A dual photoredox and gold-catalysed cross-coupling reaction of alkynyltrimethylsilanes and aryldiazonium tetrafluoroborates is described. The reaction proceeds through visible-light mediated oxidative addition of aryldiazoniums, transmetalation of alkynyltrimethylsilanes and aryl-alkynyl reductive elimination. Exclusive selectivity for silyl-substituted alkynes is observed, with no reactivity observed for terminal alkynes

Kim, Suhong

Rojas-Martin, Jaime

Toste, F Dean

English

eScholarship - University of California

UC BerkeleyUC Berkeley Previously Published WorksTitleVisible light-mediated gold-catalysed carbon(sp 2 )–carbon(sp) cross-couplingPermalinkhttps://escholarship.org/uc/item/6w20r1qbJournalChemical Science, 7(1)ISSN2041-6520AuthorsKim, SuhongRojas-Martin, JaimeToste, F DeanPublication Date2016DOI10.1039/c5sc03025k Peer reviewedeScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaChemicalScienceEDGE ARTICLEOpen Access Article. Published on 10 November 2015. Downloaded on 11/05/2018 16:24:08.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineView Journal  | View IssueVisible light-medScheme 1 Photoredox catalyst ancatalysis.aDepartment of Chemistry, University of Calfdtoste@berkeley.edubDepartamento de Qúımica Orgánica (MóduC/Francisco Tomásy Valiente 7, Cantoblanc† Electronic supplementary informa10.1039/c5sc03025kCite this: Chem. Sci., 2016, 7, 85Received 14th August 2015Accepted 26th October 2015DOI: 10.1039/c5sc03025kwww.rsc.org/chemicalscienceThis journal is © The Royal Society of Ciated gold-catalysed carbon(sp2)–carbon(sp) cross-coupling†Suhong Kim,a Jaime Rojas-Martinab and F. Dean Toste*aA dual photoredox and gold-catalysed cross-coupling reaction of alkynyltrimethylsilanes and aryldiazoniumtetrafluoroborates is described. The reaction proceeds through visible-light mediated oxidative addition ofaryldiazoniums, transmetalation of alkynyltrimethylsilanes and aryl–alkynyl reductive elimination. Exclusiveselectivity for silyl-substituted alkynes is observed, with no reactivity observed for terminal alkynes.IntroductionFor over a decade, homogeneous gold catalysis has beeninvestigated because it provides access to novel modes ofreactivity and enables rapid generation of complex moleculararchitectures.1 However, in contrast to other low-valent latetransition metal catalysts, the majority of gold(I) complexes areunreactive towards the oxidative addition of aryl and vinylhalides and pseudohalides.2 Consequently, most gold-catalysedcross-coupling reactions have required sacricial oxidants toaccess the +3 oxidation state.3 For example, this reactivity plat-form has been exploited in a gold-catalysed oxidative couplingof organosilanes and 1,2-alkene functionalizations (Scheme 1A).As an alternative, visible light-mediated oxidative addition ofd visible light-mediated AuI/AuIIIifornia, Berkeley, CA 94720, USA. E-mail:lo 01), Universidad Autónoma de Madrid,o, 28049 Madrid, Spaintion (ESI) available. See DOI:hemistry 2016aryldiazoniums has emerged as a method for the generation ofthe requisite gold(III) intermediates via photoredox catalysis,thereby obviating the need for sacricial oxidants.4 Thesegold(III) intermediates have been intercepted by a variety ofnucleophiles in dual catalytic photoredox–gold reactions(Scheme 1B).Although aryldiazoniums and photoredox catalysts providean efficient method to generate gold(III) intermediates, severalchallenges must be addressed in order to facilitate