same radicals (kf ≪ kBr[TsBr]). The rate constants for bromine abstraction from TsBr (kBr) can be estimated to be around 108 and 1010 M−1 s−1 at 298 K for sec-butyl and phenyl radicals, respectively, based on the following facts: (i) the rate constants for chlorine atom transfer from PhCH2SO2Cl to sec-butyl and phenyl radicals are 1.2 × 106 and 1.0 × 108 M−1 s−1, respectively, at 298 K,31 and (ii) TsBr was reported 192 times more reactive than TsCl, with respect to halogen abstraction by phenyl radical, in benzene at 333 K.32 On the other hand, the rate constant for the βfragmentation (kf) varied significant, depending on the nature of the α-substituent at the carbon-centered radical. Absolute values of kf are reported in the last column of Table 2.12 Table 2. Relative Rate Constants for the Addition of Tosyl Radical to Alkenes (karel) and the Corresponding Rate Constants for β-Elimination (kf) at 293 Ka,b

Figure 2. Plots of kd vs [acrolein] (A) and kd vs [propiolic acid] (B) for the decay of CH3SO2• radical monitored at 350 nm from the pulse radiolysis of N2O-saturated solutions containing 0.5 M 2-propanol and 10 mM CH3SO2Cl at 295 K. Insets: Time dependence of absorption at 350 nm in the presence of 10 mM acrolein (A) and propiolic acid (B).

series of alkenes bearing various functional groups was determined by competitive kinetic studies in benzene at 293 K. The photoinitiated addition of TsBr to a series of alkenes consists of the radical chain reaction illustrated in Scheme 1. Scheme 1. Propagation Steps for the Photo-Initiated Addition of TsBr to Alkenes

In benzene. bFrom ref 12. Relative rate constants for the βelimination are indicated in parentheses. cAt 295 K. a

The relative reactivities of alkenes toward the addition of tosyl radical are reported in Table 2. The concentration of TsBr was chosen in a way that the transfer of bromine from TsBr must be at least 10 times faster than the β-fragmentation. The scale of relative reactivity varies within a range from 1 to 42. The acrylic CC bond (entry 6) is 11.7 times more reactive than the allylic CC bond (entry 1), and this is consistent with the reported value of 9.5 for PhSO2• radical in CH3CN at

The relative rates were measured by competition by using the following equation: ka1/ka2 = (log[M1]0 − log[M1]t )/(log[M 2]0 − log[M 2]t ) (12)

where [M1]0 and [M2]0 are the initial concentrations of alkenes M1 and M2 and [M1]t and [M2]t are the concentrations of 7626



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room temperature.33 It is important to point out that the CC bond of 2-vinylnaphthalene (entry 8) is 1.5 and 3.6 times more reactive than that of methyl vinyl ketone (entry 7) and N,Ndiisopropylacrylamide (entry 6), respectively. The delocalization of the unpaired electron over the naphthyl and carbonyl groups in the adduct radicals increases the rate constant for the addition of tosyl radical to alkenes. On the other hand, polar effects do not play any relevant role. An opposite and more pronounced influence of the substituents on the rate of elimination of tosyl radical from the adduct radicals is evident in Table 2. Indeed, the kfrel varies from 1 (entry 8) to 333 (entry 5). It is worth noting that the variation of log(kfrel) exhibits a linear relationship with that of log (karel) for alkenes bearing substituents able to bring resonance stabilization to the carbon-centered radical (π system, electron donating heteroatom, and electron withdrawing carbonyl groups) (see Supporting Information). The stabilization energy of the β-tosyl radical is a factor of primary importance in the series. It retards the rate of β-fragmentation and accelerates the rate of addition. The linear relationship implies that the substituent effects on the stabilization of the alkene parallel the substituent effects on the stabilization of the radical in this series.34 Assuming that the rate constant for the MeSO2• radical addition to acrolein in water (eq 10/−10) is similar to the rate constant for the Ts• radical addition to methyl vinyl ketone in benzene (entry 7 in Table 2), it is possible to calculate the absolute rate constants for the addition of tosyl radical to various alkenes. This assumption is reasonable since the radical chemistry of RSO2• should be independent of the R group (see Introduction) and the solvent contribution may be limited in the addition rates. The absolute values ka range from 2 × 108 M−1 s−1 for the allylic CC bond (entry 1) to 7 × 109 M−1 s−1 for the 2-vinylnaphthalene (entry 8). It is gratifying to see that the estimated value for the addition of MeSO2• to 1-hexene in CH3CN (k = 1 × 109 M−1 s−1 at 273 K)11 is similar to the value for the addition of Ts• to CH2CHCH2Ph in benzene (k = 6 × 108 M−1 s−1 at 293 K) obtained from Table 2.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed experimental procedures, kinetic plot (Figures S1), and reactivity ratios plot (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Addresses ∥

