Catalytic metal-free Si-N cross-dehydrocoupling

Alpha-metalated N,N-dimethylbenzylamine rare-earth metal complexes and their catalytic applications

This perspective summarizes our group's extensive research in the realm of organometallic lanthanide complexes, while also placing the catalytic reactions supported by these species within the context of known lanthanide catalysis worldwide, with a specific focus on phosphorus-based catalytic reactions such as intermolecular hydrophosphination and hydrophosphinylation. α-Metalated N,N-dimethylbenzylamine ligands have been utilized to generate homoleptic lanthanide complexes, which have subsequently proven to be highly active lanthanum-based catalysts. The main goal of our research program has been to enhance the catalytic efficiency of lanthanum-based complexes, which began with initial successes in the stoichiometric synthesis of organometallic lanthanide complexes and utilization of these species in catalytic hydrophosphination reactions. Not only have these species supported traditional lanthanide catalysis, such as the hydrophosphination of heterocumulenes like carbodiimides, isocyanates, and isothiocyanates, but they have also been effective for a plethora of catalytic reactions tested thus far, including the hydrophosphinylation and hydrophosphorylation of nitriles, hydrophosphination and hydrophosphinylation of alkynes and alkenes, and the heterodehydrocoupling of silanes and amines. Each of these catalytic transformations is meritorious in its own right, offering new synthetic routes to generate organic scaffolds with enhanced functionality while concurrently minimizing both waste generation and energy consumption. Objectives: We aim for the research summary presented herein to inspire and encourage other researchers to investigate f-element based stoichiometric and catalytic reactions. Our efforts in this field began with the recognition that potassium salts of benzyldimethylamine preferred deprotonation at the α-position, rather than the ortho-position, and we wondered if this regiochemistry would be retained in the formation of lanthanide complexes. The pursuit of this simple idea led first to a series of structurally fascinating homoleptic organometallic lanthanide complexes with surprisingly good stability. Fundamental studies of the protonolysis chemistry of these complexes ultimately revealed highly versatile lanthanide-based precatalysts that have propelled a catalytic investigation spanning more than a decade. We anticipate that this summative perspective will animate the synthetic as well as biological communities to consider La(DMBA)3-based catalytic methods in the synthesis of functionalized organic scaffolds as an atom-economic, convenient, and efficient methodology. Ultimately, we envision our work making a positive impact on the advancement of novel chemical transformations and contributing to progress in various fields of science and technology.

Group I Alkoxides and Amylates as Highly Efficient Silicon-Nitrogen Heterodehydrocoupling Precatalysts for the Synthesis of Aminosilanes

Group I alkoxides are highly active precatalysts in the heterodehydrocoupling of silanes and amines to afford aminosilane products. The broadly soluble and commercially available KOt Amyl was utilized as the benchmark precatalyst for this transformation. Challenging substrates such as anilines were found to readily couple primary, secondary, and tertiary silanes in high conversions (>90 %) after only 2 h at 40 °C. Traditionally challenging silanes such as Ph3 SiH were also easily coupled to simple primary and secondary amines under mild conditions, with reactivity that rivals many rare earth and transition-metal catalysts for this transformation. Preliminary evidence suggests the formation of hypercoordinated intermediates, but radicals were detected under catalytic conditions, indicating a mechanism that is rare for Si-N bond formation.

Commercially available organolithium compounds as effective, simple precatalysts for silicon-nitrogen heterodehydrocoupling

A family of commercially available organolithium compounds were found to effectively catalyze the heterodehydrocoupling of silanes and amines under ambient conditions. Ubiquitous nBuLi (1) was utilized as the benchmark catalyst, where an array of primary, secondary, and tertiary arylsilanes were coupled to electron-donating amines, affording aminosilanes in high conversions with short reaction times. Preliminary mechanistic analysis is consistent with a nucleophilic-type system that involves the formation of a hypervalent silicon intermediate. This work underscores the accessibility of Si-N heterodehydrocoupling, with organolithium reagents emerging as some of the most straightforward and cost-effective precatalysts for this transformation.

