1Electronic characteristics of ligands. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1770
Clearly, the attachment of cyclophanic rings to a carbene ligand like 19 imparts significant changes in the ligating properties of 20–22, and the p-type orbitals are more responsive than the s-donor orbitals. The bonding properties of the ligands concur with the redox potentials of the corresponding rhodium complexes. Ligand 21 exerts by far the best and 19 the weakest p-acceptor properties. In complexes 20–22, differences are governed by strong but opposing interactions between the two aromatic planes in each cyclophane skeleton. Introducing a triazo ring in 23 reduces the electron donor capacity, whereas conversion of the five-membered ring in 19 into an ylid 24 produces the strongest carbene donor of all. The following results illustrate the effective modification of the p-acceptor properties of the auxiliary ligand. A cationic gold complex of the strong s-donor ligand 19 exclusively produces the [4 + 3] adduct in the cyclisation of an allene-diene, whereas the [4 + 2] pathway is favoured with the electron-poorer combination 20.AuCl/AgBF4, in accordance with the interpretation given above (Scheme 5). Completely different behaviour exhibited by the same two complexes is also seen during the cycloaddition of ene–allenes where exclusive discrimination between [3 + 2] and [2 + 2] adduct formation occurs.5Divergent stabilisation of contributing intermediates VI and VII depending on the nature of L. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1772 6The core structure 18, of cortistatin A produced by [4 + 3] transannular C–C couplings. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1773 Bertrand and co-workers introduced cyclic (alkyl)(amino)carbene (CAAC) ligands into gold chemistry and utilised such gold complexes in the intermolecular hydroamination of alkynes. 34Employing the two gold catalysts 25 (in combination with AgOTf) and 26 (Figure 4), they discovered the promotion of unusual hydroammoniumation and methylamination of various N-methyl-2-(2-organoethynyl)anilines by carboamination of the alkyne function. Intermediates formed during the course of these reactions could be isolated (and are described below). 35Echavarren and co-workers have been responsible for a great resurgence of the chemistry of the first ever class of carbene complexes 36 prepared by Bonati and co-workers nearly 40 years ago. 37A series of hydrogen bond supported heterocyclic carbenes (HBHCs), derived from 2-pyridyl isocyanide (Chart 4), were shown to be active catalysts in reactions of 1,6-enynes. When comparing these to gold complexes bearing noncyclic carbene ligands derived from 4-pyridyl isocyanide (D and E), significantly different results were obtained that warrant mentioning: Effective catalysts to produce dienes by skeletal rearrangement contain the 2-pyridyl group. Catalytic activity of the catalyst that contains a 4-pyridyl group is very low, presumably reflecting intermolecular blocking of the active Au(I) centre by N-coordination. Whereas previously mentioned complexes like [(IPr)AuNCPh]+ favour exo-cyclisation, endo products are produced by certain HBHCs.Extremely high activity is found in related methoxycyclisations that have been studied in methanol; here, all carbenes attached to gold are open.4A hydrogen bond supported heterocyclic carbene complex, C, and two isomeric nitrogen acyclic carbene complexes of gold, D and E. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1776 Inspired in particular by the final result mentioned above, Echavarren and co-workers then studied the skeletal rearrangement in other cyclisations of 1,6-enynes employing 12 nitrogen acyclic carbene (NAC) gold complexes (27 and 28 in Scheme 7) prepared by reaction of a range of isocyanogold complexes with diethylamine or various primary amines.38Up to three different cyclisation products were obtained from a model enyne and the product ratios depended upon the substituents on the carbene. In general, selectivity can be high and the (NAC)gold(I) complexes are at least as reactive and selective as previously reported more elaborate carbene complexes of gold(I), and the yields and rates of conversions are higher than those achieved with comparable (HBHC)Au(I) catalysts. Finally, the nucleophilic addition of dibenzoylmethane to a 1,6-enyne was investigated to determine the electrophilicity of these ligands. Highly electrophilic catalysts bearing, for example, a bulky phosphine ligand preferentially afforded the product 29 (Scheme 8) as a result of stabilisation of the cationic character of the intermediate. All the NAC ligands studied reflected a highly donating character that is instrumental in preferentially facilitating the reaction via a carbenoic intermediate to form 30. 7Preparation of nitrogen acyclic carbene (NAC) gold complexes, 27 and 28, from secondary and primary amines, respectively. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1777 8Dibenzoylmethane addition to the cyclisation product of a 1,6-enyne modulated by p-acceptor ligands attached to gold(I) to afford 29, or by strong s-donor ligands to yield isomer 30. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1778 Several isonitrile gold complexes were also converted to (NAC)gold complexes by Hashmi and co-workers. 39Intermediate coordination Vinyl complexes of gold are assumed to form transiently during the activation and subsequent nucleophilic attack of alkynes and allenes, as illustrated in simplified form in Scheme 1. After Hashmi and co-workers 40 prepared a number of (vinyl)gold complexes by following the boronic acid route developed by Partyka et al. 41, namely three groups published other examples of such complexes in 2009. Liu and Hammond 42studied the cyclisation of allenoates with trialkylphosphine or triarylphosphine gold complexes in stoichiometric quantities and prepared stable complexes of type 31 (Scheme 9). Such compounds were then successfully used as transmetallation reagents in palladium-catalysed cross-coupling. 43While investigating the ‘seemingly simple’ catalytic cycle for the cycloisomerisation of allenic arenes employing the Gagosz catalyst, [Ph3PAuNTf2] (Scheme 10), 23 Gagné and co-workers isolated the intermediate (vinyl)gold complex 32. 44 Furthermore, they established that under catalytic conditions the catalyst rests as another intermediate, 33 (Figure 5). Earlier, theoretical results obtained by the research group of Toste and Morganelli indicated that such species are involved in the cycloisomerisation of enynes. 45 Subsequently, Hashmi et al. 46developed a method to isolate vinylgold species from propargylcarboxamides that contain an internal alkyne group using the cationic gold fragment [(IPr)Au]+ (F, Chart 5). The scope of this approach could be broadened to permit (vinyl)gold formation by 5-endo-dig cyclisation from an ortho-alkynylphenol and 2-(phenylethynyl)aniline,47 affording complexes 34 and 35 (Scheme 11). 9(Vinyl)gold complex 31, prepared from allenoates. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1780 10(Vinyl)gold complex 32, isolated from an allenic arene using a Gagosz-type catalyst. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1781 5Isolated dinuclear gold complex intermediate. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1783 11(Vinyl)gold complex formation from an alkynylphenol and an ethynylaniline to furnish 34 and 35, respectively. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1784 Using (CAAC)gold(I) complexes (see above and Figure 4), Bertrand and co-workers carried out related cyclisation reactions in attempts to isolate previously postulated three-coordinated gold intermediates. 35Two cationic (vinyl)gold complexes 36 and 37 were successfully isolated and authenticated (Scheme 12). 