Cyclopropyl gold carbene

It is important to note in an overview of this chemistry that the formation of most of the products can be mechanistically explained as proceeding via cyclopropyl gold carbene intermediates. The involvement of these species has been previously proposed in enyne cyclizations with other metals and their structures have been later determined by calculations. ... 

For 1,6-enynes, the initial cyclopropyl gold carbene formed in the 5-exo-dig cyclization (equation 37) evolves to form... 

Additional evidence for the involvement of metal carbenes in these processes was obtained in the reaction of dimeric substrates with a cationic Au(I) catalyst to give disubstituted alkynes (equation 39). These reactions can be explained by isomerization of the initially formed cyclopropyl gold carbene by a [1,3] metallotropic shift, followed by intramolecular trapping of the gold carbene by the alkene. Other examples of [1,3] metallotropic shifts in gold chemistry have been observed recently. ... 

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. ... 

In the presence of external nucleophiles, the situation changes and the behavior of cyclopropyl gold carbenes as delocalized electrophiles becomes manifest. Nucleophihc attack then takes place at either the cyclopropane ring or at the carbene carbon. 

Electron-rich aromatic and heteroaromatic compounds add to 1,6-enynes in the presence of Au(I) catalysts (equations 62 and 63). This reaction leads stereospecifically to adducts related to the products in the aUcoxycychzation by nucleophihc addition on cyclopropyl gold carbene intermediates. Interestingly, in a few cases, addition of the nucleophile takes place at the carbene carbon to give adducts of the type in equation (63). ... 

Carbonyl compounds can also act as the nncleophtles in intermolecular processes with 1,6-enynes. Thus, the gold(I)-catalyzed reaction between enynes and aldehydes or acetone gives stereoselectively tricyclic compounds (equation 71). The transformation is mechanistically intriguing, as it proceeds by a rearrangement of the initially formed cyclopropyl gold carbene (the intermediate in the donble-cleavage mechanism), which is then trapped by the carbonyl compound to form the products. 

Electrophilic Au(I) complexes or their halide AuX analogoues typically cyclize enynes (I, Scheme 58) (475) by a 5-exo-dig pathway to give a variety of cycloisomerization and addition derivatives. The mechanism is proposed to involve formation of a cyclopropyl gold-carbene intermediate... 

Various other nucleophiles can be employed for the trapping of cyclopropyl gold carbenes formed by cycloisomerization of 1,6-enynes. The intermolecular hydroxycyclization or alkoxycyclization of 1,6-enynes in the presence of water or alcohols is also catalyzed by Pt(II) or Pd(II), but they proceed under milder conditions and more efficiently in the presence of cationic gold(I) catalysts. ... 

Gold(I)-catalyzed intermolecular cyclization of furans with alkynes produced phenols. Under the same conditions, 1,3-diphenylisobenzofuran reacted with alkynes forming 2,3-disubstituted indenes via a cyclopropyl gold carbene (13CEJ6581). 

The formation of the six-membered ring XXI was initially proposed to take place through cyclopropyl gold carbene XVIII via 6-endo-dig cyclization (Scheme 1.18) [27]. However, DFT calculations support a mechanism in which cyclopropyl gold carbene XVI rearranges to cation XXIX, followed by protodemetallation [165]. 

Table 1.1 Bond distances for cyclopropyl gold carbene XVI determined by DPT calculations [LAu] [LAu]...

The largest l, -enyne that has been cyclized is a 1,9-enyne, which forms a 10-membered ring in the presence of a large amount of gold and silver complex (Scheme 1.26) [200]. The cycloisomerization presumably occurs via cyclopropyl gold carbene XXXVI, which can open to carbocation XXXVII or give directly the 10-membered ring. 

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