The cyclizations of enynes substituted at the alkyne gives products of formal [4+2] cyclization with Au(I) catalysts. 1,8-Dien-3-ynes cyclize by a 5-exo-dig pathway to form hydrindanes. 1,6-Enynes with an aryl ring at the alkyne give 2,3,9,9a-tetrahydro-1H-cyclopenta[b]naphthalenes by a 5-exo-dig cyclization followed by a Friedel-Crafts-type ring expansion. A 6-endo-dig cyclization is also observed in some cases as a minor process, although in a few cases, this is the major cyclization pathway. In addition to cationic gold complexes bearing bulky biphenyl phosphines, a gold complex with tris(2,6-di-tert-butylphenyl)phosphite is exceptionally reactive as a catalyst for this reaction. This cyclization can also be carried out very efficiently with heating under microwave irradiation. DFT calculations support a stepwise mechanism for the cycloaddition by the initial formation of an anti-cyclopropyl gold(I)-carbene, followed by its opening to form a carbocation stabilized by a pi interaction with the aryl ring, which undergoes a Friedel-Crafts-type reaction.

Gold(I)-catalyzed addition of carbon nucleophiles to 1,6-enynes gives two different type of products by reaction at the cyclopropane or at the carbene carbons of the intermediate cyclopropyl gold carbenes. The 5-exo-dig cyclization is followed by most 1,6-enynes, although those bearing internal alkynes and alkenes react by the 6-endo-dig pathway. The cyclopropane versus carbene site-selectivity can be controlled in some cases by the ligand on the gold catalyst. In addition to electron-rich arenes and heteroarenes, allylsilanes and 1,3-dicarbonyl compounds can be used as the nucleophiles. In the reaction of 1,5-enynes with carbon nucleophiles, the 5-endo-dig pathway is preferred.

Dialkylbiphenylphosphane-Au(I) complexes exhibit only weak metal-arene interactions with the covering arene ring. However, the contacts in isoleptic Ag(I) and Cu(I) complexes are shorter than the limiting values of 3.03 A (Ag(I)) and 2.83 A (Cu(I)). Strong metal-arene interactions were also found in the two Ag(I) aquo complexes and in two acetonitrile--Cu(I) complexes with dialkylbiphenylphosphane ligands. Arene-Ag(I) complexes with these bulky phosphane ligands show the strongest Ag(I)--arene bonds known.

Transition-metal-catalyzed reactions of 1,6-enynes [1, 2] that proceed by the selective coordination of the metal center to the alkyne take place through exo-cyclopropyl metal carbenes or the related endo intermediates. [2, 3] In the absence of nucleophiles, enynes undergo skeletal rearrangement (Scheme 1). Based on kinetic data and DFT calculations, we recently proposed that upon coordination of Au I to the enyne I, the resulting anti-cyclopropyl gold(I) carbene II rearranges to give III, which can undergo demetalation (pathway a) to form the single cleavage rearrangement product IV. Alternatively, III may undergo a 1,2-shift (pathway b) to form a new gold carbene V, which gives double cleavage rearrangement diene VI after a-hydrogen elimination. The double cleavage rearrangement can also take place directly from intermediate II to give V. Strong evidence for the existence of metal carbenes of type II has been obtained by intramolecular trapping with alkenes in ruthenium-, [5] platinum-, [6,7] and gold-catalyzed reactions. [4a, 8] We have recently shown that the anti arrangement of the carbene and the cyclopropane accounts for the configuration observed in all these intramolecular processes. [8] We now report that carbenes II and V can be trapped intermolecularly with a variety of alkenes. [9,10] We also found the first examples of a shift of cyclopropyl gold(I) carbenes. These results support the mechanism proposed for the skeletal-rearrangement of enynes.

1,6-Enynes with electron-donating substituents at the alkyne undergo gold(I)-catalysed single cleavage skeletal rearrangement, whereas substrates with electron-withdrawing substituents evolve selectively to double cleavage rearrangement. Theoretical calculations provide a qualitative rationale for these effects, and suggest that bicyclo[3.2.0]hept-5-enes are involved as intermediates. We provide the first X-ray structural evidence for the formation of a product of this class in a cycloisomerisation of a 1,6-enyne.

Complexes [AuCl{C(NHR)(NHR 0 )}] and [AuCl{C(NHR)(NEt 2 )}] (R= t Bu, p-Tol, Xylyl, p-C 6 H 4 -COOH, p-C 6 H 4 COOEt, R 0 = Me, n Bu, i Pr, n heptyl, p-Tol) have been prepared by reaction of the corresponding isocyanogold complexes [AuCl(CNR)] with either primary amines or diethylamine. All the prepared carbenes are reactive and highly selective catalysts for skeletal rearrangement, methoxycyclization of 1,6-enynes, and other mechanistically related gold-catalyzed transformations. Overall, these easily accessible nitrogen acyclic carbene (NAC) gold complexes were not second to NHC complexes and were advantageous to obtain different products.

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Complexes [AuCl{C(NHR)(NHPy-2)}] (Py-2 ) 2-pyridyl; R ) Me, tBu, nBu, iPr, nheptyl) have been prepared in amodular way from [AuCl(CNPy-2)]. The carbene moiety has a hydrogen-bond supported heterocyclic structure similar to the nitrogen heterocyclic carbenes in the solid state, and in CH2Cl2 or acetone solution, which is open in the presence of MeOH. The compounds are good catalysts for the skeletal rearrangement of enynes, and for the methoxycyclization of enynes. In contrast, the complexes [AuCl{C(NHR)(NHPy-4)}] are scarcely active due to the blocking effect of the coordination position required for the catalysis by the nitrogen of the NHPy-4 group.