Reactivity of Ir-NSiN Complexes: Ir-Catalyzed Dehydrogenative Silylation of Carboxylic Acids

X-type silyl ligands for transition-metal catalysis

Given the critical importance of novel ligand development for transition-metal (TM) catalysis, as well as the resurgence of the field of organosilicon chemistry and silyl ligands, to summarize the topic of X-type silyl ligands for TM catalysis is highly attractive and timely. This review particularly emphasizes the unique σ-donating characteristics and trans-effects of silyl ligands, highlighting their crucial roles in enhancing the reactivity and selectivity of various catalytic reactions, including small molecule activation, Kumada cross-coupling, hydrofunctionalization, C-H functionalization, and dehydrogenative Si-O coupling reactions. Additionally, future developments in this field are also provided, which would inspire new insights and applications in catalytic synthetic chemistry.

Molecular Structures of the Silicon Pyridine-2-(thi)olates Me3Si(pyX), Me2Si(pyX)2 and Ph2Si(pyX)2 (py = 2-Pyridyl, X = O, S), and Their Intra- and Intermolecular Ligand Exchange in Solution

A series of pyridine-2-olates (pyO) and pyridine-2-thiolates (pyS) of silicon was studied in solid state and in solution. The crystal structures of Me3Si(pyO) (1a), Me3Si(pyS) (1b), Me2Si(pyO)2 (2a), Me2Si(pyS)2 (2b), Ph2Si(pyO)2 (3a) and Ph2Si(pyS)2 (3b) were determined by X-ray diffraction. For that purpose, crystals of the (at room temperature) liquid compounds 1a and 1b were grown in a capillary on the diffractometer. Compounds 1a, 1b, 2a, 2b and 3a feature tetracoordinate silicon atoms in the solid state, whereas 3b gave rise to a series of four crystal structures in which the Si atoms of this compound are hexacoordinate. Two isomers (3b1 with all-cis arrangement of the C2N2S2 donor atoms in P, and 3b2 with trans S-Si-S axis in P21/n) formed individual crystal batches, which allowed for their individual 29Si NMR spectroscopic study in the solid state (the determination of their chemical shift anisotropy tensors). Furthermore, the structures of a less stable modification of 3b2 (in C2/c) as well as a toluene solvate 3b2×(toluene) (in P) were determined. In CDCl3, the equimolar solutions of the corresponding pairs of pyO and pyS compounds (2a/2b and 3a/3b) showed substituent scrambling with the formation of the products Me2Si(pyO)(pyS) (2c) and Ph2Si(pyO)(pyS) (3c), respectively, as minor components in the respective substituent exchange equilibrium.

P-Ru-Complexes with a Chelate-Bridge-Switch: A Comparison of 2-Picolyl and 2-Pyridyloxy Moieties as Bridging Ligands

Starting from [Ru(pyO)₂(nbd)] 1 and a N,P,N-tridentate ligand (2a: PhP(pic)₂, 2b: PhP(pyO)₂) (nbd = 2,5-norbornadiene, pic = 2-picolyl = 2-pyridylmethyl, pyO = 2-pyridyloxy = pyridine-2-olate), the compounds [PhP(μ-pic)₂(μ-pyO)Ru(κ²-pyO)] (3a) and [PhP(μ-pyO)₃Ru(κ²-pyO)] (3b), respectively, were prepared. Reaction of compounds 3 with CO and CNtBu afforded the opening of the Ru(κ²-pyO) chelate motif with the formation of compounds [PhP(μ-pic)₂(μ-pyO)Ru(κ-O-pyO)(CO)] (4a), [PhP(μ-pic)₂(μ-pyO)₂Ru(CNtBu)] (5a), [PhP(μ-pyO)₄Ru(CO)] (4b) and [PhP(μ-pyO)₄Ru(CNtBu)] (5b). In dichloromethane solution, 4a underwent a reaction with the solvent, i.e., substitution of the dangling pyO ligand by chloride with the formation of [PhP(μ-pic)₂(μ-pyO)Ru(Cl)(CO)] (6a). The new complexes 3a, 4a, 5a, 5b and 6a were characterized by single-crystal X-ray diffraction analyses and multi-nuclear (¹H, ¹³C, ³¹P) NMR spectroscopy. The different coordination behaviors of related pairs of molecules (i.e., pairs of 3, 4 and 5), which depend on the nature of the P-Ru-bridging ligand moieties (μ-pic vs. μ-pyO), were also studied via computational analyses using QTAIM (quantum theory of atoms in molecules) and NBO (natural bond orbital) approaches, as well as the NCI (non-covalent interactions descriptor) for weak intramolecular interactions.

