Transition-Metal-Free Silylation of Unactivated C(sp2)–H Bonds with tert-Butyl-Substituted Silyldiazenes

Catalytic alkoxysilylation of C-H bonds with tert-butyl-substituted alkoxysilyldiazenes

Organoalkoxysilanes (e.g. R-SiMe3-n (OR') n , 1 ≤ n ≤ 3 with R = alkyl or aryl) have found various applications in synthetic chemistry and materials science because the silicon-bound alkoxy groups provide unique opportunities for further derivatization and transformations. Among the few catalytic strategies that allow the direct and intermolecular introduction of an alkoxysilyl unit onto an organic substrate, the alkoxysilylation of unactivated C-H bonds has barely been achieved despite its synthetic potential and the atom-economy it conveys. In particular, a catalytic and transition metal-free C-H silylation protocol towards this class of organosilicon compounds has yet to be reported. We herein describe the first general alkoxysilylation of (hetero)arene C(sp2)-H and benzylic C(sp3)-H bonds under ambient, transition metal-free conditions using newly-prepared tert-butyl-substituted alkoxysilyldiazenes (tBu-N[double bond, length as m-dash]N-SiMe3-n (OR') n , 1 ≤ n ≤ 3 with R' = Et, iPr or tBu) as silylating reagents and tBuOK as catalytic promoter.

Reductive C(sp2)-Si Cross-Couplings by Catalytic Sodium-Bromine Exchange

The metal-halogen exchange reaction constitutes one of the most important preparative routes towards polar organometallic reagents such as aryllithium or Grignard reagents. However, despite extensive developments over the past eight decades, this fundamental organometallic elementary step has only been exploited stoichiometrically. Against this background, we demonstrate that the sodium-bromine exchange reaction can be implemented in a catalytic setting as a mean to activate C(sp2)-Br bonds in a transition metal-free manner en route to the regioselective and general preparation of (hetero)aryl silanes. Simply treating structurally diverse (hetero)aryl bromides with N-tert-butyl-N'-silyldiazenes (tBu-N=N-Si) as silylating reagents and inexpensive sodium alkoxides as catalytic promoters yields a range of aromatic organosilicon compounds under ambient conditions. Mechanistic studies provide solid evidence for the involvement of tert-butyl sodium as the C(sp2)-Br metalating agent.

Intermolecular C-H silylations of arenes and heteroarenes with mono-, bis-, and tris(trimethylsiloxy)hydrosilanes: control of silane redistribution under operationally diverse approaches

Efficient catalytic protocols for C-H silylations of arenes and heteroarenes with sterically and electronically different hydrosiloxysilanes are disclosed. The silylations are catalyzed by a well-defined Rh-complex (1 mol%), derived from [Rh(1,5-hexadiene)Cl]2 and a bulky BINAP type ligand. This catalyst not only promotes C-Si bond formation affording the desired products in up to 95% isolated yield, but also can suppress the silane redistribution side reactions of HSiMe2(OTMS). The protocol can also be applied for the C-H silylations of more reactive HSiMe(OTMS)2 with a much lower catalyst loading (0.25 mol%) and even with sterically demanding HSi(OTMS)3. The steric bulk of the arene substituent and hydrosiloxysilane is a major factor in determining the regioselectivity and electronic effect as secondary. The current method can be performed under operationally diverse conditions: with/without a hydrogen scavenger or solvent.

Bond-Forming Processes Enabled by Silicon-Masked Aryl- and Alkyl-Substituted Diazenes

Aryl- and alkyldiimides (R-N═NH with R = aryl or alkyl) are elusive intermediates of valuable synthetic use, as they are assumed to be transient species in processes involving both carbon (with concomitant loss of N2) and nitrogen nucleophiles (with conservation of the N═N moiety). The actual compounds are fragile and as such not bench stable which is why they have not yet found the attention they deserve. Conversely, early contributions showed that the stability of the parent diimide is significantly increased by replacing the hydrogen atom by a silyl group, but the synthetic applicability of these silicon-protected aryl- and alkyldiazenes has been far less explored, in part due to the absence of general procedures for their preparation. This Perspective provides an overview of the underexplored diazene chemistry that has witnessed considerable progress in recent years and highlights the potential of this motif in a range of synthetically useful (catalytic) transformations. The rediscovered silicon-masked diazenes constitute a versatile platform possessing enhanced stability and tamed reactivity in comparison to the parent hydrogen-substituted diimides. Aryl, diazenyl, and alkyl anionic key intermediates can be selectively generated in situ under Lewis base or transition metal catalysis, giving rise to novel synthetic approaches as viable alternatives to the already existing methodologies.

