Atom Transfer Radical Addition Reactions of CCl4, CHCl3, and p

Radhika P. Nair, Tae Ho Kim, and Brian J. Frost*. Department of Chemistry, University of Nevada, Reno, Nevada 89557-0216. Received January 30, 2009...
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Organometallics 2009, 28, 4681–4688 DOI: 10.1021/om900075h

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Atom Transfer Radical Addition Reactions of CCl4, CHCl3, and p-Tosyl Chloride Catalyzed by Cp0 Ru(PPh3)(PR3)Cl Complexes Radhika P. Nair, Tae Ho Kim, and Brian J. Frost* Department of Chemistry, University of Nevada, Reno, Nevada 89557-0216 Received January 30, 2009

A series of Cp0 Ru(PR3)(PPh3)Cl complexes, where Cp0 = Cp*, Dp, Ind, Cp, Tp and PR3 = PTA, PMe3, PPh3, have been used to catalyze the atom transfer radical addition (ATRA) of various chloro substrates (CCl4, CHCl3, and TsCl) to styrene and/or hexene. The complexes Cp*Ru(PTA)(PPh3)Cl, Cp*Ru(PMe3)(PPh3)Cl, DpRu(PMe3)(PPh3)Cl, and TpRu(PMe3)(PPh3)Cl have been synthesized by ligand exchange reactions with Cp0 Ru(PPh3)2Cl and characterized by NMR spectroscopy and X-ray crystallography. An alternative synthesis for CpRu(PMe3)(PPh3)Cl and the solid-state structure of the previously reported complex IndRu(PMe3)(PPh3)Cl are also described. Among the ruthenium(II) complexes studied, Cp*Ru(PTA)(PPh3)Cl and Cp*Ru(PMe3)(PPh3)Cl were very active at 60 °C with TOF values of 1060 and 933 h-1, respectively; Cp*Ru(PPh3)2Cl was the most active for the addition of CCl4 to styrene with a TOF >960 h-1 at room temperature. Total turnovers (TTO) in excess of 80 000 for the addition of CCl4 to hexene were obtained for the Cp* complexes, making these complexes the most active and robust catalysts for ATRA reported to date. Introduction The addition of halogenated compounds to olefins was first reported in the late 1930s and is commonly referred to as Kharasch addition or atom transfer radical addition (ATRA).1 It is an effective method for C-C bond formation, and its applications in synthetic organic chemistry have been well reviewed.2-7 Transition-metal-catalyzed ATRA has gained considerable interest since the early 1970s, as it provides higher

yields and stereoselectivity.2,6,8 A wide variety of metal complexes catalyze ATRA, including those of copper,9-11 iron,10,12 nickel,6,13 and palladium.14 Ruthenium complexes, in particular, have played a significant role, with RuCl2(PPh3)3 being a highly efficient and versatile catalyst for ATRA.15-28 The diverse set of

*To whom correspondence should be addressed. Tel:+1-775-7841993. Fax: +1-775-784-6804. E-mail: [email protected]. (1) (a) Kharasch, M. S.; Engelmann, H.; Mayo, F. R. J. Org. Chem. 1938, 2, 288–302. (b) Kharasch, M. S.; Jensen, E. V.; Urry, W. H. Science 1945, 102, 128–128. (c) Kharasch, M. S.; Elwood, E. V.; Urry, W. H. J. Am. Chem. Soc. 1947, 69, 1100–1105. (d) Kharasch, M. S.; Friedlander, H. N. J. Org. Chem. 1949, 14, 239–247. (2) Minisci, F. Acc. Chem. Res. 1975, 8, 165–171. (3) Bellus, D. Pure Appl. Chem. 1985, 57, 1827–1838. (4) Iqbal, J.; Bhatia, B.; Nayyar, N. K. Chem. Rev. 1994, 94, 519–564. (5) Clark, A. J. Chem. Soc. Rev. 2002, 31, 1–11. (6) Gossage, R. A.; van de Kuil, L. A.; van Koten, G. Acc. Chem. Res. 1998, 31, 423–431. (7) Pintauer, T.; Matyjaszewski, K. Chem. Soc. Rev. 2008, 37, 1087– 1097. (8) (a) Matsumoto, H.; Nikaido, T.; Nagai, Y. Tetrahedron Lett. 1975, 11, 899–902. (b) Davis, R.; Stephens, K.; Hajek, M. J. Mol. Catal. 1994, 92, 269–276. (9) Mu~ noz-Molina, J. M.; Belderraı´ n, T. R.; Perez, P. J. Adv. Synth. Catal. 2008, 350, 2365–2372. (10) (a) de Campo, F.; Lastecoueres, D.; Verlhac, J.-B. Chem. Commun. 1998, 2117–2118. (b) de Campo, F.; Lastecoueres, D.; Verlhac, J.-B. J. Chem. Soc., Perkin Trans. 1 2000, 575–580. (11) For selected examples see: (a) Pintauer, T.; Matyjaszewski, K. Chem. Soc. Rev. 2008, 37, 1087–1097. (b) Eckenhoff, W. T.; Garrity, S. T.; Pintauer, T. Eur. J. Inorg. Chem. 2008, 563–571. (c) Eckenhoff, W. T.; Pintauer, T. Inorg. Chem. 2007, 46, 5844–5846. (d) Mu~noz-Molina, J. M.; Caballero, A.; Díaz-Requejo, M. M.; Trofimenko, S.; Belderraín, T. R.; Perez, P. J. Inorg. Chem. 2007, 46, 7725–7730. (12) For selected examples see: (a) Elzinga, J.; Hogeveen, H. J. Org. Chem. 1980, 45, 3957–3969. (b) Hayes, T. K.; Freyer, A. J.; Parvez, M.; Weinreb, S. M. J. Org. Chem. 1986, 51, 5501–5503. (c) Lee, G. M.; Weinreb, S. M. J. Org. Chem. 1990, 55, 1281–1285.

