Palladium-Catalyzed CH Fluorosilylation of 2-Phenylpyridines: Synthesis of Silafluorene Equivalents

Non-Covalent Interaction-Controlled Site-Selective C-H Transformations

Site-selective C-H transformations are important to obtain desired compounds as single products in a highly efficient manner. However, it is generally difficult to achieve such transformations because organic substrates contain many C-H bonds with similar reactivities. Therefore, the development of practical and efficient methods for controlling site selectivity is highly desirable. The most frequently used strategy is "directing group method". Although this method is highly effective and promotes site-selective reactions, it has several limitations. Our group recently reported other methods to achieve site-selective C-H transformations using non-covalent interactions between a substrate and a reagent or a catalyst and a substrate (non-covalent method). In this personal account, the background of site-selective C-H transformations, our reaction design to achieve site-selective C-H transformations, and recently reported reactions are explained.

Silver(I)-Catalyzed C4-H Amination of 1-Naphthylamine Derivatives with Azodicarboxylates at Room Temperature

A highly facile and efficient protocol for silver(I)-catalyzed C4–H amination of 1-naphthylamine derivatives with readily available azodicarboxylates utilizing picolinamide as a bidentate directing group is reported, providing an alternative strategy for the synthesis of 4-aminated 1-naphthylamine derivatives. The reaction proceeded smoothly in acetone at room temperature undergoing a self-redox process under the base and external oxidant-free conditions, affording the desired products with good to excellent yields.

Synthesis of fluorenes and their related compounds from biaryls and Meldrum's acid derivatives

A new synthetic method for preparing fluorenes from amino group-containing biaryls and Meldrum's acid derivatives was developed. The reaction proceeded without a catalyst and loss of functional groups. The corresponding six- and seven-membered cyclic products were obtained using biaryl ether and ortho-terphenyl as substrates, respectively.

Merging Photoredox Catalysis with Transition Metal Catalysis: Direct C4-H Sulfamidation of 1-Naphthylamine Derivatives

A mild and efficient protocol for the copper(I)-catalyzed C4-H sulfamidation of 1-naphthylamine derivatives with diphenylsulfonimide (NHSI) was explored at room temperature, affording the desire produces in moderate to good yields. The control experiments indicated that this visible-light-promoted reaction might proceed via a single-electron-transfer process. In addition, preliminary DFT studies for the intermediates in the catalytic cycle were also explored, indicating that the C4 site in the naphthyl ring is the most likely electrophilic reactive site and providing some exact basis for the plausible mechanism.

Activation of the Si–B interelement bond related to catalysis

Covering the past seven years, this review comprehensively summarises the latest progress in the preparation and application of Si–B reagents, including the discussion of relevant reaction mechanisms.

Cobalt-catalyzed C8–H sulfonylation of 1-naphthylamine derivatives with sodium sulfinates

A facile and efficient protocol for cobalt-catalyzed regioselective C8–H sulfonylation of 1-naphthylamine derivatives with sodium sulfinates was developed to afford sulfonylated naphthylamines in moderate to good yields.

Copper(II)-Catalyzed Direct Amination of 1-Naphthylamines at the C8 Site

A new and simple protocol for C8-H amination of 1-naphthylamine derivatives was developed using copper salt and di-tert-butyl peroxide (DTBP) as the catalyst and oxidant, respectively. This reaction proceeded smoothly under the additive and solvent-free conditions. In addition, this amination showed excellent regioselectivity and could tolerate various functional groups.

