3-Silaazetidine: An Unexplored yet Versatile Organosilane Species for Ring Expansion toward Silaazacycles

Catalytic Asymmetric Construction of C- and Si-Stereogenic Silacyclopentanes via Hydrosilylation of Arylmethylenecyclopropanes

Silacycles have exhibited significant potential for application in the fields of medicinal chemistry, agrochemistry, and materials science. Accordingly, the development of effective methods for synthesizing these compounds has attracted increasing attention. Here, we report an efficient Cu-catalyzed enantioselective hydrosilylation of arylmethylenecyclopropanes with hydrosilanes, that allows the rapid assembly of various enantioenriched carbon- and silicon-stereogenic silacyclopentanes in good yields with excellent enantioselectivities and diastereoselectivities under mild conditions. Further stereospecific transformation of the Si-H bond on the chiral silicon center expands the diversity of these C- and Si-stereogenic silacyclopentanes.

The role of silicon in drug discovery: a review

This review aims to highlight the role of silicon in drug discovery. Silicon and carbon are often regarded as being similar with silicon located directly beneath carbon in the same group in the periodic table. That being noted, in many instances a clear dichotomy also exists between silicon and carbon, and these differences often lead to vastly different physiochemical and biological properties. As a result, the utility of silicon in drug discovery has attracted significant attention and has grown rapidly over the past decade. This review showcases some recent advances in synthetic organosilicon chemistry and examples of the ways in which silicon has been employed in the drug-discovery field.

Enantioselective Copper-Catalyzed Sequential Hydrosilylation of Arylmethylenecyclopropanes

Despite impressive advances in the construction of enantioenriched silacarbocycles featuring silicon-stereogenic centers via a selection of well-defined sila-synthons, the development of a more convenient and economic method with readily available starting materials is significantly less explored and remains a considerable challenge. Herein, we report the first example of copper-catalyzed sequential hydrosilylation of readily accessible methylenecyclopropanes (MCPs) and primary silanes, affording an efficient and convenient route to a wide range of chiral silacyclopentanes bearing consecutive silicon- and carbon-stereogenic centers with excellent enantio- and diastereoselectivities (generally ≥98 % ee, >25 : 1 dr). Mechanistic studies reveal that these reactions combine copper-catalyzed intermolecular ring-opening hydrosilylation of aryl MCPs and intramolecular asymmetric hydrosilylation of the resultant Z/E mixture of homoallylic silanes.

Ligand-Controlled Regiodivergent Ring Expansion of Benzosilacyclobutenes with Alkynes en Route to Axially Chiral Silacyclohexenyl Arenes

A ligand-controlled regiodivergent and enantioselective ring expansion of benzosilacyclobutenes with internal naphthyl alkynes has been achieved by adjusting the ligand cavity size. The ligand (S)-8H-binaphthyl phosphoramidite, featuring small methyl groups on its arms, provides a spacious cavity that favors sterically demanding Si-Csp3 ring expansion, predominantly yielding axially chiral (S)-1-silacyclohexenyl arenes. In contrast, the ligand (R)-spiro phosphoramidite, with bulky t-Bu groups on its arms, offers a compact cavity that facilitates less sterically demanding Si-Csp2 ring expansion, leading primarily to axially chiral (S)-2-silacyclohexenyl arenes. Density functional theory calculations delineate distinct mechanistic pathways for each ring expansion route and elucidate their regio- and enantioselectivity.

Ruthenium-Catalyzed Sequential Hydrosilylation/Dehydrogenation and C-H Silylation: Synthesis of Six-Membered Indole Silacycles and Pyrrole Silyl Ether Cycles

Selective dehydrogenative C-H silylation is one of the most powerful tools to synthesize silacycles. Herein, we developed Ru-catalyzed sequential hydrosilylation/C-H silylation of allyl-indoles and dehydrogenative O-H/C-H silylation of pyrrole phenols. Both six-membered indole silacycles and pyrrole silyl ether cycles were successfully synthesized with good functional group tolerance. Furthermore, the RuHCl(CO)(PPh3)3 catalyst exhibited high reaction compatibility in hydrosilylation of alkene, dehydrogenative O-H silylation, and C-H silylation.