Sustainable preparation of aminosilane monomers, oligomers, and polymers through Si-N dehydrocoupling catalysis

This article covers historical and recent efforts to catalyse the dehydrocoupling of amines and silanes, a direct method for Si-N bond formation that offers hydrogen as a byproduct. In some applications, this transformation can be used as a sustainable replacement for traditional aminosilane synthesis, which demands corrosive chlorosilanes while generating one equivalent of ammonium salt waste for each Si-N bond that is formed. These advantages have driven the development of Si-N dehydrocoupling catalysts that span the periodic table, affording mechanistic insight that has led to advances in efficiency and selectivity. Given the divergence in precursors being used, characterization methods being relied on, and applications being targeted, this article highlights the formation of monomeric aminosilanes separately from oligomeric and polymeric aminosilanes. A recent study that allowed for the manganese catalysed synthesis of perhydropolysilazane and commercial chemical vapor deposition precursors is featured, and key opportunities for advancing the field of Si-N dehydrocoupling catalysis are discussed.

Structure-Reactivity Relationships in Borane-Based FLP-Catalyzed Hydrogenations, Dehydrogenations, and Cycloisomerizations

ConspectusThe activation of molecular hydrogen by main-group element catalysts is an extremely important approach to metal-free hydrogenations. These so-called frustrated Lewis pairs advanced within a short period of time to become an alternative to transition metal catalysis. However, deep understanding of the structure-reactivity relationship is far less developed compared to that of transition metal complexes, although it is paramount for advancing frustrated Lewis pair chemistry.In this Account, we provide detailed insight into how Lewis acidity and Lewis basicity correlate to reactivity. The reactivity of frustrated Lewis pairs will be systematically discussed in context with selected reactions. The influence of major electronic modifications of the Lewis pairs is correlated with the ability to activate molecular hydrogen, to channel reaction kinetics and reaction pathways, or to achieve C(sp³)-H activations.First, we will describe how we entered this emerging field of research after quickly realizing that information was lacking on how the reactivity changes with modification of the frustrated Lewis pair. This led us to the development of a qualitative and quantitative structure-reactivity relationship in metal-free imine hydrogenations. The imine hydrogenation was utilized as the model reaction to experimentally determine the activation parameters of the FLP-mediated hydrogen activation for the first time. This kinetic study revealed autoinduced catalytic profiles when Lewis acids weaker than tris(pentafluorophenyl)borane were applied, opening up to study the Lewis base dependency within one system. With this knowledge of the interplay between Lewis acid strength and Lewis basicity, we developed methods for the hydrogenation of densely functionalized nitroolefins, acrylates, and malonates. Here, the reduced Lewis acidity needed to be counterbalanced by a suitable Lewis base to ensure efficient hydrogen activation. The opposite measure was necessary for the hydrogenation of unactivated olefins. For these, comparably less electron-releasing phosphanes were required to generate strong Brønsted acids by hydrogen activation. These systems displayed highly reversible hydrogen activation even at temperatures as low as -60 °C. A systematic study of these systems enabled the development of acceptorless dehydrocouplings of amines with silanes and dehydrogenations of aza-heterocycles by C(sp³)-H activations. Furthermore, the C(sp³)-H and π-activation was utilized to achieve cycloisomerizations by carbon-carbon and carbon-nitrogen bond formations. Lastly, new frustrated Lewis pair systems featuring weak Lewis bases as active components in the hydrogen activation were developed for the reductive deoxygenation of phosphane oxides and carboxylic acid amides.

B(C6 F5 )3 -Catalyzed Regioselective Ring Opening of Cyclic Amines with Hydrosilanes

Opening the ring of cyclic amines by regioselective fission of one of the carbon-nitrogen bonds greatly expands the repertoire of available nitrogen-containing skeletons. Unlike approaches starting from cyclic tertiary amines, methods that can directly open secondary amines are still scarce. The present work discloses an efficient reductive ring opening of either of these cyclic amines using PhSiH₃ under B(C₆ F₅ )₃ catalysis. By this, the direct transformation of unstrained cyclic amines into the corresponding acyclic amines is achieved in a simple one-pot operation. A stepwise mechanism proceeding through the intermediacy of silylammonium ions followed by reductive cleavage of a carbon-nitrogen bond was experimentally verified.