Synthesis, structures and photophysical properties of hexacoordinated organosilicon compounds with 2-(2-pyridyl)phenyl groups

We synthesised novel hexacoordinated organosilicon compounds with two 2-(2-pyridyl)phenyl groups. Single-crystal X-ray structure analyses indicated that Lewis acid-base interactions exist between the silicon atom and two nitrogen atoms of the pyridine rings, and that hexacoordinated organosilicon compounds have slightly distorted octahedral structures in the solid state. The hexacoordinated organosilicon compounds are stable in air, water, heat, acids, and bases. The fluorescent quantum yield increased dramatically and a significant red-shift of the maximum fluorescence wavelength was observed with the introduction of amino groups on the 2-(2-pyridyl)phenyl aromatic rings. The fluorescence colours of a hexacoordinated organosilicon compound with two amino groups can be switched by protonation and deprotonation (neutralisation) of the amino groups.

Recent progress of transition metal-catalysed regioselective C–H transformations based on noncovalent interactions

Recent advances in the transition metal-catalysed regioselective C–H transformations controlled by noncovalent interactions between substrates and reagents or ligands are summarised.

C-H addition reactivity of 2-phenylpyridine and 2,2'-bipyridine with pentaphenylborole

The reactions of pentaphenylborole with pyridines with a pendent aryl group in the 2-position, specifically 2-phenylpyridine and 2,2'-bipyridine, readily afforded adducts. Upon thermolysis, the adducts converted to ortho C-H addition products with the two substituents introduced onto the diene on the boracyclic carbon atoms adjacent to boron in a syn fashion. Photolysis of the syn 2-phenypyridine product led to isomerization to the anti conformer which did not readily occur for the 2,2'-bipyridine complex. Such C-H bond reactivity is uncommon for main group elements and provides insight into methodologies to access boron frameworks.

Ir-Catalyzed Enantioselective, Intramolecular Silylation of Methyl C-H Bonds

We report highly enantioselective intramolecular, silylations of unactivated, primary C(sp³)-H bonds. The reactions form dihydrobenzosiloles in high yields with excellent enantioselectivities by functionalization of enantiotopic methyl groups under mild conditions. The reaction is catalyzed by an iridium complex generated from [Ir(COD)OMe]₂ and chiral dinitrogen ligands that we recently disclosed. The C-Si bonds in the enantioenriched dihydrobenzosiloles were further transformed to C-Cl, C-Br, C-I, and C-O bonds in final products. The potential of this reaction was illustrated by sequential C(sp³)-H and C(sp²)-H silylations and functionalizations, as well as diastereoselective C-H silylations of a chiral, natural-product derivative containing multiple types of C-H bonds. Preliminary mechanistic studies suggest that C-H cleavage is the rate-determining step.

Synthesis and Properties of Sila[n]helicenes via Dehydrogenative Silylation of C-H Bonds under Rhodium Catalysis

Use of a rhodium catalyst with (R)-(S)-BPPFA ligand allows efficient synthesis of sila[n]helicenes via dehydrogenative silylation of C-H bonds. By selecting the proper ligands, the current method provides the ability to prepare unsymmetrical sila[n]helicene derivatives without any oxidants. The resulting sila[6]helicene is a rare example of a five-membered ring-fused [6]helicene, which was isolated as a single pure enantiomer without substituents on the terminal benzene rings.

Regioselectivity in C-H activation: reagent control in cyclometallation of 2-(1-naphthyl)-pyridine

2-(1-Naphthyl)-pyridine () possesses sp(2) C-H bonds in both the γ- and δ-positions and is therefore a suitable substrate for studying the cyclometallation selectivity with different reagents and conditions. Such selectivity studies are reported. Based on deuterium-exchange experiments it is concluded that cycloruthenation with RuCl2(p-cymene) dimer is reversible with kinetic and thermodynamic preference for γ-substitution. Electrophilic cycloborylation, on the other hand, shows unusual δ-substitution. The previously published cyclopalladation and cycloauration of the substrate was studied in detail and was shown to be irreversible; they proceed under kinetic control and give γ- and δ-substitution for palladium and gold, respectively.