carbon–carbon cross-coupling via these species. First, aryldiazoniumsalts are highly electrophilic and reactive towards many classesof nucleophilic coupling partners. Aryldiazoniums are generallydecomposed under Kumada and Negishi coupling conditionsand do not tolerate ligand additives and stoichiometric basesemployed in the majority of Suzuki, Sonogashira and Hiyamacoupling reactions.5 Second, as visible light-mediated oxidativeaddition of aryldiazoniums is proposed to proceed throughradical intermediates, nucleophilic coupling partners that canparticipate in single electron transfer, such as organotin andorganotriuoroborates compounds, are problematic.6 Inspiredby the oxidative coupling of organosilanes, we envisioned thatthe gold(III) complexes generated via photoredox catalysis mightundergo transmetalation with organosilanes, generating anintermediate poised for carbon–carbon bond formationthrough reductive elimination.7,8Results and discussionsWhile aryldiazonium tetrauoroborates have been shown toreact with halo-, azido-, allyl- and thiotrimethylsilanes, theirreactions with alkynyltrimethylsilanes has not been previouslyreported.9 Additionally, previous reports of organosilane trans-metallation with gold lead us to hypothesize that the trans-metallation of alkynyltrimethylsilanes to gold(III) intermediatesmight result in productive cross-coupling. On the basis of thishypothesis, we examined the combined gold/photoredoxcoupling of 1 equivalent of aryldiazonium salt 1a and 1 equiv-alent of alkynyltrimethylsilane 2.Chem. Sci., 2016, 7, 85–88 | 85Table 2 Scope of aryldiazonium tetrafluoroboratesReaction conditions: 1a–l (0.24 mmol, 1.2 equiv.), 2 (0.2 mmol, 1.0equiv.), Ph3PAuCl (10 mol%), Ru(bpy)3(PF6)2 (2 mol%), acetonitrile(2 mL), N2 atmosphere, visible light, rt for 3 h. Isolated yields.Chemical Science Edge ArticleOpen Access Article. Published on 10 November 2015. Downloaded on 11/05/2018 16:24:08.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineEvaluation of reaction conditions showed that the combi-nation of Ar3PAuCl and Ru(bpy)3(PF6)2, in acetonitrile gave thehighest yield of aryl alkyne 3a (Table 1). Changing either thesolvent or the photoredox catalyst proved detrimental to theyield of the desired product. With respect to the ligands on thegold catalyst, both electronic and steric factors impacted theefficiency of the cross-coupling. For example, electron-decienttriarylphosphine ligands resulted in signicantly lower yield of3a (Table 1, entry 10). The sterics of the ligand showed an evenmore dramatic effect on reaction yield. The reaction conductedwith (p-tol)3PAuCl provided 72% yield of the desired product,while the much more hindered (o-tol)3PAuCl-catalysed reactiononly provided 17% yield of 3a (Table 1, entry 11 and 12). Theyield of product was lower when tricyclohexylphosphinegold(I)chloride was used as catalyst instead of triphenylphoshine-gold(I) chloride (Table 1, entry 13). Gold(I) complexes of dia-lkylbiarylphosphine were also examined, but all showed lessthan 5% yield due to the combined effect of dialkyl groups andthe steric effect of biaryl groups. Finally, no coupling productwas observed in the absence of the ruthenium catalyst (entry 14)or when the reaction was conducted in the dark (entry 15).With an optimized catalyst system in hand, the scope withrespect to the aryldiazonium coupling partner was examined(Table 2). The yields of alkynylation reactions of electron-pooraryldiazonium