Gold η²-Coordination to Unsaturated and Aromatic Hydrocarbons: The Key Step in Gold-Catalyzed Organic Transformations

Abstract

For a long time, π-complexes of gold in any of its oxidation states were considered elusive species of uncertain stoichiometry, low stability, and limited relevance. The recent upsurge of research activities in homo-and heterogeneous catalysis of reactions by gold and its compounds has now provided detailed and extensive information on almost all molecular prototypes with Au 0 , Au þ , and Au 3þ bound to alkenes, alkynes, and even arenes with low η 2 -hapticity (or, in special cases, η 1 and η 3 ). This extremely important progress in organogold chemistry is reviewed here with an emphasis on the hard experimental evidence and the structure and energy characteristics of gold π-coordination in molecular systems.

Au-Catalyzed Biaryl Coupling To Generate 5- to 9-Membered Rings: Turnover-Limiting Reductive Elimination versus π-Complexation

“…The structures of the largest rings (5ah, 5ai, entries 17,18) were confirmed by single crystal X-ray diffraction.Although a number of other intramolecular direct arylation strategies have been reported, 10 the gold-catalyzed cyclization displays excellent functional group tolerance, proceeds at low reaction temperatures, uses readily prepared TMS-bearing starting materials, and yields 5-9 membered ring products bearing a diverse range of substitution patterns, from CF3 to highly electron donating (Scheme 2). For the majority of examples, 1-2 mol% Au allowed complete conversion, at 27°C in a reasonable timescale; with some substrates substantially lower loadings were possible.…”

“…This eliminates C-H cleavage from consideration as the turnover-rate limiting step. As noted by Schmidbaur, "η 2 -coordination to unsaturated and aromatic hydrocarbons is the key step in gold-catalyzed organic transformations", 17 and gold(III) has a long history of interacting with arenes. For example, it has been known for over 80 years that anhydrous AuCl3 reacts readily with aromatic hydrocarbons leading C-H auration of the arene.…”

“…18 However, to date, all attempts to experimentally prepare stable molecules demonstrating Auarene interactions have been unsuccessful. 17 Nonetheless, such 2 -arene -complexes species have been explored computationally, 19 and are implicated as precursors to C-H auration. In our previous studies, 9h this intermolecular -complexation event was deduced to be turnover-rate limiting, and the relative rates of arylation a series of arenes shown to correlate with known stabilities of reference, Ag--arene complexes and HF/BF3 Wheland-intermediates.…”

“…Scheme 7. NEEDS EDITING Mechanism of Au-catalyzed cyclization, 4  5, with turnover-rate limiting step (TLS) proceeding via a mono-aryl (17) or diaryl (14) gold resting state, depending on tether-length (n), and electronic influence of substituents (R Reductive elimination fastest via 3-coordinate 14 rather than 4-coordinate 15.…”

Supra-molecular synthons based on gold…π(arene) interactions

“…4a, have been developed [27], as such species are crucial in many catalytic applications of gold [28]. X-ray crystal structure determination shows the π-allene ligand to be coordinated at the least congested side (Fig.…”