Gold cations: generation

The acyl cation (acylium ion, oxocarbonium ion) is a resonance hybrid of the two main contributing structures 192 and 192a. The importance of structure 192a is indicated, for instance, by the high frequency carbonyl absorption (2200-2300 cm-1) observed in acyl cations generated from acyl halides and Lewis acids (Olah et al., 1962, 1963 Bethell and Gold, 1967). 

Other nucleophiles that have been used in this context are acetylides (alkynes). The addition of those to iminium cations generated in situ from aldehydes and secondary amines accomplishes a gold(III)-catalyzed three-component coupling for the synthesis of propargylamines, as can be observed in equation (124). The reactions are performed in water or in tetrahydrofuran (THF) when supported catalysts are employed.Chiral prolinol derivatives as... 

The easiest reactions are those in which the nucleophile is the gold-activated species. Examples of this are Au(I)-catalyzed carbene and nitrene transfers (equations 142 and 143) that convert olefins into cyclopropanes or aziridines, respectively. In the carbene transfer, ethyl diazoacetate is the source of carbene and the active NHC-gold cationic catalyst is generated by chloride abstraction with sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate NaBAT4. The cyclopropanation is competitive with other carbene insertions with active C H or N H bonds present in the substrate. For the aziridinations of olefins, nitrene formation is accomplished by the oxidation of sulfonamides with PhI(OAc)2 and the catalyst of choice is a gold-(I) triflate with a terpyridine ligand. 

Acetylenic ketones are viable substrates as well for gold-catalyzed cyclizations to furans. Whereas alk-3-yn-l-ones readily cyclize to substituted furans in the presence of gold(I) or gold(III) chloride (possibly via isomerization to a-allenyl ketones see Section 5.2), the corresponding transformation of alk-4-yn-l-ones takes place in the presence of the cationic gold catalyst generated in situ from PhsPAuCl and AgOTf in toluene (Scheme 4-83). The cyclization is accelerated by... 

More recently, a new method of assembling multilayers of PB on surfaces has been described.110 In contrast to the familiar process of self-assembly, which is spontaneous and leads to single monolayers, directed assembly is driven by the experimenter and leads to extended multilayers. In a proof-of-concept experiment, the generation of multilayers of Prussian blue (and the mixed Fein/Run analog ruthenium purple) on gold surfaces by exposing them alternately to positively charged ferric cations and [Fe(CN)6]4- or [Ru(CN)f,]4 anions has been demonstrated.110... 

By media variables we mean the solvent, electrolyte, and electrodes employed in electrochemical generation of excited states. The roles which these play in the emissive process have not been sufficiently investigated. The combination of A vV-dimethylformamide, or acetonitrile, tetra-n-butylammonium perchlorate and platinum have been most commonly reported because they have been found empirically to function well. Despite various inadequacies of these systems, however, relatively little has been done to find and develop improved conditions under which emission could be seen and studied. Electrochemiluminescence emission has also been observed in dimethyl sulfite, propylene carbonate, 1,2-dimethoxyethane, trimethylacetonitrile, and benzonitrile.17 Recently the last of these has proven very useful for stabilizing the rubrene cation radical.65,66 Other electrolytes that have been tried are tetraethylam-monium bromide and perchlorate1 and tetra-n-butylammonium bromide and iodide.5 Emission has also been observed with gold,4 mercury,5 and transparent tin oxide electrodes,9 but few studies have yet been made1 as to the effects of electrode construction and orientation on the emission character. 

Since the metal center in gold complexes usually displays acid character, Au- -NM contacts will be more easily formed as the basicity of the non-metal increases. Thus, there are many gold- -halogen weak interactions between cationic gold complexes and halides or anions of which the halogen forms part, but most are individual contacts between ions that do not generate supramolecular structures. For example, about 50% of Au- -Cl contacts are isolated cation- -anion interactions. 

Reactions by Other Nucleophiles As in the case of the formal cycloadditions of alkenes to allyl cations, the addition of alkenes to gold(I)-activated allenes generates intermediates that determine which cycloaduct formed. Based on this hypothesis, Toste et al. recently developed enantiorich bicycle-[3.2.0] structures by [2+2]-cycloaddition reaction catalyzed by chiral biarylphosphinegold(I) complexes [51]. 

Research in this area was developed further by Li et at. using 1,3-dienes 64 for the gold-catalyzed annulation of phenols and naphtols [58, 59]. These generated various dihydrobenzofuran derivatives. The best yields were achieved when the catalytic system included enough AuCl3 and silver salt to remove halogen atoms and deliver cationic gold. 

Alkali metal anions have also been generated as a result of cryptand stabilization of the corresponding cation. Cryptands were found to enhance the solubility of zerovalent alkali metals in various organic solvents.156-157 Initially, the solutions apparently contain the cryptate cation and solvated electrons together with free ligand. When more metal is dissolved, metal anions, M , are formed.158 Dye and co-workers have isolated gold-colored crystals of [Na+ c 2.2.2]Na 159160 and the crystal structure has been determined.161,162 Anion clusters such as Sb] , Pb2 and Sn," have been isolated as crystalline salts of the [2.2.2] cryptate counterion [2.2.2].162,163... 

The ability of the hydrophilic PAA core of the amphiphilic core-shell brushes to coordinate with different metal cations can be used for the synthesis of novel nanosized organic/inorganic hybrids or for the generation of gold clusters or cobalt nanowire [126]. 

The first examples of hydration of alkynes catalyzed by gold salts were reported in 1976 by Thomas." Later, Utimoto found that the reaction could be carried out with lower amounts of catalyst." Tanaka and coworkers reported a general hydration of alkynes using cationic Au(I) complexes generated in situ by protonolysis of [AuMe(PPh3)] as depicted in equations (2) and (3). Markovnikov-type addition is observed in all cases. Other complexes of Au(I) and Au(III) have proved to be effective in this reaction. The somewhat related gold-catalyzed addition of HCl to alkynes is an industrial process for the generation of vinyl chloride. " ... 

In the same manner, dimethylsulfide substitution proved to be a practical procedure for the preparation of NHC-gold complexes. Good yields were obtained for the synthesis of monocarbene Au(I) complexes (104) from [(Me2S)AuCl] and one equivalent of free NHC (Scheme 17) iHios jjjg steric hindrance of NHCs was found to be of critical importance, since NHCs with smaller nitrogen substituents such as Mes, wo-propyl or n-butyl led to a lower yield and formation of a byproduct identified as the biscarbene complex [(NHC)2Au]+ Cl. Fortunately, the silver-mediated transmetallation was an efficient alternative route for these NHCs (see Section 2.6). When the same reaction was performed with two equivalents of in situ generated NHCs, the cationic biscarbene complex (105) [(NHC)2Au]X (X = Cl, Br, orPFe) was obtained as the only product. ... 

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