Boron-Catalyzed Aromatic C-H Bond Silylation with Hydrosilanes

Metal-free catalytic C-H silylation of a series of aromatic compounds such as N,N-disubstituted anilines with various hydrosilanes has been achieved for the first time using commercially available B(C6F5)3 as a catalyst. This protocol features simple and neutral reaction conditions, high regioselectivity, wide substrate scope (up to 40 examples), Si-Cl bond compatibility, and no requirement for a hydrogen acceptor.

Silylations of Arenes with Hydrosilanes: From Transition-Metal-Catalyzed C¢X Bond Cleavage to Environmentally Benign Transition-Metal-Free C¢H Bond Activation

The construction of carbon-silicon bonds is highlighted as an exciting achievement in the field of organosilicon chemistry and green chemistry. Recent developments in this area will enable the sustainable chemical conversion of silicon resources into synthetically useful compounds. Especially, the catalytic silylation through C¢H bond activation without directing groups and hydrogen acceptors is one of the most challenging topics in organic chemistry and green chemistry. These remarkable findings on catalytic silylation can pave the way to a more environmentally benign utilization of earth-abundant silicon-based resources in synthetic chemistry.

Modular Approach to Reductive C(sp2)-H and C(sp3)-H Silylation of Carboxylic Acid Derivatives through Single-Pot, Sequential Transition Metal Catalysis

We report a modular approach to catalytic reductive Csp2-H and Csp3-H silylation of carboxylic acid derivatives encompassing esters, ketones, and aldehydes. Choice of either an Ir(I)/Rh(I) or Rh(I)/Rh(I) sequence leads to either exhaustive reductive ester or reductive ketone/aldehyde silylation, respectively. Notably, a catalyst-controlled direct formation of doubly reduced silyl ethers is presented, specifically via Ir-catalyzed exhaustive hydrosilylation. The resulting silyl ethers undergo Csp2-H and benzylic Csp3-H silylation in a single vessel.

Deprotonative C-H silylation of functionalized arenes and heteroarenes using trifluoromethyltrialkylsilane with fluoride

A highly selective C-H silylation reaction of functionalized arenes and heteroarenes was developed using Ruppert-Prakash reagent (TMSCF3) activated by alkali metal fluoride. TMSCF3 is considered to play dual roles as a precursor of a mild base and also as a silicon electrophile. The silylation is compatible with sensitive functional groups such as halogen and nitro groups.

2,2′-Disilylazobenzenes featuring double intramolecular nitrogen⋯silicon coordination: a photoisomerizable fluorophore

Fluorescent (E)-4,4′-dimethyl-2,2′-bis(trimethylsilyl)azobenzene was synthesized.

Reductive arene ortho-silanolization of aromatic esters with hydridosilyl acetals

The design and application of a single-pot, reductive arene C–H bond silanolization of esters for synthesis of ortho-formyl arylsilanols.

Rhodium-catalysed synthesis of multi-substituted silylindenes from aryl alkynes and hydrosilanes via C–H bond activation

We successfully developed rhodium-catalysed synthesis of multi-substituted silylindenes from 2 equivalents of aryl alkynes and 1 equivalent of hydrosilanes in moderate to good yields via C–H bond activation for the first time.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.

Metal controlled regioselectivity in the cyclometallation of 2-(1-naphthyl)-pyridine

Cyclometallations of 2-(1-naphthyl)-pyridine with gold and palladium salts proceed with completely orthogonal regioselectivities.

Iridium-catalyzed regioselective silylation of aromatic and benzylic C-H bonds directed by a secondary amine

Reported herein is an iridium-catalyzed, regioselective silylation of the aromatic C-H bonds of benzylamines and the benzylic C-H bonds of 2,N-dialkylanilines. In this process, (hydrido)silyl amines, generated in situ by dehydrogenative coupling of benzylamine or aniline with diethylsilane, undergo selective silylation at the C-H bond γ to the amino group. The products of this silylation are suitable for subsequent oxidation, halogenation, and cross-coupling reactions to deliver benzylamine and arylamine derivatives.