Copper-Catalyzed Enantio- and Diastereoselective Addition of Silicon Nucleophiles to 3,3-Disubstituted Cyclopropenes

Recent advances and perspectives in synthetic applications of silylboronates as silyl radical precursors

Silylboronates, as powerful and versatile reagents, have been widely used in synthetic chemistry over the past few decades, due to their ability to incorporate silicon and boron atoms into organic molecules. With the rapid development of radical chemistry, the use of silylboronates as silyl radical precursors has recently become a research focus in organic synthesis. Significant achievements have been made in the synthetic applications of silylboronates as silyl radical sources for various C-Si and C-X bond forming transformations. This review summarizes these recent advances, discusses their advantages and limitations, and illustrates the synthetic chances still open for further research and applications in this emerging area.

Cellulose supported TiO2/Cu2O for highly asymmetric conjugate addition of α,β-unsaturated compounds in aqueous phase

A series of new kind green cellulose-supported bimetallic TiO2/Cu2O (Cell@TiO2/Cu2O) catalytic materials were obtained by in-situ reduction method employing cellulose as the carrier. The effects of metal percentage composition on the morphology and construction of the catalytic materials were systematically investigated. The Cell@TiO2/Cu2O were characterized by FT-IR, TG, XPS, SEM, TEM, EDS, and the element content was obtained by elemental analysis. Then, the achieved catalytic materials were applied to the chiral borylation reaction of α,β-unsaturated compounds, including nitrile compounds, esters, and α,β-unsaturated ketones. Remarkably, this approach provides an efficient strategy to gain an important class of chiral organic boron compounds with target chiral products in high yields as well as enantioselectivities. Besides, the Cell@TiO2/Cu2O could be easily recycled and effectively reused. This work constructed bimetallic TiO2/Cu2O on cellulose as a newly catalyst to obtain chiral boron compounds in aqueous phase.

Rapid Access to Chiral Spiro[2.3] Lactams: Stereoselective Hydroborylation and Hydrosilylation and Remote Control of Axial Chirality by Copper-Catalyzed Desymmetrization of Spirocyclopropenes

Chiral spirocyclopropyl β-lactams are common motifs in bioactive compounds and pharmaceuticals. Here we disclose a diastereoselective and enantioselective hydroborylation and hydrosilylation of spirocyclopropenes, via a Cu-catalyzed desymmetrization strategy, for the rapid preparation of enantio-enriched spirocyclopropyl β-lactams. The efficient desymmetrization strategy allows the remote control of axial chirality, offering the borylated and silylated products bearing central, spiro, and axial chirality. The combination of multichiral elements would provide a novel motif for biological evaluation in potential drug discovery.

Copper(I)-Catalyzed Asymmetric Hydrophosphination of 3,3-Disubstituted Cyclopropenes

Herein, a copper(I)-catalyzed asymmetric hydrophosphination of 3,3-disubstituted cyclopropenes is reported. It provides a series of phosphine derivatives in high to excellent diastereo- and enantioselectivities. The methodology enjoys broad substrate scope on both 3,3-disubstituted cyclopropenes and diarylphosphines. The high stereoselectivity is attributed to both the high stability of the Cu(I)-(R,R)-QUINOXP* complex in the presence of stoichiometric HPPh2 and the produced phosphines, and the high-performance asymmetric induction of the Cu(I)-(R,R)-QUINOXP* complex. Finally, the method is used for the synthesis of new chiral phosphine-olefin compounds built on a cyclopropane skeleton, one of which serves as a wonderful ligand in Rh-catalyzed asymmetric conjugate addition of phenylboronic acid to various α,β-unsaturated compounds.

Copper-Catalyzed Enantioselective Desymmetric Protosilylation of Prochiral Diynes: Access to Optically Functionalized Tertiary Alcohols

In this protocol, a copper-catalyzed desymmetric protosilylation of prochiral diynes was developed. The corresponding products were obtained in moderate to high yields and enantiomeric ratios. This approach provides a simple method for synthesizing functionalized chiral tertiary alcohols in the presence of a chiral pyridine-bisimidazoline (Pybim) ligand.