(13) For selected examples see: (a) van de Kuil, L. A.; Grove, D. M.; Gossage, R. A.; Zwikker, J. W.; Jenneskens, L. W.; Drenth, W.; van Koten, G. Organometallics 1997, 16, 4985–4994. (b) Kleij, A. W.; Gossage, R. A.; Klein Gebbink, R. J. M.; Brinkmann, N.; Reijerse, E. J.; Kragl, U.; Lutz, M.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 2000, 122, 12112–12124. (c) Pandarus, V.; Zargarian, D. Organometallics 2007, 26, 4321–4334. (14) For selected examples see: (a) Tsuji, J.; Sato, K.; Nagashima, H. Chem. Lett. 1981, 1169–1170. (b) Motoda, D.; Kinoshita, H.; Shinokubo, H.; Oshima, K. Adv. Synth. Catal. 2002, 344, 261–265. (c) Dneprovskii, A. S.; Ermoshkin, A. A.; Kasatochkin, A. N.; Boyarskii, V. P. Russ. J. Org. Chem. 2003, 39, 933–946. (15) Matsumoto, H.; Nakano, T.; Nagai, Y. Tetrahedron Lett. 1973, 14, 5147–5150. (16) For examples see: Severin, K. Curr. Org. Chem. 2006, 10, 217– 224 and references therein. (17) Quebatte, L.; Thommes, K.; Severin, K. J. Am. Chem. Soc. 2006, 128, 7440–7441. (18) (a) Quebatte, L.; Scopelliti, R.; Severin, K. Angew. Chem., Int. Ed. 2004, 43, 1520–1524. (b) Quebatte, L.; Solari, E.; Scopelliti, R.; Severin, K. Organometallics 2005, 24, 1404–1406. (19) Quebatte, L.; Scopelliti, R.; Severin, K. Eur. J. Inorg. Chem. 2005, 3353–3358. (20) Lundgren, R. J.; Rankin, M. A.; McDonald, R.; Stradiotto, M. Organometallics 2008, 27, 254–258. (21) (a) Tutusaus, O.; Delfosse, S.; Demonceau, A.; Noels, A. F.; Vi~ nas, C.; Teixidor, F. Tetrahedron Lett. 2003, 44, 8421–8425. (b) Tutusaus, O.; Vi~nas, C.; Nu~nez, R.; Teixidor, F.; Demonceau, A.; Delfosse, S.; Noels, A. F.; Mata, I.; Molins, E. J. Am. Chem. Soc. 2003, 125, 11830– 11831. (22) Richel, A.; Delfosse, S.; Cremasco, C.; Delaude, L.; Demonceau, A.; Noels, A. F. Tetrahedron Lett. 2003, 44, 6011–6015. (23) For selected examples see: (a) De Clercq, B.; Verpoort, F. Catal. Lett. 2002, 83, 9–13. (b) De Clercq, B.; Verpoort, F. J. Organomet. Chem. 2003, 672, 11–16. (24) Oe, Y.; Uozumi, Y. Adv. Synth. Catal. 2008, 350, 1771–1775. (25) Thommes, K.; Ic-li, B.; Scopelliti, R.; Severin, K. Chem. Eur. J. 2007, 13, 6899–6907. (26) Simal, F.; Wlodarczak, L.; Demonceau, A.; Noels, A. F. Tetrahedron. Lett. 2000, 41, 6071–6074.

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Scheme 1

Scheme 2

ruthenium catalysts for ATRA includes bimetallic ruthenium complexes,18 cationic systems supported by phosphines19 or chelating P,N ligands,20 η5-carborane complexes,21 carbenecontaining compounds,22,23 and resin-supported catalysts.24 Recent developments in ATRA chemistry have focused on half-sandwich Ru(II) and Ru(III) phosphine complexes bearing η6-arene or η5-cyclopentadienyl type ligands.16-20,22,24-28 A general mechanism for transition-metal-catalyzed ATRA is represented in Scheme 1.29,30 A detailed mechanism of the RuCl2(PPh3)3-catalyzed ATRA has been described by Davis and co-workers.29 Initiation presumably involves the dissociation of a phosphine ligand, generating a coordinatively unsaturated RuII complex, which in turn activates the carbon-halogen bond, yielding a RuIII-X species and carbon-centered radical. The latter adds to the olefin, generating a new carbon radical. Transfer of a halogen atom from the metal halide to the carbon radical yields the 1:1 Kharasch product and regenerates the RuII catalyst. In the presence of a large excess of olefin, atom transfer radical polymerization (ATRP) can occur, as observed by Sawamoto31 and Matyjaszewski.32 ATRA and ATRP are mechanistically similar, and several metal complexes are known to catalyze both reactions.11a,33 Azobis(isobutyronitrile) (AIBN) has recently been employed in Ru-catalyzed ATRA reactions by Severin and co-workers, who have suggested that AIBN allows for the reduction of Cp*RuIII(PPh3)Cl2 back to the catalytically active Cp*RuII(PPh3)Cl.17 Recently, this methodology has been applied with great success to copper-catalyzed ATRA reactions.11b,11c Our group has described the synthesis, characterization, and activity toward transfer hydrogenation of a series of Cp0 Ru(PTA)(PR3)X complexes, where 5 5 Cp0 = η5-C5Me5 (Cp*), η -C8H9 (Dp), η -C9H7 (Ind), 5 η -C5H5 (Cp), PR3=1,3,5-triaza-7-phosphadamantane (PTA),

PPh3, and X = Cl, H.34 As part of our continued interest in ruthenium phosphine chemistry,34,35 we report here the efficiency of series of Cp0 Ru(PR3)(PPh3)Cl (PR3 = PTA, PMe3, PPh3) complexes as catalysts for the addition of CCl4, CHCl3, and p-tosyl chloride (TsCl) to styrene and hexene in the presence of AIBN (Scheme 2).

(27) Simal, F.; Wlodarczak, L.; Demonceau, A.; Noels, A. F. Eur. J. Org. Chem. 2001, 2689–2695. (28) (a) De Clercq, B.; Verpoort, F. Tetrahedron Lett. 2001, 42, 8959– 8963. (b) Quebatte, L.; Haas, M.; Solari, E.; Scopelliti, R.; Nguyen, Q. T.; Severin, K. Angew. Chem., Int. Ed. 2005, 44, 1084–1088. (c) Richel, A.; Demonceau, A.; Noels, A. F. Tetrahedron Lett. 2006, 47, 2077–2081. (d) Borguet, Y.; Richel, A.; Delfosse, S.; Leclerc, A.; Delaude, L.; Demonceau,  A. Tethedron Lett. 2007, 48, 6334–6338. (e) Díaz-Alvarez, A. E.; Crochet, P.; Zablocka, M.; Duhayon, C.; Cadierno, V.; Majoral, J.-P. Eur. J. Inorg. Chem. 2008, 786–794. (29) (a) Bland, W. J.; Davis, R.; Durrant, J. L. A. J. Organomet. Chem. 1985, 280, 397–406. (b) Bland, W. J.; Davis, R.; Durrant, J. L. A. J. Organomet. Chem. 1984, 267, C45–C48. (30) Matyjaszewski, K. L. Curr. Org. Chem. 2002, 6, 67. (31) Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28, 1721. (32) Wang, J. S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117, 5614. (33) (a) Simal, F.; Demonceau, A.; Noels, A. F. Tetrahedron. Lett. 1999, 40, 5689–5693. (b) Opstal, T.; Verpoort, F. New J. Chem. 2003, 27, 257–262.

