Gold-Mediated Insertion of Oxygen into Silicon–Silicon Bond: An Original Au(I)/Au(III) Redox Sequence

Abstract

The diphosphine−disilane iPr 2 (o-C 6 H 4 )-SiMe 2 SiMe 2 (o-C 6 H 4 )PiPr 2 reacts with AuCl(SMe 2 ) via coordination of the two phosphines and oxidative addition of the σ-Si−Si bond. The ensuing bis(silyl) gold(III) complex has been unequivocally authenticated by NMR spectroscopy at −60°C. Upon heating, it evolves cleanly to give a disiloxane gold(I) complex that has been fully characterized, including by X-ray diffraction analysis. Oxidation of the disilane proceeds via an original Au(I)/Au(III) redox sequence. According to 18 O labeling experiments, both water and dioxygen are competent oxygen sources. Oxidative addition of the σ-Si−Si bond to form a bis(silyl) gold(III) complex seems to be a prerequisite for the disilane → disiloxane conversion to occur. O ver the past decade, homogeneous catalysis with gold complexes has attracted considerable interest and it has been applied to a wide range of organic transformations with very high, often unprecedented efficiency and selectivity. 1 The vast majority of these reactions are based on the unique carbophilic Lewis acidity of gold complexes that is typically used to activate alkynes, alkenes, allenes, etc. toward nucleophilic attack. Recently, the scope and versatility of homogeneous gold catalysis has been further extended, in particular toward oxidative coupling and addition reactions (in the presence of external oxidants) 2 as well as C−C bond forming reactions using dual Au−Pd catalysis. 3 In parallel with these very rapid and important synthetic achievements, increasing efforts have been made over the past few years to shed light into the underlying mechanistic issues. The recent characterization of some putative intermediates involved in gold activation of π systems 4 (side-on complexes, 5 vinyl gold complexes, 6 geminal digold complexes 7 ) has provided valuable information and contributed to refine mechanistic proposals. Comparatively, the precise factors involved in controlling two-electron redox and dual catalysis with gold complexes remain less understood, but some insight has started to be gained into key transformations (oxidative addition/reductive elimination, transmetalation, etc.) 8,9 In that general context, we have recently shown, thanks to phosphine chelating assistance, that gold can undergo unexpectedly facile oxidative addition of σ-Si−Si bonds, affording original bis(silyl) gold(III) complexes. 10,11 Upon varying the substituents at the phosphorus anchors, we have incidentally found that the disilane moiety can be converted into a disiloxane moiety (oxygen insertion) via a Au(I)/Au(III) sequence. These results are reported herein. Labeling experiments have been performed to identify the possible source of the oxygen atom, and the role of gold in the disilane → disiloxane conversion has been probed.As previously reported, 10 the diphosphine−disilane ligand 1a readily reacts with [AuCl(SMe 2 )] in dichloromethane at −78°C via coordination of the two phosphines and oxidative addition of the σ-Si−Si bond at gold (Scheme 1). The ensuing bis(si...

Gold‐Catalyzed Cross‐Coupling Reactions: An Overview of Design Strategies, Mechanistic Studies, and Applications

“…[383,385] Chem.E ur.J.2020, 26,1442 -1487 www.chemeurj.org 2019 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Au/Pd alloy with equal amountso fA ua nd Pd is desirable for carrying out Ullmann coupling, decreasing the Pd content imparts it with better activityt oward the homocoupling of aryl boronic acids. [395] They reasonedb ased on previous reports that have shown the ability of Au I for activating the SiÀSi bond, [243,245] and the hydrolysis and alcoholysis of disilanes. [394] 7.4.…”

“…bonds could be activated by Au I under milder conditions. [245] Oxidative addition of Au I into SnÀSn bonds has been reported recently for diphosphine-distannane systems (Scheme 47 a). [242] The highly electron-deficient thiolatel igands stabilized the Au III complex formed.…”

“…The reaction typically proceeded at low temperatures;i nsomecases, even at À80 8C. [245] Scheme47. They also monitored the relevance of counterions in the reaction mechanisms.…”

“…Through as eries of articles, Amgoune, Bourissou, and co-workers have established methodologies for mild oxidative addition of disilanes to Au I complexes. [243][244][245] They used the design principle of starting with the disilane reactants containing one or two phosphine ligandsi ns uitable positions, which can cushion or act as buttresses with Au in the transition state and later form bonds with the metal in thep roduct (Scheme 46 a). The reaction typically proceeded at low temperatures;i nsomecases, even at À80 8C.…”

Fundamental Elementary Steps in Gold Chemistry

“…Reaction of water or oxygen with a related, in situ prepared bis(silyl)gold(III) complex featuring a chloride anion resulted in the reduction of the metal center to give a diphosphine gold(I) complex featuring a net insertion of an oxygen atom in the SiSi bond (Scheme 3.14) [57]. This sequence presents a rare example of a gold (I)-gold(III)-gold(I) redox process.…”