A Pd(ii)-catalyzed homo-coupling of Au(i)-aryls is reported. The reaction is driven by a Pd(0)/Au(i) redox reaction that generates a gold mirror and Pd(ii), and illustrates one of the challenges for developing dual catalytic Au–Pd systems

Gagné, Michel R.

Weber, Dieter

PubMed

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Pd(0)/Au(I) redox incompatibilities as revealed by Pd-catalyzedhomo-coupling of arylgold(I)-complexeswz†,‡Dieter Weber and Michel R. Gagné*Caudill Laboratories, Department of Chemistry, University of North Carolina at Chapel Hill,Chapel Hill, NC 27599-3290, USAAbstractA Pd(II)-catalyzed homo-coupling of Au(I)-aryls is reported. The reaction is driven by a Pd(0)/Au(I) redox reaction that generates a gold mirror and Pd(II), and illustrates one of the challenges fordeveloping dual catalytic Au–Pd systems.Pd(0)/Pd(II) catalyzed coupling reactions rely on five basic elementary steps: oxidativeaddition, reductive elimination, migratory insertion, β-elimination, and transmetallation.1With the exception of transmetallation, gold complexes rarely facilitate these basictransformations, a partial consequence of their high barriers for Au(I)/Au(0) and Au(III)/Au(I)redox transitions2 along with their preference for low coordination numbers. Despite thesedifferences Au(I)/Au(III) catalyzed cross-coupling reactions have been reported3,4 under theaction of strong oxidants like selectfluor,5 PhI(OAc)2,6 or peroxy acids.7 Mechanisticstudies by Toste on systems utilizing selectfluor suggest fascinating bimolecular reductiveelimination mechanistic possibilities.8,9Fueled by the seemingly orthogonal reactivity modes of Au and Pd catalysts, are suggestionsof tantalizing new cross-coupling schemes that sequence the reactivities of Au and Pd.Reports of at least partial success in interfacing Au- and Pd catalysis in this manner havebeen published by Blum,10 Hashmi,11 Sarkar,12 and Sestelo.13 In particular, Blum hasreported an impressive example of bimetallic catalysis wherein a gold-vinyl intermediateobtained from an allene activation reaction could be intercepted by an in situ formedpalladium π-allyl species to perform a vinyl-allyl coupling that was catalytic in both goldand palladium.14 We report herein that undesirable redox reactions between Pd(0) and Au(I)can interfere with the normal operation of several putative catalytic cycles that could couplethe reactivity of Au and Pd. These observations may explain the difficulty of developingreactions that are catalytic in both metals.Motivated by the potential of utilizing aryl and vinyl electrophiles to model the reactivity ofC(sp2)–Au intermediates in Au(I)-catalysis, we studied the palladium-catalyzed cross-coupling of isolated vinyl and arylgold(I)-complexes with aryl-triflates.15 Unfortunately,attempts to couple complexes A16 and B17 (Fig. 1) and TolOTf with various Pd-catalystsfailed.18 TLC and GC-MS analysis indicated that at best only traces of the desired cross-coupled products were obtained (24 h).†This article is part of a ChemComm web-based themed issue on new advances in catalytic C–C bond formation via late transitionmetals‡Electronic supplementary information (ESI) available: Experimental details, NMR, and GC-MS data.This journal is © The Royal Society of Chemistry 2011*mgagne@unc.edu.NIH Public AccessAuthor ManuscriptChem Commun (Camb). Author manuscript; available in PMC 2012 September 07.Published in final edited form as:Chem Commun (Camb). 2011 May 14; 47(18): 5172–5174. doi:10.1039/c1cc11055a.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptIn contrast, the same reaction conditions converted arylgold(I)-complex 1a into a variety ofbiaryl products (eqn (1)). The reaction progress was accompanied by the formation of brownparticles, which gradually led to a gold mirror on the glass surface. Once TLC indicatedcomplete conversion, GC-MS showed that three biaryl products, 2, 3, and 4, were generatedin an 8 : 85 : 7 ratio, respectively.19 The structure of 4 was confirmed by 1H NMR.