Kinetics and Mechanism of the Gold-Catalyzed Hydroamination of 1,1-Dimethylallene with N-Methylaniline

The mechanism of the intermolecular hydroamination of 3-methylbuta-1,2-diene ( 1 ) with N-methylaniline ( 2 ) catalyzed by (IPr)AuOTf has been studied by employing a combination of kinetic analysis, deuterium labelling studies, and in situ spectral analysis of catalytically active mixtures. The results of these and additional experiments are consistent with a mechanism for hydroamination involving reversible, endergonic displacement of N-methylaniline from [(IPr)Au(NHMePh)]⁺ ( 4 ) by allene to form the cationic gold π-C1,C2-allene complex [(IPr)Au(η²-H₂C=C=CMe₂)]⁺ ( I ), which is in rapid, endergonic equilibrium with the regioisomeric π-C2,C3-allene complex [(IPr)Au(η²-Me₂C=C=CH₂)]⁺ ( I′ ). Rapid and reversible outer-sphere addition of 2 to the terminal allene carbon atom of I′ to form gold vinyl complex (IPr)Au[C(=CH₂)CMe₂NMePh] ( II ) is superimposed on the slower addition of 2 to the terminal allene carbon atom of I to form gold vinyl complex (IPr)Au[C(=CMe₂)CH₂NMePh] ( III ). Selective protodeauration of III releases N-methyl-N-(3-methylbut-2-en-1-yl)aniline ( 3 a ) with regeneration of 4 . At high conversion, gold vinyl complex II is competitively trapped by an (IPr)Au⁺ fragment to form the cationic bis(gold) vinyl complex {[(IPr)Au]₂[C(=CH₂)CMe₂NMePh]}⁺ ( 6 ).     

Competitive Gold‐Activation Modes in Terminal Alkynes: An Experimental and Mechanistic Study

The competition between π‐ and dual σ,π‐gold‐activation modes is revealed in the gold(I)‐catalyzed heterocyclization of 1‐(o‐ethynylaryl)urea. A noticeable effect of various ligands in gold complexes on the choice of these activation modes is described. The cationic [Au(IPr)]⁺ (IPr=2,6‐bis(diisopropylphenyl)imidazol‐2‐ylidene) complex cleanly promotes the π activation of terminal alkynes, whereas [Au(PtBu₃)]⁺ favors intermediate σ,π species. In this experimental and mechanistic study, which includes kinetic and cross‐over experiments, several σ‐gold, σ,π‐gold, and other gold polynuclear reaction intermediates have been isolated and identified by NMR spectroscopy, X‐ray diffraction, or MALDI spectrometry. The ligand control in the simultaneous or alternative π‐ and σ,π‐activation modes is also supported by deuterium‐labeling experiments.     

{Au(IPr)}2(μ-OH)]X complexes: synthetic, structural and catalytic studies

The synthesis of a series of dinuclear gold hydroxide complexes has been achieved. These complexes of type [{Au(IPr)}₂(μ‐OH)]X (X=BF₄, NTf₂, OTf, FABA, SbF₆; IPr=2,6‐bis(disopropylphenyl)imidazol‐2‐ylidene; NTf₂=bis(trifluoromethanesulfonyl)imidate; OTf=trifluoromethanesulfonate; FABA=tetrakis(pentafluorophenyl)borate) are easily formed in the presence of water and prove highly efficient in the catalytic hydration of nitriles. Their facile formation in aqueous media suggests they are of relevance in gold‐catalyzed reactions involving water. Additionally, a series of [Au(IPr)(NCR)][BF₄] (R=alkyl, aryl) complexes was synthesized as they possibly occur as intermediates in the catalytic reaction mechanism. ¹H and ¹³C NMR data as well as key bond lengths obtained by X‐ray diffraction studies are compared and reveal an interesting structure–activity relationship. The collected data indicate a negligible effect of the nature of the nitrile on the reactivity of [Au(L)(NCR)][X] complexes in catalysis.     

