Silyl-Heck Reactions for the Preparation of Unsaturated Organosilanes

Synergistic Palladium/Lewis Acid-Catalyzed Regio- and Stereo-divergent Bissilylation of Alkynoates: Scope, Mechanism, and Origin of Selectivity

Transition metal-catalyzed bissilylation reactions of alkynes with disilane reagents have become one of the most straightforward and efficient protocols to rapidly produce structurally diverse alkenyl silicon derivatives. In these reactions, the utilization of unsymmetrical disilane reagents provided the possibilities for reactivity enhancement as well as the synthetic merits in contrast to symmetrical disilane reagents. However, a major yet challenging objective is achieving precise control over the selectivity including the regioselectivity and the cis/trans-selectivity. Herein we realized the first divergent bissilylation of alkynoates with our developed air-stable disilane reagent 8-(2-substituted-1,1,2,2-tetramethyldisilanyl)quinoline (TMDQ) by means of synergistic Pd/Lewis acid catalytic system. The catalytic system precisely dictates the selectivity, resulting in the divergent synthesis of 1,2-bissilyl alkenes. The power of these 1,2-bissilyl alkenes serving as the key synthetic intermediates has been clearly demonstrated by rapid construction of diverse motifs and densely functionalized biologically active compounds. In addition, the origins of the switchable selectivities were well elucidated by experimental and computational studies on the reaction mechanism and were mainly attributed to different ligand steric effects, the use of the specific disilane reagent TMDQ and the different coordination modes of different Lewis acid with alkynoates.

Diverse Synthesis of Alkenylsilanes via Pd-Catalyzed Alkenyl C-H Silylation

Here, we disclose a general approach for the diverse synthesis of alkenylsilanes in a highly efficient, stereoselective, and atom-economic manner by leveraging the palladium-catalyzed disilylation reaction of 2-bromostyrene derivatives with hexamethyldisilane, which is suitable for the preparation of a series of disubstituted, trisubstituted, and tetrasubstituted alkenylsilanes. Furthermore, the resulting tetrasubstituted alkenylsilanes could be readily transformed into the corresponding diarylated benzosiloles, which have been proven to be a potential AIE material and a fluorene material.

Oxidative Addition of Silicon-Chloride Bonds to a Zerovalent Ruthenium Center and Direct Generation of an Ethylene Insertion Complex

The oxidative addition of a silicon-chloride (Si-Cl) bond to a metal center can be a key reaction step in coordinative silicon chemistry, but this reaction is seldom observed. Herein, we report direct oxidative addition of the Si-Cl bonds of dimethyldichlorosilane (Me₂SiCl₂) and cyclotrimethylenedichlorosilane [(CH₂)₃SiCl₂] to low-valent ruthenium complexes, yielding the 16e^- chloro(organosilyl)ruthenium complexes [N₃]Ru(Cl)(SiMe₂Cl) (4a) and [N₃]Ru(Cl)(SiCl(CH₂)₃) (4b) ([N₃] = 2,6-(MesN═CMe)₂C₅H₃N; Mes = 1,3,5-trimethylphenyl; Me = methyl). The reversible reaction of 4a with ethylene yields an 18e^- ethylene adduct, in which an ethylene is subsequently inserted into a ruthenium-silicon (Ru-Si) bond to produce the 16e^- complex [N₃]Ru(Cl)(CH₂CH₂SiMe₂Cl) (7). This study provides a good example of the direct generation of an ethylene insertion product, which is an important intermediate in the catalytic reduction of unsaturated molecules.

Palladium-Catalyzed anti-Carbosilylation of Alkynes to Access Isoquinolinone-Containing Exocyclic Vinylsilanes

A Pd-catalyzed trans-selective carbosilylation reaction of alkynes has been developed. The trans-vinylpalladium species, generated through intramolecular syn-carbopalladation of alkynes and subsequent cis-trans isomerization, were captured by hexamethyldisilane to form multisubstituted vinylsilanes. This reaction provides a useful strategy for the stereoselective synthesis of isoquinolinone-containing exocyclic tetrasubstituted vinylsilanes.

Iron-catalyzed para-selective C–H silylation of benzamide derivatives with chlorosilanes

This paper developed the para-selective silylation of benzamide derivatives with chlorosilanes using FeCl₂ catalysis.

Synthesis of Unsaturated Silyl Heterocycles via an Intramolecular Silyl-Heck Reaction

We report the synthesis of unsaturated silacycles via an intramolecular silyl-Heck reaction. Using palladium catalysis, silicon electrophiles tethered to alkenes cyclize to form 5- and 6-membered silicon heterocycles. The effects of alkene substitution and tether length on the efficiency and regioselectivity of the cyclizations are described. Finally, through the use of an intramolecular tether, the first examples of disubstituted alkenes in silyl-Heck reactions are reported.