Catalytic Access to Diastereometrically Pure Four- and Five-Membered Silyl-Heterocycles Using Transborylation

Silyl-heterocycles offer a unique handle to expand and explore chemical space, reactivity, and functionality. The shortage of catalytic methods for the preparation of diverse and functionalized silyl-heterocycles however limits widespread exploration and exploitation. Herein the borane-catalyzed intramolecular 1,1-carboboration of silyl-alkynes has been developed for the synthesis of 2,3-dihydrosilolyl and silylcyclobut-2-enyl boronic esters. Successful, catalytic carboboration has been achieved on a variety of functionally diverse silyl-alkynes, using a borane catalyst and transborylation-enabled turnover. Mechanistic studies, including 13C-labelling, computational studies, and single-turnover experiments, suggest a reaction pathway proceeding by 1,2-hydroboration, 1,1-carboboration, and transborylation to release the alkenyl boronic ester product and regenerate the borane catalyst.

Palladium-Catalyzed Skeletal Rearrangement of Substituted 2-Silylaryl Triflates via 1,5-C-Pd/C-Si Bond Exchange

A palladium-catalyzed skeletal rearrangement of 2-(2-allylarylsilyl)aryl triflates has been developed to give highly fused tetrahydrophenanthrosilole derivatives via unprecedented 1,5-C-Pd/C-Si bond exchange. The reaction pathways can be switched toward 4-membered ring-forming C(sp2 )-H alkylation by tuning the reaction conditions to give completely different products, fused dihydrodibenzosilepin derivatives, from the same starting materials. The inspection of the reaction conditions revealed the importance of carboxylates in promoting the C-Pd/C-Si bond exchange.

Strain-Driven, Non-Catalysed Ring Expansion of Silicon Heterocycles

Silacycles are ubiquitous building blocks. Small silacycles can typically be expanded catalytically. A silirane, silirene and phosphasilirene as well as a siletane and a silolene were prepared starting from the base-free bromosilylene [(dtbp Cbz)SiBr] (dtbp Cbz=1,8-bis(3,5-ditertbutylphenyl)-3,6-ditertbutylcarbazolyl). As these heterocycles were derived from a dicoordinated silylene, they are susceptible to reactions with an external base. The three-membered silacycles readily undergo non-catalysed ring expansion reactions with isonitriles yielding the related four-membered silacycles. Surprisingly, the ring-expanded derivatives of the silirane undergo up to two further isomerisation reactions, first by enamine formation and then by another ring expansion. DFT computations were utilised to gauge the scope of this reactivity pattern. Three-membered silacycles should essentially universally undergo a ring expansion with isonitriles, while for four-membered silacycles, only very few instances are predicted to accommodate more challenging kinetic requirements of this ring expansion. Larger silacycles lack the ring strain energy required for this ring expansion reaction and are not expected to be expanded.

Rhodium-Catalyzed Hydrolytic Cleavage of the Silicon-Carbon Bond of Silacyclobutanes to Access Silanols

Herein, we report the first rhodium-catalyzed hydrolytic cleavage of the silicon-carbon bond in silacyclobutanes using water as the reactant. A series of silacyclobutanes could be employed in this reaction in the presence of the Rh/BINAP complex, resulting in the corresponding silanols in good yields. Additionally, a chiral 1,1,4,4-tetraaryl-2,3-O-isopropylidene-l-threitol-derived phosphoramidite ligand could be used in this reaction to yield Si-stereogenic silanol with promising enantioselectivity.

Scalable Synthesis of Silacyclohexanones and Ready Access to Silicon Building Blocks

A simple and efficient two-step method for the synthesis of silacyclohexanones starting from bis(bromoethylsilanes) using TosMIC is presented. The prepared silacyclohexanones were transformed to nine different heterocycles with silicon incorporation. In addition, the developed methodology was used for the synthesis of a sila analogue of the HDAC6 inhibitor tubastatin A.

