Nucleophilic attack gold complexes

The excellent ability of late transition metal complexes to activate alkynes to nucleophilic attack has made them effective catalysts in hydroamination reactions. The gold(l)-catalyzed cyclizations of trichloroacetimidates 438, derived from homopropargyl alcohols, furnished 2-(trichloromethyl)-5,6-dihydro-4f/-l,3-oxazines 439 under exceptionally mild conditions (Equation 48). This method was successfully applied to compounds possessing aliphatic and aromatic groups R. With R = Ph, cyclization resulted in formation of 439 with complete (Z)-stereoselectivity <2006OL3537>. 

The coordination of gold complexes to the —C -system activates them very efficiently in order to attack a nucleophile. The carbonylic double bond can also be activated for nucleophilic addition. 

N==Ca+—Aua which makes the isocyanide ligands susceptible to nucleophilic attack and has led to formation of carbene complexes, iminoalkyl complexes and catalytic conversion of isocyanides to formamidines using alkyl or aryl isocyanide complexes of gold(I).301,402 4(y7-409-415 A review of this significant work has been published.16... 

In the Au(I) catalysis of electron-poor alkynes such as 4, the catalytically active species is likely to be a cationic ligand-stabilized gold(I) Jt-complex, as in previously reported additions of oxygen nucleophiles to alkynes [5], Gold catalysts are very soft and thus carbophilic rather than oxophilic. On the basis of this assumption a plausible mechanism can be formulated as shown in Scheme 6. The cationic or strongly polarized neutral Au(I)-catalyst coordinates to the alkyne, and nucleophilic attack of the electron-rich arene from the opposite side leads to the formation of a vinyl-gold intermediate 7 which is stereospecifically protonated with final formation of the Z-olefm 8 [2, 4]. Regioselectivity is dominated by elec-... 

In a simplified form, the nucleophilic attack to the [AuL]+-alkyne complex gives trans-alkenyl-gold complexes as intermediates (equation 1). Although simple alkyne-Au(I) complexes are usually stable only at low temperatures,a few compounds of this type have been characterized. 

Alkenes act as nucleophiles with alkynes in the presence of gold catalysts. In the most simple version of the reaction, enynes are converted with gold complexes or salts, and in the absence of nucleophiles, into rearranged dienes, cyclopropanated carbocycles, and/or bicyclic cyclobutenes. Depending on the length of the tether and the nature of the substituents, the olefin attack to the alkyne occurs in an endo or an exo fashion (equation 33). Besides, substitution at the alkene plays an important role on the regioselectivity of the nucleophilic attack. ... 

Hydroarylation can also be mediated by Au(I) and Au(III) (Scheme 33) (384). In the case of aryl substituted alkynes, the Au(III) Ji complex undergoes electrophilic aromatic substitution with the electron-rich arene to give aLkenyl-Au(III) complex, which is immediately protonated by the H generated upon C C bond formation. For the Au(I)-catalyzed hydroarylation, the cationic gold complex k coordinates the alkyne, with subsequent nucleophilic attack by the arene from the opposite face leading to an alkenyl-gold complex, which is protonated to the desired products. The nature of the reaction causes the regioselectivity of this reaction to be sensitive to electronic rather than steric factors. 

Organogold carbene compounds [Au(carbene)(CN) j iR ] can be obtained from com-plexed cyanides [Au(CN)m(C6F5) ] (n = 1 or 3, m = 1 and n = m = 2) by sequential alkylation (to form isocyanide complexes) and nucleophilic attack of amine" . Gold(III) diisocyanides [Au(CNR)2(CgF5)2]+ (R = Ph or p-tolyl), which can also be obtained by isocyanide substitution of ether in [Au(C6F5)2(OEt2)2] > react with hydrazobenzene NH(Ph)NHPh and hydrazine or phenyUiydrazine according to the reactions in Scheme 32 to furnish cyclic bis(carbene) and cyclic carbene-imidoyl compounds . ... 

Backvall has reported the gold(III)-catalyzed cycloisomerization of allene-substituted malonic esters to form p,Y-unsaturated 6-lactones [115]. For example, treatment of allenyl malonate derivative 76 with a catalytic 1 3 mixture of AUCI3 and AgSbF in acetic acid at 70 °C led to isolation of 8-lactone 77 in 99% yield (Eq. (12.41)). The transformation was restricted to substrates that contained a terminally disubstituted allenyl moiety and presumably occurs via nucleophilic attack of the carbonyl oxygen atom on a gold-complexed allene followed by acetate-mediated demethylation and protodeauration. 

These results have been followed up with a Pt(ll) pincer complex that can realize the reaction of benzamide with 5.5 bar of ethylene at 100 °C for extended reaction times to achieve the desired N-ethyl-substituted amide [160]. Stoichiometric reactions with morpholine and the observed lack of catalytic turnover with this nucleophilic amine substrate point toward nucleophilic attack of coordinated ethylene as being the operative mechanism for this system [160]. Furthermore, by taking advantage of ongoing developments in Au-catalyzed hydroamination, Widenhoefer has shown that ethylene hydroureation can be realized with a gold(I) catalyst (22) with only 4 bar of ethylene pressure at 60 °C to give ethyl-substituted cycHc urea in outstanding yields (Scheme 15.22) [127]. 

Although Bertrand and coworkers have not recently reported further investigations of this system with ammonia, a computational chemistry team from Spain has independently explored the mechanism of these cationic CAAC-gold(I) complexes in silica [182]. These investigations revealed that nucleophilic attack of the coordinated alkyne resulted in the lowest energy transition state and that excess ammonia is required to realize catalyst turnover, whereby the ammonia acts as a proton shuttle that promotes tautomerization and, ultimately, release of the product (Scheme 15.25) [182]. 

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