Experimental Section General Methods. All reactions were performed under a dry nitrogen atmosphere, using conventional Schlenk vacuum-line techniques. Reagents were purchased from commercial suppliers and used as received. Solvents were dried with molecular sieves and degassed with N2, prior to use. Cp*Ru(PPh3)2Cl,36 DpRu(PPh3)2Cl,37 IndRu(PPh3)2Cl,38 CpRu(PPh3)2Cl,39 TpRu(PPh3)2Cl,40 CpRu(PTA)(PPh3)Cl,34,41 DpRu(PTA)(PPh3)Cl,34 TpRu(PTA)(PPh3)Cl,42 IndRu(PTA)(PPh3)Cl,34 IndRu(PMe3)(PPh3)Cl,43 and 1,3,5-triaza-7-phosphaadamantane (PTA)44 were prepared as described in the literature. Kharasch reactions were carried out in standard NMR tubes or in 1.5 mL vials. The 1:1 Kharasch adduct was characterized by 1H NMR spectroscopy. Each catalytic run was repeated at least twice to ensure reproducibility. NMR spectra were recorded with a Varian NMR System 400 spectrometer. 1H NMR spectra were referenced to a residual solvent relative to tetramethylsilane (TMS). Phosphorus chemical shifts are relative to an external reference of 85% H3PO4 in D2O, with positive values downfield of the reference. Elemental analyses were performed by Midwest Microlab, LLC. X-ray crystallographic data were collected at 100((1) K on a Bruker APEX CCD diffractometer with Mo KR radiation (λ = 0.710 73 A˚) and a detector-to-crystal distance of 4.94 cm. Data collection was optimized utilizing the APEX2 (34) Mebi, C. A.; Nair, R. P.; Frost, B. J. Organometallics 2007, 26, 429–438. (35) (a) Mebi, C. A.; Frost, B. J. Inorg. Chem. 2007, 46, 7115–7120. (b) Mebi, C. A.; Frost, B. J. Organometallics 2005, 24, 2339–2346. (c) Frost, B. J.; Mebi, C. A. Organometallics 2004, 23, 5317–5323. (36) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112, 5166–5175. (37) Kirss, R. U.; Ernst, R. D.; Arif, A. M. J. Organomet. Chem. 2004, 689, 419–428. (38) Oro, L. A.; Ciriano, M. A.; Campo, M.; Foces-Foces, C.; Cano, F. H. J. Organomet. Chem. 1985, 289, 117–131. (39) Joslin, F. L.; Mague, J. T.; Roundhill, D. M. Organometallics 1991, 10, 521–524. (40) Coucouvanis, D. Inorg. Synth. 2002, 33, 206. (41) Romerosa, A.; Campos-Malpartida, T.; Lidrissi, C.; Saoud, M.; Serrano-Ruiz, M.; Peruzzini, M.; Garrido-Cardenas, J. A.; Garcı´ aMaroto, F. Inorg. Chem. 2006, 45, 1289–1298. (42) (a) Bola~ no, S.; Bravo, J.; Castro, J.; Rodrı´ guez-Rocha, M. M.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L.; Gonsalvi, L.; Peruzzini, M. Eur. J. Inorg. Chem. 2007, 5523–5532. (b) García-Fernandex, A.; Díez,  Sanchez, J.; Gamasa, M. P.; Lastra, E. Polyhedron 2008, 27, J.; Manteca, A.; 1214–1228. (43) Gamasa, M. P.; Gimeno, J.; Gonzalez-Bernardo, C.; MartinVaca, B. M. Organometallics 1996, 15, 302–308. (44) (a) Daigle, D. J. Inorg. Synth. 1998, 32, 40–45. (b) Daigle, D. J.; Pepperman, A. B.Jr.; Vail, S. L. J. Heterocycl. Chem. 1974, 11, 407–408.

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Table 1. Crystallographic Data and Structural Refinement Details for Cp*Ru(PTA)(PPh3)Cl, Cp*Ru(PMe3)(PPh3)Cl, IndRu(PMe3)(PPh3)Cl, and TpRu(PMe3)(PPh3)Cl

empirical formula formula wt color T (K) wavelength (A˚) cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z Dcalcd (Mg/m3) abs coeff (mm-1) F(000) cryst size (mm3) 2θ range (deg) index ranges no. of rflns collected no. of indep rflns Rint no. of data/restraints/params GOF on F2 final R indices (I > 2σ(I)) R indices (all data) CCDC no.

Cp*Ru(PTA)(PPh3)Cl (1)

Cp*RuCl(PMe3)(PPh3) (6)

IndRuCl(PMe3)(PPh3) (8)

TpRuCl(PMe3)(PPh3) (10)

C34H42ClN3P2Ru 691.17 orange 100(2) 0.710 73 monoclinic P21/n 11.6045(2) 14.8894(3) 18.4456(4) 90 102.0290(10) 90 3117.12(11) 4 1.473 0.720 1432 0.27  0.13  0.04 1.77-27.50 -12 e h e 15 -19 e k e 15 -23 e l e 13 34 336 7153 0.0507 7153/0/360 1.026 R1 = 0.0364 wR2 = 0.0749 R1 = 0.0518 wR2 = 0.0797 699360

C31H39ClP2Ru 610.08 orange 100(2) 0.710 73 monoclinic P21/n 11.3789(2) 14.3410(2) 18.3738(2) 90 107.3420(10) 90 2862.03(7) 4 1.416 0.771 1264 0.16  0.07  0.06 1.83-28.28 -15 e h e 15 -19 e k e 19 -19 e l e 24 31 805 6867 0.0432 6867/0/324 1.094 R1 = 0.0301 wR2 = 0.0631 R1 = 0.0434 wR2 = 0.0808 713868

C30H31ClP2Ru 590.01 red 100(2) 0.710 73 triclinic P1 12.3176(2) 13.1046(2) 17.1601(3) 80.5320(10) 82.1640(10) 89.4570(10) 2706.45(8) 4 1.448 0.813 1208 0.16  0.12  0.10 1.58-27.64 -15 e h e 16 -17 e k e 15 -22 e l e 22 51 558 12 486 0.0390 12 486/0/619 1.087 R1 = 0.0289 wR2 = 0.0675 R1 = 0.0448 wR2 = 0.0827 713866

C30H34N6BClP2Ru 687.90 green 100(2) 0.710 73 monoclinic P21/n 12.3342(2) 20.1362(3) 13.1231(2) 90 105.6100(10) 90 3139.09(8) 4 1.456 0.717 1408 0.19  0.07  0.02 1.90-26.00 -15 e h e 12 -24 e k e 24 -14 e l e 16 27 640 5785 0.0662 5785/0/373 1.038 R1 = 0.0420 wR2 = 0.0781 R1 = 0.0704 wR2 = 0.0844 713867

software with a 0.5° rotation about ω between frames and an exposure time of 10 s per frame. Data integration, correction for Lorentz and polarization effects, and final cell refinement were performed using SAINTPLUS and corrected for absorption using SADABS. The structures were solved using direct methods followed by successive least-squares refinement on F2 using the SHELXTL 6.10 software package.45 All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were placed in calculated positions. Crystallographic data and data collection parameters are given in Table 1. A complete list of bond lengths and angles may be found in the Supporting Information. Synthesis of Cp*Ru(PTA)(PPh3)Cl (1). Cp*Ru(PPh3)2Cl (0.300 g, 0.37 mmol) and PTA (0.0588 g, 0.37 mmol) were dissolved in 70 mL of CH2Cl2 and refluxed under nitrogen for 2 h. The solution was cooled to room temperature, filtered, and pulled dry under vacuum. The resulting solid was washed with water followed by diethyl ether (3  10 mL), affording 169 mg of 1 as an orange powder (65% yield). 1H NMR (400 MHz, CDCl3): δ 7.62-7.33 (m, 15H, Ph); 4.39, 4.27 (AB quartet, 2 JHAHB = 13 Hz, 6H, NCH2N); 3.95, 3.36 (AB quartet, 2JHAHB = 15 Hz, 6H, PCH2N); 1.34 (s, 15H, CH3). 31P{1H} NMR (162 MHz, CDCl3): δ 47.6 (d, PPh3, 2JPP = 40.4 Hz); -38.9 (d, PTA, 2 JPP = 40.4 Hz). Anal. Calcd for C34H42N3P2RuCl: C, 59.08; H, 6.12; N, 6.08. Found: C, 59.28; H, 6.17; N, 5.94. Orange crystals suitable for X-ray diffraction were obtained by slow diffusion of diethyl ether into a toluene solution of 1. Synthesis of Cp*Ru(PMe3)(PPh3)Cl (6). Cp*Ru(PPh3)2Cl (0.500 g, 0.63 mmol) and excess PMe3 (200 μL, 1.93 mmol) were dissolved in 30 mL of toluene and heated in an oil bath at 35 °C for 10 min under nitrogen. The toluene was then evaporated (45) (a) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112–122. (b) Sheldrick, G. M. SHELXTL: Structure Determination Software Suite, Version 6.10; Bruker AXS: Madison, WI, 2001.

