Abderrahmane Amgoune

Reactivity of Gold Complexes towards Elementary Organometallic Reactions

For a while, the reactivity of gold complexes was largely dominated by their Lewis acid behavior. In contrast to the other transition metals, the elementary steps of organometallic chemistry-oxidative addition, reductive elimination, transmetallation, migratory insertion-have scarcely been studied in the case of gold or even remained unprecedented until recently. However, within the last few years, the ability of gold complexes to undergo these fundamental reactions has been unambiguously demonstrated, and the reactivity of gold complexes was shown to extend well beyond π-activation. In this Review, the main achievements described in this area are presented in a historical context. Particular emphasis is set on mechanistic studies and structure determination of key intermediates. The electronic and structural parameters delineating the reactivity of gold complexes are discussed, as well as the remaining challenges.

Gold–Silane and Gold–Stannane Complexes: Saturated Molecules as σ‐Acceptor Ligands

The discovery that saturated molecules may form s complexes by side-on coordination of a s bond to a transition metal represents a major breakthrough in transition-metal chemistry. Over the years, considerable progress has been made in the understanding of this bonding situation. The coordination and activation of s bonds involving Group 14 elements (E = C, Si, Ge, Sn, Pb) is at the forefront of developments in this area. An increasing variety of complexes A and B 7] (Scheme 1) featuring side-on coordinated s(E H) and s(E E) bonds have been isolated, and the key factors governing the delicate balance between dissociation and oxidative addition have been progressively identified. The common feature and believed prerequisite for the coordination of saturated molecules free of lone pairs to transition metals is the superposition of ligand!metal donation (from a filled s orbital of the ligand to an empty d orbital of the metal) and metal!ligand back-donation (from a filled d orbital of the metal to an empty s* orbital of the ligand). Aiming at identifying new types of metal–ligand interactions, and stimulated by our work on Group 13 Lewis acids as acceptor ligands, we recently became interested in complexes of type C, in which a saturated Group 14 element could behave as an end-on, s-acceptor ligand toward a transition metal. Heavier Group 14 elements such as silicon and tin are known to readily form hypervalent compounds through donor!acceptor interactions with organic Lewis bases. A related situation is envisioned in complexes C, with a transition metal acting as a Lewis base. Such donor! acceptor interactions between transition metals and silanes or stannanes have been invoked in a few highly strained complexes on the basis of relatively short M E distances. Furthermore, the presence of a Pd!Sn dative bond was recently evidenced structurally and theoretically in a palladastannatrane cage complex supported by four methimazolyl groups. Herein, we report the straightforward synthesis and complete characterization of three gold complexes supported by diphosphino silane and stannane ligands. The presence of metal!silane and metal!stannane interactions in these complexes has been substantiated spectroscopically, structurally, and theoretically, thus providing unambiguous evidence for the existence of complexes of type C. From our previous studies on Group 13 Lewis acids, the use of two phosphine buttresses ligated by ortho-phenylene spacers was considered as a good compromise in order to support, but not impose, the coordination of the Group 14 element. We thus targeted the two complexes [o{(iPr2P)C6H4}2E(Ph)FAuCl] 2 (E = Si) and 4 (E = Sn). The Scheme 1. Complexes A–C featuring Group 14 saturated molecules coordinated to transition metals (E= C, Si, Ge, Sn, Pb).

Facile oxidative addition of aryl iodides to gold(I) by ligand design: bending turns on reactivity.

Thanks to rational ligand design, the first gold(I) complexes to undergo oxidative addition of aryl iodides were discovered. The reaction proceeds under mild conditions and is general. The ensuing aryl gold(III) complexes have been characterized by spectroscopic and crystallographic means. DFT calculations indicate that the bending induced by the diphosphine ligand plays a key role in this process.

Ring-opening polymerization with Zn(C6F5)2-based Lewis pairs: original and efficient approach to cyclic polyesters.

Dual systems combining Zn(C6F5)2 with an organic base (an amine or a phosphine) promote the controlled ring-opening polymerization of lactide and ε-caprolactone. The Lewis pairs cooperate to activate the monomers, affording well-defined high molecular weight cyclic polyesters. Efficient chain-extension gives access to cyclic block copolymers.

Activation of Aryl Halides at Gold(I): Practical Synthesis of (P,C) Cyclometalated Gold(III) Complexes

Taking advantage of phosphine chelation, direct evidence for oxidative addition of Csp(2)-X bonds (X = I, Br) to a single gold atom is reported. NMR studies and DFT calculations provide insight into this unprecedented transformation, which gives straightforward access to stable (P,C) cyclometalated gold(III) complexes.

A dual organic/organometallic approach for catalytic ring-opening polymerization.

A dual catalytic system combining an original cationic zinc complex and a tertiary amine is shown to promote efficiently the polymerization of lactide under mild conditions.

Oxidative Addition of Carbon–Carbon Bonds to Gold

The oxidative addition of strained CC bonds (biphenylene, benzocyclobutenone) to DPCb (diphosphino-carborane) gold(I) complexes is reported. The resulting cationic organogold(III) complexes have been isolated and fully characterized. Experimental conditions can be adjusted to obtain selectively acyl gold(III) complexes resulting from oxidative addition of either the C(aryl)C(O) or C(alkyl)C(O) bond of benzocyclobutenone. DFT calculations provide mechanistic insight into this unprecedented transformation.