salts were generally higher than those of reac-tions with electron-rich aryldiazonium coupling partners. Itshould be noted that the intrinsic instability of ortho-substituted aryldiazonium salts limited their use in this trans-formation (Table 2, entry 3k and 3l). It is also noteworthy that allhalogen substitutions on the aryldiazonium coupling partnerTable 1 Optimization of reaction conditionsEntry Gold cat. Photoredox cat. Solvent Yielda1 Ph3PAuCl Ru(bpy)3(PF6)2 CH3CN 702 Ph3PAuCl Ru(bpy)3(PF6)2 Acetone 393 Ph3PAuCl Ru(bpy)3(PF6)2 CH3NO2 154 Ph3PAuCl Ru(bpy)3(PF6)2 DMF 105 Ph3PAuCl Ru(bpy)3(PF6)2 MeOH 186 Ph3PAuCl Ru(bpy)3(PF6)2 EtOH 47 Ph3PAuCl Ir(ppy)3 CH3CN 218 Ph3PAuCl Ir(ppy)2(dtbbpy)PF6 CH3CN 459 (p-MeOPh)3PAuCl Ru(bpy)3(PF6)2 CH3CN 7310 (p-CF3Ph)3PAuCl Ru(bpy)3(PF6)2 CH3CN 4311 (p-tol)3PAuCl Ru(bpy)3(PF6)2 CH3CN 7212 (o-tol)3PAuCl Ru(bpy)3(PF6)2 CH3CN 1713 Cy3PAuCl Ru(bpy)3(PF6)2 CH3CN 4814 Ph3PAuCl — CH3CN 015b Ph3PAuCl Ru(bpy)3(PF6)2 CH3CN 0Reaction conditions: 4-uorobenzenediazonium tetrauoroborate 1a(0.2 mmol, 1 equiv.), 1-trimethylsilyl-2-phenylacetylene 2 (0.2 mmol, 1equiv.), gold catalyst (5 mol%), photoredox catalyst (1 mol%), solvent(2 mL), N2 atmosphere, visible light, rt for 6 h.a GC yields. b Thereaction was run in the dark. bpy ¼ 2,20-bipyridine, ppy ¼ 2-phenylpyridine,dtbbpy ¼ 4,40-di-tert-butyl-2,20-bipyridine.86 | Chem. Sci., 2016, 7, 85–88were preserved during the coupling reaction. Bromo- andiodoaryldiazonium salts were readily coupled and leavinghalides intact for use in further reactions (Table 2, 3b and 3j);this chemoselectivity is challenging under typical palladium-catalysed Sonogashira coupling reactions.10The difference in reactivity of electron-rich and -poor aryl-diazoniums may be the result of competing reaction pathways.Oxidative addition is believed to proceed through single elec-tron reduction of aryldiazoniums and this process is preferredwith electron-poor aryldiazonium salts.4 On the other hand, thegeneration of aryl cations, through loss of dinitrogen, is facilewith electron-rich aryldiazoniums. Correspondingly, weobserved trace amount of desired product when benzenedia-zonium tetrauoroborate was heated at 60 C for 9 hours in thepresence of 1-trimethylsilyl-2-(4-methoxy)phenylacetylene;however the same reactivity was not observed with 4-uo-rophenyldiazonium salts on the same conditions.The scope of alkynyltrimethylsilanes was also investigated(Table 3). Both aryl- and alkylethynyltrimethylsilanes were coupledwith modest to high product yields. o-Isopropylphenyl, biphenylyland naphthylalkynyltrimethylsilanes participated in the gold-cat-alysed coupling (Table 3, 3m, 3n and 3o). Potentially sensitivebenzylic and propargylic C–H and C–X bonds were well toleratedunder the reaction conditions (Table 3, 3q, 3aa and 3s–v). Addi-tionally, alkynyltrimethylsilane 3w was prepared in 78% yieldfrom the coupling of aryldiazonium salt 1d and bis(trimethylsilyl)acetylene, providing a compliment to using trimethylsilylacetylenein a traditional Sonogashira coupling (Table 3, 3w).Another interesting feature of this reaction is the effect of thesilyl group on yield (Table 4). When trimethylsilylacetylene wasThis journal is © The Royal Society of Chemistry 2016Table 3 Scope of alkynyltrimethylsilanesReaction conditions: 1d (0.24 mmol, 1.2 equiv.), 2m–aa (0.2 mmol, 1.0equiv.), Ph3PAuCl (10 mol%), Ru(bpy)3(PF6)2 (2 mol%), acetonitrile(2 mL), N2 atmosphere, visible light, rt for 3 h. Isolated yields. Ar ¼ p-nitrophenyl. a 5 equiv. of bis(trimethylsilyl)acetylene was used with 1equiv. of 1d. b GC yields.Scheme 2 Reaction with allenyltrimethylsilanes.Edge Article Chemical ScienceOpen Access Article. Published on 10 November 2015. Downloaded on 11/05/2018 16:24:08.