under vacuum, and the solid was washed with methanol (3  10 mL), affording 220 mg of 6 as a yellow powder (57% yield). 1 H NMR (400 MHz, CD2Cl2): δ 7.61-7.28 (m, 15H, Ph); 1.28 (s, 15H, Cp(CH3)5); 1.06 (d, 9H, P(CH3)3, 2JPH = 8.4 Hz). 31 P{1H} NMR (162 MHz, CD2Cl2): δ 47.5 (d, PPh3, 2JPP = 44.1 Hz); 1.1 (d, PMe3, 2JPP = 44.1 Hz). Anal. Calcd for C31H39P2RuCl: C, 61.03; H, 6.44; N, 0.00. Found: C, 60.96; H, 6.49; N, 0.04. Orange crystals suitable for X-ray diffraction were obtained by slow cooling of a solution of 6 in hot methanol. Synthesis of DpRu(PMe3)(PPh3)Cl (7). A solution of DpRu(PPh3)2Cl (0.600 g, 0.78 mmol) and PMe3 (200 μL, 1.93 mmol) in toluene (50 mL) was heated in an oil bath at 35 °C for 2 h under nitrogen. The solvent was evaporated under vacuum, and the resulting solid was dissolved in CH2Cl2 and purified by column chromatography on silica. The band eluted with dichloromethane was collected and dried under vacuum, yielding 270 mg of 7 as an orange crystalline solid (60% yield). 1H NMR (400 MHz, CD2Cl2): δ 7.56-7.32 (m, 15H, Ph); 4.03, 3.63, 3.15 (s, 3H, Cp), 2.50-2.10 (m, 6H, Cp(CH2)3); 1.17 (d, 9H, P(CH3)3, 2JPH = 8.8 Hz). 31P{1H} NMR (162 MHz, CD2Cl2): δ 49.1 (d, PPh3, 2JPP = 46.8 Hz); 0.5 (d, PMe3, 2JPP = 46.8 Hz). Anal. Calcd for C29H33P2RuCl: C, 60.05; H, 5.73; N, 0.00. Found: C, 60.08; H, 5.69; N, 0.06. Synthesis of CpRu(PMe3)(PPh3)Cl (9). CpRu(PMe3)(PPh3)Cl was synthesized by an alternative method from that reported.46 A mixture of CpRu(PPh3)2Cl (1.0 g, 1.38 mmol) and PMe3 (300 μL, 2.90 mmol) in 80 mL of toluene was heated in an oil bath at 35 °C for 30 h under nitrogen. The reaction mixture was dried under vacuum. The resulting solid residue was dissolved in CH2Cl2 and purified by column chromatography on silica. The fraction eluted with dichloromethane was collected and dried under vacuum, affording 9 as a yellow crystalline solid (46) Bruce, M. I.; Wong, F. S.; Skelton, B. W.; White, A. H. J. Chem. Soc., Dalton Trans. 1981, 1398–1405.

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(370 mg, 50% yield). 1H NMR data for CpRu(PMe3)(PPh3)Cl obtained by this method are identical with those reported in the literature.46 31P{1H} NMR (162 MHz, CD2Cl2): δ 48.6 (d, PPh3, 2 JPP = 38.2 Hz); 2.1 (d, PMe3, 2JPP = 38.2 Hz). Synthesis of TpRu(PMe3)(PPh3)Cl (10). A mixture of TpRu(PPh3)2Cl (1.150 g, 1.31 mmol) and PMe3 (300 μL, 2.90 mmol) in 80 mL of toluene was heated in an oil bath at 35 °C for 4 h under nitrogen. The solvent was evaporated under vacuum, and the residue was dissolved in CH2Cl2 and purified by column chromatography on silica. The band eluted with dichloromethane was collected and dried under vacuum, yielding 400 mg of 10 as a yellow-green crystalline solid (45% yield). 1H NMR (400 MHz, CD2Cl2): δ 7.98 (1H, pyrazolyl); 7.65-7.21 (m, 18H, PPh3 and pyrazolyl); 6.60 (1H, pyrazolyl); 6.18 (1H, pyrazolyl); 5.78 (1H, pyrazolyl); 5.70 (1H, pyrazolyl); 5.57 (1H, pyrazolyl); 1.10 (d, 9H, P(CH3)3, 2JPH = 8.0 Hz). 31P{1H} NMR (162 MHz, CD2Cl2): δ 50.4 (d, PPh3, 2JPP = 35.1 Hz); 10.4 (d, PMe3, 2 JPP = 35.1 Hz). Anal. Calcd for C31H39P2RuCl: C, 52.38; H, 4.98; N, 12.22. Found: C, 52.47; H, 4.89; N, 12.00. Green crystals suitable for X-ray diffraction were obtained by slow evaporation of a dichloromethane solution of 10. Catalytic Experiments.47 ATRA of CCl4 to Styrene/Hexene. In a 1.5 mL vial 1 mol % of catalyst (0.0138 mmol), 5 mol % of AIBN (11.3 mg, 0.069 mmol), styrene (158 μL, 1.38 mmol) or hexene (171 μL, 1.38 mmol), CCl4 (533 μL, 5.52 mmol), and hexamethylbenzene as an internal standard (5 mg, 0.031 mmol) were added. Toluene-d8 was added to bring the total volume to 1 mL. The resulting solution was sealed in an NMR tube and heated at 60 °C in an oil bath. The formation of product was monitored by 1H NMR spectroscopy at predetermined intervals. ATRA of p-TsCl to Styrene. In a 1.5 mL vial 1 mol % of catalyst (0.0044 mmol), 5 mol % of AIBN (4 mg, 0.022 mmol), styrene (50 μL, 0.44 mmol), p-TsCl (99 mg, 0.52 mmol), and hexamethylbenzene as an internal standard (5 mg, 0.031 mmol) were added. Toluene-d8 was added to bring the total volume to 1 mL. The resulting solution was sealed in an NMR tube and heated at 60 °C in an oil bath. The formation of product was monitored by 1H NMR spectroscopy at predetermined intervals. ATRA of CHCl3 to Styrene. In a 1.5 mL vial 1 mol % of catalyst (0.0138 mmol), 5 mol % of AIBN (11.3 g, 0.069 mmol), styrene (158 μL, 1.38 mmol), and hexamethylbenzene as an internal standard (5 mg, 0.031 mmol) were added. The reaction mixture was brought to a total volume of 1 mL with the addition of CHCl3 and heated at 60 °C in an oil bath. Aliquots of the reaction mixture (∼30 μL) were diluted with CDCl3 and analyzed by 1H NMR spectroscopy.

