Pd-catalyzed synthesis of allylic silanes from allylic ethers

Stereoselective Synthesis of Tri- and Tetrasubstituted Allylsilanes via Copper-Catalyzed Decarboxylative Silylation of Vinylethylene Carbonates

Herein, a stereoselective copper-catalyzed decarboxylative silylation of readily available vinylethylene carbonates (VECs) with PhMe2Si-Bpin is reported, affording a wide range of tri- and tetrasubstituted allylsilanes in moderate to high yields with E-selectivity. This protocol was characterized by high stereoselectivity, broad substrate scope, operational simplicity, and mild reaction conditions, which were amenable to diverse derivatizations and gram-scale synthesis.

Decarboxylative Allylation of Silanecarboxylic Acids Enabled by Organophotocatalysis

Herein we present a visible-light-facilitated transition-metal-free allylic silylation reaction under mild conditions. This protocol is enabled by an inexpensive organophotocatalyst and provides efficient and concise synthetic routes to substituted allylsilanes, particularly from readily available allyl sulfones and stable silanecarboxylic acids as silyl radical precursors. Further investigations reveal that this strategy is also generally compatible with vinyl sulfones to access vinylsilanes. The silver catalytic system opens up an alternative entry to realize the decarboxylative allylation of silanecarboxylic acids.

Diverse Alkyl-Silyl Cross-Coupling via Homolysis of Unactivated C(sp3)-O Bonds with the Cooperation of Gold Nanoparticles and Amphoteric Zirconium Oxides

Since C(sp³)-O bonds are a ubiquitous chemical motif in both natural and artificial organic molecules, the universal transformation of C(sp³)-O bonds will be a key technology for achieving carbon neutrality. We report herein that gold nanoparticles supported on amphoteric metal oxides, namely, ZrO₂, efficiently generated alkyl radicals via homolysis of unactivated C(sp³)-O bonds, which consequently promoted C(sp³)-Si bond formation to give diverse organosilicon compounds. A wide array of esters and ethers, which are either commercially available or easily synthesized from alcohols participated in the heterogeneous gold-catalyzed silylation by disilanes to give diverse alkyl-, allyl-, benzyl-, and allenyl silanes in high yields. In addition, this novel reaction technology for C(sp³)-O bond transformation could be applied to the upcycling of polyesters, i.e., the degradation of polyesters and the synthesis of organosilanes were realized concurrently by the unique catalysis of supported gold nanoparticles. Mechanistic studies corroborated the notion that the generation of alkyl radicals is involved in C(sp³)-Si coupling and the cooperation of gold and an acid-base pair on ZrO₂ is responsible for the homolysis of stable C(sp³)-O bonds. The high reusability and air tolerance of the heterogeneous gold catalysts as well as a simple, scalable, and green reaction system enabled the practical synthesis of diverse organosilicon compounds.

Direct synthesis and applications of solid silylzinc reagents

The increased synthetic utility of organosilanes has motivated researchers to develop milder and more practical synthetic methods. Silylzinc reagents, which are typically the most functional group tolerant, are notoriously difficult to synthesize because they are obtained by a pyrophoric reaction of silyllithium, particularly Me3SiLi which is itself prepared by the reaction of MeLi and disilane. Furthermore, the dissolved LiCl in silylzinc may have a detrimental effect. A synthetic method that can avoid silyllithium and involves a direct synthesis of silylzinc reagents from silyl halides is arguably the simplest and most economical strategy. We describe, for the first time, the direct synthesis of PhMe2SiZnI and Me3SiZnI reagents by employing a coordinating TMEDA ligand, as well as single crystal XRD structures. Importantly, they can be obtained as solids and stored for longer periods at 4 °C. We also demonstrate their significance in cross-coupling of various free alkyl/aryl/alkenyl carboxylic acids with broader functional group tolerance and API derivatives. The general applicability and efficiency of solid Me3SiZnI are shown in a wide variety of reactions including alkylation, arylation, allylation, 1,4-addition, acylation and more.

Selective Synthesis of Z-Cinnamyl Ethers and Cinnamyl Alcohols through Visible Light-Promoted Photocatalytic E to Z Isomerization

A photocatalytic E to Z isomerization of alkenes using an iridium photosensitizer under mild reaction conditions is disclosed. This method provides scalable and efficient access to Z-cinnamyl ether and allylic alcohol derivatives in high yields with excellent stereoselectivity. Importantly, this method also provides a powerful strategy for the selective synthesis of Z-magnolol and honokiol derivatives possessing potential biological activity.

