Axial Ligand Enables Synthesis of Allenylsilane through Dirhodium(II) Catalysis

Described herein is a dirhodium(II)-catalyzed silylation of propargyl esters with hydrosilanes, using tertiary amines as axial ligands. By adopting this strategy, a range of versatile and useful allenylsilanes can be achieved with good yields. This reaction not only represents a SN2'-type silylation of the propargyl derivatives bearing a terminal alkyne moiety to synthesize allenylsilanes from simple hydrosilanes, but also represents a new application of dirhodium(II) complexes in catalytic transformation of carbon-carbon triple bond. The highly functionalized allenylsilanes that are produced can be transformed into a series of synthetically useful organic molecules. In this reaction, an intriguing ON-OFF effect of the amine ligand was observed. The reaction almost did not occur (OFF) without addition of Lewis base amine ligand. However, the reaction took place smoothly (ON) after addition of only catalytic amount of amine ligand. Detailed mechanistic studies and density functional theory (DFT) calculations indicate that the reactivity can be delicately improved by the use of tertiary amine. The fine-tuning effect of the tertiary amine is crucial in the formation of the Rh-Si species via a concerted metalation deprotonation (CMD) mechanism and facilitating β-oxygen elimination.

Cu-Catalyzed Regioselective Silylation of Chloro-Substituted Allenyl-Bdan

A copper-catalyzed efficient regioselective silylation reaction of chloro-substituted allenyl-Bdan was developed. Under mild reaction conditions, allenyl and propargyl silane compounds can be selectively obtained in moderate to high yields by adjusting the bases and solvents used in the reactions. This study offers direct and efficient methods for synthesizing multifunctionalized allenyl and propargyl silane compounds from the same initial material of chloro-substituted allenyl-Bdan.

Advances in disilylation reactions to access cis/trans-1,2-disilylated and gem-disilylated alkenes

Organosilane compounds are widely used in both organic synthesis and materials science. Particularly, 1,2-disilylated and gem-disilylated alkenes, characterized by a carbon-carbon double bond and multiple silyl groups, exhibit significant potential for subsequently diverse transformations. The versatility of these compounds renders them highly promising for applications in materials, enabling them to be valuable and versatile building blocks in organic synthesis. This review provides a comprehensive summary of methods for the preparation of cis/trans-1,2-disilylated and gem-disilylated alkenes. Despite notable advancements in this field, certain limitations persist, including challenges related to regioselectivity in the incorporation and chemoselectivity in the transformation of two nearly identical silyl groups. The primary objective of this review is to outline synthetic methodologies for the generation of these alkenes through disilylation reactions, employing silicon reagents, specifically disilanes, hydrosilanes, and silylborane reagents. The review places particular emphasis on investigating the practical applications of the C-Si bond of disilylalkenes and delves into an in-depth discussion of reaction mechanisms, particularly those reactions involving the activation of Si-Si, Si-H, and Si-B bonds, as well as the C-Si bond formation.

Nickel-Catalyzed Reductive Cross-Coupling of Propargylic Acetates with Chloro(vinyl)silanes: Access to Silylallenes

Because of their various reactivities, propargyl acetates are refined chemical intermediates that are extensively applied in pharmaceutical synthesis. Currently, reactions between propargyl acetates and chlorosilanes may be the most effective method for synthesizing silylallenes. Nevertheless, owing to the adaptability and selectivity of substrates, transition metal catalysis is difficult to achieve. Herein, nickel-catalyzed reductive cross-coupling reactions between propargyl acetates and substituted vinyl chlorosilanes for the synthesis of tetrasubstituted silylallenes are described. Therein, metallic zinc is a crucial reductant that effectively enables two electrophilic reagents to selectively construct C(sp2)-Si bonds. Additionally, a Ni-catalyzed reductive mechanism involving a radical process is proposed on the basis of deuteration-labeled experiments.

Cobalt-Promoted Electroreductive Cross-Coupling of Prop-2-yn-1-yl Acetates with Chloro(vinyl)silanes

An electroreductive cross-coupling of prop-2-yn-1-yl acetates with chloro(vinyl)silanes for producing tetrasubstituted silylallenes is developed. The method enables the formation of a new C─Si bond through the cathodic reduction formation of the silyl radical, radical addition across the C≡C bond, the alkenyl anion intermediate formation, and deacetoxylation and represents a mild, practical route to the synthesis of silylallenes. Mechanistic studies reveal that CoCl2 acts as the mediator to promote the formation of the alkenyl anion intermediate via electron transfer.

Copper-Catalyzed Reaction of 2,3-Allenols with Silylzinc Reagents: Access to 2-Silyl-1,3-butadienes

Reaction of 2,3-allenols with PhMe₂SiZnCl or Ph₂MeSiZnCl under catalysis of IPrCuCl or SIPrCuCl was carried out, affording 2-silyl-1,3-butadienes. Secondary and tertiary 2,3-allenols could be used as coupling partners. Reaction of secondary 2,3-allenols gave (E)-2-silyl-1,3-butadienes as the only products.

Expedient cobalt-catalyzed stereospecific cascade C-N and C-O bond formation of styrene oxides with hydrazones

Cobalt-catalyzed cascade C-N and C-O bond formation of epoxides with hydrazones is described to furnish oxadiazines using air as an oxidant. The catalyst plays a dual role as a Lewis acid followed by a redox catalyst to accomplish the C-H/O-H cyclization. Optically active styrene oxide can be reacted enantiospecifically (>99% ee).