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Figure 1. Cp0 Ru(PR3)(PPh3)Cl complexes used as catalyst precursors for atom transfer radical addition. Table 2. 31P{1H} Chemical Shift (ppm) and Coupling Constants (Hz) for the Series of Cp0 Ru(PR3)(PPh3)Cl Complexes (Cp0 = Cp*, Dp, Cp, Ind, Tp; PR3 = PTA, PMe3) in CD2Cl2 31

complex a

Cp*Ru(PTA)(PPh3)Cl (1) DpRu(PTA)(PPh3)Cl (2)34 IndRu(PTA)(PPh3)Cl (3)

34

CpRu(PTA)(PPh3)Cl (4)34,41 a

TpRu(PTA)(PPh3)Cl (5)

42

Cp*Ru(PMe3)(PPh3)Cl (6) DpRu(PMe3)(PPh3)Cl (7) IndRu(PMe3)(PPh3)Cla (8)43 CpRu(PMe3)(PPh3)Cl (9) TpRu(PMe3)(PPh3)Cl (10)

Results and Discussion 0

The series of Cp Ru(PR3)(PPh3)Cl complexes 1-10 (Figure 1) have been synthesized in good yields by ligand exchange of the Cp0 Ru(PPh3)2Cl complexes 11-15 with the appropriate PR3 ligand (PTA or PMe3). The synthesis and characterization of CpRu(PTA)(PPh3)Cl,41 DpRu(PTA)(PPh3)Cl,34 IndRu(PTA)(PPh3)Cl,34 TpRu(PTA)(PPh3)Cl,42 CpRu(PMe3)(PPh3)Cl,46 IndRu(PMe3)(PPh3)Cl,43 Cp*Ru(PPh3)2Cl,36 DpRu(PPh3)2Cl,37 IndRu(PPh3)2Cl,38 CpRu(PPh3)2Cl,39 and TpRu(PPh3)2Cl40 have been previously reported in the literature. We report here the new compounds Cp*Ru(PTA)(PPh3)Cl, Cp*Ru(PMe3)(PPh3)Cl, DpRu(PMe3)(PPh3)Cl, and TpRu(PMe3)(PPh3)Cl, along with the solid-state structure of IndRu(PMe3)(PPh3)Cl and an alternative synthesis for CpRu(PMe3)(PPh3)Cl. We have obtained Cp*Ru(PTA)(PPh3)Cl (1) in modest yield (47) For ease of comparison we have, in general, followed the experimental procedure from ref 17 for all catalytic reactions.

a

P{1H} NMR

47.6 (PPh3) -38.9 (PTA) 43.2 (PPh3) -39.6 (PTA) 50.8 (PPh3) -22.6 (PTA) 48.4 (PPh3) -34.5 (PTA) 46.9 (PPh3) -34.2 (PTA) 47.5 (PPh3) 1.1 (PMe3) 49.1 (PPh3) 0.5 (PMe3) 50.1 (PPh3) 16.3 (PMe3) 48.6 (PPh3) 2.1 (PMe3) 50.4 (PPh3) 10.4 (PMe3)

2

JPP, Hz

40.4 45.2 42.8 45.0 33.1 44.1 46.8 44.8 38.2 35.1

In CDCl3.

(65%) by refluxing equimolar amounts of PTA and Cp*Ru(PPh3)2Cl in toluene. Complexes 6, 7, 9, and 10 were prepared by heating (35 °C) Cp0 Ru(PPh3)2Cl with excess PMe3 in toluene. The compounds 1, 6, 7, and 10 were characterized by elemental analysis and NMR spectroscopic techniques (1H and 31P{1H} NMR). In addition, the structures of 1, 6, 8, and 10 were further confirmed by X-ray crystallography (vide infra). The 31P{1H} NMR spectra of the series of Cp0 Ru(PR3)(PPh3)Cl complexes each consist of two doublets due to coupling of the inequivalent phosphorus nuclei: 2Jpp = 40.4 Hz (1), 44.1 Hz (6), 46.8 Hz (7), 38.2 Hz (9), 35.1 Hz (10) (Table 2). The PTA and PMe3 ligands are more sensitive to the metal environment than PPh3, as evidenced by the range of 31P chemical shifts for PTA (Δppm ≈ 16; -22.6 to -38.9 ppm) and PMe3 (Δppm ≈ 16; 0.5-16.3 ppm) versus PPh3 (Δppm ≈ 7.5; 43.2-50.8 ppm).

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Figure 2. Thermal ellipsoid (50% probability) representation of Cp*Ru(PTA)(PPh3)Cl (1) with the atomic numbering scheme. Hydrogen atoms have been omitted for clarity. Selected bond lengths (A˚) and angles (deg): Ru1-Cl1=2.4494(6); Ru1P1=2.3008(7); Ru1-P2=2.2853(7); Ru1-Cp*cent=1.868; P1Ru1-P2=94.81(2); P1-Ru1-Cl1=84.22(2); P2-Ru1-Cl1 = 92.14(2).

Figure 3. Thermal ellipsoid (50% probability) representation of Cp*Ru(PMe3)(PPh3)Cl (6) with the atomic numbering scheme. Hydrogen atoms have been omitted for clarity. Selected bond lengths (A˚) and angles (deg): Ru1-Cl1 = 2.4583(6); Ru1-P1 = 2.3006(7); Ru1-P2 = 2.3049(7); Ru1-Cp*cent = 1.870; P1-Ru1-P2 = 96.78(2); P1-Ru1-Cl1 = 83.82(2); P2Ru1-Cl1 = 92.75(2).

Structures of Cp0 Ru(PPh3)(PR3)Cl Complexes (Cp0 =Cp*, Ind, Tp; PR3=PTA, PMe3). Orange plates of Cp*Ru(PTA)(PPh3)Cl (1) suitable for X-ray diffraction were obtained by slow diffusion of diethyl ether into a toluene solution of 1. A thermal ellipsoid representation of 1 is depicted in Figure 2. Complex 1 exhibits a classic piano-stool geometry with one PTA, PPh3, a chloride, and a η5-Cp* ring occupying the six coordination sites around the ruthenium atom. The Ru-Cl (2.4494(6) A˚) and Ru-P (2.3008(7), 2.2853(7) A˚) bond distances are comparable to the Cp, Dp, and Ind analogues.34 The P-Ru-P bond angle, 94.81(2)°, is similar to that of the Cp analogue and is

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Figure 4. Thermal ellipsoid (50% probability) representation of IndRu(PMe3)(PPh3)Cl (8) with the atomic numbering scheme. Hydrogen atoms have been omitted for clarity. Selected bond lengths (A˚) and angles (deg): Ru1-Cl1=2.4399(6); Ru1P1=2.2608(7); Ru1-P2=2.3077(6); Ru1-Cp*cent=1.895; P1Ru1-P2=98.72(2); P1-Ru1-Cl1=87.03(2); P2-Ru1-Cl1= 92.54(2).

