Alkyne-gold

Hydration and Hydroalkoxylation of Alkynes Gold compounds were first applied to catalyze these types of reactions by Utimoto et al. in 1991, when they studied the use of Au(III) catalysts for the effective activation of alkynes. Previously, these reactions were only catalyzed by palladium or platinum(II) salts or mercury(II) salts under strongly acidic conditions. Utimoto et al. reported the use of Na[AuCI41 in aqueous methanol for the hydration of alkynes to ketones [13]. 

The possibility of using gold complexes for both therapy and imaging has also been explored in the case of a series of alkyne gold(l) complexes such as 47 (Scheme 11.9) [138]. This complex is water soluble, thanks to the presence of the l,3,5-triaza-7-phosphaadamantane (PTA) ligand, and liuninescent with a Aem = 486 nm (Aexc = 384 nm) in the solid state at room temperature. Intracellular distribution studies of complex 47 in A2780 ovarian carcinoma cells revealed that the complex was efficiently and rapidly internalized. 

DFT calculations have been carried out to shed some light on the mechanism for cyclobutene formation. No direct pathway for the formation of cyclobutene 11-14 from flnf/-exo-cyclopropyl gold carbene 11-13 was found (Scheme 3.9). In contrast, iyw-exo-cyclopropyl gold carbene 11-13 forms cyclobutene 11-14 by a cyclopropane ring expansion. The formation of iyn-exo-cyclopropyl gold carbene 11-13 has been postulated to occur by a iyn-type attack of the alkene to the alkyne gold moiety in 11-15. However the anti attack is more favorable the syn attack could compete if the substitution at the alkene and/or the alkyne does not favor the skeletal rearrangement [Ref. 15 in Chap. 1]. 

BENZANNULATION OF ENYNE WITH ALKYNE GOLD-CATALYZED BENZANNULATION REACTION... 

A number of alkyne-gold complexes have been structurally characterized [17-21] and studied in solution [22-25]. Well-characterized complexes of gold(l) with aUcenes are also known [26 2] and their structures have been studied in solution [38, 39, 43, 44]. The solid state structures of cationic aUene—gold(I) [45] and diene-gold(I) [46] have also been determined. 

Unlike in the case of [M(CO)6] (M = Cr, Mo, W) and certain Ru(II) complexes, which activate alkynes via vinylidene metal complexes [102, 107-111], gold complexes promote reactions of alkynes by the formation of electrophilic ri -alkyne-gold(l) complexes [1-13]. 

Mechanistically, formation of products of exo-trig and endo-tng cyclization can be explained by the attack of the nucleophiles to cyclopropyl gold(l) carbene intermediates XXVI at carbons a or ft to form products 86 or 87 (Scheme 47), similarly to that found for Pt(ll) ([235], an example of gold-catalyzed methoxycy-clization of an allenene [237]). In the first step, the alkene reacts with the alkyne-gold(I) complex in an electrophilic addition process. 

A unique method to generate the pyridine ring employed a transition metal-mediated 6-endo-dig cyclization of A-propargylamine derivative 120. The reaction proceeds in 5-12 h with yields of 22-74%. Gold (HI) salts are required to catalyze the reaction, but copper salts are sufficient with reactive ketones. A proposed reaction mechanism involves activation of the alkyne by transition metal complexation. This lowers the activation energy for the enamine addition to the alkyne that generates 121. The transition metal also behaves as a Lewis acid and facilitates formation of 120 from 118 and 119. Subsequent aromatization of 121 affords pyridine 122. 

Fig. 2.12 Silver, gold and platinum complexes with monodentate NHC ligands as catalysts for the diboration of alkenes and alkynes...

The NHCs have been used as ligands of different metal catalysts (i.e. copper, nickel, gold, cobalt, palladium, rhodium) in a wide range of cycloaddition reactions such as [4-1-2] (see Section 5.6), [3h-2], [2h-2h-2] and others. These NHC-metal catalysts have allowed reactions to occur at lower temperature and pressure. Furthermore, some NHC-TM catalysts even promote previously unknown reactions. One of the most popular reactions to generate 1,2,3-triazoles is the 1,3-dipolar Huisgen cycloaddition (reaction between azides and alkynes) [8]. Lately, this [3h-2] cycloaddition reaction has been aided by different [Cu(NHC)JX complexes [9]. The reactions between electron-rich, electron-poor and/or hindered alkynes 16 and azides 17 in the presence of low NHC-copper 18-20 loadings (in some cases even ppm amounts were used) afforded the 1,2,3-triazoles 21 regioselectively (Scheme 5.5 Table 5.2). 

Hashmi, A.S.K. (2003) Homogeneous Gold-Catalysts and Alkynes A Successful Liaison. Gold Bulletin, 36, 3-9. 

Fedchenfeld, H. and Weaver, M.J. (1989) Binding of alkynes to silver, gold, and underpotential deposited silver electrodes as deduced by surface-enhanced Raman spectroscopy. The Journal of Physical Chemistry, 93, 4276—4282. 

Zhang, S., Chandra, K.L. and Gorman, C.B. (2007) Self-Assembled Monolayers of Terminal Alkynes on Gold. Journal of the American Chemical Society, 129, 4876-4877. 

Recently, the three-component couplings of a-oxyaldehydes, alkynes, and amines in water were investigated by using gold, silver, and copper... 

Unsubstituted alkyne gives dinuclear gold(I) acetylides [Au2 (C=C)(PR3)2],2098,2189,2190,2211,2212 and polyines also yield similar type of compounds. Polynuclear gold(I) acetylides with... 

Gold(I) complexes of the type [AuCl(PPh3)ra] (n= 1, 2) or [Au(N03)(PPh3)] show an excellent performance towards oxidative addition carbonylation or aromatic amines to form corresponding carbamates, and also towards the carbonilation of aliphatic amines to produce either alkylureas or formamides.2552,2553 Cationic gold(I) compounds of the type [AuL]+ where L = phosphine, phosphite, or arsine are excellent catalysts for the addition of alcohols to alkynes.2554... 

Gold as an efficient catalyst has widely been used in furan synthesis. One example is shown below, in which the double hydroarylation of unactivated alkyne using 2-methylfuran afforded a difuranylmethane derivative in a moderate yield <06EJOC4340>. 

Addition Reactions of Alkynes Catalyzed by Gold Complexes 266... 

Complexes of Alkenes, Alkynes, and Arenes with Gold Compounds 299... 

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