Substituent Effects in the Cyclization of Yne-Diols Catalyzed by Gold Complexes Featuring L2/Z-Type Diphosphinoborane Ligands

Sulfonium cations as versatile strongly π-acidic ligands

More than a century old, sulfonium cations are still intriguing species in the landscape of organic chemistry. On one hand they have found broad applications in organic synthesis and materials science, but on the other hand, while isoelectronic to the ubiquitous tertiary phosphine ligands, their own coordination chemistry has been neglected for the last three decades. Here we report the synthesis and full characterization of the first Rh(i) and Pt(ii) complexes of sulfonium. Moreover, for the first time, coordination of an aromatic sulfonium has been established. A thorough computational analysis of the exceptionally short S–Rh bonds obtained attests to the strongly π-accepting nature of sulfonium cations and places them among the best π-acceptor ligands available today. Our calculations also show that embedding within a pincer framework enhances their π-acidity even further. Therefore, in addition to the stability and modularity that these frameworks offer, our pincer complexes might open the way for sulfonium cations to become powerful tools in π-acid catalysis.

Back to the scene: while isolobal to the ubiquitous tertiary phosphines, sulfonium cations as ligands were neglected for decades. This work revives the coordination chemistry of these species showing their potential as ligands for π-acid catalysis.

[Development of Gold-catalyzed Reaction Utilizing Electron Acceptability of Z-type Ligand]

The interaction between transition metals and ligands is important for catalytic reactions. The ligands are largely dominated by the covalent X-type (hydride, alkyl and halogen) and/or dative L-type ligands (e.g., P, N, CO, olefin, etc.). Therefore, the interaction of the Z-type ligands (B, Al and Si, etc.) with transition metals is emerging as a new concept for the reactivity of the metal center. Recently, we developed the synthesis of the gold complex Au(DPB)X (DPB=diphosphine-borane) featuring the Z-type ligand, and their catalytic reaction. The gold catalysts showed a high activity compared to the general catalysts (without Z-ligand) for the various cyclization reactions due to the electron-withdrawing effect of the Z-ligand on the coordinating gold center. In this review, first the structure analysis of the synthesized Au→Z complex is introduced in detail, and second, the catalytic reactions based on the alkyne activation are described.

Evaluation of Pd→B Interactions in Diphosphinoborane Complexes and Impact on Inner-Sphere Reductive Elimination

The dative Pd→B interaction in a series of ^RDPB^R’ Pd⁰ and Pd^II complexes (^RDPB^R’=(o‐PR₂C₆H₄)₂BR’, diphosphinoborane) was analyzed using XRD, ¹¹B NMR spectroscopy and NBO/NLMO calculations. The borane acceptor discriminates between the oxidation state Pd^II and Pd⁰, stabilizing the latter. Reaction of lithium amides with [(^RDPB^R’)Pd^II(4‐NO₂C₆H₄)I] chemoselectively yields the C−N coupling product. DFT modelling indicates no significant impact of Pd^II→B coordination on the inner‐sphere reductive elimination rate.

Catalysis using transition metal complexes featuring main group metal and metalloid compounds as supporting ligands

Recent development in catalytic application of transition metal complexes having an M-E bond (E = main group metal or metalloid element), which is stabilized by a multidentate ligand, is summarized. Main group metal and metalloid supporting ligands furnish unusual electronic and steric environments and molecular functions to transition metals, which are not easily available with standard organic supporting ligands such as phosphines and amines. These characteristics often realize remarkable catalytic activity, unique product selectivity, and new molecular transformations. This perspective demonstrates the promising utility of main group metal and metalloid compounds as a new class of supporting ligands for transition metal catalysts in synthetic chemistry.

Rhodium-Catalyzed C-H Activation Enabled by an Indium Metalloligand

Rhodium complexes with an indium metalloligand were successfully synthesized by utilizing a pyridine-tethered cyclopentadienyl ligand as a support for an In-Rh bond. The indium metalloligand dramatically changes the electronic and redox properties of the rhodium metal, thereby enabling catalysis of sp² C-H bond activation.

Reaction selectivity of homochiral versus heterochiral intermolecular reactions of prochiral terminal alkynes on surfaces

Controlling selectivity between homochiral and heterochiral reaction pathways on surfaces remains a great challenge. Here, competing reactions of a prochiral alkyne on Ag(111): two-dimensional (2D) homochiral Glaser coupling and heterochiral cross-coupling with a Bergman cyclization step have been examined. We demonstrate control strategies in steering the reactions between the homochiral and heterochiral pathways by tuning the precursor substituents and the kinetic parameters, as confirmed by high-resolution scanning probe microscopy (SPM). Control experiments and density functional theory (DFT) calculations reveal that the template effect of organometallic chains obtained under specific kinetic conditions enhances Glaser coupling between homochiral molecules. In contrast, for the reaction of free monomers, the kinetically favorable reaction pathway is the cross-coupling between two heterochiral molecules (one of them involving cyclization). This work demonstrates the application of kinetic control to steer chiral organic coupling pathways at surfaces.