Figure 5. Thermal ellipsoid (50% probability) representation of TpRu(PMe3)(PPh3)Cl (10) with the atomic numbering scheme. Hydrogen atoms have been omitted for clarity. Selected bond lengths (A˚) and angles (deg): Ru1-Cl1=2.4046(8); Ru1P1 = 2.2993(11); Ru1-P2 = 2.3125(9); Ru1-N1 = 2.097(3); Ru1-N3 = 2.162(3); Ru1-N5 = 2.132(3); P1-Ru1-P2 = 96.31(3); P1-Ru1-Cl1=94.98(3); P2-Ru1-Cl1=90.66(3).

slightly smaller than that reported for the Dp and Ind analogues.34 Thermal ellipsoid representations of the solid-state structures of Cp*Ru(PMe3)(PPh3)Cl (6), IndRu(PMe3)(PPh3)Cl (8), and TpRu(PMe3)(PPh3)Cl (10), along with the atomic numbering schemes, are depicted in Figures 3-5, respectively.

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Table 3. Selected Bond Lengths (A˚) and Angles (deg) for a Series of Cp0 Ru(PPh3)(PR3)Cl Complexes (Cp0 = Cp*, Ind, Tp and PR3 = PTA, PMe3)a

Ru-Cl Ru-P1 Ru-P2 Ru-Cp0 cent P1-Ru-P2 P1-Ru-Cl P2-Ru-Cl Cp-ML2 a

Cp*Ru(PTA)(PPh3)Cl (1)

Cp*Ru(PMe3)(PPh3)Cl (6)

IndRu(PMe3)(PPh3)Cl (8)

TpRu(PMe3)(PPh3)Cl (10)

2.4494(6) 2.3008(7) 2.2853(7) 1.868 94.81(2) 84.22(2) 92.14(2) 51.9

2.4583(6) 2.3006(7) 2.3049(7) 1.870 96.78(2) 83.82(2) 92.75(2) 62.7

2.4399(6) 2.2608(7) 2.3077(6) 1.895 98.72(2) 87.03(2) 92.54(2) 51.6

2.4046(8) 2.2993(11) 2.3125(9) 96.31(3) 94.98(3) 90.66(3)

See the Supporting Information for further details.

Table 4. Atom Transfer Radical Addition of CCl4 to Styrene Catalyzed by the Cp0 Ru(PR3)(PPh3)Cl Complexes 1-10a entry

cat.

1d 2 3e 4f 5 6 7 8 9 10 11 12 13

Cp*Ru(PTA)(PPh3)Cl (1) Cp*Ru(PTA)(PPh3)Cl (1) Cp*Ru(PTA)(PPh3)Cl (1) Cp*Ru(PTA)(PPh3)Cl (1) DpRu(PTA)(PPh3)Cl (2) IndRu(PTA)(PPh3)Cl (3) CpRu(PTA)(PPh3)Cl (4) TpRu(PTA)(PPh3)Cl (5) Cp*RuCl(PMe3)(PPh3) (6) DpRuCl(PMe3)(PPh3) (7) IndRuCl(PMe3)(PPh3) (8) CpRuCl(PMe3)(PPh3) (9) TpRuCl(PMe3)(PPh3) (10)

substrate

TOFb (h-1)

time (h)

yieldc (%)

CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CCl4

33 1060 415 444 24.1 12.6 2.4

4 0.37 0.63 1.25 3.5 8 28 75 1.25 2 6 11 31

>99 >99 >99 >99 >99 >99 61 27 >99 >99 >99 95 46

933 51.1 20.0 8.7

a All reactions were performed in toluene-d8 at 60 °C using 1 mol % of catalyst and 5 mol % of AIBN; the ratio of CCl4 to styrene is 4:1. b TOF is defined as (mol of product)/((mol of cat.) h), calculated at 50-65% conversion of styrene. c The yield is determined by 1H NMR spectroscopy of product versus internal standard. dAt 25 °C. e Without AIBN. f 10 mol % PPh3.

Table 5. Atom Transfer Radical Addition of CCl4 to Styrene Catalyzed by Cp0 Ru(PR3)2Cl Complexes 11-18 (PR3 = PPh3, PTA)a entry 1d 2 3 4 5 6e 7e 8e

cat. Cp*Ru(PPh3)2Cl (11) DpRu(PPh3)2Cl (12) IndRu(PPh3)2Cl (13) CpRu(PPh3)2Cl (14) TpRu(PPh3)2Cl (15) Cp*Ru(PTA)2Cl (16) DpRu(PTA)2Cl (17) CpRu(PTA)2Cl (18)

Table 6. TTO Values Obtained at 0.005 mol % Catalyst Loading for the Addition of CCl4 to Styrene Catalyzed by Cp0 Ru(PR3)(PPh3)Cl Complexes (PR3 = PTA, PMe3, PPh3)a

substrate TOFb (h-1) time (h) yieldc (%)

entry

CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CCl4 CCl4

1 2 3

>960 389.2 234.4 61.8 44.1

0.1 1.25 1.75 2.75 55 2.5 24 24

96 >99 >99 >99 38 >99 31 5

All reactions were performed in toluene-d8 at 60 °C using 1 mol % of catalyst and 5 mol % of AIBN; the ratio of CCl4 to styrene is 4:1. b TOF is defined as (mol of product)/((mol of cat.) h), calculated at 50-65% conversion of styrene. c The yield is determined by 1H NMR spectroscopy of product versus internal standard. d Room temperature, TOF calculated at 96% yield. e For solubility reasons, the reactions were performed in CD2Cl2. a

Orange crystals of 6 suitable for X-ray diffraction were obtained by slowly cooling a hot methanol solution of 6. X-ray-quality crystals of 8 and 10 were obtained by layering a dichloromethane solution of 8 with acetone and slowly evaporating a dichloromethane solution of 10, respectively. All three piano-stool complexes consist of a Cp0 ancillary ligand (η5-Cp*, Ind, η3-Tp) coordinated to ruthenium along with one PPh3, PMe3, and a chloride ligand. The Ru-Cl and Ru-P bond distances are comparable and are observed to be in the ranges 2.404-2.458 and 2.260-2.312 A˚, respectively (Table 3). The P1-Ru-Cl bond angle for 10 (94.98°) is larger than that for the Cp* and Ind analogues (83.82, 87.03°). Catalytic Activity. Current ruthenium catalysts for ATRA, such as Cp0 Ru(PPh3)2Cl, are active but have exhibited

cat.

TTOb

time (h)

yieldc (%)

Cp*Ru(PTA)(PPh3)Cl (1) Cp*Ru(PMe3)(PPh3)Cl (6) Cp*Ru(PPh3)2Cl (11)

11 600 6 000 16 400

24 24 24

58 30 82

a All reactions were performed in toluene-d8 at 60 °C with 0.005 mol % of catalyst and 5 mol % of AIBN ; the ratio of CCl4 to styrene is 4:1. b TTO is defined as (mol of product)/(mol of cat.). c NMR yield determined by 1H NMR spectroscopy of product versus internal standard.

limited total turnovers (TTO). For example, Cp*RuCl(PPh3)2 and Cp*RuCl2(PPh3) catalyzed addition of CCl4 to styrene with TTO values of 1600-170026 and 13 200,17 respectively. We hypothesized that replacement of one labile PPh3 ligand with a more strongly bound ligand, such as PTA or PMe3, would increase stability and therefore longevity of the catalyst. Complexes 1-10 were examined as catalysts for the addition of CCl4 to styrene (Table 4). Complex 1, with the electron-rich Cp* ligand, exhibited the highest activity, with >99% conversion in 22 min at 60 °C (entry 2, Table 4). A TOF value of 1060 h-1 and TTO of 11 600 was achieved with 1; these values are comparable to the highest ones reported for a Ru ATRA catalyst (TOF = 1500 h-1, TTO = 9000).21b Even at room temperature catalyst 1 provided >99% conversion in 4 h (TOF = 33 h-1; Table 4, entry 1). Trends in catalytic activity for compounds 1-10 toward addition of CCl4 to styrene correlate with the electron-donating ability of the Cp0 ligand: Cp*. Dp>Ind> Cp> Tp. These results indicate that Cp0 ligands play a significant role in the catalyst performance. For slow reactions (>10 h), although complete