Mechanistic insights into reductive deamination with hydrosilanes catalyzed by B(C6F5)3: A DFT study

Selective defunctionalization of synthetic intermediates is a valuable approach in organic synthesis. Here, we present a theoretical study on the recently developed B(C6F5)3/hydrosilane-mediated reductive deamination reaction of primary amines. Our computational results provide important insights into the reaction mechanism, including the active intermediate, the competing reactions of the active intermediate, the role of excess hydrosilane, and the origin of chemoselectivity. Moreover, the study on the substituent effect of hydrosilane indicated a potential way to improve the efficiency of the reductive deamination reaction.

N-Silylamines in catalysis: synthesis and reactivity

A summary of the catalytic synthesis and reactivity of N-silylated amines is presented. Dehydrocoupling of amines with silanes, hydrosilylation of imines and dealkenylative coupling of amines with vinylsilanes are three ways to achieve their catalytic syntheses. The resultant N-silylamines serve as substrates in a variety of reactions, including C-N and C-C bond forming reactions, and are preferred in transformations because of the facile Si-N hydrolytic cleavage to reveal free amine products upon reaction completion. This review highlights the distinct electronic properties of N-silyl amines, N-silyl imines and N-silyl enamines that result in complementary reactivity to that of parent non-silyl variants.

Activation of Si-H and B-H bonds by Lewis acidic transition metals and p-block elements: same, but different

In this Perspective we discuss the ability of transition metal complexes to activate and cleave the Si-H and B-H bonds of hydrosilanes and hydroboranes (tri- and tetra-coordinated) in an electrophilic manner, avoiding the need for the metal centre to undergo two-electron processes (oxidative addition/reductive elimination). A formal polarization of E-H bonds (E = Si, B) upon their coordination to the metal centre to form σ-EH complexes (with coordination modes η1 or η2) favors this type of bond activation that can lead to reactivities involving the formation of transient silylium and borenium/boronium cations similar to those proposed in silylation and borylation processes catalysed by boron and aluminium Lewis acids. We compare the reactivity of transition metal complexes and boron/aluminium Lewis acids through a series of catalytic reactions in which pieces of evidence suggest mechanisms involving electrophilic reaction pathways.

Introducing the Catalytic Amination of Silanes via Nitrene Insertion

The direct functionalization of Si–H bonds by the nitrene insertion methodology is described. A copper(I) complex bearing a trispyrazolylborate ligand catalyzes the transfer of a nitrene group from PhI=NTs to the Si–H bond of silanes, disilanes, and siloxanes, leading to the exclusive formation of Si–NH moieties in the first example of this transformation. The process tolerates other functionalities in the substrate such as several C–H bonds and alkyne and alkene moieties directly bonded to the silicon center. Density functional theory (DFT) calculations provide a mechanistic interpretation consisting of a Si–H homolytic cleavage and subsequent rebound to the Si-centered radical.

B(C6F5)3-Catalyzed Reductive Denitrogenation of Benzonitrile Derivatives

A B(C₆F₅)₃-catalyzed reductive denitrogenation of aromatic nitriles is reported, achieving the metal-free transformation of a cyano into a methyl group in a single synthetic operation. Tris(phenylsilyl)amine is liberated as the nitrogen-containing byproduct. On the basis of control experiments as well as a nuclear magnetic resonance spectroscopic analysis, an S_N1-type mechanism involving a trisilylammonium ion as a key intermediate is proposed.