Synthetic strategies to access silacycles

In comparison with all-carbon parent compounds, the incorporation of Si-element into carboskeletons generally endows the corresponding sila-analogues with unique biological activity and physical-chemical properties. Silacycles have recently shown promising application potential in biological chemistry, pharmaceuticals industry, and material chemistry. Therefore, the development of efficient methodology to assemble versatile silacycles has aroused increasing concerns in the past decades. In this review, recent advances in the synthesis of silacycle-system are briefly summarized, including transition metal-catalytic and photocatalytic strategies by employing arylsilanes, alkylsilane, vinylsilane, hydrosilanes, and alkynylsilanes, etc. as starting materials. Moreover, a clear presentation and understanding of the mechanistic aspects and features of these developed reaction methodologies have been high-lighted.

Nickel-Catalyzed Reductive [4 + 1] Sila-Cycloaddition of 1,3-Dienes with Dichlorosilanes

Transition-metal-catalyzed sila-cycloaddition has been a promising tool for accessing silacarbocycle derivatives, but the approach has been limited to a selection of well-defined sila-synthons. Herein, we demonstrate the potential of chlorosilanes, which are industrial feedstock chemicals, for this type of reaction under reductive nickel catalysis. This work extends the scope of reductive coupling from carbocycle to silacarbocycle synthesis and from single C-Si bond formation to sila-cycloaddition reactions. The reaction proceeds under mild conditions and shows good substrate scope and functionality tolerance, and it offers new access to silacyclopent-3-enes and spiro silacarbocycles. The optical properties of several spiro dithienosiloles as well as structural variations of the products are demonstrated.

Catalytic Asymmetric Dehydrogenative Si-H/N-H Coupling: Synthesis of Silicon-Stereogenic Silazanes

Despite growing progress in the construction of silazanes, the catalytic asymmetric synthesis of silicon-stereogenic silazanes is significantly less explored and remains a considerable challenge. Herein, we report a highly enantioselective synthesis of silicon-stereogenic silazanes via catalytic dehydrogenative coupling of dihydrosilanes with anilines. The reaction readily produces a wide range of chiral silazanes and bis-silazanes in excellent yields and stereoselectivities (up to 99% ee). Further utility of this process is demonstrated by the construction of polycarbosilazanes featuring configurational main chain silicon-stereogenic chirality. In addition, the straightforward transformation of the enantioenriched silazanes delivers various chiral silane compounds in a stereospecific fashion, illustrating their potential utilities as synthons for the synthesis of novel silicon-containing functional molecules.

Intramolecular Ring Expansion of 3-Silaazetidine with Alkynes Enabled by Pd-Catalyzed Si-C Bond Activation

An intramolecular ring expansion of in situ formed 3-silaazetidine with internal alkynes has been developed via Pd-catalyzed Si-C bond activation. The reaction gives rise to 6,5- and 6,6-fused bicyclic 1,3-azasilines, in which the silicon atom locates at the ring junction position.

Palladium-catalyzed synthesis of 4-sila-4H-benzo[d][1,3]oxazines by intramolecular Hiyama coupling

A palladium-catalyzed synthesis of 4-sila-4H-benzo[d][1,3]oxazines, silicon-switched analogs of biologically relevant 4H-benzo[d][1,3]oxazines, was developed by the intramolecular Hiyama coupling of 3-amido-2-(arylsilyl)aryl triflates.

Sila-spirocyclization involving unstrained C(sp3)-Si bond cleavage

C - Si Bond cleavage is one of the key elemental steps for a wide variety of silicon-based transformations. However, the cleavage of unstrained Si-C(sp3) bonds catalyzed by transition metal are still in their infancy. They generally involve the insertion of a M - C(sp2) species into the C - Si bond and consequent intramolecular C(sp2)‒Si coupling to exclusively produce siloles. Here we report the Pd-catalyzed sila-spirocyclization, in which the Si-C(sp3) bond is activated by the insertion of a M - C(sp3) species and followed by the formation of a new C(sp3)‒Si bond, allowing the construction of diverse spirosilacycles. This reactivity mode, which is strongly supported by DFT calculations may open an avenue for the Si-C(sp3) bond cleavage and silacycle synthesis.

