The resting state of the gold(I)-catalyzed hydroarylation of 1 changes in the presence of Ag+, with silver free catalysts resting at the dinuclear gold structure 5 and Ag+ containing solutions resting at a heteronuclear species like 6. Adventitious Ag+ (typically from LAuCl activation) can therefore intercept key organogold intermediates and effect the catalysis even when it does not effect the reaction in Au free control experiments

Gagné, Michel R.

Weber, Dieter

PubMed

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Dinuclear Gold–Silver Resting States May Explain Silver Effectsin Gold(I)-CatalysisDieter Weber and Michel R. Gagné*Caudill Laboratories, Department of Chemistry, University of North Carolina at Chapel Hill,Chapel Hill, North Carolina 27599-3290AbstractThe resting state of the gold(I)-catalyzed hydroarylation of 1 changes in the presence of Ag+, withsilver free catalysts resting at the dinuclear gold structure 5 and Ag+ containing solutions resting ata heteronuclear species like 6. Adventitious Ag+ (typically from LAuCl activation) can thereforeintercept key organogold intermediates and effect the catalysis even when it does not effect thereaction in Au free control experiments.Silver salts are commonly used to activate metal halides for catalytic transformations, andthis strategy is especially useful in homogeneous gold catalysis.1 After control experimentsdemonstrating that the AgX (X = BF4, OTf, NTf2, etc.) activator does not catalyze thereaction of interest, the silver salts are often used in excess to ensure quantitative activationof the precatalyst.2 Despite proving that the silver salts themselves are unreactive, there areexamples of gold(I)-catalyzed reactions where Ag+ effected either activity or selectivity,3but models for accommodating such observations are not well appreciated in the goldcatalysis community. We report herein observations that point to Ag+ ions interceptingAu(I) catalytic intermediates and subsequently effecting catalyst speciation and reactionkinetics; a structural model is also suggested.In a recent investigation of the gold(I)-catalyzed hydroarylation of 1 to 2,4 we noted that thecatalyst rested at the digold vinyl intermediate 5,5 which could additionally be synthesizedby the combination of Ph3PAuNTf2 (4) and the monoaurated vinylgold(I)-complex 3(Scheme 1). Based on the doubly aurated aryl compounds of Schmidbaur, 5 was formulatedas an out of plane Au–C–Au 3-center-2-electron bond with a stabilizing d10 closed shellaurophilic interaction.6 Motivated by the observation of Ag+-effects in gold(I)-catalysis andthe known examples of auro-argentophilic closed shell interactions in polymetalliccomplexes, we examined the effect of Ag+ on the catalyst speciation.7mgagne@unc.edu.Supporting Information Available: Synthetic procedures, characterization, and additional spectroscopic data. This material is availablefree of charge via the Internet at http://pubs.acs.org.NIH Public AccessAuthor ManuscriptOrg Lett. Author manuscript; available in PMC 2010 August 2.Published in final edited form as:Org Lett. 2009 November 5; 11(21): 4962–4965. doi:10.1021/ol902116b.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptAs expected, the catalyst generated in situ from Ph3PAuCl/AgNTf2 (5 mol %/25 mol %)was found to be effective for the conversion of 1 to 2, while the AgNTf2 activator was muchless reactive at RT.8 Monitoring the reactions by NMR, however, revealed a significantlydifferent behavior from Ph3PAuNTf2 (4) alone.9 Instead of 5 being observed using 4, theAg+-containing catalyst formulations rested at a new structure with a singlet in the 31P NMRat δ = 41 ppm (Figure 1a) and regenerated 4 at the end of the reaction. This compound, 6,could be independently synthesized by reacting a mixture of 4 and AgNTf2 with 1 in thepresence of 2,6-di-tert-butylpyridine, which acts to arrest protodemetalation.10 Theresonances of this new species (Figure 1b) exactly matched those observed in the 1H and 31PNMR of the in situ monitored catalytic reactions. Figure 1c and d additionally display