Syntheses, X-ray Crystal Structures, and Solution Behavior of Monomeric, Cationic, Two-Coordinate Gold(I) π-Alkene Complexes

Abstract

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 insights and functionalization of alkynes in homogeneous gold catalysis.

“…As expected, hydrogen bond acceptors with lower gold affinity (e.g., DMPU, DMF) [141][142][143][144][145][146][147][148] enhanced the rate of reaction (Scheme 14b), and ionic hydrogen bond acceptors (e.g., Bu4N Other research groups reported that certain compounds improved the chemical yields in selected gold catalyzed reactions. 153,154 One notable example is the …”

“…Some research groups have also speculated that the deactivation of the gold catalyst is due to disproportionation. 76,77 However, no direct experimental evidence has been reported to support these statements. We decided to investigate the actual gold valence change in the decay process experimentally.…”

“…[68][69][70][71][72][73][74] In most gold catalyzed reactions, catalyst loadings in the 1-5% range are the norm. [14][15][16][17][18] One reason for the low turnover numbers (TON) is the relatively fast decay of cationic gold and its conversion into non-reactive species (e.g., Au [75][76][77][78][79] Despite this drawback, there has been no comprehensive systematic investigations on the decay of homogeneous gold catalysis, a stark contrast with the extensive work reported on catalyst deactivation in heterogeneous catalysis. 80 In this chapter, we answer some fundamental and intriguing questions on gold(I) decay, such as the effects of starting material, nucleophile, solvent, counterion and additives, and we offer a probable pathway for gold(I) decay.…”

α‐ und β‐Hydridabstraktion in Gold(I)‐alkylkomplexen

“…[8][9][10] Der CAAC-Gold(I)-n-butylkomplex 2 konnte in hoher Ausbeute (97 %) ausgehend von CAAC-Gold(I)-chlorid 1 durch einfachen Ligandenaustausch mit n-Butyllithium erhalten werden (Schema 2). Das Carbensignal ist im 13 C{ 1 H}-NMR-Spektrum von Komplex 2 signifikant tieffeldverschoben (2: 266.8, 1: 237.2 ppm), wie dies durch den Austausch des Chlorids gegen einen elektropositiven Substituenten zu erwarten ist; die Struktur von 2 wurde außerdem rçntgenographisch ermittelt. [11] Die Reaktion von 2 mit Triphenylcarbenium-tetrafluoroborat verläuft in CH 2 Cl 2 rasch, was anhand der schnellen Entfärbung festgestellt werden kann.…”

“…Vergleicht man die Strukturparameter von 5 mit denen analoger NHC- [13] (NHC = N-heterocyclisches Carben) oder…”