Metal-catalysed C-C bond formation at cyclopropanes

Cyclopropanes are important substructures in natural products and pharmaceuticals. Although traditional methods for their incorporation rely on cyclopropanation of an existing scaffold, the advent of transition-metal catalysis has enabled installation of functionalized cyclopropanes using cross-coupling reactions. The unique bonding and structural properties of cyclopropane render it more easily functionalized in transition-metal-catalysed cross-couplings than other C(sp³) substrates. The cyclopropane coupling partner can participate in polar cross-coupling reactions either as a nucleophile (organometallic reagents) or as an electrophile (cyclopropyl halides). More recently, single-electron transformations featuring cyclopropyl radicals have emerged. This Review will provide an overview of transition-metal-catalysed C-C bond formation reactions at cyclopropane, covering both traditional and current strategies, and the benefits and limitations of each.

Copper-Catalyzed Highly Enantioselective Addition of a Silicon Nucleophile to 3-Substituted 2H-Azirines Using an Si-B Reagent

3-Substituted 2H-azirines can be considered strained cyclic ketimines, and highly enantioselective addition reactions of silicon nucleophiles to either acyclic or cyclic ketimines have been elusive so far. The present work closes this gap for those azirines by means of a copper-catalyzed silylation using a silyl boronic ester as a latent silicon nucleophile. The resulting C-silylated, unprotected (N-H) aziridines are obtained in high yields and with excellent enantioselectivities and can be further converted into valuable compounds with hardly any erosion of the enantiomeric excess.

Copper-Catalyzed Enantioselective Hydrosilylation of gem-Difluorocyclopropenes Leading to a Stereochemical Study of the Silylated gem-Difluorocyclopropanes

Optically active cyclopropanes have been widely investigated especially from the views of pharmaceutical and agrochemical industries, and substituting one of the methylenes with the difluoromethylene unit should be promising for developing novel biologically relevant compounds and functional materials. In this paper, the copper‐catalyzed enantioselective hydrosilylation of gem‐difluorocyclopropenes to provide the corresponding chiral gem‐difluorocyclopropanes is presented. The use of copper(I) chloride, chiral ligands including bidentate BINAPs and monodentate phosphoramidites, and silylborane Me₂PhSi‐Bpin accompanying sodium tert‐butoxide in methanol was appropriate for the enantioselective hydrosilylation of the strained C=C double bond, and the resultant chiral difluorinated three‐membered ring was unambiguously characterized. Subsequent activation of the silyl groups in enantio‐enriched gem‐difluorocyclopropanes showed substantial reduction of the enantiopurity, indicating cleavage of the distal C−C bond leading to the transient acyclic intermediates.

Catalytic regio- and stereoselective silicon–carbon bond formations on unsymmetric gem-difluorocyclopropenes by capture of silyl metal species

A highly regioselective silylation of unsymmetric gem-difluorocyclopropenes was achieved by the capture of in-situ formed silyl metal intermediates, which gave structurally diverse silyldifluorocyclopropanes with good yields and stereoselectivity.

Silylcyclopropanes by Selective [1,4]-Wittig Rearrangement of 4-Silyl-5,6-dihydropyrans

4-Silyl-5,6-dihydropyrans undergo remarkably selective [1,4]-Wittig rearrangements to give silylcyclopropanes in good yields. The selectivity is independent of the silyl group, but it is influenced by the electronic character of the migrating center. Electron-rich and electron-neutral (hetero)aryl groups and aliphatic substituents at the migrating center lead to exclusive [1,4]-migration, whereas electron-deficient aryl groups predominantly afford [1,2]-Wittig products.

Silylative aromatization of p-quinone methides under metal and solvent free conditions

A base-mediated silylation reaction leading to benzyl silanes has been developed. Under transition-metal and solvent free conditions, the silylation of a wide array of p-quinone methides is achieved using a Cs₂CO₃ catalyst in yields up to 96%. Carboxylation of the as-obtained organosilane with gaseous CO₂ provides a new synthetic protocol for the preparation of carboxylic acid.