Two thiourea‐containing gold(I) complexes

The crystal structures of two salts of bis­(thio­urea)­gold(I) complexes, namely bis­(thio­urea‐κS)­gold(I) chloride, [Au(CH₄N₂S)₂]Cl, (I), and bis­[bis­(thio­urea‐κS)­gold(I)] sulfate, [Au(CH₄N₂S)₂]₂SO₄, (II), have been determined. The chloride salt, (I), is isomorphous with the corresponding bromide salt, although there are differences in the bonding. The Au^I ion is located on an inversion centre and coordinated by two symmetry‐related thio­urea ligands through the lone pairs on their S atoms [Au—S 2.278 (2) Å and Au—S—C 105.3 (2)°]. The sulfate salt, (II), crystallizes with four independent [Au(CH₄N₂S)₂]⁺ cations per asymmetric unit, all with nearly linear S—Au—S bonding. The cations in (II) have similar conformations to that found for (I). The Au—S distances range from 2.276 (3) to 2.287 (3) Å and the Au—S—C angles from 173.5 (1) to 177.7 (1)°. These data are relevant in interpreting different electrochemical processes where gold–thio­urea species are formed.     

The Bis(ferrocenyl)phosphenium Ion Revisited

The bis(ferrocenyl)phosphenium ion, [Fc₂P]⁺, reported by Cowley et al. (J. Am. Chem. Soc. 1981 , 103, 714–715), was the only claimed donor‐free divalent phosphenium ion. Our examination of the molecular and electronic structure reveals that [Fc₂P]⁺ possesses significant intramolecular Fe???P contacts, which are predominantly electrostatic and moderate the Lewis acidity. Nonetheless, [Fc₂P]⁺ undergoes complex formation with the Lewis bases PPh₃ and IPr to give the donor–acceptor complexes [Fc₂P(PPh₃)]⁺ and [Fc₂P(IPr)]⁺ (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazole‐2‐ylidene).     

Decarboxylation of aromatic carboxylic acids by gold(I)-N-heterocyclic carbene (NHC) complexes

A recently isolated gold(I) complex, [Au(IPr)(OH)], permits the transformation of carboxylic acids to the corresponding decarboxylated gold(I)-aryl complex without the use of silver co-catalyst under mild reaction conditions.     

Synthesis,Characterization, and Reactivity of Cationic Gold Diarylallenylidene Complexes

Methoxide abstraction from gold acetylide complexes of the form (L)Au[η¹‐C≡CC(OMe)ArAr′] (L=IPr, P(^tBu)₂(ortho‐biphenyl); Ar/Ar′=C₆H₄X where X=H, Cl, Me, OMe) with trimethylsilyl trifluoromethanesulfonate (TMSOTf) at −78 °C resulted in the formation of the corresponding cationic gold diarylallenylidene complexes [(L)Au=C=C=CArAr′]⁺ OTf⁻ in ≥85±5 % yield according to ¹H NMR analysis. ¹³C NMR and IR spectroscopic analysis of these complexes established the arene‐dependent delocalization of positive charge on both the C1 and C3 allenylidene carbon atoms. The diphenylallenylidene complex [(IPr)Au=C=C=CPh₂]⁺ OTf⁻ reacted with heteroatom nucleophiles at the allenylidene C1 and/or C3 carbon atom.     

Synthesis and crystal structure of 2‐amino‐3‐hydroxypyridinium dioxido(pyridine‐2,6‐dicarboxylato‐κ3O2,N,O6)vanadate(V) and its conversion to nanostructured V2O5

2‐Amino‐3‐hydroxypyridinium dioxido(pyridine‐2,6‐dicarboxylato‐κ³O²,N,O⁶)vanadate(V), (C₅H₇N₂O)[V(C₇H₃NO₄)O₂] or [H(amino‐3‐OH‐py)][VO₂(dipic)], (I), was prepared by the reaction of VCl₃ with dipicolinic acid (dipicH₂) and 2‐amino‐3‐hydroxypyridine (amino‐3‐OH‐py) in water. The compound was characterized by elemental analysis, IR spectroscopy and X‐ray structure analysis, and consists of an anionic [VO₂(dipic)]⁻ complex and an H(amino‐3‐OH‐py)⁺ counter‐cation. The V^V ion is five‐coordinated by one O,N,O′‐tridentate dipic dianionic ligand and by two oxide ligands. Thermal decomposition of (I) in the presence of polyethylene glycol led to the formation of nanoparticles of V₂O₅. Powder X‐ray diffraction (PXRD) and scanning electron microscopy (SEM) were used to characterize the structure and morphology of the synthesized powder.     