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article Onlinecoupled with 1d to generate terminal alkyne 3x, the yield was15% with none of the corresponding silylacetylene 3w (Table 3).This observation suggests that, under the current base freeTable 4 Effect of counteranion and silyl groupEntryCounteranion(X)Silyl group(Y) Yielda1 BF4 TMS 702 TsO TMS 683 PF6 TMS 244 BF4 TES 615 BF4 TBDMS 106 BF4 TIPS 107 BF4 TBDPS 18 BF4 H 3Reaction conditions: aryldiazonium salt (0.2 mmol, 1 equiv.),phenylethynylsilane (0.2 mmol, 1 equiv.), Ph3PAuCl (5 mol%),Ru(bpy)3(PF6)2 (1 mol%), acetonitrile (2 mL), N2 atmosphere, visiblelight, rt for 6 h.a GC yields.This journal is © The Royal Society of Chemistry 2016reaction conditions, the silyl group plays a critical role in thecoupling. To investigate this role, a set of reactions was per-formed by varying counteranions of aryldiazonium salts andsilyl group identities of alkynylsilanes. The yield of couplingproduct was diminished as the steric hindrance of silyl groupwas increased (Table 4, entry 1 and entries 4–7). While bothtetrauoroborate and tosylate diazonium salts proved efficientcoupling partners, the yield was diminished when hexa-uorophosphate salts were used.11 Moreover, when the corre-sponding terminal alkyne was used instead ofalkynyltrimethylsilanes, only 3% of the coupling product wasobserved (Table 4, entry 8).12 Taken together, these resultssuggests that transmetallation from the organosilane is criticalfor high efficiency of the current coupling reaction.Other organotrimethylsilanes were tested under the opti-mized reaction conditions. While the reactions of aryl andvinylsilanes were complicated by competing reactions,13,14 thegold/photoredox-catalysed coupling of allenyltrimethylsilane 4provided propargylic compound 5 as the sole coupling product(Scheme 2). The product from aryl–allenyl reductive eliminationand the other aryl–propargylic coupling isomer were preparedindependently (Scheme 2, 6 and 7). We examined whether 6 wasan intermediate that underwent isomerization to 5 facilitated byvisible light and the photoredox catalyst. However, no isomer-ization was observed of 6 to either 5 or 7 under the reactionconditions. Therefore, the formation of 5 is best rationalized bytransmetalation of 4 to the gold(III) intermediate, followed by1,3-migration and aryl–propargyl reductive elimination.ConclusionsIn summary, we have demonstrated that trimethylsilylalkynesand aryldiazonium tetrauoroborates can be coupled via dualphotoredox and gold catalysis strategy. The reaction proceedsunder mild conditions and shows excellent functional grouptolerance, including aryl halides that may be reactive undertraditional coupling conditions. This process compliments theSonogashira coupling, especially when aryl–alkynyl coupling isrequired under non-basic conditions and may present anadvantage when TMS-alkynes are available rather than corre-sponding terminal alkynes.AcknowledgementsWe are grateful for nancial support from the NIHGMS (RO1GM073932). Jaime Rojas-Martin thanks MINECO for the EEBB-Chem. Sci., 2016, 7, 85–88 | 87Chemical Science Edge ArticleOpen Access Article. Published on 10 November 2015. Downloaded on 11/05/2018 16:24:08.  