12Cyclic (alkyl)(amino)carbene gold(I) complexes 25 and 26 (compare with Figure 4) in the preparation of vinyl(gold) complexes 36 and 37 from compounds with alkyne and amine functionalities. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1785 Although they are often postulated as intermediates in homogeneous gold catalysis, very few gold carbene complexes that are not stabilised by one or two adjacent heteroatoms have been isolated and described in detail. 48,49 The formation of a free cationic carbene complex, [(L)Au=CHPh]+, 38 (featuring the same ligand L as in catalyst 6 with R = t-Bu), during the annulation of 1,6-enynes that carry OMe substituents at the propargyl position (Scheme 13), has been disclosed, and convincingly verified. 50The formation of the carbene complex is a consequence of a rare retro-cyclopropanation. Following a well-established route to prepare various carbene complexes of metals, including the Grubbs catalyst, 51,52 Fürstner and co-workers combined substituted cyclopropanes with a labile gold complex as shown in Scheme 14. 53 Both products obtained can best be described as thermally interconvertible geometric isomers of oxocarbenium cations with C=C double bonds located a to the gold atom and with their positive charges stabilised by two oxygen atoms. 13Formation of an unusual carbene complex, 38, during annulation of methoxy-containing 1,6-enynes. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1786 14Oxocarbenium cations, 39, obtained stoichiometrically from cyclopropanes and a cationic gold fragment. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1787 Note that the intermediate oxonium compound that was identified during the stoichiometric Hammond synthesis of gold-vinyl complexes 42 is related to Fürstner’s cationic compounds. Although not specifically mentioned by the former authors, both their intermediate compound and the various products can also be represented by carbene complex mesomeric structures (Scheme 15). The relatively high 13C NMR chemical shift (d 205) for the coordinated carbon atom in the one fully described neutral product concurs with at least some carbocationic carbene character.54 Unfortunately, the corresponding value for the intermediate has not been reported. Such ligated carbons in Fürstner’s compounds resonate at d values > 210. This result should not eclipse the fact that other experimental and physical data for these cationic compounds are fully compatible with those of gold-vinyl structures, as discussed above. 15Resonance structures for intermediate and product in the Hammond synthesis of vinyl(gold) complexes. http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1788 After metal slippage 55,56 and nucleophilic attack during cycloisomerisation of enynes (Scheme 1) the issue that emerges 57,58 is not whether so-called carbene complexes are formed. Aminocarbene complexes of the Fischer-type also carry significant positive charges on their nitrogen atoms and their C–N bonds exhibit significant barriers of rotation suggesting preponderance of a zwitterionic state, and still they are generally classified as carbene complexes. Questions of both a more fundamental and a practical nature remain: To what extent should metal–carbon double bond character be invoked when explaining reactivity and transition or intermediate state stabilisation or, alternatively, to what extent is the cationic charge delocalised? How could the different mesomeric contributions that constitute the delocalised representation in Figure 6 (compare again Scheme 1) be logically and selectively enhanced, and expressed, by the tailoring of ancillary ligands to allow discrimination between different available catalysed pathways while considering different modes of activation? As we have reported, important progress has already been made in this respect. Could suitable model gold complexes without heteroatoms in their alkyne-derived ligands or analogues thereof be prepared