Copper-Catalyzed Enantioselective Reductive Aldol Reaction of α,β-Unsaturated Carboxylic Acids to Ketones: Silanes as Activator and Transient Protecting Group

We have developed the first enantioselective reductive aldol reaction of unprotected α,β-unsaturated carboxylic acids by employing a copper/bisphosphine catalyst. The reaction features in situ protection and activation of an α,β-unsaturated carboxylic acid by a hydrosilane. The copper enolate formed in situ reacts with a ketone to afford the β-hydroxy carboxylic acid with excellent enantioselectivity (up to 99% ee). The corresponding gram-scale reaction with a low catalyst loading and the derivatization of the β-hydroxy carboxylic acids highlight the practicality of this transformation.

Origin of the Ir-Si bond shortening in Ir-NSiN complexes

The Ir-Si bond distances reported for Ir-(fac-κ3-NSiNOPy) and Ir-(fac-κ3-NSiN4MeOPy) species (NSiNOPy = bis(pyridine-2-yloxy)methylsilyl and NSiN4MeOPy = bis(4-methyl-pyridine-2-yloxy)methylsily) are in the range of 2.220-2.235 Å. These values are in the lowest limit of the Ir-Si bond distances found in the Cambridge Structural Database (CSD). To understand the origin of such remarkable shortening, a computational study of the bonding situation of representative examples of Ir-(fac-κ3-NSiN) species has been carried out. It is found that the Ir-Si bond can be described as an electron-sharing (i.e. covalent) bond. Despite that, this bond is highly polarized and as a result, the contribution of the electrostatic attractions to the bonding is rather significant. Indeed, there exists a linear relationship (R2 = 0.97) between the Ir-Si bond distance and the extent of the computed electrostatic interactions, which indicates that the ionic contribution to the bonding is mainly responsible for the observed Ir-Si bond shortening.

Selective reduction of formamides to O-silylated hemiaminals or methylamines with HSiMe2Ph catalyzed by iridium complexes

The reaction of (4-methyl-pyridin-2-iloxy)ditertbutylsilane (NSitBu-H, 1) with [IrCl(coe)2]2 affords the iridium(iii) complex [Ir(H)(Cl)(κ2-NSitBu)(coe)] (2), which has been fully characterized including X-ray diffraction studies. The reaction of 2 with AgCF3SO3 leads to the formation of species [Ir(H)(CF3SO3)(κ2-NSitBu)(coe)] (3). The iridium complexes 2 and 3 are effective catalysts for the reduction of formamides with HSiMe2Ph. The selectivity of the reduction process depends on the catalyst. Thus, by using complex 2, with a chloride ancillary ligand, it has been possible to selectively obtain the corresponding O-silylated hemiaminal by reaction of formamides with one equivalent of HSiMe2Ph, while complex 3, with a triflate ligand instead of chloride, catalyzed the selective reduction of formamides to the corresponding methylamine.

Ir-catalyzed selective reduction of CO2 to the methoxy or formate level with HSiMe(OSiMe3)2

Ir-NSi-based catalysts allow controlling the selective reduction of CO₂ with HSiMe(OSiMe₃)₂ to afford methoxysilane or silyl formate.