A conjugated microporous polymer as a recyclable heterogeneous ligand for highly efficient regioselective hydrosilylation of allenes

Pyridines containing adjacent C[triple bond, length as m-dash]C bonds were utilized as ligand units and integrated into the skeleton of conjugated microporous polymers. The resultant Pd-CMP-1 was first applied as a highly efficient heterogeneous catalytic system for Pd-catalyzed allene hydrosilylation towards a wide range of allenes to produce branched allylsilanes with high regioselectivity. The ligand units of the polymer, along with the confinement effect of the porous structure, jointly regulated the regioselectivity. The parts-per-million (ppm) levels of Pd, coordinated with the recyclable heterogeneous ligand, show promise for industrial applications. This work opens a new front of using CMP as an intriguing platform for developing highly efficient catalysts to control the regioselectivities in allene hydrosilylation.

Facile hydrogenation of cinnamaldehyde to cinnamyl ether by employing a highly re-usable “dip-catalyst” containing Pt nanoparticles on a green support

We report the fabrication of a ‘dip-catalyst’ based on Pt nanoparticles on jute stalks as a green support and its catalytic application in cinnamyl alkyl ether synthesis.

Access to branched allylsilanes by nickel-catalyzed regioselective hydrosilylation of allenes

A Ni(acac)₂/Xantphos-catalyzed hydrosilylation of allenes has been developed.

Synthesis of Functionalized 1,3-Butadienes via Pd-Catalyzed Cross-Couplings of Substituted Allenic Esters in Water at Room Temperature

An environmentally responsible, mild method for the synthesis of functionalized 1,3-butadienes is presented. It utilizes allenic esters of varying substitution patterns, as well as a wide range of boron-based nucleophiles under palladium catalysis, generating sp-sp², sp²-sp², and sp²-sp³ bonds. Functional group tolerance measured via robustness screening, along with room temperature and aqueous reaction conditions highlight the methodology's breadth and potential utility in synthesis.

Cobalt-catalyzed regioselective stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes

The first transition metal-catalyzed stereoconvergent Markovnikov 1,2-hydrosilylation of (E/Z)-dienes was effectively achieved with excellent E-selectivities using a cobalt catalyst.

We report the first stereoconvergent Markovnikov 1,2-hydrosilylation of conjugated dienes using catalysts generated from bench-stable Co(acac)₂ and phosphine ligands. A wide range of E/Z-dienes underwent this Markovnikov 1,2-hydrosilylation in a stereoconvergent manner, affording (E)-allylsilanes in high isolated yields with high stereoselectivities (E/Z = >99 : 1) and high regioselectivities (b/l up to > 99 : 1). Mechanistic studies revealed that this stereoconvergence stems from a σ–π–σ isomerization of an allylcobalt species generated by the 1,4-hydrometalation of Z-dienes. In addition, a cobalt catalyst that can only catalyze the hydrosilylation of the E-isomer of an (E/Z)-diene was identified, which allows the separation of the (Z)-isomer from an isomeric mixture of (E/Z)-dienes. Furthermore, asymmetric hydrosilylation of (E)-1-aryl-1,3-dienes was studied with Co(acac)₂/(R)-difluorphos and good enantioselectivities (er up to 90 : 10) were obtained.

Highly efficient regioselective hydrosilylation of allenes using a [(3IP)Pd(allyl)]OTf catalyst; first example of allene hydrosilylation with phenyl- and diphenylsilane

A [(3IP)Pd(allyl)]OTf complex was shown to have high activity and regioselectively for allene hydrosilylation employing a wide range of silanes.

Silyl-Heck Reactions for the Preparation of Unsaturated Organosilanes

Few methods exist to directly install silyl functionality onto olefins. This Synpacts highlights the state of the art of the silyl-Heck reaction and our recently developed conditions for preparing allyl and vinyl silanes from terminal alkenes using this method.

"Designer"-Surfactant-Enabled Cross-Couplings in Water at Room Temperature

New methodologies are discussed that allow for several commonly used transition-metal-catalyzed coupling reactions to be conducted within aqueous micellar nanoparticles at ambient temperatures.

