Transition Metal Catalyzed Hiyama Cross-Coupling: Recent Methodology Developments and Synthetic Applications

Hiyama cross-coupling is a versatile reaction in synthetic organic chemistry for the construction of carbon-carbon bonds. It involves the coupling of organosilicons with organic halides using transition metal catalysts in good yields and high enantioselectivities. In recent years, hectic progress has been made by researchers toward the synthesis of diversified natural products and pharmaceutical drugs using the Hiyama coupling reaction. This review emphasizes the recent synthetic developments and applications of Hiyama cross-coupling.

Stabilization of the Pd–NHC framework with 1,2,4-triazol-5-ylidene ligands toward decomposition in alkaline media

New NHC ligands containing a base-ionizable RNH substituent at the C3 atom of the 1,2,4-triazole ring provide superior stability of the Pd–NHC bond against cleavage in strong alkaline media.

Enhanced π-back-donation resulting in the trans labilization of a pyridine ligand in an N-heterocyclic carbene (NHC) PdII precatalyst: a case study

The molecular structure of the benzimidazol-2-ylidene–PdCl2–pyridine-type PEPPSI (pyridine-enhanced precatalyst, preparation, stabilization and initiation) complex {1,3-bis[2-(diisopropylamino)ethyl]benzimidazol-2-ylidene-κC 2}dichlorido(pyridine-κN)palladium(II), [PdCl2(C5H5N)(C23H40N4)], has been characterized by elemental analysis, IR and NMR spectroscopy, and natural bond orbital (NBO) and charge decomposition analysis (CDA). Cambridge Structural Database (CSD) searches were used to understand the structural characteristics of the PEPPSI complexes in comparison with the usual N-heterocyclic carbene (NHC) complexes. The presence of weak C—H...Cl-type hydrogen-bond and π–π stacking interactions between benzene rings were verified using NCI plots and Hirshfeld surface analysis. The preferred method in the CDA of PEPPSI complexes is to separate their geometries into only two fragments, i.e. the bulky NHC ligand and the remaining fragment. In this study, the geometry of the PEPPSI complex is separated into five fragments, namely benzimidazol-2-ylidene (Bimy), two chlorides, pyridine (Py) and the PdII ion. Thus, the individual roles of the Pd atom and the Py ligand in the donation and back-donation mechanisms have been clearly revealed. The NHC ligand in the PEPPSI complex in this study acts as a strong σ-donor with a considerable amount of π-back-donation from Pd to Ccarbene. The electron-poor character of PdII is supported by π-back-donation from the Pd centre and the weakness of the Pd—N(Py) bond. According to CSD searches, Bimy ligands in PEPPSI complexes have a stronger σ-donating ability than imidazol-2-ylidene ligands in PEPPSI complexes.

Crystal structure of 1-butyl-3-{2-[(indan-5-yl)amino]-2-oxoeth-yl}-1H-imidazol-3-ium chloride

In the cation of the title mol-ecular salt, C18H24N3O+·Cl-, an intra-molecular C-H⋯O hydrogen bond stabilizes the almost coplanar orientation of the aromatic ring of the indane unit and the amide plane. In the crystal, the packing is dominated by inter-molecular C-H⋯Cl hydrogen-bonding inter-actions that result in the formation of slab-like structures propagating along [010]. The slabs are linked by weak C-H⋯O inter-actions, forming layers lying parallel to (100). The methyl-ene carbon atom of the indanyl substituent is disordered over two positions with a refined occupancy ratio of 0.84 (2):0.16 (2). The crystal studied was refined as a twin with matrix [1 0 0.9, 0 0, 0 0 ]; the resulting BASF value is 0.30.

In situ generated and stabilized Pd nanoparticles by N2,N4,N6-tridodecyl-1,3,5-triazine-2,4,6-triamine (TDTAT) as a reactive and efficient catalyst for the Suzuki–Miyaura reaction in water

The Suzuki–Miyaura coupling reaction was efficiently catalyzed by the in situ generated and stabilized Pd nanoparticles employing TDTAT in water.

Continuous flow Negishi cross-couplings employing silica-supported Pd-PEPPSI–IPr precatalyst

The synthesis of a triethoxysilyl functionalisedPd-PEPPSI–IPrcomplex preparedviaazide–alkyne cycloaddition is described.

Fluoride-free Hiyama coupling by palladium abnormal N-heterocyclic carbene complexes

A series of Pd complexes of the 1,2,3-triazole based abnormal NHC ligands of the type (a-NHC)PdI₂(L) [L = NC₅H₅ and PPh₃] successfully catalyzed the fluoride-free Hiyama coupling in air.

Rollover cyclometalation pathway in rhodium catalysis: dramatic NHC effects in the C-H bond functionalization

Organometallic chelates are readily obtained upon coordination of metal species to multidentate ligands. Because of the robust structural nature, chelation frequently serves as a driving force in the molecular assembly and chemical architecture, and they are used also as an efficient catalyst in numerous reactions. Described herein is the development of a Rh(NHC) catalytic system for the hydroarylation of alkenes and alkynes with 2,2'-bipyridines (bipy) and 2,2'-biquinolines; the most representative chelating molecules. Initially generated (bipy)Rh(NHC) chelates become labile because of the strong trans-effect of N-heterocyclic carbenes, thus weakening a rhodium-pyridyl bond, which is trans to the bound NHC. Subsequent rollover cyclometalation leads to the C-H bond activation, eventually giving rise to double functionalization of chelate molecules. Density functional calculations are in good agreement with our mechanistic proposal based on the experimental data. The present study elucidated for the first time the dramatic NHC effects on the rollover cyclometalation pathway enabling highly efficient and selective bisfunctionalization of 2,2'-bipyridines and 2,2'-biquinolines.

π-Face donation from the aromatic N-substituent of N-heterocyclic carbene ligands to metal and its role in catalysis

In this work, we calculate the redox potential in a series of Ir and Ru complexes bearing a N-heterocyclic carbene (NHC) ligand presenting different Y groups in the para position of the aromatic N-substituent. The calculated redox potentials excellently correlate with the experimental ΔE(1/2) potentials, offering a handle to rationalize the experimental findings. Analysis of the HOMO of the complexes before oxidation suggests that electron-donating Y groups destabilize the metal centered HOMO. Energy decomposition of the metal-NHC interaction indicates that electron-donating Y groups reinforce this interaction in the oxidized complexes. Analysis of the electron density in the reduced and oxidized states of representative complexes indicates a clear donation from the C(ipso) of the N-substituents to an empty d orbital on the metal. In case of the Ru complexes, this mechanism involves the Ru-alkylidene moiety. All of these results suggest that electron-donating Y groups render the aromatic N-substituent able to donate more density to electron-deficient metals through the C(ipso) atom. This conclusion suggests that electron-donating Y groups could stabilize higher oxidation states during catalysis. To test this hypothesis, we investigated the effect of differently donating Y groups in model reactions of Ru-catalyzed olefin metathesis and Pd-catalyzed C-C cross-coupling. Consistent with the experimental results, calculations indicate an easier reaction pathway if the N-substituent of the NHC ligand presents an electron-donating Y group.