This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.View Article OnlineFPI fellowship. We thank Mark D. Levin, William J. Wolf andMatthew S. Winston for helpful discussions.Notes and references1 For recent reviews on gold catalysis: (a) A. S. K. Hashimi, Acc.Chem. Res., 2014, 47, 864; (b) C. Obradors andA. M. Echavarren, Chem. Commun., 2014, 50, 16; (c)D. Garayalde and C. Nevado, ACS Catal., 2012, 2, 1462; (d)M. Rudolph and A. S. K. Hashmi, Chem. Soc. 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Soc., 2013,135, 5505; (b) M. N. Hopkinson, B. Sahoo and F. Glorius, Adv.Synth. Catal., 2014, 356, 2794; (c) X. Z. Shu, M. Zhang, Y. He,H. Frei and F. D. Toste, J. Am. Chem. Soc., 2014, 136, 5844; (d)Y. He, H. Wu and F. D. Toste, Chem. Sci., 2015, 6, 1194.5 A. Roglans, A. Pla-Quintana and M. Moreno-Manas, Chem.Rev., 2006, 106, 4622.6 For single-electron transmetalation oforganotriuoroborates (a) J. C. Tellis, D. N. Primer andG. A. Molander, Science, 2014, 345, 433; (b) D. N. Primer,88 | Chem. Sci., 2016, 7, 85–88I. Karakaya, J. C. Tellis and G. A. Molander, J. Am. Chem.Soc., 2015, 137, 2195.7 W. J. Wolf, M. S. Winston and F. D. Toste, Nat. Chem., 2013,6, 159.8 For cross-coupling reactions of alkynyltrimethylsilanes, (a)S. A. Lermontov, I. M. Rakov, N. S. Zerov and P. J. Stang,Tetrahedron Lett., 1996, 37, 4051; (b) G. W. Kabalka,L. Wang and R. M. Pagni, Tetrahedron, 2001, 57, 8017; (c)S. Kang, H. Ryu and Y. Hong, J. Chem. Soc., Perkin Trans. 1,2007, 736; (d) C. Yang and S. P. 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Chem.,1999, 1875; (b) G. Fabrizi, A. Goggiamani, A. Sferrazza andS. Cacchi, Angew. Chem., Int. Ed., 2010, 49, 4067; (c)B. Panda and T. K. Sarkar, Chem. Commun., 2010, 46, 3131;(d) X. Wu, H. Neumann and M. Beller, Chem. Commun.,2011, 47, 7959; (e) This challenge was overcome byemploying o-benzenedisulfonimide as a counteranion. ;M. Barbero, S. Cadamuro and S. Dughera, Eur. J. Org.Chem., 2014, 598. (f) While preparing a manuscript, gold-catalysed cross-coupling of aryldiazonium salts withterminal alkynes and boronic acids was published. ; R. Cai,M. Lu, E. Y. Aguilera, Y. Xi, N. G. Akhmedov, J. L. Petersen,H. Chen and X. Shi, Angew. Chem., Int. Ed., 2015, 54, 8772.11 Aer the completion of coupling reaction withtetrauoroborate salts, stoichiometric amounts oftrimethylsilyl uoride were observed by 1H NMR.12 To examine the robustness of this reaction towards terminalalkynes, 1 equiv. of phenylacetylene and 1 equiv. ofphenylethynyltrimethylsilane were reacted with 1 equiv. 1asurprisingly, less than 5% of 3a was detected.13 When (Z)-styryltrimethylsilane was used the correspondingZ-product was obtained from gold-catalysed cross couplingand E-product was obtained from uncatalysed addition ofthe aryldiazonium to the vinylsilane (see: A. Benkeser,E. W. Bennet and R. A. Hickner, J. Am. Chem. Soc., 1957,79, 6253.) Aer consumption of the starting material,photoredox promoted isomerization of E-stilbene to Z-stilbene occurred.14 In the case aryltrimethylsilanes, further optimization isrequired due to the formation of signicant amounts ofcompeting homo-coupling reactions.This journal is © The Royal Society of Chemistry 2016

Visible light-mediated gold-catalysed carbon(sp 2 )–carbon(sp) cross-coupling

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Visible light-mediated gold-catalysed carbon(sp2)-carbon(sp) cross-coupling.

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