Reversible C-H bond silylation with a neutral silicon Lewis acid

The silicon-carbon bond is a valuable linchpin for synthetic transformations. However, installing Si-C functionalities requires metalated C-nucleophiles, activated silicon reagents (silylium ions, silyl radicals, and silyl anions), or transition metal catalysis, and it occurs irreversibly. In contrast, spontaneous C-H silylations with neutral silanes leading to anionic silicates, and their reversible deconstruction, are elusive. Herein, the CH-bond silylation of heterocycles or a terminal alkyne is achieved by reaction with bis(perfluoro(N-phenyl-ortho-amidophenolato))silane and 1,2,2,6,6-pentamethylpiperidine. Computational and experimental insights reveal a frustrated Lewis pair (FLP) mechanism. Adding a silaphilic donor to the ammonium silicate products induces the reformation of the C-H bond, thus complementing previously known irreversible C-H bond silylation protocols. Interestingly, the FLP "activated" N-methylpyrrole exhibits "deactivated" features against electrophiles, while a catalytic functionalization is found to be effective only in the absence of a base.

Dual photoredox nickel-catalyzed silylation of aryl/heteroaryl bromides using hydrosilanes

Dual Ni and Ir catalysis enables the preparation of arylsilanes having a (TMS)3Si substituent from the corresponding aryl bromides and (TMS)3SiH at 30 °C using visible-light irradiation. This protocol avoids strong bases, high temperature and air and moisture sensitive silyl reagents, providing the expected arylsilanes in moderate to good yields. The reaction was shown to proceed through a silyl radical, likely generated by hydrogen atom abstraction from (TMS)3SiH by a bromide radical.

Base-promoted Conia-ene cyclization of propargyl amides

We report a tBuOK-promoted synthesis of 1,3-dihydro-2H-pyrrol-2-one and 4-methylenepyrrolidin-2-one systems via Conia-ene like intramolecular cyclization. The method features extremely short reaction times (5 min) and mild reaction conditions (rt), enabling the trapping of a propargyl unit by an amide enolate. An intriguing anionic chain mechanism is at work, which can trigger the isomerization of an exo-alkene giving access to the otherwise elusive endo-product.

Diastereoselective synthesis of tetrahydropyranes via Ag(I)-initiated dimerization of cinnamyl ethers

We present herein the first synthesis of tetrahydropyranes promoted by a silver salt. Cinnamyl ethers undergo a formal dimerization affording the target heterocycle via sequential C-O bond cleavage/C-H bond functionalization. The cascade allows one to assemble three new bonds and to establish up to four stereocenters. The reaction likely proceeds through a cationic manifold that forms the target in a diastereoselective fashion.

Synthesis of Non-Symmetric Azoarenes by Palladium-Catalyzed Cross-Coupling of Silicon-Masked Diazenyl Anions and (Hetero)Aryl Halides

The photoswitchable motif of azobenzenes is of great importance across the life and materials sciences. This maintains a constant demand for their efficient synthesis, especially that of non-symmetric derivatives. We disclose here a general strategy for their synthesis through an unprecedented C(sp2 )-N(sp2 ) cross-coupling where functionalized aryl-substituted diazenes masked with a silyl group are employed as diazenyl pronucleophiles. These equivalents of fragile diazenyl anions couple with a diverse set of (hetero)aryl bromides under palladium catalysis with no loss of dinitrogen. The competing denitrogenative biaryl formation is fully suppressed. The reaction requires only a minimal excess, that is 1.2 equivalents, of the diazenyl component. By this, a broad range of azoarenes decorated with two electron-rich/deficient aryl groups can be accessed in a predictable way with superb functional-group tolerance.

3-Alkyl-2-pyridyl Directing Group-Enabled C2 Selective C-H Silylation of Indoles and Pyrroles via an Iridium Catalyst

An iridium-catalyzed, directing group-enabled site selective intra- and intermolecular silylation of indoles and pyrroles with hydrosilanes has been developed under ligand-free conditions. Fine-tuning of the removable 3-alkyl-2-pyridyl directing group was found to be crucial for achieving high yields for C2-silylated indole and pyrrole products. Moreover, the scalability was demonstrated, and further transformations of the silylation products were achieved.

Visible Light-Induced Hydrosilylation of Electron-Deficient Alkenes by Iron Catalysis

Herein, we reported a method for iron-catalyzed, visible-light-induced hydrosilylation reactions of electron-deficient alkenes to produce value-added silicon compounds. Alkenes bearing functional groups with different steric properties were suitable substrates, as were derivatives of structurally complex natural products. Mechanistic studies showed that chlorine radicals generated by iron-catalyzed ligand-to-metal charge transfer in the presence of lithium chloride promoted the formation of silyl radicals.