(1)A simple mechanistic scheme for the formation of 2 is shown in cycle C, Scheme 1.Whereas cross-coupling to 2 is initiated by oxidative addition of TolOTf, catalytic homo-coupling of 1a to 3 requires a separate oxidant (cycle D, Scheme 1) to regenerate Pd(II) afterreductive elimination of 3. Since no external oxidant was added (with the exception ofTolOTf which would facilitate cross-coupling) further studies were initiated to gain insight.As shown in Table 1, variation of the Pd catalyst did not significantly increase the amount of2 (Table 1, entries 1–8), though reaction times did vary from catalyst to catalyst: Pd(II)sources tended to be faster. Control experiments showed that when no Pd catalyst wasadded, no conversion of 1a to coupled products was observed by TLC (Table 1, entries 9–11). Note: thermolysis of 1a in the GC injector port leads to 3.19The source of 3 and 4 was investigated by examining the role of each reaction component.When TolOTf was omitted, only 3 and 4 were formed (Table 2, entry 1), which clearlyshowed that TolOTf did not act as the oxidant for the formation of 3. Monitoring thereaction with Pd(OAc)2 by TLC indicated that 1a was consumed within 5 h (Table 2, entry2), and that an initial burst20 of coupled products was followed by a slower but steadyreaction. Using Pd2(dba)3 (Table 2, entry 3), the initial burst was not observed and thereaction proceeded slowly. A slightly faster rate could be regained using Pd2(dba)3 incombination with TolOTf, but the reaction time was still not as rapid as when a Pd(II) sourcewas used. These data suggest that it is a Pd(II) species that first engages Au(I) to producehomo-coupled products. Since the aryl groups of PAr3 ligands are known to transfer in Ni,Pd, and Pt reductive elimination reactions,21 the possibility that 4 was phosphine derivedwas tested by utilizing (p-tol)3PAuAr (1b) (Table 2, entries 4 and 5). Not surprisingly, asignificant amount of 2 was observed. When Ph-Xphos (L1, Fig. 2) was utilized as thephosphine ligand for Pd a small amount of 4 was also observed, pointing to two ligandsources for the aryl group.In the case of the non-phosphine NHC complexes (IPr-NHC)AuAr (1c) and (IMes-NHC)AuAr (1d) (Ar = 3,5-Xyl) only an initial burst of homo-coupling was observed, withneither reaction progressing beyond this point (Table 2, entries 6 and 7). This experimentsuggests that either the PAr3 ligand is necessary to facilitate the reaction or that NHCligands sufficiently stabilize the Au(I) state to inhibit the redox process. In the absence ofadded phosphine ligand Pd(OAc)2 fully converted 1a within 2 h at 58 °C (Table 2, entry 8).Conversely, 3.1 equivalents of PPh3 completely inhibit the conversion to 3 (Table 2, entry9), supporting the notion that stabilizing Au(I) either through NHC's or excess phosphine caninhibit the redox reaction.22Weber and Gagné Page 2Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptWe also attempted to elucidate whether the reaction was homogenous or heterogeneous witha Hg drop test.23 Unfortunately, this led to an instant decomposition of 1a as previouslyobserved by Grandberg while studying the redox potential of gold complexes on Hgelectrodes (eqn (2)).24(2)The catalytic coupling of two nucleophiles requires an oxidant. Based on the Grandbergredox precedent, the observed gold mirror, and the experimental results we propose thatreduction of Au(I) to Au(0) accounts for the necessary redox equivalents. As outlined in stepa of Scheme 2, two equivalents of Au(I) are consumed to regenerate Pd(II) with the Au(0)eventually forming the gold mirror.25 Since the Hg drop experiment failed, the role of Pdand/or Au nanoparticles cannot be eliminated from consideration.In conclusion, a Pd-catalyzed homo-coupling of arylgold(I)-complexes has been observed.While of limited applicability in synthesis, these results have illuminated a point ofincompatibility between these two metals and suggested strategies for overcoming theseshortcomings. Since Au-mirrors are relatively common occurrences in gold-catalyzedreactions, these results demonstrate how two elements with orthogonal reactivity profilescan be redox incompatible and thus impede the development of dual Au(I)/Pd(0) catalysts.Financial support is gratefully acknowledged from the Fulbright Foreign Student Program(DW) and the National Institute of General Medicine (GM-60578).Supplementary MaterialRefer to Web version on PubMed Central for supplementary material.Notes and references1. Wu X-F, Anbarasam P, Neumann H, Beller M. Angew. Chem., Int. Ed. 2010; 49:9047–9050.2. Hashmi ASK, Blanco MC, Fischer D, Bats JW. Eur. J. Org. Chem. 2006:1387–1389.3. (a) Wegner HA. Chimia. 