Observing Initial Steps in Gold‐Catalyzed Alkyne Transformations by Utilizing Bodipy‐Tagged Phosphine–Gold Complexes

The Pd‐catalyzed reactions of 3‐chloro‐bodipy with R₂PH (R=Ph, Cy) provide nonfluorescent bodipy–phosphines 3‐PR₂–bodipy 3 a (R=Ph) and 3 b (R=Cy; quantum yield Φ<0.001). Metal complexes such as [AgCl( 3 b )] and [AuCl( 3 b )] were prepared and shown to display much higher fluorescence (Φ=0.073 and 0.096). In the gold complexes, the level of fluorescence was found to be qualitatively correlated with the electron density at gold. Consequently, the fluorescence brightness of [AuCl( 3 b )] increases when the chloro ligand is replaced by a weakly coordinating anion, whereas upon formation of the electron‐rich complex [Au(SR)( 3 b )] the fluorescence is almost quenched. Related reactions of [AuCl( 3 b )] with [Ag]ONf)] (Nf= nonaflate) and phenyl acetylenes enable the tracking of initial steps in gold‐catalyzed reactions by using fluorescence spectroscopy. Treatment of [AuCl( 3 b )] with [Ag(ONf)] gave the respective [Au(ONf)( 3 b )] only when employing more than 2.5 equivalents of silver salt. The reaction of the “cationic” gold complex with phenyl acetylenes leads to the formation of the respective dinuclear cationic [{( 3 b )Au}₂(CCPh)]⁺ and an increase in the level of fluorescence. The rate of the reaction of [Au(ONf)( 3 b )] with PhCCH depends on the amount of silver salt in the reaction mixture; a large excess of silver salt accelerates this transformation. In situ fluorescence spectroscopy thus provides valuable information on the association of gold complexes with acetylenes.     

The Effect of Hard and Soft Donors on Structural Motifs in (Isocyanide)gold(I) Complexes

Treatment of the (isocyanide)gold(I) species LAuCl (L=^tBuNC, 2,6‐Me₂C₆H₃NC) with 4‐mercaptobenzoic acid in the presence of NaOMe yields the complexes [Au(4‐SC₆H₄CO₂H)L] in good yield. Reaction of LAuCl with 2‐HSQn (Qn=quinoline) and 2‐HSPy (Py=pyridine) under the same conditions provides the thiolato compounds [Au(2‐SQn)L] and [Au(2‐SPy)L], respectively. A structural investigation of the pyridylthiolato compound revealed chains of molecules with alternating medium and long Au−Au interactions. Treatment of this compound with HBF₄ results in the cationic species [Au(2‐HSPy)(2,6‐Me₂C₆H₃NC)]⁺ as the BF₄⁻ salt. The same product is obtained on reaction of [AuCl(2,6‐Me₂C₆H₃NC)] with AgOTf followed by HSPy. Treatment of the gold(I) halide compounds LAuCl (L=^tBuNC, 2,6‐Me₂C₆H₃NC) with potassium 1,3,4‐thiadiazole‐2,5‐dithiolate (KSSSK) leads to the isolation of dinuclear thiolatogold complexes [(AuL)₂(SSS)]. These products go on to form insoluble polymers through loss of isocyanide on standing in solution. A single crystal of [{Au(^tBuNC)}₂(SSS)] was obtained and the subsequent structural analysis revealed one of the most complicated networks known based solely on aurophilic interactions. A good comparison to the ‘soft' S‐donation of the thiolato ligands was provided by the synthesis of a number of nitratogold(I)complexes with the anion bound through the ‘hard' O‐donor. Reaction of ⁱPrNC and CyNC with Au(tht)Cl provided the complexes [AuCl(ⁱPrNC)] and [AuCl(CyNC)], respectively. These compounds were found to yield the respective nitrato species [Au(NO₃)ⁱPrNC)] and [(Au(NO₃)(CyNC)] on treatment with AgNO₃. The nitrato complexes yielded single crystals enabling a structural investigation to be carried out. While [Au(NO₃)(CyNC)] has a more conventional structure with dimers aligned into strings with alternating short and long aurophilic bonding, [Au(NO₃)(ⁱPrNC)] has a unique structure based on strings of alternating, corner‐sharing Au₆ and Au₈ units with short Au−Au contacts in edge‐sharing Au₃ triangles.     