thecorresponding spectra for the isolated intermediates 5 and 3, respectivley.Analysis of the 1H NMR of 6 indicated a complex with a PPh3 to carbocyclic frameworkratio of one. Both vinyl protons additionally were found to couple to one phosphorus nuclei(Hsyn: 4JPH = 5 Hz; Hanti: 4JPH = 14 Hz), and like 5 and previously reported multimetalatedcompounds, the quaternary bimetalated sp2 carbon showed no signal in the 13C NMR,perhaps a result of the quadrupole moment of gold.6a High resolution mass spectrometryshowed an m/z of 899.1046 (calcd 899.0966) for the [M–NTf2]+ ion with the isotope patternexpected for a Au–Ag containing compound.Inspection of X-ray data for numerous simple Au–Ag complexes, e.g. A,12b suggests thatAg+ likely binds to the vinyl carbon and the Au ion without significantly perturbing thegold-vinyl structure.12 In other words, the 3-center-2-electron interaction is not equal andthe stronger C–Au bond dominates the structure. Since only a single diastereomer isobserved, one of the diastereofaces must be preferentially populated (i.e. 6a or 6b).13Since the putative role of 6 is to react with acid to give 2 and regenerate 4 and AgNTf2, theacid stability of 6 was tested. Compound 6 was unchanged in the presence of 5 equivalentsof AcOH,5 but did provide traces of 2 with 35 equivalents over 48 h. One equivalent of thestronger acid HNTf2 (generated in situ during catalysis) resulted in a rapid protodeaurationto form 2. When treated with Br−, precipitation of AgBr drives the conversion of 6 to themonoaurated goldvinyl 3 (Scheme 2).The rapid and clean reaction of 3, a putative reaction intermediate, with 2 equivalents ofAgNTf2 to form 6, suggests that this process may be viable under catalytic conditions, andthus act to trap intermediate Au(I)-compounds. Similarly, the dinuclear gold intermediate 5reacted with 3 equivalents of AgNTf2 to provide the mixed metal species 6 showing thateven preformed digold structures can be driven to the heterometallic structure.14Such a silver-for-gold substitution seems only slightly downhill as it was found in catalyticexperiments that the resting state monitonically shifted from 6 to 5 as the Au/Ag ratio wasWeber and Gagné Page 2Org Lett. Author manuscript; available in PMC 2010 August 2.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author Manuscriptincreased. The change in speciation was especially dramatic when gold was added asPh3PAuCl, which simultaneously decreased the Ag+ concentration by precipitation of AgCl(Figure 2 and 3). Under suitably balanced conditions it is therefore possible to detect bothdigold and mixed gold/silver resting states.15The effect of excess AgNTf2 on the kinetics of the cyclization was also investigated. At aAu(I) loading of 5 mol %, the rate with a 3-fold excess of Ag+ was significantly retardedrelative to the gold only experiment (Figure 4). Unexpected was the increased rate of 1consumption as the reaction progressed.We show herein that even if the reactants are untouched by Ag+, silver ions can stillinfluence gold-catalyzed reactions by intercepting key organogold intermediates to formdinuclear intermediates with their own unique reactivity. A structural model based onexisting Au/Ag cluster chemistry literature is proposed, which may rationalize known Ag+effects.Supplementary MaterialRefer to Web version on PubMed Central for supplementary material.AcknowledgmentsWe thank J. Jacob Byrne (University of North Carolina at Chapel Hill) for samples of 1. Financial support isgratefully acknowledged from the Fulbright Foreign Student Program (for D.W.) and the National Institute ofGeneral Medicine (GM-60578).References1. Selected review articles on gold catalysis: (a) Hashmi ASK. Gold Bull 2004;37:51. (b) HashmiASK, Hutchings GJ. Angew Chem, Int Ed 2006;45:7896. (c) Hashmi ASK. Chem Rev2007;107:3180. [PubMed: 17580975] (d) Hashmi ASK. Catal Today 2007;122:211. (e) Li Z,Brouwer C, He C. Chem Rev 2008;108:3239. [PubMed: 18613729] (f) Widenhoefer RA. Chem—Eur J 2008;14:5382. (g) Gorin DJ, Sherry BD, Toste FD. Chem Rev 2008;108:3351. [PubMed:18652511] (h) Arcadi