2009; 63:44–48.(b) Garcia P, Malacria M, Aubert C, Gandon V,Fensterbank L. Chem. Cat. Chem. 2010; 2:493–497.For short reviews on oxidative couplingreactions with gold, see:4. (a) Lauterbach T, Livendahl M, Rosellón A, Espinet P, Echavarren AM. Org. Lett. 2010; 12:3006–3009. [PubMed: 20515017] (b) Corma A, Juárez R, Boronat M, Sánchez F, Iglesias M, Garcí H.Chem. Commun. 2011; 47:1446–1448.Palladium impurities could play the key role in gold-onlycatalyzed coupling reactions, especially if no oxidant is present:5. (a) Hashmi ASK, Ramamurthi TD, Rominger F. J. Organomet. Chem. 2009; 694:592–597.(b) CiuL, Zhang G, Zhang L. Bioorg. Med. Chem. Lett. 2009; 19:3884–3887. [PubMed: 19362834] (c)Zhang G, Peng Y, Ciu L, Zhang L. Angew. Chem., Int. Ed. 2009; 48:3112–3116.(d) Zhang G, CiuL, Wang Y, Zhang L. J. Am. Chem. Soc. 2010; 132:1447–1475.(e) Brenzovich WE Jr. Brazeau J-F,Toste FD. Org. Lett. 2010; 12:4728–4731. [PubMed: 21028911] (f) Ball LT, Green M, Lloyd-JonesGC, Russell CA. Org. Lett. 2010; 12:4724–4727. [PubMed: 20879724] (g) Hopkins MN, Tessier A,Salsbury A, Giuffredi GT, Combettes LE, Gee AD, Gouverneur V. Chem. Eur. J. 2010; 16:4739–4743. [PubMed: 20340120] (h) Hashmi ASK, Ramamurthi TD, Todd MH, Tsang AS-K, Graf K.Aust. J. Chem. 2010; 63:1619–1626.see also6. (a) Kar A, Mangu N, Kaiser HM, Beller M, Tse MK. Chem. Commun. 2008:386–388.(b) Kar A,Mangu N, Kaiser HM, Tse MK. J. Organomet. Chem. 2009; 694:524–537.7. Wegner HA, Ahles S, Neuburger M. Chem.–Eur. J. 2008; 14:11310–11313. [PubMed: 19006147]8. (a) Melhado AD, Brenzovich WE Jr. Lackner AD, Toste FD. J. Am. Chem. Soc. 2010; 132:8885–8887. [PubMed: 20557048] (b) Brenzovich WE Jr. Benitez D, Lackner AD, Shunatona HP,Weber and Gagné Page 3Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptTkatchouk E, Goddard WA III, Toste FD. Angew. Chem., Int. Ed. 2010; 49:5519–5522.(c) MankadNP, Toste FD. J. Am. Chem. Soc. 2010; 132:12859–12861. [PubMed: 20726525] (d) Komiya S,Albright TA, Hoffmann R, Kochi JK. J. Am. Chem. Soc. 1976; 98:7255–7265.for a similarproposal, see:and references therein.9. Scott VJ, Labinger JA, Bercaw JE. Organometallics. 2010; 29:4090–4096.10. (a) Shi Y, Ramgren SD, Blum SA. Organometallics. 2009; 28:1275–1277.(b) Shi Y, Peterson SM,Haberaecker WW III, Blum SA. J. Am. Chem. Soc. 2008; 130:2168–2169. [PubMed: 18229930]11. (a) Hashmi ASK, Lotschütz C, Döpp R, Rudolph M, Ramamurthi TD, Rominger F. Angew.Chem., Int. Ed. 2009; 48:8243–8246.(b) Hashmi ASK, Döpp R, Lotschütz C, Rudolph M, RiedelD, Rominger F. Adv. Synth. Catal. 2010; 352:1307–1314.12. (a) Panda B, Sarkar TK. Chem. Commun. 2010; 46:3131–3133.(b) Panda B, Sarkar TK.Tetrahedron Lett. 2010; 51:301–305.13. Pena-López M, Ayán-Varela M, Sandareses LA, Sestelo JP. Chem.–Eur. J. 2010; 16:9905–9909.[PubMed: 20564296]14. Shi Y, Roth KE, Ramgren SD, Blum SA. J. Am. Chem. Soc. 2009; 131:18022–18023. [PubMed:19929002]15. Sestelo has coupled p-acetylphenyl triflate with in situ formed organometallic gold compounds(from RLi and Ph3PAuCl). Our attempts to couple isolated Au-complexes 1a or PhAuPPh3 withTolOTf or p-acetylphenyl-triflate under these published conditions were unsuccessful (see thesupporting information‡ and ref. 13).16. (a) Weber D, Tarselli MA, Gagné MR. Angew. Chem., Int. Ed. 2009; 48:5733–5736.(b) Weber D,Gagneé MR. Org. Lett. 2009; 11:4962–4965. [PubMed: 19807117]17. (a) Liu L-P, Xu B, Mashuta MS, Hammond GB. J. Am. Chem. Soc. 2008; 130:17642–17643.[PubMed: 19055329] (b) Liu L-P, Hammond GB. Chem.