Crystal structure and Hirshfeld surface analysis of N-{[diphen-yl(vin-yl)sil-yl]meth-yl}-2-methyl-propan-2-ammonium chloride

N-{[Diphen-yl(vin-yl)sil-yl]meth-yl}-2-methyl-propan-2-amine, C19H25NSi, is a newly synthesized secondary amino-methyl-silane that can be used, for example, to study carboli-thia-tion reactions of vinyl-silanes. Because the neutral compound did not crystallize well, the hydro-chloride salt, C19H26NSi+·Cl-, was formed, in which the two chloride ions in the asymmetric unit have crystallographic site symmetry. An unusually long Si-C bond of 1.9117 (10) Å is observed in the cation, which may be ascribed to electronic effects due to the β N+ species. In the crystal, the cations and anions are linked by N-H⋯Cl hydrogen bonds to generate [001] chains. To further investigate the inter-molecular inter-actions, a Hirshfeld surface analysis was performed, which showed that H⋯H, C⋯H/H⋯C and H⋯Cl/Cl⋯H contacts contribute 70.4, 20.0 and 8.3%, respectively.

(4 + 2) Annulation of Cl-NH3+CH2SiMe2CH2Cl and Propynones for the Synthesis of 1,3-Azasilinones

A useful 1,3-N,Si reagent (Cl^-NH₃⁺CH₂SiMe₂CH₂Cl) and its (4 + 2) annulation with propynones have been developed. The (4 + 2) annulation is promoted by NaHCO₃ via an intermolecular N-1,4-addition/intramolecular alkylation process, leading to 1,3-azasilinones in good yields. Diverse functionalization of the alkene, carbonyl, and nitrogen moieties on the 1,3-azasilinone has been demonstrated, showcasing the potential of the approach in the synthesis of bioactive molecules containing silaazacycles.

Visible-Light-Induced, Palladium-Catalyzed Annulation of 1,3-Dienes to Construct Vinyl N-Heterocycles

Herein, a photoinduced palladium-catalyzed annulation of 1,3-dienes with bifunctional halognated alkylamines has been developed, offering a facile route to access a broad range of vinylpyrrolidines. The reactivity profile of this protocol was able to be readily manipulated to assemble vinylpyrrolidine and vinlysilaazacycle. Remarkably, the utility of this strategy was further illustrated in the construction of complex and biologically important molecules as well as the diversity-oriented transformations of the resulting product.

Synthetic Approaches for the Construction of Five- and Six-Membered Silaazacycles

Silaazacycles (or azasilacycles), containing both nitrogen and silicon atoms, are appealing ring structures in the development of silicon-containing functional molecules. The development of general and efficient methods towards these motifs has therefore attracted considerable attention from synthetic chemists. This short review intends to highlight representative advances in the synthesis of five- and six-membered silaazacycles.

1 Introduction

2 Five-Membered Silaazacycles

2.1 Five-Membered Silaazacycles bearing a 1,2-N/Si Moiety

2.2 Five-Membered Silaazacycles bearing a 1,3-N/Si Moiety

3 Six-Membered Silaazacycles

3.1 Six-Membered Silaazacycles bearing a 1,2-N/Si Moiety

3.2 Six-Membered Silaazacycles bearing a 1,3-N/Si Moiety

3.3 Six-Membered Silaazacycles bearing a 1,4-N/Si Moiety

4 Conclusion

(3 + 2)-Annulation of 1,3-N,Si-tetraorganosilane reagents TsHNCH2SiBnR1R2 with arynes for efficient synthesis of 3-silaindolines

1,3-N,Si-Tetraorganosilane reagents TsHNCH2SiBnR1R2 were developed as robust synthons to prepare 3-silaindolines via a Cs2CO3-promoted (3 + 2)-annulation reaction with arynes.

Synthesis of Silacyclohexanones from Divinylsilanes and Allylamines by a Rh-Catalyzed Cyclization

An efficient synthesis of silacyclohexanones bearing a variety of silyl substituents has been developed by a [Rh(coe)₂Cl]₂/PCy₃-catalyzed cyclization of divinylsilanes with Jun's allylamine. The silacyclohexanones can be oxidized with DDQ to give the corresponding silacyclohexadienones, which are further transformed into silicon analog of 2-deoxystreptamine or exo-alkylidenesilacyclohexadienes.