A. Chem Rev 2008;108:3266. [PubMed: 18651778] (i) Jiménez-Núñez E,Echavarren AM. Chem Rev 2008;108:3326. [PubMed: 18636778] (j) Bongers N, Krause N. AngewChem, Int Ed 2008;47:2178. (k) Gorin DJ, Toste FD. Nature 2007;446:395. [PubMed: 17377576](l) Fürstner A, Davies PW. Angew Chem, Int Ed 2007;46:3410. (m) Hashmi ASK, Rudolph M.Chem Soc Rev 2008;37:1766. [PubMed: 18762826]2. Selected example of a silver and gold catalyzed reaction: (a) Godet T, Vaxelaire C, Michel C, MiletA, Belmont P. Chem—Eur J 2007;13:5632.Selected review articles on silver-catalyzed reactions:(b) Halbes-Letinois U, Weibel JM, Pale P. Chem Soc Rev 2007;36:759. [PubMed: 17471400] (c)Weibel JM, Blanc A, Pale P. Chem Rev 2008;108:3149. [PubMed: 18616324] (d) Naodovic M,Yamamoto H. Chem Rev 2008;108:3132. [PubMed: 18637695] (e) Álvarez-Corral M, Muñoz-Dorado M, Rodríguez-García I. Chem Rev 2008;108:3174. [PubMed: 18630971] (f) Yamamoto Y.Chem Rev 2008;108:3199. [PubMed: 18611053] (g) Patil NT, Yamamoto Y. Chem Rev2008;108:3395. [PubMed: 18611054]3. See for example: (a) Li H, Widenhoefer RA. Org Lett 2009;11:2671. [PubMed: 19514795] (b)Tarselli MA, Chianese AR, Lee SJ, Gagné MR. Angew Chem, Int Ed 2007;46:6670.4. For selected examples of allene activation by Au(I) catalysts, see: (a) Hashmi ASK, Schwarz L,Choi JH, Frost TM. Angew Chem, Int Ed 2000;39:2285. (b) Liu L, Xu B, Mashuta MS, HammondGB. J Am Chem Soc 2008;130:17642. [PubMed: 19055329] (c) Zhang Z, Liu C, Kinder RE, HanX, Qian H, Widenhoefer RA. J Am Chem Soc 2006;128:9066. [PubMed: 16834380] (d) Liu Z,Wasmuth AS, Nelson SG. J Am Chem Soc 2006;128:10352. [PubMed: 16895385] (e) Park C, LeePH. Org Lett 2008;10:3359. [PubMed: 18613690] (f) Toups KL, Liu GT, Widenhoefer RA. JOrganomet Chem 2009;694:571. [PubMed: 20305794] (g) Gockel B, Krause N. Org Lett2006;8:4485. [PubMed: 16986931]Weber and Gagné Page 3Org Lett. Author manuscript; available in PMC 2010 August 2.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author Manuscript5. Weber D, Tarselli MA, Gagné MR. Angew Chem, Int Ed 2009;48:5733.6. These Au–Au interactions are estimated to be worth ~10 kcal mol−1. (a) Porter KA, Schier A,Schmidbaur H. Organometallics 2003;22:4922.Selected review articles on aurophilicity: (b)Schmidbaur H, Schier A. Chem Soc Rev 2008;37:1931. [PubMed: 18762840] (c) Schmidbaur H.Gold Bull 2000;1:33. (d) Schmidbaur H. Chem Soc Rev 1995;24:391.See also: (e) Pyykkö P.Angew Chem, Int Ed 2004;43:4412. (f) Pyykkö P. Inorg Chim Acta 2005;358:4113. (g) Pyykkö P.Chem Soc Rev 2008;37:1967. [PubMed: 18762842] (h) Nesmeyanov AN, Perevalova ÉG,Grandberg KI, Lemenovskii DA. Izv Akad Nauk SSSR, Ser Khim 1974;5:1124.7. Selected review on auro-metallophilic interactions: Bardaji M, Laguna A. Eur J Inorg Chem2003:3069.See ref 12 for selected examples of auro-argentophilic interactions.8. Tarselli MA, Gagné MR. J Org Chem 2008;73:2439. [PubMed: 18278940]9. Ph3PAuNTf2 is a silver-free catalyst: Mézailles N, Ricard L, Gagosz F. Org Lett 2005;7:4133.[PubMed: 16146370]10. In the presence of AgNTf2, full conversion of 1 to 6 required only 1 equivalent of 4. In contrast 2equivalents of 4 were needed for full conversion of 1 to 5 when no silver salt was present, see ref5. Resin-bound 2,6-di-tert-butylpyridine was used for synthetic applications.11. We presume that Ag+ is either coordinated by a weak neutral ligand (e.g. substrate) or byNTf2−. 19F NMR data was not indicative. X-ray quality crystals have not been forthcoming.12. Examples of Au–Ag complexes with a bridging carbon: (a) Contel M, Garrido J, Gimeno MC,Jones PG, Laguna A, Laguna M. Organometallics 1996;15:4939. (b) Contel M, Jiménez J, JonesPG, Laguna A, Laguna M. J Chem Soc, Dalton Trans 1994:2515. (c) Fernández EJ, Hardacre C,Laguna A, Lagunas MC, López-de-Luzuriaga JM, Monge M, Montiel M, Olmos ME, Puelles RC,Sánchez-Forcada E. Chem—Eur J 2009;15:6222. (d) Fernández EJ, Gimeno MC, Laguna A,López-de-Luzuriaga JM, Monge M, Pyykkö P, Sundholm D. J Am Chem Soc 2000;122:7287. (e)Fernández EJ, Laguna A, López-de-Luzuriaga JM, Montiel M, Olmos ME, Pérez J, Puelles RC.Organometallics 