Nature of the Chain Propagation in the Photostimulated Reaction of 1

Nature of the Chain Propagation in the Photostimulated Reaction of 1-Bromonaphthalene with Sulfur-Centered Nucleophiles Luciana C. Schmidt, Juan E. Argu¨ello,* and Alicia B. Pen˜e´n˜ory* INFIQC, Departamento de Quı´mica Orga´ nica, Facultad de Ciencias Quı´micas, UniVersidad Nacional de Co´ rdoba, Ciudad UniVersitaria, 5000 Co´ rdoba, Argentina [email protected]; [email protected] ReceiVed December 14, 2006

The reactivity of -SC(NH)NH2 (1), MeCOS- (2), and PhCOS- (3) toward 1-naphthyl radicals was studied in DMSO. The photostimulated reaction of anions 1, 2, and 3 with 1-bromonaphthalene (4) after quenching with MeI renders 1-(methylthio)naphthalene (6) as a main product together with bis(1-naphthyl) sulfide (7) and naphthalene (5). The thioacetate ion (2) and thiobenzoate ion (3) were unreactive toward 4 as electron-donor under photostimulation; however, in the presence of potassium tert-butoxide anion (entrainment conditions), they gave the mentioned products 5, 6, and 7, after the addition of MeI. Quenching of the triplet state of 4 was assigned as the photoinduced initiation step, with a rate constant value of (4.6 ( 0.5) × 108 M-1 s-1 for tert-butoxide anion and a rough estimated value of (8 ( 7) × 107 M-1 s-1 for anion 1. By using hydrogen abstraction from DMSO as the competitive reaction, the absolute rate constants for the addition of anions 1, 2, and 3 to 1-naphthyl radicals have been determined to be 1.0 × 109, 1.2 × 109, and 3.5 × 109 M-1 s-1, respectively. This reactivity order is in agreement with the stability of the resulting radical anions (ArNu)•- (10-12)•-. The inhibition experiments of the photoinduced substitution reaction in the presence of radical scavengers and the global quantum yield higher than the unity are evidence of a radical chain mechanism for these substitution reactions by anions 1 and 2. Anion 3 adds to the 1-naphthyl radical, but is neither able to initiate nor to keep the propagation cycle. Evaluation of the electron-transfer driving forces for the reaction between (ArNu)•- and 4 together with the absence of a chain reaction for the anion 3 indicate that the propagation in the proposed mechanism is given by an acid-base reaction between the radical ‚C(O)Me or ‚C(NH)NH2 (13) and a base.

Introduction Radical nucleophilic substitution involving electron-transfer (ET) steps or SRN1 mechanism has proven to be a versatile mechanism for replacing an adequate leaving group at the ipso position by a nucleophile. This process has a considerably broad scope in relation to substrates and nucleophiles, and the most relevant results as well as synthetic applications have * Corresponding authors. Phone: (+54) 351-4334170/73. Fax: (+54) 3514333030/4334174.

been widely reviewed.1 The mechanism of this reaction is a chain process with radicals and radical anions as intermediates, whose initiation step involves an ET to the substrate. The general accepted pathways for the propagation cycle are outlined in Scheme 1. Although the most extensively used initiation method is a photoinduced ET (PET) from a charged nucleophile to the substrate, there are not many studies on its mechanism. These PETs can be accomplished in one of the following pathways: (1) photoexcitation of a charge-transfer complex (CTC) formed 10.1021/jo062569+ CCC: $37.00 © 2007 American Chemical Society