Department of Pharmaceutical Chemistry, 2095 Constant Avenue, University of Kansas, Lawrence, Kansas 66047, United States. ⊥ Department of Biochemistry, University of WisconsinMadison, Madison, WI 53726, United States. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The support and sponsorship by COST Action CM0603 on “Free Radicals in Chemical Biology (CHEMBIORADICAL)” are kindly acknowledged.

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REFERENCES

(1) Chatgilialoglu, C.; Bertrand, M. P.; Ferreri, C. In S-Centered Radicals, Alfassi, Z. B., Ed.; Wiley: Chichester, 1999; pp 312−354. (2) Bertrand, M. P.; Ferreri, C. In Radicals in Organic Synthesis; Renaud, P.; Sibi, M., Eds.; Wiley-VCH: Weinheim, 2001; Vol. 2; pp 485−503. (3) Taniguchi, T.; Idota, A.; Ishibashi, H. Org. Biomol. Chem. 2011, 9, 3151−3153. (4) Zeng, X.; Ilies, L.; Nakamura, E. Org. Lett. 2012, 14, 954−956. (5) Rouquet, G.; Robert, F.; Méreau, R.; Castet, F.; Landais, Y. Chem.Eur. J. 2011, 17, 13904−13911. (6) Liautard, V.; Robert, F.; Landais, Y. Org. Lett. 2011, 13, 2658− 2661. (7) James, P.; Schenk, K.; Landais, Y. J. Org. Chem. 2006, 71, 3630− 3633. (8) Tanaka, N.; Sato, E.; Matsumoto, A. Org. Biomol. Chem. 2011, 9, 3753−3758. (9) Tanaka, N.; Sato, E.; Matsumoto, A. Macromolecules 2011, 44, 9125−9137. (10) Alabugin, I. V.; Timokhin, V. I.; Abrams, J. N.; Manoharan, M.; Abrams, R.; Ghiviriga, I. J. Am. Chem. Soc. 2008, 130, 10984−10995. (11) Gozdz, A. S.; Maslak, P. J. Org. Chem. 1991, 56, 2179−2189. (12) Timokhin, V. I.; Gastaldi, S.; Bertrand, M. P.; Chatgilialoglu, C. J. Org. Chem. 2003, 68, 3532−3537. (13) Mortensen, A.; Skibsted, L. H.; Sampson, J.; Rice-Evans, C.; Everett, S. A. FEBS Lett. 1997, 418, 91−97. (14) Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. O. C. J. Chem. Soc., Perkin Trans 2 1979, 770−775. (15) Chatgilialoglu, C.; Gilbert, B. C.; Norman, R. O. C. J. Chem. Soc., Perkin Trans 2 1980, 1429−1436. (16) Chatgilialoglu, C.; Griller, D.; Guerra, M. J. Phys. Chem. 1987, 91, 3747−3750. (17) Alberti, A.; Chatgilialoglu, C.; Guerra, M. J. Chem. Soc., Perkin Trans 2 1986, 1179−1182. (18) (a) Chatgilialoglu, C.; Griller, D.; Kanabus-Kaminska, J. M.; Lossing, F. P. J. Chem. Soc., Perkin Trans 2 1994, 357−360. (b) Laarhoven, L. J. J.; Mulder, P.; Wayner, D. D. M. Acc. Chem. Res. 1999, 32, 342−349. (19) Tamba, M.; Dajka, K.; Ferreri, C.; Asmus, K.-D.; Chatgilialoglu, C. J. Am. Chem. Soc. 2007, 129, 8716−8723. (20) Houman, A.; Hamed, E. M. Phys. Chem. Chem. Phys. 2012, 14, 113−124. (21) Rosen, B. M.; Percec, V. Chem. Rev. 2009, 109, 5069−5119.