Controlling selectivity between homochiral and heterochiral reaction pathways on surfaces is intriguing but challenging. Here, the authors demonstrate strategies in steering the reactions of prochiral terminal alkynes between the homochiral and heterochiral pathways by tuning the precursor substituents and the kinetic parameters.

Silver halide complexes of a borane/bis(phosphine) ligand

Silver halide complexes of a borane/bis(phosphine) ligand have been prepared and characterized. With AgF, the borane abstracts fluoride, resulting in a zwitterionic complex. With AgCl, AgBr, and AgI, the halide stays coordinated to Ag, with little to no Ag-B interaction.

Unsaturated vicinal frustrated phosphane/borane Lewis pairs as ligands in gold(i) chemistry

The P/B FLP 6, formed by the 1,1-hydroboration of dimesitylphosphino(trimethylsilyl)acetylene with HB(C6F5)2 forms the Au(i)X complexes 9a (X: Cl) and 10a (X: NTf2), respectively. Both show marked AuB interactions. The P/B FLP isomer 8, featuring the bulky SiMe3 substituent at the carbon adjacent to boron forms gold complexes 9b and 10b, both of which show weaker AuB interactions. Complex 10a was employed in the catalytic hydroamination of a series of alkynes with p-toluidine. Complexes 9 and 10 were characterized by X-ray diffraction.

Synthesis and ligand substitution reactions of κ4-B,S,S′,S′′-ruthenaboratranes

Ruthenaboratranes of the form [Ru(CO)L{κ⁴-B(mt)₃}] (mt = N-methimazolyl) arise via substitution of the PPh₃ ligand in [Ru(CO)(PPh₃){κ⁴-B(mt)₃}] by L (L = PMe₂Ph, PMe₃, P(OMe)₃, P(OEt)₃, P(OPh)₃) or reactions of [RuCl(R)(CO)L_n] (R = Ph, CHCHPh; n = 2, L = PCy₃; n = 3, L = P(OMe)₃, PMe₂Ph) with Na[HB(mt)₃].

Highly efficient palladium-catalysed carbon dioxide hydrosilylation employing PMP ligands

A series of zero-valent Pd complexes featuring PMP ligands (M = Li^I, Cu^I, Zn^II) is reported. An excellent activity in chemoselective CO₂ hydrosilylation producing silyl formate is observed (TOF_1/2 = 3000 h⁻¹).

Modulating the σ-Accepting Properties of an Antimony Z-type Ligand via Anion Abstraction: Remote-Controlled Reactivity of the Coordinated Platinum Atom

In search of ligand platforms, which can be used to remotely control the catalytic activity of a transition metal, we have investigated the coordination noninnocence of ambiphilic L₂/Z-type ligands containing a trifluorostiborane unit as a Lewis acid. The known dichlorostiboranyl platinum complex (( o-(Ph₂P)C₆H₄)₂SbCl₂)PtCl (1) reacts with TlF in the presence of acetonitrile (MeCN) and cyclohexyl isocyanide (CyNC) to afford the trifluorostiborane platinum complexes 2 ((( o-(Ph₂P)C₆H₄)₂SbF₃)Pt-NCMe) and 3 ((( o-(Ph₂P)C₆H₄)₂SbF₃)Pt-CNCy), respectively. Formation of these complexes, which results from a redistribution of the halide ligands about the dinuclear core, affects the nature of the Pt-Sb bond. The latter switches from covalent in 1 to polar covalent (or dative) in 2 and 3 where the trifluorostiborane moiety engages the platinum center in a Pt → Sb interaction. The polarity of the Pt-Sb bond can be modulated further by abstraction of an antimony-bound fluoride ligand using B(C₆F₅)₃. These reactions afford the cationic complexes [(( o-(Ph₂P)C₆H₄)₂SbF₂)Pt-NCMe]⁺ ([5]⁺) and [(( o-(Ph₂P)C₆H₄)₂SbF₂)Pt-CNCy]⁺ ([6]⁺) which have been isolated as [BF(C₆F₅)₃]^- salts. These complexes possess a highly Lewis acidic difluorostibonium moiety, which exerts an intense draw on the electron density of the platinum center. As a result, the latter becomes significantly more electrophilic. In the case of [5]⁺, which contains a labile acetonitrile ligand, this increased electrophilicity translates into increased carbophilicity as reflected by the ability of this complex to promote enyne cyclization reactions. These results demonstrate that the coordination noninnocence of antimony Z-ligands can be used to adjust the catalytic activity of an adjoining metal center.

Activation of disulfide bond cleavage triggered by hydrophobization and lipophilization of functionalized dihydroasparagusic acid

Introduction of a hydrophilic group into dihydroasparagusic acid (DHAA) indicated higher reduction ability of disulfide in protein and lower air oxidation.