Manipulating micellar environments for enhancing transition metal-catalyzed cross-couplings in water at room temperature

The remarkable effects of added salts on the properties of aqueous micelles derived from the amphiphile PTS are described. Most notably, Heck reactions run in the presence of NaCl lead to couplings on aryl bromides in water at room temperature. Olefin cross- and ring-closing metathesis reactions run in the presence of small amounts of pH-lowering KHSO(4) are also accelerated, another phenomenon that does not apply to typical processes in organic media. These salt effects allow, in general, for synthetically valuable C-C bond-forming processes to be conducted under environmentally benign conditions. Recycling of the surfactant is also demonstrated.

TPGS-750-M: a second-generation amphiphile for metal-catalyzed cross-couplings in water at room temperature

An environmentally benign surfactant (TPGS-750-M), a diester composed of racemic α-tocopherol, MPEG-750, and succinic acid, has been designed and readily prepared as an effective nanomicelle-forming species for general use in metal-catalyzed cross-coupling reactions in water. Several "name" reactions, including Heck, Suzuki-Miyaura, Sonogashira, and Negishi-like couplings, have been studied using this technology, as have aminations, C-H activations, and olefin metathesis reactions. Physical data in the form of DLS and cryo-TEM measurements suggest that particle size and shape are key elements in achieving high levels of conversion and, hence, good isolated yields of products. This new amphiphile will soon be commercially available.

Palladium-catalyzed oxidative allylic C-H silylation

Palladium-catalyzed allylic C-H silylation was performed with use of hexamethyldisilane as the silyl source. These C-H functionalization reactions occur only in the presence of hypervalent iodine reagents or other strong oxidants and proceed with excellent regioselectivity, providing the linear allylic isomer of the allylsilane products. In demonstrating the first oxidative allylic C-H silylation of alkenes, this study marks an important advance for the catalytic C-H functionalization method.

Exploration of new C-O electrophiles in cross-coupling reactions

Since their development in the 1970s, cross-coupling reactions catalyzed by transition metals have become one of the most important tools for constructing both carbon-carbon and carbon-heteroatom bonds. Traditionally, organohalides were widely studied and broadly used as the electrophile, both in the laboratory and in industry. Unfortunately, the high cost, environmental toxicity, and sluggish preparation often associated with aryl halides can make them undesirable for the large-scale syntheses of industrial applications. However, with the further development of catalytic systems, and particularly of the ligands contained therein, a variety of electrophiles have now been successfully applied to cross-coupling reactions. Oxygen-based electrophiles have attracted much attention due to their ready availability from phenol and carbonyl compounds. Initially, aryl and alkenyl triflates were used in cross-coupling reactions due to their high reactivity; however, low moisture stability and high cost hampered their application. Later, with the development of highly efficient catalytic systems, the less reactive sulfonates and phosphates were successfully employed in cross-coupling reactions. Although they have higher stability and can be easily prepared, low atom economy remains an obstacle to their broader utility. Our group has worked to directly apply the abundant and readily available oxygen-containing compounds, such as phenols, alcohols, ethers, and carbonyl compounds, to cross-coupling reactions. In this Account, we describe our recent efforts in transition-metal-catalyzed cross-coupling reactions of new O-based electrophiles via C-O bond activation. We began by developing the methylation of aryl methyl ethers and benzyl methyl ethers via Ni-catalyzed selective C-O bond cleavage. With the refined Ni-based catalytic system, we further applied aryl/alkenyl carboxylates and carbamates to Suzuki-Miyaura, Negishi, and Kumada-Tamao-Corriu reactions to construct various biaryl scaffolds and highly substituted alkenes. To further improve the carbon atom economy, we developed the diaryl sulfates as one-by-one electrophiles (that is, both aryl groups are used in the reaction). Most recently, we have achieved the first successful cross-coupling reaction of magnesium naphtholates with aryl Grignard reagents. These results extend aryl and benzyl ethers, aryl and alkenyl carboxylates/carbamates, and magnesium naphtholates as novel electrophiles in cross-coupling reactions. More importantly, these studies contribute to our better understanding the intrinsic nature of C-O bonds, which were traditionally considered "inert" but clearly show enormous synthetic potential with the proper catalysts.

Synthesis of alpha-arylated allylsilanes through palladium-catalyzed gamma-selective allyl-aryl coupling

A palladium-catalyzed gamma-selective allyl-aryl coupling between gamma-silylated allylic esters and arylboronic acids produced alpha-arylated allylsilanes with E-alkene geometry. The reaction tolerated various functional groups in both the arylboronic acids and the allylic esters and afforded functionalized allylsilanes. The reaction of optically active allylic esters took place with excellent alpha-to-gamma chirality transfer with syn stereochemistry to give chiral allylsilanes.