CONCLUSIONS

In the present study unambiguous evidence is provided for an electron-transfer between α-hydroxyl alkyl radicals and MeSO2Cl to afford MeSO2• via the radical anion of sulfonyl chloride and similarly to the reaction of hydrated electrons with MeSO2Cl to give the sulfonyl radical.19 This is a new entry to sulfonyl radical formation and complementary to the reported reaction of oxidation of sulfinic acid by hydroxyl radicals in aqueous solutions.35,36 The absolute rate constants for the addition of MeSO2• to acrolein and propiolic acid are measured, the former one being a near-diffusion-limited reaction, and the reverse rate constants could be estimated. The reactivity of p-CH3C6H4SO2• radical toward a series of alkenes bearing various functional groups was also determined. The rate constants of the addition of tosyl radical to alkenes vary more than one-order of magnitude, the faster one being close to the diffusion control due to the stabilization of the adduct radical. Due to the current interest in sulfonyl radical reactivity, this work can contribute to the tuning of reaction conditions for improving selectivity and foster new applications. 7627



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(22) Wang, C.; Russell, G. A. J. Org. Chem. 1999, 64, 2346−2352. (23) Truce, W. E.; Heuring, D. L.; Wolf, G. C. J. Org. Chem. 1974, 39, 238−244. (24) Emmi, S. S.; D’Angelantonio, M.; Poggi, G.; Beggiato, G.; Camaioni, N.; Geri, A.; Martelli, A.; Pietropaolo, D.; Zotti, G. Res. Chem. Intermed. 1998, 24, 1−14. (25) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513−886. (26) Ross, A. B.; Mallard, W. G.; Helman, W. P.; Buxton, G. V.; Huie, R. E.; Neta, P. NDRL-NIST Solution Kinetics Database, version 3.0; National Institute of Standards and Technology: Gaithersburg, MD, 1998. (27) Asmus, K.-D.; Möckel, H.; Henglein, A. J. Phys. Chem. 1973, 77, 1218−1221. (28) Jiang, D.; Barata-Vallejo, S.; Golding, B. T.; Ferreri, C.; Chatgilialoglu, C. Org. Biomol. Chem. 2012, 10, 1102−1107. (29) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637−1755. (30) Horowitz, A. Int. J. Chem. Kinet. 1976, 8, 709−723. (31) Chatgilialoglu, C. J. Org. Chem. 1986, 51, 2871−2873. (32) Bennett, J. E.; Brunton, G.; Gilbert, B. C.; Whittall, P. E. J. Chem. Soc., Perkin Trans. 2 1988, 1359−1364. (33) Da Silva Correa, C. M. M.; Oliveira, M. A. B. C. S. J. Chem. Soc., Perkin Trans 2 1983, 711−715. (34) For treatment of the non polar resonance effect, see: Zhang, K.C.; Fu, Y.; Lin, B.-L.; Liu, L.; Cheng, Y.-H.; Guo, Q.-X. J. Mol. Struct.− Theochem. 2003, 625, 199−205. (35) Sehested, K.; Holcman, J. J. Radiat. Phys. Chem. 1996, 47, 357− 360. (36) Flyunt, R.; Makogon, O.; Schuchmann, M. N.; Asmus, K.-D.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 2 2001, 787−792.

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Kinetic Studies on the Formation of Sulfonyl Radicals and Their

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