Rachel E. M. Brooner

Cationic, two-coordinate gold π complexes.

Cationic, two-coordinate gold π complexes that contain a phosphine or N-heterocyclic supporting ligand have attracted considerable attention recently owing to the potential relevance of these species as intermediates in the gold-catalyzed functionalization of C-C multiple bonds. Although neutral two-coordinate gold π complexes have been known for over 40 years, examples of the cationic two-coordinate gold(I) π complexes germane to catalysis remained undocumented prior to 2006. This situation has changed dramatically in recent years and well-defined examples of two-coordinate, cationic gold π complexes containing alkene, alkyne, diene, allene, and enol ether ligands have been documented. This Minireview highlights this recent work with a focus on the structure, bonding, and ligand exchange behavior of these complexes.

Direct Observation of a Cationic Gold(I)–Bicyclo[3.2.0]hept‐1(7)‐ene Complex Generated in the Cycloisomerization of a 7‐Phenyl‐1,6‐enyne

F rstner first posited that these outcomes were consistent with the intermediacy of metal-stabilized nonclassical cyclopropylmethyl-, cyclobutyl-, and homoallylic carbocations/ carbenes accessed through attack of the C=C moiety on a metal-complexed C C bond (Scheme 1), and mechanistic thought in this area has evolved largely within this conceptual framework. The involvement of nonclassical carbocations/ carbenes is supported by a wealth of indirect experimental evidence, including trapping experiments, isotopic labeling studies, and stereochemical analyses, and through numerous computational studies. 2] Absent, however, is direct experimental evidence regarding the structure and reactivity of these cationic complexes, as no organometallic intermediate has been observed spectroscopically in any of these transformations. Platinum(II), gold(I), 13] and rhodium(II) complexes catalyze the cycloisomerization of 7-aryl-1,6-enynes (A, R = Ar) to form vinylcyclopentenes C and/or bicyclo[3.2.0]hept-6-enes E (Scheme 1). Directly implicated in these and related 16] transformations is the strained bicyclo[3.2.0]hept-1(7)-ene species I, presumably generated through 6-endo-cyclization followed by 1,2-alkyl migration from cyclopropyl carbene II and consumed either by ring opening to form B and/or 1,3-hydrogen migration to form E (Scheme 1). Possible contributors to I include the metalated cyclobutyl carbenium ion I a, the p-alkene complex I b, and the metallacyclopropane complex I c. In this context, it appeared likely that contribution of the latter forms would engender stability to I not realized in the corresponding cyclopropyl carbene intermediates II or III, such that I might represent a local minimum on the reaction coordinate. Indeed, here we report the selective generation, spectroscopic characterization, and reactivity analysis of a gold–bicyclo[3.2.0]hept-1(7)-ene complex formed in the gold-catalyzed cycloisomerization of a 7-phenyl-1,6-enyne. Toward detection of a reactive bicyclo[3.2.0]hept-1(7)-ene complex, we investigated the gold(I)-catalyzed conversion of the 7-phenyl-1,6-enyne 1 into the 6-phenylbicyclo[3.2.0]hept6-ene 2 reported by Echavarren (Scheme 2). In an initial experiment, a 1:1:1 mixture of 1, [LAuCl] (L = P(tBu)2(obiphenyl)), and AgSbF6 in CD2Cl2 was mixed thoroughly at 78 8C. P and H NMR analysis at 80 8C showed formation of an approximately 9:1 mixture of p-alkene and p-alkyne Scheme 1. Ligandand substrate-dependent pathways for enyne cycloaddition catalyzed by electrophilic noble-metal complexes.

Synthesis and Study of Cationic, Two‐Coordinate Triphenylphosphine– Gold–π Complexes

Cationic, two-coordinate triphenylphosphine-gold(I)-π complexes of the form [(PPh₃)Au(π ligand)]⁺SbF₆⁻ (π ligand=4-methylstyrene, 1∙SbF₆), 2-methyl-2-butene (3∙SbF₆), 3-hexyne (6∙SbF₆), 1,3-cyclohexadiene (7∙SbF₆), 3-methyl-1,2-butadiene (8∙SbF₆), and 1,7-diphenyl-3,4-heptadiene (10∙SbF₆) were generated in situ from reaction of [(PPh₃)AuCl], AgSbF₆, and π ligand at -78 °C and were characterized by low-temperature, multinuclear NMR spectroscopy without isolation. The π ligands of these complexes were both weakly bound and kinetically labile and underwent facile intermolecular exchange with free ligand (ΔG(≠) ≈9 kcal mol(-1) in the case of 6∙SbF₆) and competitive displacement by weak σ donors, such as trifluoromethane sulfonate. Triphenylphosphine-gold(I)-π complexes were thermally unstable and decomposed above -20 °C to form the bis(triphenylphosphine) gold cation [(PPh₃)₂Au]⁺SbF₆⁻ (2∙SbF₆).

Tandem gold/silver-catalyzed cycloaddition/hydroarylation of 7-aryl-1,6-enynes to form 6,6-diarylbicyclo[3.2.0]heptanes.

Mixtures of [{PCy2(o-biphenyl)}AuCl] and AgSbF6 catalyze the tandem cycloaddition/hydroarylation of 7-aryl-1,6-enynes with electron-rich arenes to form 6,6-diarylbicyclo[3.2.0]heptanes in good yield under mild conditions. Experimental observations point to a mechanism involving gold-catalyzed cycloaddition followed by silver-catalyzed hydroarylation of a bicyclo[3.2.0]hept-1(7)-ene intermediate.