Peptide Bond Formation of Amino Acids by Transient Masking with Silylating Reagents

A one-pot peptide bond-forming reaction has been developed using unprotected amino acids and peptides. Two different silylating reagents, HSi[OCH(CF₃)₂]₃ and MTBSTFA, are instrumental for the successful implementation of this approach, being used for the activation and transient masking of unprotected amino acids and peptides at C-termini and N-termini, respectively. Furthermore, CsF and imidazole are used as catalysts, activating HSi[OCH(CF₃)₂]₃ and also accelerating chemoselective silylation. This method is versatile as it tolerates side chains that bear a range of functional groups, while providing up to >99% yields of corresponding peptides without any racemization or polymerization.

Mechanistic insights into the dearomative diborylation of pyrazines: a radical or non-radical process?

The mechanisms of the dearomative diborylation of pyrazines were investigated via a combination of density functional theory calculations and experimental studies.

Modification of silicone resins by Si–N cross-dehydrocoupling with perfect thermal stability and mechanical performance

A series of new types of modified hydrosilanes were synthesized, using hydrosilanes and nitrogen heterocycles by Si–N cross-dehydrocoupling catalyzed by Ru3CO12.

Silicon-Nitrogen Bond Formation via Heterodehydrocoupling and Catalytic N-Silylation

Silicon-nitrogen bond formation is an important subfield in main group chemistry, and catalysis is an attractive route for efficient, selective formation of these bonds. Indeed, heterodehydrocoupling and N-silylation offer facile methods for the synthesis of small molecules through the coupling of primary, secondary, and tertiary silanes with N-containing substrates such as amines, carbazoles, indoles, and pyrroles. However, the reactivity of these catalytic systems is far from uniform, and critical issues are often encountered with product selectivity, conversions, substrate scope, catalyst activation, and in some instances, competing side reactions. Herein, a catalogue of catalysts and their reactivity for Si-N heterodehydrocoupling and N-silylation are reported.

Dichotomous Si-H Bond Activation by Alkoxide and Alcohol in Base-Catalyzed Dehydrocoupling of Silanes

The activation of silanes in dehydrogenative coupling with alcohols under general base catalysis was studied experimentally (using multinuclear NMR, IR, and UV-visible spectroscopies) and computationally (at DFT M06/6-311++G(d,p) theory level) on the example of Ph_4-nSiH_n (n = 1-3) interaction with (CF₃)₂CHOH in the presence of Et₃N. The effect of the phenyl groups' number and H^- substitution by the electron-withdrawing (CF₃)₂CHO^- group on Si-H bond hydricity (quantified as hydride-donating ability, HDA) and Lewis acidity of silicon atom (characterized by maxima of molecular electrostatic potential) was accessed. Our results show the coordination of Lewis base (Y = Me₃N, ROH, OR^-) leads to the increased hydricity of pentacoordinate hypervalent Ph_4-nSi(Y)H_n complexes and a decrease of the reaction barrier for H₂ release. The formation of tertiary complexes [Ph_4-nSi(Y)H_n]···HOR is a critical prerequisite for the dehydrocoupling with alkoxides being ideal activators. The latter can be external or internal, generated by in situ HOR deprotonation. The mutual effect of tetrel interaction and dihydrogen bonding in tertiary complexes (RO^-)Ph_4-nSiH_n···HOR leads to dichotomous activation of Si-H bond promoting the proton-hydride transfer and H₂ release.

Reductive Deamination with Hydrosilanes Catalyzed by B(C6 F5 )3

The strong boron Lewis acid tris(pentafluorophenyl)borane B(C6 F5 )3 is known to catalyze the dehydrogenative coupling of certain amines and hydrosilanes at elevated temperatures. At higher temperature, the dehydrogenation pathway competes with cleavage of the C-N bond and defunctionalization is obtained. This can be turned into a useful methodology for the transition-metal-free reductive deamination of a broad range of amines as well as heterocumulenes such as an isocyanate and an isothiocyanate.

Dual reactivity of B(C6F5)3 enables the silylative cascade conversion of N-aryl piperidines to sila-N-heterocycles: DFT calculations

A theoretical study reveals that the dual reactivity of B(C₆F₅)₃ enables the unique silylative cascade conversion of N-aryl piperidines to bridged sila-N-heterocycles.