Synthesis,Characterization, and Reactivity of Cationic Gold Diarylallenylidene Complexes

Methoxide abstraction from gold acetylide complexes of the form (L)Au[η¹‐C≡CC(OMe)ArAr′] (L=IPr, P(^tBu)₂(ortho‐biphenyl); Ar/Ar′=C₆H₄X where X=H, Cl, Me, OMe) with trimethylsilyl trifluoromethanesulfonate (TMSOTf) at ?78 °C resulted in the formation of the corresponding cationic gold diarylallenylidene complexes [(L)Au=C=C=CArAr′]⁺ OTf^? in ≥85±5 % yield according to ¹H NMR analysis. ¹³C NMR and IR spectroscopic analysis of these complexes established the arene‐dependent delocalization of positive charge on both the C1 and C3 allenylidene carbon atoms. The diphenylallenylidene complex [(IPr)Au=C=C=CPh₂]⁺ OTf^? reacted with heteroatom nucleophiles at the allenylidene C1 and/or C3 carbon atom.     

Cationic NHC-gold(I) complexes: Synthesis, isolation, and catalytic activity

The reaction of [(NHC)AuCl] complexes (NHC = N-heterocyclic carbene) with a chloride abstractor of the type AgX, where X is a non-coordinating anion, led, in the presence of a neutral coordinating solvent S, to a series of cationic gold(I) complexes of formulae [(NHC)Au(S)]X. Hence, different cationic NHC-gold(I) species bound to acetonitrile, pyridine, 2-Br-pyridine, 3-Br-pyridine, norbornadiene, and THF could be synthesized and characterized by ¹H and ¹³C NMR spectroscopies. Among these, the results of X-ray diffraction studies for [(IPr)Au(NCMe)]SbF₆, [(IAd)Au(NCMe)]PF₆, [(IPr)Au(pyr)]PF₆, [(IPr)Au(2-Br-pyr)]PF₆, [(IPr)Au(3-Br-pyr)]PF₆ are discussed. As special feature, the structure of [(IPr)Au(2-Br-pyr)]PF₆ presented a secondary interaction between the gold and bromine atoms. Additionally, while attempting to obtain crystals of [(IPr)Au(nbd)]PF₆, we crystallized a decomposition product featuring a very rare anion as bridging ligand with formulae [(μ-PF₄)((IPr)Au)₂]PF₄. The observation of a possible P-F bond activation has important implications for cationic Au-based homogeneous catalysis. Finally, we compared the catalytic activities of the different cationic [(NHC)Au(S)]X complexes in the allylic acetate rearrangement reaction and notably observed the inertness of pyridine-based catalysts.     

Study of the Interaction of Some Potential Anticancer Gold(III) Complexes with Biologically Important Thiols Using NMR,UV–Vis,and Electrochemistry

The interaction of gold(III) complexes [Au(en)Cl₂]Cl, [Au(en)₂]Cl₃, [Au(cis‐DACH)Cl₂]Cl, and [Au(cis‐DACH)₂]Cl₃ (en = ethylenediamine, DACH = cis‐1,2‐diaminocyclohexane) with biologically important thiols, such as glutathione (GSH), dl ‐penicillamine (PSH), mercaptoacetic acid (MAA), and N‐(2‐mercaptopropionyl)glycine (MPG), has been studied using ¹H, ¹³C NMR, UV–vis spectroscopy and electrochemistry in aqueous solution. Kinetic data revealed that the reactivity of their substitution reaction followed the order: [Au(en)Cl₂]⁺ > [Au(en)₂]³⁺ > [Au(cis‐DACH)Cl₂]⁺ > [Au(cis‐DACH)₂]³⁺. The thiol reactivity increased with decreasing its size, viz. MAA ≫ MPG > PSH > GSH. Square wave stripping voltammetry displayed peaks for Au(III) and Au(I) at +0.875 V and +1.4 V respectively. The interaction of the complexes with thiols resulted in reduction of gold(III) to gold(I) and thiol ligands (RSH) were oxidized to disulfide (RSSR).     

Hydride Transfer to Gold: Yes or No? Exploring the Unexpected Versatility of Au⋅⋅⋅H−M Bonding in Heterobimetallic Dihydrides