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Published on Web 03/10/2007

Chain Propagation in the Reaction of 1-Bromonaphthalene SCHEME 1

SCHEME 2

between the nucleophile and the substrate followed by ET;2-6 (2) homolytic cleavage of the C-X bond;3 (3) ET from an excited nucleophile to the substrate;7,8 (4) ET from a nucleophile to an excited substrate;9 and (5) photoejection of an electron from an excited nucleophile.10 For nucleophiles that are unreactive at initiation but quite reactive at propagation, the addition of minute amounts of another nucleophile able to initiate the reaction increases the generation of intermediates. This allows the less reactive initiation-nucleophile to start its own propagation. The process thus can afford the substitution of a poor electron-donor nucleophile through an entrainment reaction, which is also a support for the chain nature of the reaction. Because the SRN1 is a chain mechanism, the overall reactivity depends on the efficiency of the initiation, propagation, and termination steps. The initiation step may be slow; nevertheless, for the process to work efficiently, the chain propagation must be fast and feasible to allow for long chains to build up. The magnitude of the chain length of any SRN1 reaction can easily be derived from the overall quantum yield by determining the initiation quantum yield. The latter could be obtained when the propagation cycle is eliminated or reduced significantly in comparison with the initiation step. For instance, a quantum (1) (a) Rossi, R. A.; Pierini, A. B.; Pen˜e´n˜ory, A. B. Chem. ReV. 2003, 103, 71-167. (b) Rossi, R. A.; Pierini, A. B.; Santiago, A. N. In Organic Reactions; Paquette, L. A., Bittman, R., Eds.; John Wiley & Sons, Inc.: New York, 1999; Vol. 54, Chapter 1, pp 1-271. (c) Rossi, R. A.; Pierini, A. B.; Pen˜e´n˜ory, A. B. Recent Advances in the SRN1 Reaction of Organic Halides. In The Chemistry of Functional Group; Patai, S., Rappoport, Z., Eds.; Wiley: Chichester, 1995; Suppl. D2, Chapter 24, pp 1395-1485. (d) Rossi, R. A.; Pen˜e´n˜ory, A. B. In CRC Handbook of Organic Photochemistry and Photobiology, 2nd ed.; Horspool, W. M., Lenci, F., Eds.; CRC Press Inc.: Boca Raton, FL, 2003; Chapter 47, pp 47-1-47-24. (e) Save´ant, J. M. Acc. Chem. Res. 1993, 26, 455-461. (f) Kornblum, N. Aldrichimica Acta 1990, 23, 71-78. (g) Rossi, R. A.; Pen˜e´n˜ory, A. B. Curr. Org. Synth. 2006, 3, 121-158. (2) Fox, M. A.; Younathan, J.; Fryxell, G. E. J. Org. Chem. 1983, 48, 3109-3112. (3) Hoz, S.; Bunnett, J. F. J. Am. Chem. Soc. 1977, 99, 4690-4688. (4) Argu¨ello, J. E.; Pen˜e´n˜ory, A. B.; Rossi, R. A. J. Org. Chem. 2000, 65, 7175-7182. (5) Wu, B.; Zeng, F.; Ge, M.; Cheng, X.; Wu, G. Sci. China 1991, 34, 777; Chem Abstr. 1992, 116, 58463h. (6) Cheng, C.; Stock, L. M. J. Org. Chem. 1991, 56, 2436-2443. (7) Argu¨ello, J. E.; Pen˜e´n˜ory, A. B. J. Org. Chem. 2003, 68, 23622368. (8) Tolbert, L. M.; Siddiqui, S. J. Org. Chem. 1984, 49, 17441751. (9) Ivanov, V. L.; Eggert, L.; Kuzmin, M. G. High Energy Chem. 1987, 21, 284; Chem. Abstr. 1988, 109, 169594b. (10) Ahbala, M.; Hapiot, P.; Houmam, A.; Jouini, M.; Pinson, J.; Save´ant, J.-M. J. Am. Chem. Soc. 1995, 117, 11488-11498.

FIGURE 1. UV-vis spectra of 4 and anion 1 in DMSO.

yield of 0.127 was determined for substitution of neophyl iodide by the enolate anion of acetophenone.4 The quantum yield of initiation, evaluated by the suppression of the propagation steps with the presence of the radical trap di-tert-butylnitroxide (DTBN), has a value of 10-3. On this basis, the chain length or Φpropagation obtained for the reaction is ca. 127. This is the first report of Φpropagation for an SRN1 process, demonstrating that overall quantum yields below 1 cannot be taken as a criterion against a chain. We have described for the first time the reactivity of sulfurcentered nucleophiles such as thiourea anion11 and thioacetate anion12 in photoinduced aromatic SRN1 as a “one-pot” method for the synthesis of several sulfur aromatic compounds from moderate to good yields. The use of commercially available, easily handled thiourea, potassium tert-butoxide, and thioacetate salts, and the mild conditions (room temperature under nitrogen atmosphere in DMSO), make this process a very convenient methodology for obtaining aryl thiols, alkyl aryl sulfides, symmetrical or unsymmetrical diaryl sulfides, and diaryl disulfides (Scheme 2). Moreover, with hydrogen abstraction from DMSO as a clock reaction, the absolute rate constants for the addition of S2-, thiourea, and benzene thiolate anions to 1-naphthyl radicals have been measured. The values obtained are 0.5 × 109, 1.0 × 109, and 5.1 × 109 M-1 s-1, respectively.13 Even though the reactivity of thiourea and thioacetate anions in the reactions with aryl halides is quite similar, these nucleophiles show a very different behavior at the initiation step.11,12 The advantage of these reactions for the synthesis of sulfur compounds together with the scarce information available on the photoinduced initiation and the propagation steps have prompted us to study the mechanism of these photostimulated reactions. Therefore, we report herein the reactivity of thiourea (1), thioacetate (2), and thiobenzoate (3) anions versus 1-naphthyl radicals and a comparative and detailed study of the photoinduced electron-transfer pathway at the initiation level and their reactivity in the propagation steps. We have also measured the quantum yields and the chain length for the propagation cycle of these SRN1 reactions and provided evidence for the identity of the chain carriers. (11) Argu¨ello, J. E.; Schmidt, L. C.; Pen˜e´n˜ory, A. B. Org. Lett. 2003, 5, 4133-4136. (12) Schmidt, L. C.; Rey, V.; Pen˜e´n˜ory, A. B. Eur. J. Org. Chem. 2006, 2210-2214. (13) Argu¨ello, J. E.; Schmidt, L. C.; Pen˜e´n˜ory, A. B. ArkiVoc 2003, Part (x), 411-419.