B(C6F5)3-Catalyzed C-C Coupling of 1,4-Naphthoquinones with the C-3 Position of Indole Derivatives in Water

An atom-economical and environmentally benign approach for the synthesis of indole-substituted 1,4-naphthoquinones from indoles and 1,4-naphthoquinones using readily available Lewis acidic B(C₆F₅)₃ in water and with the recycling of water and part of the catalyst is reported. The reaction proceeded through the B(C₆F₅)₃-catalyzed C(sp²)-H and C(sp²)-H bond coupling of 1,4-naphthoquinones with the C-3 position of indole derivatives in water. This methodology provides a facile protocol for the synthesis of some new indole-substituted 1,4-naphthoquinones in satisfactory yields and with a broad substrate scope. When compared to known methods for the synthesis of indole-substituted 1,4-naphthoquinones, this protocol is practical and efficient and does not require a transition-metal catalyst or toxic organic solvents. In addition, we utilized a simple filtration process for complete recycling of the solvent and the part of the catalyst in each reaction cycle.

Synthesis of structured polysiloxazanes via a Piers-Rubinsztajn reaction

A series of siloxazanes were successfully prepared by a Piers-Rubinsztajn reaction between methoxydisilazanes and the corresponding hydrosilanes. Polysiloxazanes with narrow dispersion were also synthesized from methoxydisilazanes and Si-H terminated oligosiloxanes. The possible interaction mechanism between tris(pentafluorophenyl)borane and the methoxydisilazane was investigated.

B(C6F5)3-catalyzed dehydrogenative cyclization of N-tosylhydrazones and anilines via a Lewis adduct: a combined experimental and computational investigation

Tris(pentafluorophenyl)borane-catalyzed dehydrogenative-cyclization of N-tosylhydrazones with aromatic amines has been disclosed. This metal-free catalytic protocol is compatible with a range of functional groups to provide both symmetrical and unsymmetrical 3,4,5-triaryl-1,2,4-triazoles. Mechanistic experiments and density functional theory (DFT) studies suggest an initial Lewis adduct formation of N-tosylhydrazone with B(C6F5)3 followed by sequential intermolecular amination of the borane adduct with aniline, intramolecular cyclization and frustrated Lewis pair (FLP)-catalyzed dehydrogenation for the generation of substituted 1,2,4-triazoles.

A self-catalyzed reaction of 1,2-dibenzoyl-o-carborane with hydrosilanes - formation of new hydrofuranes

The activation of Si-H bonds is a very important transformation both in organic and inorganic chemistry. Herein we report that 1,2-dibenzoyl-o-carborane (1) reacts with Si-H bonds, yielding new hydrofurane-type products. The mechanism of this Si-H bond activation was studied both experimentally and by DFT calculations, and supposedly proceeds in an FLP-type manner.

Synthesis of hydrosilanes via Lewis-base-catalysed reduction of alkoxy silanes with NaBH4

Hydrosilanes were synthesized by reduction of alkoxy silanes with BH₃ in the presence of hexamethylphosphoric triamide (HMPA) as a Lewis-base catalyst. The reaction was also achieved using an inexpensive and easily handled hydride source NaBH₄, which reacted with EtBr as a sacrificial reagent to form BH₃in situ.

Comparative DFT study of metal-free Lewis acid-catalyzed C–H and N–H silylation of (hetero)arenes: mechanistic studies and expansion of catalyst and substrate scope