The potential for coordination and H-transfer from Cp₂MH₂ (M=Zr, W) to gold(I) and gold(III) complexes was explored in a combined experimental and computational study. [(L)Au]⁺ cations react with Cp₂WH₂ giving [(L)Au(κ²-H₂WCp₂)]⁺ (L=IPr ( 1 ), cyclic (alkyl)(amino)carbene ( 2 ), PPh₃ ( 3 ) and Dalphos-Me ( 4 ) [IPr=1,3-bis(diisopropylphenyl)imidazolylidene; Dalphos-Me=di(1-adamantyl)-2-(dimethylamino)phenyl-phosphine], while [Au(DMAP)₂]⁺ (DMAP=p-dimethylaminopyridine) affords the C₂-symmetric [Au(κ-H₂WCp₂)₂]⁺ ( 5 ). The Dalphos complex 4 can be protonated to give the bicationic adduct 4 H, showing Au^I⋅⋅⋅H⁺−N hydrogen bonding. The gold(III) Lewis acid [(C^N−CH)Au(C₆F₅)(OEt₂)]⁺ binds Cp₂WH₂ to give an Au-H-W σ-complex. By contrast, the pincer species [(C^N^C)Au]⁺ adds Cp₂WH₂ by a purely dative W→Au bond, without Au⋅⋅⋅H interaction. The biphenylyl-based chelate [(C^C)Au]⁺ forms [(C^C)Au(μ-H)₂WCp₂]⁺, with two 2-electron-3-centre W−H⋅⋅⋅Au interactions and practically no Au−W donor acceptor contribution. In all these complexes, strong but polarized W−H bonds are maintained, without H-transfer to gold. On the other hand, the reactions of Cp₂ZrH₂ with gold complexes led in all cases to rapid H-transfer and formation of gold hydrides. Relativistic DFT calculations were used to rationalize the striking reactivity and bonding differences in these heterobimetallic hydride complexes along with an analysis of their characteristic NMR parameters and UV/Vis absorption properties.     

Acridine N-Heterocyclic Carbene Gold(I) Compounds: Tuning from Yellow to Blue Luminescence

The synthesis and the luminescence features of three gold(I)-N-heterocyclic carbene (NHC) complexes are presented to study how the n-alkyl group can influence the luminescence properties in the crystalline state. The mononuclear gold(I)-NHC complexes, [( L¹ )Au(Cl)] ( 1 ), [( L² )Au(Cl)] ( 2 ), and [( L³ )Au(Cl)] ( 3 ) were isolated from the reactions between [(tht)AuCl] and corresponding NHC ligand precursors, [N-(9-acridinyl)-N’-(n-butyl)-imidazolium chloride, ( L¹ .HCl)], [N-(9-acridinyl)-N’-(n-pentyl)-imidazolium chloride, ( L² .HCl)] and [N-(9-acridinyl)-N’-(n-hexyl)-imidazolium chloride, ( L³ .HCl)]. Their single-crystal X-ray analysis reveals the influence of the n-alkyl groups on solid-state packing. A comparison of the luminescence features of 1 – 3 with n-alkyl substituents is explored. The molecules 1 – 3 depicted blue emission in the solution state, while the yellow emission (for 1 ), greenish-yellow emission (for 2 ), and blue emission (for 3 ) in the crystalline phase. This paradigm emission shift arises from n-butyl to n-pentyl and n-hexyl in the crystalline state due to the carbon-carbon rotation of the n-alkyl group, which tends to promote unusual solid packing. Hence n-alkyl group adds a novel emission property in the crystalline state. Density Functional Theory and Time-Dependent Density Functional Theory calculations were carried out for monomeric complex, N-(9-acridinyl)-N’-(n-heptyl)imidazole-2-ylidene gold(I) chloride and dimeric complex, N-(9-acridinyl)-N’-(n-heptyl)imidazole-2-ylidene gold(I) chloride to understand the structural and electronic properties.     

The march of progress: structures of the adducts of potassium dicyanidoaurate(I) with 2,2′‐bipyridyl (redetermination) and 1,10‐phenanthroline

The structure of the 2,2′‐bipyridyl adduct poly[(μ₂‐2,2′‐bipyridyl‐κ³N ,N ′:N )di‐μ₃‐cyanido‐κ⁶C :N :N‐gold(I)potassium(I)], [AuK(CN)₂(C₁₀H₈N₂)]_n , ( 1 ) (space group P 2₁), has been redetermined [previous determination: Jones et al. (1980). Acta Cryst . B 36 , 160–162]. The bipyridyl ligands coordinate only the potassium ion, which has a coordination sphere consisting of seven N‐atom donors; gold(I) remains in the form of linear dicyanidoaurate(I) ions. The extended structure consists of layers in which the Au^I atoms form chains parallel to the short a axis, with Au…Au contacts of 3.7286 (1) Å, whereas the chains of potassium ions, which are also parallel to a , are bridged by bipyridyl and dicyanidoaurate residues. The analogous 1,10‐phenanthroline adduct, namely poly[di‐μ₃‐cyanido‐κ⁶C :N :N‐(μ₂‐1,10‐phenanthroline‐κ³N ,N ′:N )gold(I)potassium(I)], [AuK(CN)₂(C₁₂H₈N₂)]_n , ( 2 ), crystallizes as nonmerohedral twins in the space group C 2/c . The packing is closely related to that of ( 1 ), but the chains are now parallel to the short b axis and the layers are parallel to (10). The two independent Au^I atoms occupy special positions on inversion centres and twofold axes; the Au…Au contacts are 3.6771 (2) Å.     