A novel and efficient synthetic protocol is reported for the synthesis of benzyl silanes from readily available silylborane and p-quinone methides using 5% cesium carbonate under solvent-free conditions.

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.

Beyond Carbon: Enantioselective and Enantiospecific Reactions with Catalytically Generated Boryl- and Silylcopper Intermediates

Catalytic asymmetric C-C bond formation with alkylcopper intermediates as carbon nucleophiles is now textbook chemistry. Related chemistry with boron and silicon nucleophiles where the boryl- and accordingly silylcopper intermediates are catalytically regenerated from bench-stable pronucleophiles had been underdeveloped for years or did not even exist until recently. Over the past decade, asymmetric copper catalysis employing those main-group elements as nucleophiles rapidly transformed into a huge field in its own right with an impressive breadth of enantioselective C-B and C-Si bond-forming reactions, respectively. Its current state of the art does not have to shy away from comparison with that of boron's and silicon's common neighbor in the periodic table, carbon. This Outlook is not meant to be a detailed summary of those manifold advances. It rather aims at providing a brief conceptual summary of what forms the basis of the latest exciting progress, especially in the area of three-component reactions and cross-coupling reactions.

Cobalt-Catalyzed Diastereoselective and Enantioselective Hydrosilylation of Achiral Cyclopropenes

A mild diastereoselective and enantioselective cobalt-catalyzed hydrosilylation reaction of achiral cyclopropenes has been developed. In this protocol, various substituted cyclopropenes and arylsilanes were transformed, in the presence of readily available chiral cobalt complex, into silylcyclopropanes with high selectivities.

Copper-Catalyzed Enantioselective and Exo-Selective Addition of Silicon Nucleophiles to 7-Oxa- and 7-Azabenzonorbornadiene Derivatives

An enantioselective formal hydrosilylation of 7-oxa- and 7-azabenzonorbornadiene derivatives is reported. The exo-selective addition of the silicon moiety across these strained alkenes is achieved under copper catalysis, employing Si-B reagents as silicon pronucleophiles in the presence of an alkoxide base. No ring opening is observed. While successful for those substrates, the same procedure cannot be applied to benzonorbornadiene, norbornadiene, or norbornene.

Recent advances in Cu-catalyzed C(sp3)-Si and C(sp3)-B bond formation

Numerous reactions generating C-Si and C-B bonds are in focus owing to the importance of incorporating silicon or boron into new or existing drugs, in addition to their use as building blocks in cross-coupling reactions en route to various targets of both natural and unnatural origins. In this review, recent protocols relying on copper-catalyzed sp3 carbon-silicon and carbon-boron bond-forming reactions are discussed.

Copper-catalyzed non-directed hydrosilylation of cyclopropenes: highly diastereoselective synthesis of fully substituted cyclopropylsilanes

The non-directed hydrosilylation of cyclopropenes with earth-abundant and environmentally benign base metal catalysis was described.

Recent developments in cyclopropene chemistry

As readily accessible strained carbocycles, cyclopropenes show a diverse range of reactivities, and a lot of novel and useful transformations have been developed.

Copper-catalyzed asymmetric silyl addition to alkenyl-substituted N-heteroarenes

Asymmetric conjugate addition of PhMe₂SiBPin to a wide range of N-heteroaryl alkenes proceeded in the presence of a copper catalyst coordinated with a chiral phosphoramidite ligand to afford useful β-silyl N-heteroarenes in high yields and ees.

Copper-Catalyzed Enantio- and Diastereoselective Addition of Silicon Nucleophiles to 3,3-Disubstituted Cyclopropenes

A highly stereocontrolled syn-addition of silicon nucleophiles across cyclopropenes with two different geminal substituents at C3 is reported. Diastereomeric ratios are excellent throughout (d.r.≥98:2) and enantiomeric excesses usually higher than 90 %, even reaching 99 %. This copper-catalyzed C-Si bond formation closes the gap of the direct synthesis of α-chiral cyclopropylsilanes.