and studied in order to establish the contribution (or activity) of different canonical (or tautomeric) structures? Protodeactivation plays a key role in gold-mediated catalytic processes and catalyst regeneration. Experiments to determine the relative kinetic basicities of a series of neutral gold compounds were conducted by Roth and Blum.59 The gold-coordinated carbon basicities conform to the trend sp3 < sp < sp2 in (Ph3P)AuX with X equal to CH3 < t-BuC≡C < Ph < vinyl, which is different from the normal basicity trend of the related anions based on their pKb values (sp << sp2 < sp3). 60 Care should however be exercised in drawing such comparisons because of the important role that the solvent might play. Notably, precatalysts bearing the NHC ligand IPr, protodeaurate nearly twice as effectively as their phosphine analogues. Despite the unexplained results obtained for some heteroatom-containing (vinyl)gold compounds, these findings offer useful information applicable to the future design of catalysts. Conclusions Confining ourselves mostly to intramolecular addition reactions catalysed by gold(I) compounds, impressive developments in clarifying the consecutive changes in gold coordination during the total process have been described. Many more opportunities and challenges exist and these have to be addressed before the outcome of reactions through tuning of auxiliary ligands can be predicted with confidence. Total manipulation of such reactions will only be possible if the reaction mechanisms and, hence, the important intermediates that form are known, and their metal–ligand bonding is understood on a fundamental level. The presently developed frameworks and hypotheses should be rigorously challenged experimentally and theoretically in attempts to alter and refine them. Meticulous kinetic investigations should be a priority. Acknowledgements Financial support (to HGR) from the Oppenheimer Memorial Fund is gratefully acknowledged. 1.SchmidbaurHSchierA.Gold η2-coordination to unsaturated and aromatic hydrocarbons: The key step in gold-catalyzed organic transformations.201029327doi:10.1021/om900900u2.SchmidbaurHSchierA.Organogold chemistry. In: Crabtree RH, Mingos DMP, editors220062513083.SalviNBelpassiLTarantelliFOn the Dewar-Chatt-Duncanson model for catalytic gold(I) complexes20101672317239PMid:204680424.GorinDJTosteFDRelativistic effects in homogeneous gold catalysis2007446395403doi:10.1038/nature05592PMid:173775765.HashmiASKGold catalyzed organic reactions200710731803211doi:10.1021/cr000436xPMid:175809756.ArcadiAAlternative synthetic methods through new developments in catalysis by gold200810832663325doi:10.1021/cr068435dPMid:186517787.LiZBrouwerCHeCGold catalyzed organic transformations200810832393265doi:10.1021/cr068434lPMid:186137298.GorinDJSherryBDTosteFDLigand effects in homogeneous Au catalysis200810833513378doi:10.1021/cr068430gPMid:18652511PMCid:27548039. Jiménez-NúñezEEchavarrenAMGold-catalyzed cycloisomerizations of enynes: A mechanistic perspective200810833263350doi:10.1021/cr0684319PMid:1863677810. WidenhoeferRARecent developments in enantioselective gold(I) catalysis20081453825391doi:10.1002/chem.200800219PMid:1844203111.BelmontPParkerESilver and gold catalysis for cycloisomerization reactions200960756089doi:10.1002/ejoc.20090079012.FürstnerAGold and platinum catalysis – a convenient tool for generating molecular complexity20093832083221doi:10.1039/b816696jPMid:1984735213.NevadoC.Gold catalysis: Recent developments and future trends201064247251doi:10.2533/chimia.2010.24714.SenguptaSShiXRecent advances in asymmetric gold catalysis20102609619 doi:10.1002/cctc.201000070 15.GarciaPMalacriaMAubertCGandonVFensterbankLGold-catalysed cross-couplings: New opportunities for C–C bond formation20102493497doi:10.1002/cctc.200900319 16.De MendozaPEchavarrenAMSynthesis of arenes and heteroarenes by hydroarylation reactions catalyzed by electrophilic metal