2006;25:4307. (f) Fernández EJ, Laguna A, López-de-Luzuriaga JM, Monge M,Montiel M, Olmos ME, Rodríguez-Castillo M. Organometallics 2006;25:3639. (g) Ruiz J, RieraV, Vivanco M, García-Granda S, García-Fernández A. Organometallics 1992;11:4077. (h) VicenteJ, Chicote MT, Alvarez-Falcón MM, Jones PG. Organometallics 2005;24:4666. (i) Schuster O,Monkowius U, Schmidbaur H, Ray SS, Krüger S, Rösch N. Organometallics 2006;25:1004. (j)Wei QH, Zhang LY, Yin GQ, Shi LX, Chen ZN. J Am Chem Soc 2004;126:9940. [PubMed:15303866] (k) Xie ZL, Wei QH, Zhang LY, Chen ZN. Inorg Chem Commun 2007;10:1206. (l)Wei QH, Yin GQ, Zhang LY, Chen ZN. Organometallics 2006;25:4941. (m) Hussain MS,Mazhar-Ul-Haque, Abu-Salah OM. J Cluster Sci 1996;7:167. (n) Mazhar-Ul-Haque, Horne W,Abu-Salah OM. J Crystallogr Spectrosc Res 1992;22:421.Other examples: (o) Crespo O, GimenoMC, Laguna A, Larraz C, Villacampa MD. Chem—Eur J 2007;13:235. (p) Catalano VJ, MooreAL. Inorg Chem 2005;44:6558. [PubMed: 16156614] (q) Catalano VJ, Etogo AO. Inorg Chem2007;46:5608. [PubMed: 17547393] (r) Catalano VJ, Etogo AO. J Organomet Chem2005;690:6041. (s) Vicente J, Chicote MT, Lagunas MC. Inorg Chem 1993;32:3748. (t) CatalanoVJ, Horner SJ. Inorg Chem 2003;42:8430. [PubMed: 14658896] (u) Rawashdeh-Omary MA,Omary MA, Fackler JP Jr. Inorg Chim Acta 2002;334:376. (v) Fernández EJ, Laguna A, López-de-Luzuriaga JM, Monge M, Pyykkö P, Runeberg N. Eur J Inorg Chem 2002:750.13. Alternatively the two diastereofaces could be rapidly interconverting through a doubly argentedintermediate or transition state. Laguna has recently reported a cluster with just such a bondingmodel, see refs 12e and 12f.14. Auro-argentophilic interactions are stronger than Au-Au interactions due to the introduction of adipolar interaction: Catalano VJ, Malwitz MA, Etogo AO. Inorg Chem 2004;43:5714. [PubMed:15332824]15. After adding >15 mol % 4 some of it was also observed in the NMR, as was (Ph3P)2AuNTf2, seeref 4b.Weber and Gagné Page 4Org Lett. Author manuscript; available in PMC 2010 August 2.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptFigure 1.Comparison of 1H (top) and 31P NMR (bottom) data. (a) Catalysis of 1 with 5 mol % 4 inthe presence of 20 mol % AgNTf2 in CD2Cl2. (b) Compound 6 trapped with 2,6-di-tert-butylpyridine in CD2Cl2. (c) Isolated 5 in CD2Cl2. (d) Isolated 3 in CDCl3.Weber and Gagné Page 5Org Lett. Author manuscript; available in PMC 2010 August 2.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptFigure 2.Change in catalyst resting state from 6 (Au–Ag) to 5 (Au–Au) upon additions of Ph3PAuClto a solution of 0.1 mmol of 1 and 25 mol % AgNTf2 in CD2Cl2.Weber and Gagné Page 6Org Lett. Author manuscript; available in PMC 2010 August 2.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptFigure 3.Change in catalyst resting state from 6 (Au–Ag) to 5 (Au–Au) upon additions of 4 to asolution of 0.1 mmol 1 and 15.5 mol % AgNTf2 in CD2Cl2.Weber and Gagné Page 7Org Lett. Author manuscript; available in PMC 2010 August 2.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptFigure 4.ln [1] versus time [s] for the conversion to 2 by 1H NMR; 0.1 mmol 1, 5 mol %Ph3PAuNTf2 (4), and 0.05 mmol hexamethylbenzene (internal standard) in 0.5 mL CD2Cl2;6.2 mg (16 mol %) AgNTf2 was added as a solid (see Supporting Information for details).Weber and Gagné Page 8Org Lett. Author manuscript; available in PMC 2010 August 2.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptScheme 1.Catalyst Resting State 5 Was Observed for the Gold(I)-Catalyzed IntramolecularHydroarylation of 1 to 2Weber and Gagné Page 9Org Lett. Author manuscript; available in PMC 2010 August 2.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptScheme 2.Reactions of Intermediate 6aa For experimental details, see the Supporting Information.Weber and Gagné Page 10Org Lett. Author manuscript; available in PMC 2010 August 2.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author Manuscript

Dinuclear Gold−Silver Resting States May Explain Silver Effects in Gold(I)-Catalysis

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