Cross-Coupling of Heteroatomic Electrophiles

At the advent of cross-coupling chemistry, carbon electrophiles based on halides or pseudohalides were the only suitable electrophilic coupling partners. Almost two decades passed before the first cross-coupling reaction of heteroatom-based electrophiles was reported. Early work by Murai and Tanaka initiated investigations into silicon electrophiles. Narasaka and Johnson pioneered the way in the use of nitrogen electrophiles, while Suginome began the exploration of boron electrophiles. The chemistry reviewed within provides perspective on the use of heteroatomic electrophiles, specifically silicon-, nitrogen-, boron-, oxygen-, and phosphorus-based electrophiles in transition-metal catalyzed cross-coupling. For the purposes of this review, a loose definition of cross-coupling is utilized; all reactions minimally proceed via an oxidative addition event. Although not cross-coupling in a traditional sense, we have also included catalyzed reactions that join a heteroatomic electrophile with an in situ generated nucleophile. However, for brevity, those involving hydroamination or C-H activation as a key step are largely excluded. This work includes primary references published up to and including October 2018.

A Bench-Stable, Single-Component Precatalyst for Silyl-Heck Reactions

Studies of the silyl-Heck reaction aimed at identifying active palladium complexes have revealed a new species that is formed in situ. This complex has been identified as the palladium iodide dimer, [(JessePhos)PdI₂]₂, which has been found to be a competent single-component precatalyst for the silyl-Heck reaction. This complex is easily prepared and is temperature, moisture, and air stable. Additionally, this precatalyst provides higher activity and greater reproducibility compared to previous systems.

Stereoselective Synthesis of Z-Vinylsilanes via Palladium-Catalyzed Direct Intermolecular Silylation of C(sp2)-H Bonds

An efficient and convenient palladium-catalyzed direct intermolecular silylation of C(sp²)-H bonds by using disilanes as the silicon source with the assistance of a readily removable bidentate directing group is reported. This strategy provided a regio- and stereoselective protocol for exclusive synthesis of Z-vinylsilanes with reasonable to excellent yields and good functional group compatibility. Silylation of the isolated palladacycle intermediate revealed the Z-stereoselective pathway. Moreover, the practicality and effectiveness of this method were illustrated by a gram-scale experiment and further functionalization of the silylation product.

Direct Synthesis of Alkenyl Boronic Esters from Unfunctionalized Alkenes: A Boryl-Heck Reaction

We report the first example of a boryl-Heck reaction using an electrophilic boron reagent. This palladium-catalyzed process allows for the conversion of terminal alkenes to trans-alkenyl boronic esters using commercially available catecholchloroborane (catBCl). In situ transesterification allows for rapid access to a variety of boronic esters, amides, and other alkenyl boron adducts.

Simplified Preparation of Trialkylvinylsilanes via the Silyl-Heck Reaction Utilizing a Second Generation Catalyst

Recently we reported a second-generation ligand, bis(3,5-di-tert-butylphenyl)(tert-butyl)phosphine, for the preparation of allyl silanes using the silyl-Heck reaction. We now show that this new ligand also provides superior reactivity in the preparation of vinylsilanes from styrene derivatives. For the first time, this new ligand provides exceptionally high yields of trialkylvinylsilanes using a widely available palladium pre-catalyst, Pd₂(dba)₃. Finally, we demonstrate that this new catalyst system is able to form more highly decorated all carbon substituted vinylsilanes that have been shown to possess superior reactivity in oxidation and cross coupling reactions.

Rational design of a second generation catalyst for preparation of allylsilanes using the silyl-Heck reaction

Using rational ligand design, we have developed of a second-generation ligand, bis(3,5-di-tert-butylphenyl)(tert-butyl)phosphine, for the preparation of allylsilanes using the palladium-catalyzed silyl-Heck reaction. This new ligand provides nearly complete suppression of starting material alkene isomerization that was observed with our first-generation catalyst, providing vastly improved yields of allylsilanes from simple alkene starting materials. The studies quantifying the electronic and steric properties of the new ligand are described. Finally, we report an X-ray crystal structure of a palladium complex resulting from the oxidative addition of Me₃SiI using an analogous ligand that provides significant insight into the nature of the catalytic system.

The First Example of Nickel-Catalyzed Silyl-Heck Reactions: Direct Activation of Silyl Triflates Without Iodide Additives

For the first time, nickel-catalyzed silyl-Heck reactions are reported. Using simple phosphine-supported nickel catalysts, direct activation of silyl triflates has been achieved. These results contrast earlier palladium-catalyzed systems, which require iodide additives to activate silyl-triflates. These nickel-based catalysts exhibit good functional group tolerance in the preparation of vinyl silanes, and unlike earlier systems, allows for the incorporation of trialkylsilanes larger than Me₃Si.