Cationic gold complexes, enyne

Activation of the triple bond of enynes with electrophilic metal derivatives, especially cationic gold complexes, platinum salts such as PtCl2, and ruthenium derivatives, has been reviewed.117 These catalysts make possible nucleophilic addition of the double... 

Treatment of the 1,5-enyne 66 with a cationic gold complex caused cycliza-tion to generate the spirocyclic carbene complex 67 (Scheme 7.26) [35]. 1,2-Alkyl migration followed to furnish the tricyclic compound 68. 

Selective activation of alkyne functions of enynes to give products either of alkoxy-cyclization or of exo- and endo-skeletal rearrangement can be achieved by using alkynophilic cationic gold(I) complexes. The endocyclic cyclization catalysed by gold(I) proceeds via a mechanism different from those known for Pd(II), Hg(II), or Rh(I) catalysts.118... 

The Conia-ene reaction of P-ketoesters bearing a pendant triple bond involves the cycloisomerization of an enyne formed by enolization. This transformation is efficiently catalyzed by cationic gold(I) complexes and afforded cyclopentane derivatives with excellent yields and moderate to good diastereoselectivities (Scheme 4-40). Acetylenic silyl enol ethers or imines react in an analogous manner. By this method, iodoalkynes were converted into iodocyclopentenes, which are highly useful in natural product synthesis. 

There are many examples of preparing cyclopentane structures from enynes by gold-catalyzed carbocyclization reactions. Toste et al. have reported that Au(l)-phosphine complexes act as superior catalysts for isomerization of 1,5-enynes to bicyclo[3.1.0]hexenes [124]. For example, treatment of 1,5-enyne (86) with 1 mol% of PhsP-AuPFfi in dichloromethane at room temperature results in formation of cyclopropane-fused cyclopentene (87) in 99% yield (Scheme 18.30). 1,6-Enynes also undergo similar cycloisomerization to five-membered cyclic compounds under the influence of cationic gold(l) catalysts [125, 126], Hydroxylated enynes are versatile precursors for cyclopentenones by gold-catalyzed cycloisomerization... 

By using cationic gold(I) complexes, bicyclo[4.2.0]oct-6-ene II-IO or II-ll are obtained starting from 1,7-enynes II-9a or II-9b (Scheme 3.7) [Ref. 15 in Chap. 1 ]. Ring opening to form 1,3-dienes does not occur even after heating at 120-150 °C. 

Nieto-Oberhuber, C., Munoz, M. R, Bunuel, E., Nevado, C., Cardenas, D. J., Echavarren, A. M. (2004). Cationic gold(I) complexes highly alkynophilic catalysts for the exo- and endo-cyclization of enynes. Angewandte Chemie - International Edition, 43,2402-2406. 

In 2010, Malacria and coworkers developed a novel Au-catalyzed cyclization of 1,6-enyne 9, furnishing allene-substituted tetrahydrofuran 11 in excellent yield. The proposed mechanism involves an initial enyne cyclization to generate the cationic gold-vinyl complex 10 and subsequent unexpected [l,5]-hydride shift. Deuteration experiments support the [l,5]-hydride shift and rule out the inter-molecular scrambling. This cascade reaction is unusual because the cyclization precedes followed by an uncommon [l,5]-hydride shift (Scheme 12.5) [9]. 

The formation of these products can be rationalized as shown in equation (46). Thus, upon complexation of Au(I) to the aUcyne in the reacting enyne, an endo cyclization occurs to form a cyclopropyl gold carbene, which can be trapped by a nucleophile. However, the lack of stereospecificity observed in some of these reactions, suggests the formation of open six-membered ring cations as intermediates, which can be intercepted by an O-Boc group.3-Hydroxy-1,5-allenynes react similarly with Pt(II) catalysts. ... 

Starting from tetrahydrocyclopenta[f)]furan-2-one 342, enyne 343, the substrate for the domino reaction, was prepared in 12 steps and with an overall yield of 45%. Exposure of 343 to the electron-rich gold(I) complex (t-Bu)2P(o-biphenyl)AuCl at room temperature afforded cis-hydrindanone 344 in 78% yield as a single stereoisomer (Scheme 14.54). The postulated mechanism involved Au(I) activation of the alkyne to initiate the cationic olefin cyclization of 346 to give carbocation 347, which then underwent a pinacol rearrangement to the final product 344. An originally attempted Lewis acid-catalyzed domino Prins/pinacol rearrangement of... 

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