Coordination Chemistry of Gold Catalysts in Solution: A Detailed NMR Study

Coordination chemistry of gold catalysts bearing eight different ligands [L=PPh₃, JohnPhos (L2), Xphos (L3), DTBP, IMes, IPr, dppf, S‐tolBINAP (L8)] has been studied by NMR spectroscopy in solution at room temperature. Cationic or neutral mononuclear complexes LAuX (L=L2, L3, IMes, IPr; X=charged or neutral ligand) underwent simple ligand exchange without giving any higher coordinate complexes. For L2AuX the following ligand strength series was determined: MeOH?hex‐3‐yne <MeCN≈OTf^??Me₂S<2,6‐lutidine<4‐picoline<CF₃CO₂^?≈DMAP<TMTU<PPh₃<OH^?≈Cl^?. Some heteroligand complexes DTBPAuX exist in solution in equilibrium with the corresponding symmetrical species. Binuclear complexes dppf(AuOTf)₂ and S‐tolBINAP(AuOTf)₂ showed different behavior in exchange reactions with ligands depending on the ligand strength. Thus, PPh₃ causes abstraction of one gold atom to give mononuclear complexes LLAuPPh₃⁺ and (Ph₃P)_nAu⁺, but other N and S ligands give ordinary dicationic species LL(AuNu)₂²⁺. In reactions with different bases, LAu⁺ provided new oxonium ions whose chemistry was also studied: (DTBPAu)₃O⁺, (L2Au)₂OH⁺, (L2Au)₃O⁺, (L3Au)₂OH⁺, and (IMesAu)₂OH⁺. Ultimately, formation of gold hydroxide LAuOH (L=L2, L3, IMes) was studied. Ligand‐ or base‐assisted interconversions between (L2Au)₂OH⁺, (L2Au)₃O⁺, and L2AuOH are described. Reactions of dppf(AuOTf)₂ and S‐tolBINAP(AuOTf)₂ with bases provided more interesting oxonium ions, whose molecular composition was found to be [dppf(Au)₂]₃O₂²⁺, L8(Au)₂OH⁺, and [L8(Au)₂]₃O₂²⁺, but their exact structure was not established. Several reactions between different oxonium species were conducted to observe mixed heteroligand oxonium species. Reaction of L2AuNCMe⁺ with S^2? was studied; several new complexes with sulfide are described. For many reversible reactions the corresponding equilibrium constants were determined.     