–Asian J. 2009; 4:1230–1236. [PubMed:19472292]18. Dooleweerdt K, Fors BP, Buchwald SL. Org. Lett. 2010; 12:2350–2353. [PubMed: 20420379] andreferences therein.19. GC-MS and GC data could not be used to determine the yield of incomplete reactions, sincearylgold(I)-complexes pass through silica or florisil and homo-couple in the hot injector port (seethe supporting information‡).20. The initial burst was qualitatively observed by TLC. It is best explained by a fast first turnover of aPd(II) source.21. (a) Vicente J, Bermuũdez MD, Escribano J. Organometallics. 2010; 10:3380–3384.(b) NakamuraA, Otsuka S. Tetrahedron Lett. 1974:463–466.and references therein22. The exact nature of the inhibiting effect of PPh3 is unclear, but we note that when it is in excessnot even a single turnover occurs.23. Anton DR, Crabtree RH. Organometallics. 1983; 2:855–859.24. Rakhimov RD, Butin KP, Grandberg KI. J. Organomet. Chem. 1994; 464:253–260.25. Hirner JJ, Blum SA. Organometallics. 2011; 30:1299–1302.Notable observations include redoxchemistry between Au(I) and Ni(0) to generate paramagnetic Ni(I) (i.e. single electron processes)and gold mirrors. We are unable to confirm the presence of Pd(I) intermediates in the presentchemistry.Weber and Gagné Page 4Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptFig. 1.Structures of Au-vinyl complexes A and B.Weber and Gagné Page 5Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptFig. 2.Various ligands utilized in attempted cross-coupling.Weber and Gagné Page 6Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptScheme 1.Mechanistic schemes for cross- and homo-coupling of 1a.Weber and Gagné Page 7Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptScheme 2.Proposed mechanism for the homo-coupling of 1a.Weber and Gagné Page 8Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptWeber and Gagné Page 9Table 1Ligand screen and control experimentsaEntryAdditive[Pd]LTime2:3:4 f1TolOTfPd(OAc)2L1<19h8:85:72TolOTfPd(OAc)2L3<19h2:84:143TolOTfPd(OAc)2L424 hd1:86:13g4TolOTfdppfPdCl 2bNone<19h11:74:155TolOTfPd2(dba)3L119 hd13:73:14g6TolOTfPd2(dba)3L324 hd5:83:12g7TolOTfPd2(dba)3L424 hd7:81:12g8TolOTfPEPPSI-IPrcNone<29h1:88:129TolOTfNoneL119 he—10TolOTfNoneNone19 he—11NoneNoneNone19 he—a Reaction conditions: 5—10 mol% [Pd]/L; 2.5 equiv. TolOTf; 10 mg (17.7 μmol) of 1a.b dppf = 1,1'-bis(diphenylphosphino)-ferrocene.c PEPPSI-IPr = [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene](3-chloropyridyl)palladium(II) dichloride.d Incomplete conversion to coupled product was indicated by TLC; after the indicated period of time, the reactions was analyzed by GC-MS which thermally converts unreacted 1a to 3.e No conversion indicated by TLC.f Product ratio determined by GC-MS.Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptWeber and Gagné Page 10g See ref. 19.Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.NIH-PA Author ManuscriptNIH-PA Author ManuscriptNIH-PA Author ManuscriptWeber and Gagné Page 11Table 2No addition of TolOTf and variation of substrateaEntrySubstrate[Pd]L'Time2:3:4 f11aPd(OAc)2L1<19h0:76:2421aPd(OAc)2L1<5h0:75:2531aPd2(dba)3L119 hc0:98:2g41aPd(OAc)2L15 hc19:80:1g51aPd(OAc)2L25 hc18:82:0g61cPd(OAc)2L15 hd—71aPd(OAc)2L112 hd—81aPd(OAc)2None<2h0:83:17h91aPd(OAc)2PPhs3b12 he—a Reaction conditions: 5—10 mol% [Pd]/L; 10 mg (17.7 μmol) of la.b 3.1 equivalents of PPh 3 vs. 1a were added.c Incomplete conversion to coupled product was indicated by TLC; after the indicated period of time, the reactions was analyzed by GC-MS which thermally converts 1 to 3.d Only an initial burst to coupled product was observed.e No conversion indicated by TLC.f Product ratio determined by GC-MS.g See ref. 19 .h An 0:84:16 mixture (NMR ratio) of products was isolated by flash column chromatography on silica (45% yield of 3, and a 5% yield of 4).Chem Commun (Camb). Author manuscript; available in PMC 2012 September 07.

Pd(0)/Au(i) redox incompatibilities as revealed by Pd-catalyzed homo-coupling of arylgold(i)-complexes

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Pd(0)/Au(i) redox incompatibilities as revealed by Pd-catalyzed homo-coupling of arylgold(i)-complexes

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