Ni-Catalyzed Decarboxylative Silylation of Alkynyl Carbonates: Access to Chiral Allenes via Enantiospecific Conversions

A Ni-mediated decarboxylative silylation of alkynyl cyclic carbonates used as versatile propargylic surrogates is reported affording a wide range of highly substituted 2,3- and 3,4-allenol products in good yields. The formal cross-coupling between a tentative intermediate Ni(allenyl) and the silyl reagent was further extended to enantiospecific conversions providing access to chiral allene synthons. This protocol marks the first Ni-catalyzed propargylic silylation proceeding through an S_N2' manifold.

Palladium-Catalyzed Silylation of 2,3-Allenols with Unactivated Disilanes: Access to 2-Silyl-1,3-butadienes and α-Silyl-β-hydroxy Vinylsilanes

Regioselective silylation of 2,3-allenols with disilanes was carried out under catalysis of Pd₂dba₃/P(o-MeOC₆H₄)₃. In the presence of Cs₂CO₃, the reaction achieved 2-silyl-1,3-dienes. Reaction of 1-aryl-2,3-allenols gave the products with excellent Z/E selectivity and E-isomers as the major species. Reaction of α-alkylallenols or α-alkyl-α-aryl-allenols resulted in products with moderate Z/E selectivity and E-isomers are also major. Without a base, the reaction produced α-silyl-β-hydroxyl vinylsilanes, which were converted to 2-silyl-1,3-dienes upon treatment with Cs₂CO₃.

Synergistic Pd/Cu-catalysed regio- and stereoselective borylation of allenylic carbonates

A simple Pd/Cu-catalyzed borylation of allenylic carbonates with B₂Pin₂ was developed using a cheap P(OEt)₃ ligand. Under mild neutral conditions, 2-boryl 1,3-butadienes were obtained selectively in moderate to high yields. Furthermore, the use of different diboron reagents was also feasible in the reaction.

Generation of Axially Chiral Fluoroallenes through a Copper-Catalyzed Enantioselective β-Fluoride Elimination

Herein we report the copper-catalyzed silylation of propargylic difluorides to generate axially chiral, tetrasubstituted monofluoroallenes in both good yields (27 examples >80%) and enantioselectivities (82-98% ee). Compared to previously reported synthetic routes to axially chiral allenes (ACAs) from prochiral substrates, a mechanistically distinct reaction has been developed: the enantiodiscrimination between enantiotopic fluorides to set an axial stereocenter. DFT calculations and vibrational circular dichroism (VCD) suggest that β-fluoride elimination from an alkenyl copper intermediate likely proceeds through a syn-β-fluoride elimination pathway rather than an anti-elimination pathway. The effects of the C1-symmetric Josiphos-derived ligand on reactivity and enantioselectivity were investigated. Not only does this report showcase that alkenyl copper species (like their alkyl counterparts) can undergo β-fluoride elimination, but this elimination can be achieved in an enantioselective fashion.

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.

Controllable regio- and stereo-selective coupling reactions of homoallenylboronates

Unprecedented palladium-catalysed regio- and stereo-selective coupling reactions of homoallenylboronates with (hetero)aryl iodides, allyl bromides and alkynyl bromides in aqueous solution were successfully developed.

Cu-Catalyzed Synthesis of Tetrasubstituted 2,3-Allenols through Decarboxylative Silylation of Alkyne-Substituted Cyclic Carbonates

An efficient and mild Cu-catalyzed protocol has been developed for the decarboxylative silylation of alkyne-functionalized cyclic carbonate substrates affording 2,3-allenols featuring four different substituents. This practical methodology gives access to a wide scope of tetrasubstituted functionalized allenes in excellent yields.

Copper-catalyzed regiodivergent 1,4- and 1,6-conjugate silyl addition to diendioates: access to functionalized allylsilanes

A copper-catalyzed regioselective 1,4- and 1,6-conjugate addition of a silyl reagent to diendioates was established. Various 1,4- and 1,6-protosilylation products were obtained in good yields and with high regioselectivity via tuning the ligands used in the reactions. This protocol has provided a simple and efficient method for the synthesis of multisubstituted functionalized allylsilanes.

Divergent Protosilylation and Protoborylation of Polar Enynes

Copper-catalyzed divergent conjugate protosilylation and protoborylation of polar enynes were developed. The corresponding β-boryldienoates and β-silyldienotes were obtained in moderate to good yields and with good stereoselectivity. In this protocol, novel cascade double protoborylation/protodeboronation processes of polar enynoates enabled access of the useful trisubstituted vinylboronates in up to 80% yield and with up to 98:2 Z/ E ratio. Moreover, divergent transformations of the products thus obtained were also investigated.

Rhodium(III)-Catalyzed Redox-Neutral C-H Activation/Annulation of N-Aryloxyacetamides with Alkynyloxiranes: Synthesis of Highly Functionalized 2,3-Dihydrobenzofurans

Alkynyloxiranes have been employed for the first time as effective coupling partners in Cp*Rh^III-catalyzed C-H functionalization reactions. Their annulation with N-aryloxyamides then offers a redox-neutral and efficient synthesis of functionalized 2,3-dihydrobenzofurans bearing an exocyclic E-allylic alcohol and a tetrasubstituted carbon center in good yields with a broad substrate scope. The products can be easily converted to molecules with more complexity, which provides an opportunity for rapid assembly of structurally diverse heterocycles.