Direct selective dehydrogenative silylation of thiophenes, pyridines, indoles and anilines to synthesize silyl-substituted aromatic compounds catalyzed by metal-free Lewis acids was achieved recently. However, there is still insufficient mechanistic data for these transformations. Using density functional theory calculations, we conducted a detailed investigation of the mechanism of the B(C₆F₅)₃-catalyzed dehydrogenative silylation of N-methylindole, N,N-dimethylaniline and N-methylaniline. We successfully located the most favourable reaction pathways that can explain the experimental observations notably well. The most favourable pathway for B(C₆F₅)₃-catalyzed C–H silylation of N-methylindole includes nucleophilic attack, proton abstraction and hydride migration. The C–H silylation of N,N-dimethylaniline follows a similar pathway to N-methylindole rather than that proposed by Hou's group. Our mechanism successfully explains that the transformations of N-methylindoline to N-methylindole produce different products at different temperatures. For N-methylaniline bearing both N–H and para-phenyl C–H bonds, the N–H silylation reaction is more facile than the C–H silylation reaction. Our proposed mechanism of N–H silylation of N-methylaniline is different from that proposed by the groups of Paradies and Stephan. Lewis acids Al(C₆F₅)₃, Ga(C₆F₅)₃ and B(2,6-Cl₂C₆H₃)(p-HC₆F₄)₂ can also catalyze the C–H silylation of N-methylindole like B(C₆F₅)₃, but the most favourable pathways are those promoted by N-methylindoline. Furthermore, we also found several other types of substrates that would undergo C–H or N–H silylation reactions under moderate conditions. These findings may facilitate the design of new catalysts for the dehydrogenative silylation of inactivated (hetero)arenes.

We investigated the mechanism of the dehydrosilylation of (hetero)arenes and extended the scope of the silylation catalysts and substrates.

NHC-Copper-Catalyzed Asymmetric Dearomative Silylation of Indoles

We report an asymmetric dearomative silylation reaction of 3-acylindoles using a chiral NHC-copper(I) complex as catalyst to afford a range of cyclic α-aminosilanes with high regio- and enantioselectivity. Initial mechanistic studies revealed that the observed high diastereoselectivity was attributed to the facile epimerization of the 3-acyl group. We also demonstrated that the product could be used as a versatile precursor for the synthesis of functionalized indolines in high enantiomeric purity.

Catalytic Access to Bridged Sila- N-heterocycles from Piperidines via Cascade sp3 and sp2 C-Si Bond Formation

Described herein is the development of an unprecedented route to bridged sila- N-heterocycles via B(C₆F₅)₃-catalyzed cascade silylation of N-aryl piperidines with hydrosilanes. Mechanistic studies indicated that an outer-sphere ionic path is operative to involve three sequential catalytic steps having N-silyl piperidinium borohydride as a resting species: (i) dehydrogenation of the piperidine ring, (ii) β-selective hydrosilylation of a resultant enamine intermediate, and (iii) intramolecular dehydrogenative sp² C-H silylation.

FLP-Catalyzed Transfer Hydrogenation of Silyl Enol Ethers

Herein we report the first catalytic transfer hydrogenation of silyl enol ethers. This metal free approach employs tris(pentafluorophenyl)borane and 2,2,6,6-tetramethylpiperidine (TMP) as a commercially available FLP catalyst system and naturally occurring γ-terpinene as a dihydrogen surrogate. A variety of silyl enol ethers undergo efficient hydrogenation, with the reduced products isolated in excellent yields (29 examples, 82 % average yield).

Boron-Catalyzed O-H Bond Insertion of α-Aryl α-Diazoesters in Water

A catalytic, metal-free O-H bond insertion of α-diazoesters in water in the presence of B(C₆F₅)₃· nH₂O (2 mol %) was developed, affording a series of α-hydroxyesters in good to excellent yields. The reaction features easy operation and wide substrate scope, and importantly, no metal is needed as compared with the conventional methods. Significantly, this approach further expands the applications of B(C₆F₅)₃ under water-tolerant conditions.

B(C6F5)3-Catalyzed Regioselective Deuteration of Electron-Rich Aromatic and Heteroaromatic Compounds

Deuterium labeled compounds find widespread application in life science. Herein, the deuteration of electron-rich (hetero)aromatic compounds employing B(C₆F₅)₃ as the catalyst and D₂O as the deuterium source is reported. This protocol is highly efficient, simply manipulated, and successfully applied in the deuteration of 23 substrates including natural neurotransmitter-like melatonin. It is assumed that the weakening of the O-D bond ultimately results in the formation of electrophilic D⁺.