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Figure 6. Kinetics plot for the addition of CCl4 to styrene (2), yielding the Kharasch product ((), (1,3,3,3-tetrachloropropyl)benzene, catalyzed by Cp*Ru(PTA)(PPh3)Cl and followed by 1H NMR spectroscopy. Conditions: 60 °C, 1 mol % of catalyst, 5% of AIBN. Table 7. Atom Transfer Radical Addition of p-TsCl to Styrene Catalyzed by Complexes 1-10a entry 1 2 3 4 5 6 7 8 9 10 11

cat. Cp*Ru(PTA)(PPh3)Cl (1) DpRu(PTA)(PPh3)Cl (2) IndRu(PTA)(PPh3)Cl (3) CpRu(PTA)(PPh3)Cl (4) TpRu(PTA)(PPh3)Cl (5) Cp*RuCl(PMe3)(PPh3) (6) DpRuCl(PMe3)(PPh3) (7) IndRuCl(PMe3)(PPh3) (8) CpRuCl(PMe3)(PPh3) (9) TpRuCl(PMe3)(PPh3) (10) Cp*Ru(PPh3)2Cl (11)

substrate TsCl TsCl TsCl TsCl TsCl TsCl TsCl TsCl TsCl TsCl TsCl

TOFb (h-1) 45.5 23.6 53.4 20.6 1.2 32.4 9.6 15.8 131

time (h) 3.8 5.8 4.8 7 52 5 12 29 6 75 2.5

yieldc (%) 96 95 90 95 66 97 96 33 99 40 96

All reactions were performed in toluene-d8 at 60 °C using 1 mol % of catalyst and 5 mol % of AIBN; the ratio of TsCl to styrene is 1.2:1. b TOF is defined as (mol of product)/((mol of cat.) h), calculated at 50-65% conversion of styrene. c The yield is determined by 1H NMR spectroscopy of product versus internal standard (hexamethylbenzene, 0.031 mmol). a

conversion of styrene was observed, the yield of the monoadduct was considerably lower (Table 4, entries 7, 8, and 13). This difference may be attributed to the formation of side products such as oligomers/polymers. It has been reported previously that the reactivity of the catalyst is dependent on the Ru-P bond strength and largely independent of the electronics of the phosphine.26 In our case, Cp0 Ru(PMe3)(PPh3)Cl complexes 7-10 showed enhanced reactivity toward ATRA reactions as compared to their PTA counterparts, which corresponds to the electron-donating ability of the phosphines (PMe3 > PTA).48 Exceptions were the most electron rich Cp* complexes 1 and 6; the addition of CCl4 to styrene catalyzed by 6 was slightly slower than that for 1 (TOF = 933 h-1 versus TOF = 1060 h-1; Table 4, entries 2 and 9). For comparison purposes, we investigated the activity of Cp0 Ru(PPh3)2Cl complexes (11-15) toward the addition of CCl4 to styrene under the same conditions as complexes 1-10 (Table 5, entries 1-5; compounds 11, 13, and 14 have been previously reported as ATRA catalysts26,27). The Cp0 Ru(PPh3)2Cl complexes 11-15 all exhibited superior (48) (a) Tolman, C. A. Chem. Rev. 1977, 77, 313–348. (b) Darensbourg, D. J.; Robertson, J. B.; Larkins, D. L.; Reibenspies, J. H. Inorg. Chem. 1999, 38, 2473–2481.

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Figure 7. Kinetics plot for the addition of p-TsCl to styrene, yielding the Kharasch product (() catalyzed by IndRu(PTA)(PPh3)Cl (8) and followed by 1H NMR spectroscopy. Conditions: 60 °C, 1 mol % of 8, 5% of AIBN.

performance relative to their mixed-phosphine counterparts. We also tested the catalytic activity of the substitionally inert Cp0 Ru(PTA)2Cl complexes 16-18, of which only the Cp* complex exhibited any significant reactivity (Table 5, entries 6-8). The reactivity order was Cp0 Ru(PPh3)2Cl > Cp0 Ru(PR3)(PPh3)Cl>Cp0 Ru(PTA)2Cl. These results suggest that the dissociation of the phosphine is crucial in the reaction. Substitution of one of the two labile PPh3 ligands in Cp0 Ru(PPh3)2Cl with a strongly bound phosphine, such as PTA or PMe3, lead to reduced activity of the mixed-phosphine complexes 1-10. The Cp*Ru(PPh3)2Cl-catalyzed reaction was remarkably fast; a 96% yield was obtained after only 6 min at room temperature (TOF > 960 h-1), as compared to 99% yield after 4 h for 1 (TOF 33 h-1). Using 0.005 mol % of 11, a TTO of 16 400 was obtained after 24 h at 60 °C (Table 6, entry 3). Reducing the catalyst loading of 1 from 1 mol % to 0.005 mol %, we obtained a TTO of 11 600 in 24 h, which is slightly lower than that reported for Cp*Ru(PPh3)Cl217 (Table 6, entry 1). The PMe3 analogue 6, at 0.005 mol % catalyst loading, provided a TTO of 6 000 after 24 h (Table 6, entry 2). The plot of [styrene] and Kharasch product formation versus time for the reaction of CCl4 with styrene catalyzed by 1 (Figure 6) contains a sigmoidal curve, suggesting an initiation step. Similar plots for the addition of CCl4 to styrene with a variety of Cp0 Ru(PR3)(PPh3)Cl complexes are available in the Supporting Information, and all show various degrees of induction. The induction is presumably due to a combination of phosphine dissociation and activation of the C-Cl bond. It is likely that C-Cl bond activation is the more important component, as the p-TsCl reactions (vide infra) show no induction period.49,50 The potential importance of phosphine dissociation was examined by addition of excess PPh3 (10 mol %) to the catalytic mixture. As expected, PPh3 inhibits the reaction rate, although the reaction still proceeds to >99% conversion (Table 4, entry 4). This is in contrast to the case for other catalysts, where the (49) We have found that phosphine (PPh3) dissociation in these complexes is slow in the absence of either AIBN or CCl4. Phosphine dissociation is almost certainly necessary for catalysis; however, the presence of radicals greatly increases the rate of dissociation. Harkreader, J. L.; Frost, B. J. Unpublished results. (50) A reviewer suggested that the lack of an induction period for the addition of TsCl to styrene may be attributed to a reaction of TsCl with PPh3. (See Watanabe, Y.; Mase, N.; Tateyama, M.; Toru, T. Tetrahedron: Asymmetry 1999, 10, 737-745.)

