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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.