complexes201082801820doi:10.1351/PAC-CON-09-10-0617. WuJKrollPRaska DiasHVGold(I) chloride coordinated 3-hexyne200948423425doi:10.1021/ic8020854PMid:19072612 18. FlüggeSAnoopAGoddardRThielWFürstnerAStructure and bonding in neutral and cationic 14-electron gold alkyne π-complexes20091585588565doi:10.1002/chem.200901062PMid:1956914119.NechaevMSRayonVMFrenkingGEnergy partitioning analysis of the bonding in ethylene and acetylene complexes of group 6, 8 and 11 metals200410831343142doi:10.1021/jp031185+ 20. Garcia-MotaMCabelloNMaserasFEchavarrenMPerez-RamirezJLopezNSelective homogeneous and heterogeneous gold catalysis with alkynes and alkenes: Similar behaviour and different origin2008916241629doi:10.1002/cphc.200800246PMid:1853721921.AkanaJBhattacharyyaKXMüllerPSadighiJReversible C–F bond formation and the Au-catalyzed hydrofluorination of alkynes.J Am Chem Soc200712977367737doi:10.1021/ja0723784PMid:1754740922.GagoszFRecent developments in gold catalysis20096517571767doi:10.1016/j.tet.2008.12.04023.MezaillesNRicardLGagoszFPhosphine gold(I) bis(trifluoromethanesulfonyl)-imidate complexes as highly efficient and air-stable catalysts for the cycloisomerization of enynes2005741334136doi:10.1021/ol0515917PMid:1614637024.HooperTNGreenMRussellCACationic Au(I) complexes: Synthesis, structure and reactivity20104623132315doi:10.1039/b923900fPMid:2023494325.HashmiSAKLoosALittmannAet al.Gold(I) complexes of KITPHOS monophosphines: Efficient cycloisomerization catalysts2009351576582doi:10.1002/adsc.200800681 26.Nieto-OberhuberCPérez-GalánPHerrero-GómezEet al.Gold(I)-catalyzed intramolecular [4 + 2] cycloadditions of arylalkynes or 1,3-enynes with alkenes: Scope and mechanism2008130269279doi:10.1021/ja075794xPMid:1807617027.TrilloBLópezFMontserratSet alGold-catalyzed [4C + 3C] intramolecular cycloaddition of allene-dienes: Synthetic protocol and mechanistic implications20091533363339doi:10.1002/chem.200900164PMid:19229922 28.MauléonPZeldinRMGonzálezAZTosteFDLigand-controlled access to [4 + 2] and [4 + 3] cycloadditions in gold-catalyzed reactions of allene-dienes200913163486349doi:10.1021/ja901649sPMid:19378998PMCid:271163829.AlonsoITrilloBLópezFet alGold catalyzed [4C + 2C] cycloadditions of allenedienes, including an enantioselective version with new phosphoramidite-based catalysts: Mechanistic aspects of the divergence between [4C + 3C] and [4C + 2C] pathways20091311302013030doi:10.1021/ja905415rPMid:1969793630.BenitezDTkatchoukEGonzálezAZGoddard (III) WA, Toste FD. On the impact of steric and electronic properties of ligands on gold(I)-catalyzed cycloaddition reactions20091147984801doi:10.1021/ol9018002PMid:19780543PMCid:278358331.AlcarazoMStorkSAnoopAThielWFürstnerASteering the surprisingly modular π-acceptor properties of N-heterocyclic carbenes: Implications for gold catalysis20104925422546doi:10.1002/anie.200907194PMid:2020954932.GonzálezATosteFDGold(I)-catalyzed enantioselective [4 + 2]-cycloaddition of allene-dienes201012200203doi:10.1021/ol902622bPMid:19961192PMCid:279890633. GungBWCraftDTBaileyLNKirschbaumKGold-catalyzed transannular [4 + 3] cycloaddition reactions201016639644doi:10.1002/chem.200902185PMid:19937623 34. ZengXFreyGDKinjoRDonnadieuBBertrandGSynthesis of a simplified version of stable, bulky and rigid cyclic alkyl(amino) carbenes and catalytic activity of ensuing gold(I) complex in the three-component preparation of 1,2-dihydroquinoline derivatives200913186908696doi:10.1021/ja902051m PMid:19456108PMCid:2724870 35ZengXKinjoRDonnadieuBBertrandGSerendipitous discovery of the catalytic hydroammoniumation and methylamination of alkynes20104994294536.BartholoméCRamiroZPérez-GalánPet alGold(I) complexes with hydrogen-bond supported heterocyclic carbenes as active catalysts in reactions of 1,6-enynes2008471139111397doi:10.1021/ic801446vPMid:1894717837.MinghettiGBonatiFBis(carbene) complexes of gold(I) and gold(III)197354C62C6338.BartoloméCRamiroZGarcia-CuadradoDet alNitrogen acyclic