Stable Anticancer Gold(III)–Porphyrin Complexes: Effects of Porphyrin Structure

In the design of physiologically stable anticancer gold(III) complexes, we have employed strongly chelating porphyrinato ligands to stabilize a gold(III) ion [Chem. Commun. 2003 , 1718; Coord. Chem. Rev. 2009 , 253, 1682]. In this work, a family of gold(III) tetraarylporphyrins with porphyrinato ligands containing different peripheral substituents on the meso‐aryl rings were prepared, and these complexes were used to study the structure–bioactivity relationship. The cytotoxic IC₅₀ values of [Au(Por)]⁺ (Por=porphyrinato ligand), which range from 0.033 to >100 μM , correlate with their lipophilicity and cellular uptake. Some of them induce apoptosis and display preferential cytotoxicity toward cancer cells than to normal noncancerous cells. A new gold(III)–porphyrin with saccharide conjugation [Au(4‐glucosyl‐TPP)]Cl ( 2 a ; H₂(4‐glucosyl‐TPP)=meso‐tetrakis(4‐β‐D ‐glucosylphenyl)porphyrin) exhibits significant cytostatic activity to cancer cells (IC₅₀=1.2–9.0 μM ) without causing cell death and is much less toxic to lung fibroblast cells (IC₅₀>100 μM ). The gold(III)–porphyrin complexes induce S‐phase cell‐cycle arrest of cancer cells as indicated by flow cytometric analysis, suggesting that the anticancer activity may be, in part, due to termination of DNA replication. The gold(III)–porphyrin complexes can bind to DNA in vitro with binding constants in the range of 4.9×10⁵ to 4.1×10⁶ dm³ mol^?1 as determined by absorption titration. Complexes 2 a and [Au(TMPyP)]Cl₅ ( 4 a ; [H₂TMPyP]⁴⁺=meso‐tetrakis(N‐methylpyridinium‐4‐yl)porphyrin) interact with DNA in a manner similar to the DNA intercalator ethidium bromide as revealed by gel mobility shift assays and viscosity measurements. Both of them also inhibited the topoisomerase I induced relaxation of supercoiled DNA. Complex 4 a , a gold(III) derivative of the known G‐quadruplex‐interactive porphyrin [H₂TMPyP]⁴⁺, can similarly inhibit the amplification of a DNA substrate containing G‐quadruplex structures in a polymerase chain reaction stop assay. In contrast to these reported complexes, complex 2 a and the parental gold(III)–porphyrin 1 a do not display a significant inhibitory effect (<10 %) on telomerase. Based on the results of protein expression analysis and computational docking experiments, the anti‐apoptotic bcl‐2 protein is a potential target for those gold(III)–porphyrin complexes with apoptosis‐inducing properties. Complex 2 a also displays prominent anti‐angiogenic properties in vitro. Taken together, the enhanced stabilization of the gold(III) ion and the ease of structural modification render porphyrins an attractive ligand system in the development of physiologically stable gold(III) complexes with anticancer and anti‐angiogenic activities.     

Three-Coordinate Rhodium Complexes in Low Oxidation States

The isolation of simultaneously low-coordinate and low-valent compounds is a timeless challenge for preparative chemists. This work showcases the preparation and full characterization of tri-coordinate rhodium(-I) and rhodium(0) complexes as well as a rare rhodium(I) complex. Reduction of [{Rh(μ-Cl)(IPr)(dvtms)}₂] ( 1 , IPr=1,3-bis(2,6-diisopropylphenyl)imidazolyl-2-ylidene; dvtms=divinyltetramethyldisiloxane) with KC₈ gave the trigonal complexes K[Rh(IPr)(dvtms)] and [Rh(IPr)(dvtms)], whereas the cation [Rh(IPr)(dvtms)]⁺ results from their oxidation or by abstraction of chloride from 1 with silver salts. The paramagnetic Rh⁰ complex is a unique fully metal-centered radical with the unpaired electron in the d_z2 orbital. The Rh(-I) complex reacts with PPh₃ with replacement of the NHC ligand, and behaves as a nucleophile, which upon reaction with [AuCl(PPh₃)] generates the trigonal pyramidal complex [(IPr)(dvtms)Rh-Au(PPh₃)] with a metal–metal bond between two d¹⁰ metal centers.     

Competitive Gold‐Promoted Meyer–Schuster and oxy‐Cope Rearrangements of 3‐Acyloxy‐1,5‐enynes: Selective Catalysis for the Synthesis of (+)‐(S)‐γ‐Ionone and (−)‐(2S,6 R)‐cis‐γ‐Irone

We report a simple, highly stereoselective synthesis of (+)‐(S)‐γ‐ionone and (‐)‐(2S,6R)‐cis‐γ‐irone, two characteristic and precious odorants; the latter compound is a constituent of the essential oil obtained from iris rhizomes. Of general interest in this approach are the photoisomerization of an endo trisubstituted cyclohexene double bond to an exo vinyl group and the installation of the enone side chain through a [(NHC)Au^I]‐catalyzed Meyer–Schuster‐like rearrangement. This required a careful investigation of the mechanism of the gold‐catalyzed reaction and a judicious selection of reaction conditions. In fact, it was found that the Meyer–Schuster reaction may compete with the oxy‐Cope rearrangement. Gold‐based catalytic systems can promote either reaction selectively. In the present system, the mononuclear gold complex [Au(IPr)Cl], in combination with the silver salt AgSbF₆ in 100:1 butan‐2‐one/H₂O, proved to efficiently promote the Meyer–Schuster rearrangement of propargylic benzoates, whereas the digold catalyst [{Au(IPr)}₂(μ‐OH)][BF₄] in anhydrous dichloromethane selectively promoted the oxy‐Cope rearrangement of propargylic alcohols.