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Schmidt et al. TABLE 1. Reactions of Sulfur-Centered Nucleophiles with 1-Bromonaphthalene (4) in DMSOa product yield (%)c entry

“S”-

1d,e 2d,e 3d,e 4 5f 6f 7f 8f 9 10f 11f

1 2

3

conditions time (min) 180, dark 15, hν 15, hν 180, hν 15, hν 15, hν 15, hν 15, hν 90, hν 90, hν 180, hν

comp. addedb

DTBN p-DNB DTBN TEMPO

convn

5

6

7

91 25

9 2

48 9

19 8

92

3

48

34

22 39

kNu2 = kNu1, is in agreement with the stability of the resulting radical anions (ArNu)•-. Even anion 3 is the most reactive in the addition step; the global reaction is the least efficient as compared to anions 1 or 2. This can be explained taking into account the nature of the chain propagation of the reaction. The inhibition experiments of the photoinduced substitution reaction in the presence of radical scavengers and the global quantum yield higher than the unity are evidence of a radical chain mechanism for the substitution reactions by anions 1 and 2. Anion 3 adds to 1-naphthyl radical, but it is neither able to initiate the reaction nor to keep the propagation cycle. Positive values of the ET driving forces for the reaction between (ArNu)•- and 4 together with the absence of a chain reaction for the anion 3 indicate that the propagation step is given by an acid-base reaction between the radical ‚C(O)Me or ‚C(NH)NH2 and a base to afford (OdCdCH2)•- or (HNdCdNH)•that transfer their odd electron to 4, continuing the chain. Experimental Section Chemicals. t-BuOK, SC(NH2)2, MeCOSK and PhCOSH, 1-bromonaphthalene, naphthalene, thiophenol, p-dinitrobenzene, TEMPO, and DTBN were all high-purity commercial samples, which were used without further purification. DMSO was distilled under vacuum and stored over molecular sieves (4 Å). Anions 1, 3, and PhS- were generated in situ by acid-base deprotonation using t-BuOK. 2- Naphthalenethiol, acetate (10b), and 2-naphthalenethiol, benzoate (11b), were synthesized by reaction between 2-naphthalenetiol and acetyl or benzoyl chloride.36 All products are known and exhibited physical properties identical to those reported in the literature. Also, they were isolated by radial chromatography from the reaction mixture and characterized by 1H and 13C NMR and mass spectrometry. Registry no.: 1-(methylthio)naphthalene,37 [10075-72-6]; bis(1-naphthyl) sulfide,38 [607-53-4]; 1-naphthyl phenyl sulfide,39 [7570-98-1]; 2-naphthalenethiol, acetate,36 [831-23-2]; 2-naphthalenethiol, benzoate,40 [10154-60-6]. Instrumentation. Time-Resolved Absorption Spectroscopy. The laser flash photolysis system was based on a pulsed Nd:YAG (36) Penn, J. H.; Liu, F. J. Org. Chem. 1994, 59, 2608-2612. (37) Gilman, H.; Webb, F. J. J. Am. Chem. Soc. 1949, 71, 4062-4066. (38) Hauptmann, H.; Wladislaw, B. J. Am. Chem. Soc. 1950, 72, 710712. (39) Bunnett, J. F.; Creary, X. J. Org. Chem. 1974, 39, 3173-3174. (40) Chen, C.-T.; Kuo, J.-H.; Pawar, V. D.; Munot, Y. S.; Weng, S.-S.; Ku, C.-H.; Liu, C.-Y. J. Org. Chem. 2005, 70, 1188-1197.

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Schmidt et al. laser, using 266 nm radiation as the excitation wavelength. The single pulses were of ca. 10 ns duration, and the energy was ca. 20 mJ/pulse. A Xe lamp was employed as the detecting light source. The laser flash photolysis apparatus consisted of the pulsed laser, the Xe lamp, a monochromator, a photomultiplier (PMT) system, and oscilloscope. The output signal of the oscilloscope was transferred to a personal computer for study. Electrochemistry. The working electrode was a 3 mm-diameter glassy carbon electrode disk that was carefully polished and ultrasonically rinsed in absolute ethanol before use. The counter electrode was a platinum wire, and the reference electrode was an aqueous SCE electrode. All experiments have been done at 20 °C. Cyclic voltammetric data were recorded using a commercial computer-controlled potentiostat.

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Acknowledgment. We thank Prof. M. A. Miranda for LFP facilities. This work was supported in part by the Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas (CONICET), SECyT-Universidad Nacional de Co´rdoba, and FONCyT, Argentina. L.C.S. gratefully acknowledges receipt of a fellowship from CONICET. Supporting Information Available: General methods, completed Table 2, quenching of the triplet of 4 by t-BuOK, and cyclic voltammograms of 10b and 11b. This material is available free of charge via the Internet at http://pubs.acs.org. JO062569+