Cationic gold(I) complexes have recently emerged as effective catalysts for the functionalization of C-C multiple bonds. 1 With few exceptions, mechanisms involving outersphere attack of a nucleophile on a transient cationic gold π-complex have been invoked for these transformations. 1 However, although gold π-complexes have been known for over 40 years, examples of the cationic, two-coordinate gold(I) π-complexes germane to π-activation catalysis are exceedingly rare and monomeric gold(I) π-alkene complexes are unknown. [2][3][4][5] Echvarren 6 and Bertrand 7 have reported the X-ray crystal structures cationic, two coordinate gold π-arene complexes. Sadighi 8 and Bertrand 7 have reported monomeric, cationic gold(I) π-alkyne complexes, but high-resolution X-ray analysis has not been reported. 9 Toste has reported the X-ray crystal structures of multimeric, cationic, two-coordinate gold(I) π-alkyne and π-alkene complexes; however, no evidence supports the integrity of the gold(I) π-alkene bond in solution. 10 Here we report the syntheses, X-ray crystal structures, and solution behavior of monomeric, cationic, two-coordinate gold π-alkene complexes.Treatment of a methylene chloride suspension of (IPr)AuCl [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidine] and AgSbF 6 (1:1) with isobutylene at room temperature for 20 min led to isolation of [(IPr)Au(η 2 -H 2 C=CMe 2 )] + SbF 6 − (1a) in 98% yield as an air-and thermally-stable white solid that was characterized by NMR, MALDI-MS, combustion analysis, and X-ray crystallography (see below). Complexation of isobutylene to gold in solution was established by NMR, in particular by the large difference in the 13 C NMR shifts of the olefinic carbon atoms of bound [δ155.2 (s), 88.2 (t)] and free [δ142.4 (s), 110.5 (t)] isobutylene. The 1 J C=C coupling constant of the isobutylene ligand of the 13 C-isotopomer (IPr)Au(η 2 -H 2 13 C=CMe 2 ) (1a-13 C 1 ) ( 1 J C=C = 66 Hz) was diminished only slightly relative to free isobutylene ( 1 J C=C = 71 Hz), pointing to minimal deviation of the bound isobutylene from ideal sp 2 hydbridization. 11In addition to 1a, gold π-alkene complexes [(IPr)Au(η 2 -alkene)] + SbF 6 − [alkene = norbornene (1b), 2-methyl-2-butene (1c), methylenecyclohexane (1d), 2,3-dimethyl-2-butene (1e), cis-2-butene (1f), 1-hexene (1g), and 4-methylstyrene (1h)] were isolated in >80% yield and were fully characterized (Chart 1).Slow diffusion of hexane into a CH 2 Cl 2 solution of 1a at 4 °C gave colorless crystals of 1a·2CH 2 Cl 2 suitable for X-ray analysis ( Figure 1, Table 1). Complex 1a adopts a slightly distorted linear conformation with a C (carbene) -Au-alkene (centroid) angle of 172 °. The C=C bond of the isobutylene is rotated 52 degrees out of the carbene N-C-N plane, which positions E-mail: rwidenho@chem.duke.edu. Supporting Information Available: Experimental procedures, spectroscopic data, and X-ray crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org. one isobutylene methyl group near the car...

Mechanistic investigation of gold(I)-catalyzed intramolecular allene hydroalkoxylation established a mechanism involving rapid and reversible C–O bond formation followed by turnover-limiting protodeauration from a mono(gold) vinyl complex. This on-cycle pathway competes with catalyst aggregation and formation of an off-cycle bis(gold) vinyl complex.

A number of cationic gold pi-alkene complexes of the form {[P(t-Bu)(2)o-biphenyl]Au(pi-alkene)}(+)SbF(6)(-) were isolated and three were analyzed by X-ray crystallography.