Stereo‐ and Regioselective Alkyne Hydrometallation with Gold(III) Hydrides

Abstract

The hydroauration of internal and terminal alkynes by gold(III) hydride complexes [(C^N^C)AuH] was found to be mediated by radicals and proceeds by an unexpected binuclear outer‐sphere mechanism to cleanly form trans‐insertion products. Radical precursors such as azobisisobutyronitrile lead to a drastic rate enhancement. DFT calculations support the proposed radical mechanism, with very low activation barriers, and rule out mononuclear mechanistic alternatives. These alkyne hydroaurations are highly regio‐ and stereospecific for the formation of Z‐vinyl isomers, with Z/E ratios of >99:1 in most cases.

Oxidative Addition of Alkenyl and Alkynyl Iodides to a AuI Complex

“…These observations, as well as other data (see Supporting Information), are consistent with oxidative addition to give Au III complex (E)-3 a. [13] When a solution of (E)-3 a was exposed to an atmosphere of ethylene reversal to starting complex 1 was observed, demonstrating the reversibility of oxidative addition. [12] Similar behavior was observed for p-substituted variants (E)-2 b-e.…”

“…The origin of this side process was investigated using an analogue of 8 a derived from 13 C-labelled phenyliodoacetylene (see Supporting Information). 13 C NMR profiling of (12). The structure contains disordered I/Cl atoms (I/Cl 0.78/0.22 occupancies, respectively), as observed with similar complexes.…”

Cyclometalated Gold(III) Complexes: Synthesis, Reactivity, and Physicochemical Properties

“…[47] Complex 24 was prepared as ad ark yellow crystalline solid by the reaction of 15 with LiHBEt 3 in toluene at À78 8 8C. [48] This reaction seems to be mediated by radicals,a nd ab inuclear outer-sphere mechanism has been proposed to explain the formation of the observed trans insertion products 25 c. Interestingly,the reaction of 24 with the analogous complexes Au III À OAc F 14 (fast) and Au III À OH 15 (slow) produced Au II dimer 26 by reductive condensation. Ther igidity of the C^N^C pincer ligand, which suppresses the otherwise expected reductive elimina-tion to form C(sp 2 )ÀHbonds,seems to be responsible for the stability of this complex.…”

The First Gold(III) Formate: Evidence for β‐Hydride Elimination

“…[8] In 2015, Bochmann and co-workers reported the insertion of CO into ag old(III) hydroxo complex [k 3 -(C^N^C)Au(OH)]. [11] Importantly, gold(III) carboxylates and formates have been proposed as intermediates prone to undergo b-hydride elimination in gold-catalyzed WGSRs [12] and the dehydrogenation of formic acid, [13] but the isolation and direct characterization of these species to substantiate these proposals has proven remarkably challenging.I nt his context, the resulting gold hydrides have also attracted significant attention [14] although experimental validation for their participation in catalysis remains scarce. The stability of this gold(III) hydride proved to be remarkable as it was stable in the presence of air, moisture,and even acetic acid.…”

“…[10] Furthermore,t hese species were used for the hydroauration of alkynes in the presence of ar adical initiator by homolytic cleavage of the Au À Hb ond to form the corresponding trans-vinylgold(III) complexes. [11] Importantly, gold(III) carboxylates and formates have been proposed as intermediates prone to undergo b-hydride elimination in gold-catalyzed WGSRs [12] and the dehydrogenation of formic acid, [13] but the isolation and direct characterization of these species to substantiate these proposals has proven remarkably challenging.I nt his context, the resulting gold hydrides have also attracted significant attention [14] although experimental validation for their participation in catalysis remains scarce.…”