Metal-Free, Stereospecific Bis-Silylation of Functionalized Alkynes with NHC-Supported Silylaminosilylene

Synthesis and Reactivity of Base-Stabilized and Base-Free Silaimidoyl Bromides

The reactivity of the base-free bromosilylene dtbpCbzSiBr (dtbpCbz = 1,8-bis(3,5-di-tert-butylphenyl)-3,6-di-tert-butylcarbazolyl) toward carbodiimides and azides was studied in order to generate base-stabilized and base-free silaimidoyl bromides, respectively. The steric bulk of carbodiimides and azides allows control over the reactivity. While with small substituents such as tert-butyl or adamantyl, the reactions cannot be stopped at the Si═N stage, with large substituents, they lead to C-H activation in the product. The Dipp substituent (Dipp = 2,6-diisopropylphenyl) allowed the isolation of the silaimidoyl bromide dtbpCbzSi(Br)NDipp and its CNDipp-coordinated analogue. The reactivity of the Si═N double bond species was studied with respect to cycloaddition and donor exchange reactions.

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.

Facile Access to an NHC-Coordinated Silicon Ring Compound with a Si=N Group and a Two-Coordinate Silicon Atom

Reaction of the bicyclo[1.1.0]tetrasilatetraamide Si₄ {N(SiMe₃ )Dipp}₄ 1 (Dipp=2,6-diisopropylphenyl) with 5 equiv of the N-heterocyclic carbene NHC^Me4 (1,3,4,5-tetramethylimidazol-2-ylidene) affords a bifunctional carbene-coordinated four-membered-ring compound with a Si=N group and a two-coordinate silicon atom Si₄ {N(SiMe₃ )Dipp}₂ (NHC^Me4 )₂ (NDipp) 2. When 2 reacts with 0.25 equiv sulfur (S₈ ), two sulfur atoms add to the divalent silicon atom in plane and perpendicular to the plane of the Si₄ ring, which confirms the silylone character of the two-coordinate silicon atom in 2.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Cleavage of BN triple bonds by main group reagents

A silylene and a 1,2-diaminoalkyne respectively cleave the strong BN triple bond in iminoboranes.

Reactivity of an NHC-stabilized silylene towards ketones. Formation of silicon bis-enolates vs. bis-silylation of the CO bond

The reaction of NHC-stabilized silylaminosilylene 1 with enolizable ketones rapidly led to the regio- and stereoselective formation of silicon bis-enolates in excellent yields under mild conditions.

Base-stabilized silaimine and its donor-free dimer derived from the reaction of NHC-supported silylene with SiCl4

Reaction of the N-heterocyclic carbene (NHC)-stabilized silylene ArN(SiMe₃)Si(IiPr)Cl (1, Ar = 2,6-iPr₂C₆H₃, IiPr = 1,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene) with SiCl₄ resulted in the formation of three different products ArNSi(IiPr)Cl₂ (2), ArN(SiMe₃)SiCl₃ (3) and (ArNSiCl₂)₂ (4) under different conditions.

A facile access to a novel NHC-stabilized silyliumylidene ion and C–H activation of phenylacetylene

This work presents a facile access to a donor-stabilized monoarylsilyliumylidene ion and its unique reaction with a terminal alkyne.