Spontaneous Oxidative Addition of σ‐SiSi Bonds at Gold

The past decade has witnessed tremendous development in homogeneous gold catalysis. Thanks to unique soft and tunable Lewis acidity, gold(I) and gold(III) complexes have been shown to efficiently activate most types of p-bonded substrates. Besides these synthetic achievements, mechanistic issues have attracted considerable attention, and much progress has been accomplished recently thanks to the characterization of several key intermediates. With the aim of further extending the scope of gold catalysis, major interest has been devoted over the last few years to gold-mediated processes involving two-electron redox cycles. The isolobal analogy between Au(I/III) and Pd(0/II) complexes is stimulating the search for “gold versions” of palladium-catalyzed transformations, and in particular cross-couplings. Accordingly, a variety of intraand intermolecular coupling reactions have been shown to be efficiently catalyzed by gold complexes in the presence of strong exogeneous oxidants. A few studies have also suggested the ability of gold complexes to promote crosscouplings in a similar way to palladium catalysts, but possible palladium contamination has recently cast doubt on the involvement of gold complexes in these transformations. These recent contributions as a whole open a new facet of gold catalysis, and at the same time, raise fundamental questions about the properties and behavior of gold complexes. In particular, better knowledge on the basic elementary steps involved in redox cycles (oxidative addition, transmetallation, migratory insertion, b elimination, reductive elimination) is highly desirable to help further developments into this emerging area of gold catalysis. In this regard, it is striking to note that, despite the stability and availability of both Au and Au complexes, oxidative addition reactions at gold remain very elusive. In fact, the reluctance of gold to undergo oxidative addition is presumed to be an intrinsic limitation to achieve Au/Au redox cycles akin to those typically operating with Pd. Rare evidence for oxidative addition at gold dates back to the pioneering contribution of Kochi on the reaction of highly polarized sp C X bonds with alkyl gold(I) complexes. Furthermore, Bachman et al. reported in 2008 the first and to date only example of activation of an apolar s bond with gold. Fluorinated disulfides were shown to oxidatively add and reductively eliminate at gold in a reversible fashion. Seeking to gain more insight into s-bond activation and oxidative addition at gold, we became interested in extrapolating the strategy we have been developing for some years to study unusual metal–ligand interactions. Our approach consists in the use of anchoring phosphine sites, and this has been applied to support the coordination of Lewis acids and most recently s-Si Si bonds. Accordingly, the coordination of the diphosphine–disilane ligand 1 to copper and silver has been explored (Scheme 1), leading to the first structural characterization of a s complex with a coinage metal. As a further extension of this work, we were intrigued by the coordination of 1 to gold, considering three possible scenarios: 1) Formation of a s-complex upon side-on coordination of the disilane moiety, as observed with copper; 2) no participation of the s-Si Si bond to the coordination, as observed with silver; or 3) oxidative addition of the s-Si Si bond to give a bis(silyl)gold(III) complex. Oxidative addition

Hypervalent Silicon Compounds by Coordination of Diphosphine–Silanes to Gold

Coordination of ambiphilic diphosphine-silane ligands [o-(iPr(2)P)C(6)H(4)](2)Si(R)F (R=F, Ph, Me) to AuCl affords pentacoordinate neutral silicon compounds in which the metal atom acts as a Lewis base. X-ray diffraction analyses, NMR spectroscopy, and DFT calculations substantiate the presence of Au→Si interactions in these complexes, which result in trigonal-bipyramidal geometries around silicon. The presence of a single electron-withdrawing fluorine atom is sufficient to observe coordination of the silane as a σ-acceptor ligand, provided it is positioned trans to gold. The nature of the second substituent at silicon (R=F, Ph, Me) has very little influence on the magnitude of the Au→Si interaction, in marked contrast to N→Si adducts. According to variable-temperature and 2D EXSY NMR experiments, the apical/equatorial positions around silicon exchange in the slow regime of the NMR timescale. The two forms, with the fluorine atom in trans or cis position to gold, were characterized spectroscopically and the activation barrier for their interconversion was estimated. The bonding and relative stability of the two isomeric structures were assessed by DFT calculations.

Enhanced π‐Backdonation from Gold(I): Isolation of Original Carbonyl and Carbene Complexes

The specific electronic properties of bent o-carborane diphosphine gold(I) fragments were exploited to obtain the first classical carbonyl complex of gold [(DPCb)AuCO](+) (ν(CO)=2143 cm(-1) ) and the diphenylcarbene complex [(DPCb)Au(CPh2 )](+) , which is stabilized by the gold fragment rather than the carbene substituents. These two complexes were characterized by spectroscopic and crystallographic means. The [(DPCb)Au](+) fragment plays a major role in their stability, as substantiated by DFT calculations. The bending induced by the diphosphine ligand substantially enhances π-backdonation and thereby allows the isolation of carbonyl and carbene complexes featuring significant π-bond character.