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Table 8. Atom Transfer Radical Addition of CHCl3 to Styrene and of CCl4 to Hexene Catalyzed by Cp*Ru(PR3)(PPh3)Cl (PR3 = PTA, PMe3, PPh3)a entry 1d 2d 3d 3e 4e 6e

cat. Cp*Ru(PTA)(PPh3)Cl (1) Cp*Ru(PMe3)(PPh3)Cl (6) Cp*Ru(PPh3)2Cl (11) Cp*Ru(PTA)(PPh3)Cl (1) Cp*Ru(PMe3)(PPh3)Cl (6) Cp*Ru(PPh3)2Cl (11)

substrate CHCl3 CHCl3 CHCl3 CCl4 CCl4 CCl4

olefin styrene styrene styrene hexene hexene hexene

TOFb (h-1)

time (h)

yieldc (%)

28.2 62.6 272 202 53.3 100

4 3.5 2 4 7 4.5

>99 >99 93 93 >99 9255

All reactions were performed at 60 °C using 1 mol % of catalyst and 5 mol % of AIBN. b TOF is defined as (mol of product)/((mol of cat.) h), calculated at 50-65% conversion of styrene. c The yield is determined by 1H NMR spectroscopy of product versus internal standard. d 1.38 mmol of styrene, with chloroform used as both solvent and substrate. e 1.38 mmol of hexene, 5.52 mmol of carbon tetrachloride, in toluene-d8. a

reaction is almost completely inhibited upon addition of excess phosphine.27,29a Severin and co-workers have recently reported that the addition of AIBN is necessary for catalyst activity at low catalyst loading.17 They proposed that AIBN allows for the reduction of Cp0 RuIII(PR3)Cl2 back to the active catalyst, Cp0 RuII(PR3)Cl.17 In the absence of AIBN a significant decrease in the reaction rate was observed for Cp*Ru(PTA)(PPh3)Cl (TOF = 415 h-1; entry 3, Table 4), which is comparable to that of Cp*Ru(PPh3)2Cl (TOF = 400 h-1).26 We observed a slight decrease in the time required to reach >99% conversion as the [AIBN] was increased from 1 mol % to 15 mol %. Above 15 mol % of AIBN no further increase in the rate of reaction was observed.51 The RuCl2(PPh3)3-catalyzed addition of sulfonyl chlorides to olefins has been reported by Kamagita and co-workers.52 We explored the efficiency of our catalytic system toward the addition of p-toluenesulfonyl chloride (p-TsCl) to styrene (Table 7). The reactivity order for complexes 1-5 was similar to that observed for the addition of CCl4 to styrene, with the exception of catalyst 3 (Table 7, entry 3). Unlike ATRA with CCl4, the PMe3 complexes 6-10 exhibited lower reactivity toward the addition of p-TsCl to styrene as compared to their PTA analogues 1-5. Complex 3 exhibited a high TOF (53.4 h-1) at short reaction time (∼50% conversion); however, activity leveled off at 90% conversion (Figure 7).53 Of interest is the fact that reactions involving p-TsCl with styrene exhibited no induction period.51 The Cp* complexes 1, 6, and 11 were the most active catalysts among the series (1-11) for the addition of both CCl4 and p-TsCl to styrene and were therefore explored for the ATRA of the more challenging substrate, CHCl3 (Table 8, entries 1-3). Compound 11 was the most active, with a TOF of 272 h-1 and 93% yield in 2 h. Compound 6 (TOF 62.6 h-1) has a faster initial rate with respect to 1 (TOF 28.2 h-1), as evidenced by the higher TOF (obtained at 5065% conversion), with both complexes yielding >99% conversion at 60 °C in 3-4 h. Compounds 1, 6, and 11 are also active for the addition of CCl4 to 1-hexene (Table 8, entries 4-6). The Cp*Ru(PTA)(PPh3)Cl-catalyzed addition of CCl4 to hexene proceeded rapidly with a TOF of 202 h-1 as compared to the PMe3 analogue (53.3 h-1) and PPh3 analogue (100 h-1). The TTO values obtained for the addition of CCl4 to hexene were impressive; with catalyst loadings of 0.001 mol %, Cp*Ru(PTA)(PPh3)Cl (1) exhibited a TTO of 79 000 in 48 h, Cp*Ru(PMe3)(PPh3)Cl (6) was more impressive with a TTO of 88 000 after 48 h, and (51) See the Supporting Information for details. (52) Kamigata, N.; Shimizu, T. Rev. Heteroat. Chem. 1997, 17, 1–50. (53) The reaction could be brought to completion by addition of excess of p-TsCl.

Cp*Ru(PPh3)2Cl was the most active with a TTO of 92 000 after 40 h.54 Prior to this work the highest TTO value reported for ATRA was 44 000 for the addition of CCl4 to 1-hexene.17 In conclusion, we found that the air-stable and readily synthesized Cp0 Ru(PPh3)(PR3)Cl complexes (Cp0 = Cp*, Dp, Ind, Cp, Tp; PR3 = PTA, PMe3, PPh3) efficiently catalyzed the atom transfer radical addition of CCl4 and p-toluenesulfonyl chloride to styrene. The Cp* analogues also catalyzed the addition of CHCl3 to styrene and of CCl4 to hexene. Catalyst performance appears to depend mainly on the electron richness of the ruthenium center and on the nature of the phosphine ligand. The pronounced electron-donating ability of Cp* rendered Cp*Ru(PPh3)2Cl (11) an exceptional catalyst and both Cp*Ru(PTA)(PPh3)Cl (1) and its PMe3 counterpart (6) very efficient ATRA catalysts (the order of reactivity is Cp*Ru . DpRu > IndRu > CpRu >TpRu). The catalytic activity of Cp0 Ru(PTA)(PPh3)Cl and Cp0 Ru(PMe3)(PPh3)Cl, however, is significantly reduced relative to the activity of the parent Cp0 Ru(PPh3)2Cl complexes. This indicates to us that the efficiency of catalysis is hampered by replacement of PPh3 with a more strongly binding phosphine such as PTA or PMe3;a strong indication that phosphine dissociation is involved in the reaction mechanism. This was further confirmed by the lower catalytic activity of Cp0 Ru(PTA)2Cl complexes. The catalytic performance of the ruthenium complexes may be further tuned by modifications of ancillary ligands and phosphines and awaits further study.55

Acknowledgment. We gratefully acknowledge financial support from the National Science Foundation CAREER program (NSF Grant No. CHE-0645365). Financial support from the NSF is also acknowledged for the X-ray and NMR facilities (Grant Nos. CHE-0226402 and CHE-0521191). The Korea Research Foundation (KRF-2007-357-C00056) is acknowledged by T.H.K. for partial support. Supporting Information Available: 31P{1H} and 1H NMR spectra of all new compounds, kinetics plots of the catalytic reactions, tables of full bond lengths and angles for compounds 1, 6, 9, and 10 as well as crystallographic data in CIF format. This material is available free of charge via the Internet at http:// pubs.acs.org. (54) The procedure was identical with that for the other catalytic runs (Table 8, entries 4-6), except the catalyst loading was reduced to 0.001 mol %. Compound 1 yielded 79% conversion in 48 h, compound 6 provided 88% conversion in 48 h, and compound 11 yielded 92% in 40 h, as determined by 1H NMR spectroscopy. (55) Cp*Ru(PPh3)2Cl also catalyzes the reaction of CCl4 to hexene at room temperature; a 41% yield is observed in 10 min at room temperature with no further reaction observed at longer times.