gold(I) carbenes: Excellent and easily accessible catalysts in reactions of 1,6-enynes201029951956doi:10.1021/om901026m39.HashmiSAKHengstTLothschützCRomingerFNew and easily accessible nitrogen acyclic gold(I) carbenes: Structure and application in the gold-catalyzed phenol synthesis as well as the hydration of alkynes201035213151337doi:10.1002/adsc.20100012640.HashmiSAKRamamurthiTDRomingerFSynthesis, structure and reactivity of organogold compounds of relevance to homogeneous gold catalysis2009694592597doi:10.1016/j.jorganchem.2008.11.05441.PartykaDVZellerMHunterADGrayTGRelativistic functional groups: Aryl carbon-gold bond formation by selective transmetalation of boronic acids20064581888191doi:10.1002/anie.200603350PMid:1711144942.LiuL-PHammondGBReactions of cationic gold(I) with allenoates: Synthesis of stable organogold(I) complexes and mechanistic investigations on gold-catalyzed cyclizations2009412301236doi:10.1002/asia.200900091PMid:19472292 43.HashmiASKLothschützCDöppRRudolphMRamamarthiTDRomingerFGold and palladium combined for cross-coupling20094882438246doi:10.1002/anie.200902942PMid:1979021844.WeberDTarselliMAGagnéMRMechanistic surprises in the gold(I)-catalyzed intramolecular hydroarylation of allenes20094857335736doi:10.1002/anie.200902049PMid:19562821PMCid:297832945.CheongPHMorganelliPLuzungMRHoukKNTosteFDGold-catalyzed cycloisomerization of 1,5-allenynes via dual activation of an ene reaction200813045174526doi:10.1021/ja711058fPMid:18327944PMCid:2995695 46.HashmiASKSchusterARomingerFGold catalysis: Isolation of vinylgold complexes derived from alkynes20094882478249doi:10.1002/anie.200903134PMid:1976882247.HashmiASKRamamurthiTDRomingerFOn the trapping of vinylgold intermediates2010352971975doi:10.1002/adsc.201000011 48.StrasserCEStander-GroblerESchusterOCronjeSRaubenheimerHGPreparation of remote NHC complexes of rhodium(I) and gold(I) by ligand transfer200919051912doi:10.1002/ejic.20080118049.RaubenheimerHCronjeSCarbene complexes of gold: Preparation, medical application and bonding20083719982011doi:10.1039/b708636aPMid:1876284350.Solorio-AlvaradoCREchavarrenAMGold-catalyzed annulation/fragmentation: Formation of free gold carbenes by retro-cyclopropanation20101321188111883doi:10.1021/ja104743kPMid:2069852951. BingerPMüllerPBennRet al.Vinylcarbene complexes of titanocene198928610611doi:10.1002/anie.198906101 52.NguyenSTJohnsonLKGrubbsRHRing-opening metathesis polymerization (ROMP) of norbornene by a group VIII carbene complex in protic mediaJ Am Chem Soc199211439743975doi:10.1021/ja00036a053 53.SeidelGMynottRFürstnerAElementary steps of gold catalysis: NMR spectroscopy reveals the highly cationic character of a “gold carbenoid”.20094825102513doi:10.1002/anie.200806059PMid:19248070 54.SchusterOYangLRaubenheimerHGAlbrechtMBeyond conventional N-heterocyclic carbenes: Abnormal, remote, and other classes of NHC ligands with reduced heteroatom stabilization200910934453478doi:10.1021/cr8005087PMid:1933140855. RaubenheimerHGEsterhuysenMWTimoshkinAChenYFrenkingGElectrophilic addition of Ph3PAu+ to anionic alkoxy Fischer-type carbene complexes: A novel approach to metal stabilised bimetallic vinyl ether complexes20022131733181doi:10.1021/om020048g56. RaubenheimerHGEsterhuysenMWFrenkingGet al.Aurolysis of a-C-deprotonated group 6 aminocarbene and thiocarbene complexes with Ph3PAu+200645804589doi:10.1039/b607613kPMid:1701656957.FürstnerAMorencyLOn the nature of the reactive intermediates in gold-catalyzed cycloisomerization reactions20084750305033doi:10.1002/anie.200800934PMid:1850479358. HashmiSAK“High noon” in gold catalysis: Carbene versus carbocation intermediates 20084767546756doi:10.1002/anie.200802517PMid:1868317459.RothKEBlumSRelative kinetic basicities of organogold compounds20102917121716doi:10.1021/om901101f 60.SmithMBMarchJMarch’s advanced organic chemistry: Reactions, mechanisms and structure20013291http://www.sajs.co.za/index.php/SAJS/article/downloadSuppFile/459/1774