B(C6F5)3-Catalyzed Chemoselective Defunctionalization of Ether-Containing Primary Alkyl Tosylates with Hydrosilanes

CO2-Promoted and Copper-Catalyzed Dehydroxylative Coupling of Benzylic Alcohols by the NaBH4/I2 System

An efficient and CO2-promoted dehydroxylative coupling of benzylic alcohols catalyzed by ligand-free cuprous chloride has been achieved. The discovered catalytic reductive coupling reaction is a newly C-C bond-forming transformation of alcohols. Mechanistic insight is gained through control reactions.

Controllable Si-C Bond Formation from Trihydrosilanes En Route to Synthesis of 1,4-Azasilinanes with Diverse Silyl Functionalities

A B(C6F5)3-catalyzed controllable inter/intra-/intermolecular Si-C bond formation process has been developed from trihydrosilane and dienamide with alkenes, anilines, or aryl iodides. A variety of 1,4-azasilinanes have been generated with diverse exo-cyclic heteroleptic disubstitutions on silicon, thereby expanding the range of silaazacyclic rings available for the discovery of silicon-containing drugs.

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.

Precyclization Conformer Profiles of -SiR3+- and -Bcat+-Activated Linear Si-Protected Hexitols Explain Condensative Cyclization Selectivities

The condensative cyclization of sp³ C-O bonds in per-silylated hexitols is investigated by computation. Conformer searches using the Monte Carlo algorithm, followed by successively higher levels of theory (MMFF, PM3, and B3LYP), of -SiR₃⁺- and -Bcat⁺-activated substrates lead to structures primed for intramolecular chemistry. Silane activation features O4 to C1 attack, while borane activation suggests boronium ions that activate O5 to C2 reactivity. This, in conjunction with Boltzmann population analysis, parallels reported reactivity for sorbitol, mannitol, and galactitol. Calculations using the meta-hybrid M06-2X functional additionally provide free-energy profiles for each cyclization event. In most of the cases presented, precyclization conformers that position a nucleophilic oxygen less than 3.0 Å from the C-O leaving group correlate to efficient experimental reactivities. Two examples of galactitol containing bridging silyl groups are analyzed computationally, and the experimental outcomes match predictions. The computational regime presented is a step closer to providing predictive power for the reduction of per-functionalized molecules.

Borane- and Silylium-Catalyzed Difunctionalization of Carbohydrates: 3,6-Anhydrosugar Enabled 1,6-Site Selectivity

A novel diastereoselective, Lewis acid catalyzed 1,6-difunctionalization of galactose and mannose derivatives has been developed in one pot, via sequential nucleophile additions. Our studies point to the formation of a 3,6-anhydrosugar intermediate as key to the 1,6-site-selectivity. Starting material-specific reactivity occurs when competitive ring-opening C-O cleavage is possible, owed to basicity and stereoelectronic stabilization differences. Lastly, Mayr nucleophilicity parameter values helped predict which reaction conditions would be most suitable for specific nucleophiles.

Iridium-catalyzed highly stereoselective deoxygenation of tertiary cycloalkanols: stereoelectronic insights and synthetic applications

Excellent and unique diastereoselectivity is observed in the iridium-catalyzed deoxygenation of tertiary cyclohexanols and cyclopentanols. The substituent effect on the diastereoselectivity and detailed control models are analyzed case by case, using tertiary monocyclic and polycyclic cyclohexanols, bicyclic bridged cycloalkanols, and cyclopentanols as the model substrates. The selectivity is decided by the steric environment of the carbocation intermediates and is independent of the catalyst loading. Stereoelectronically, the iridium hydride approaches the carbocation in directions perpendicular to the carbocation plane. The sterically large iridium hydride delivers its hydride in the sterically least hindered direction to the carbocation. The deoxygenation has found important applications in the stereospecific arylations of sterically complex compounds. Our deoxygenation is stereochemically very different from the coupling reactions and can be used to specifically synthesize stereoisomers that are not available via cross-couplings.

Deoxygenative Divergent Synthesis: En Route to Quinic Acid Chirons

The installation of vicinal mesylate and silyl ether groups in a quinic acid derivative generates a system prone for stereoselective borane-catalyzed hydrosilylation through a siloxonium intermediate. The diversification of the reaction conditions allowed the construction of different defunctionalized fragments foreseen as useful synthetic fragments. The selectivity of the hydrosilylation was rationalized on the basis of deuteration experiments and computational studies.

Chemoselective Homologation-Deoxygenation Strategy Enabling the Direct Conversion of Carbonyls into (n+1)-Halomethyl-Alkanes

The sequential installation of a carbenoid and a hydride into a carbonyl, furnishing halomethyl alkyl derivatives, is reported. Despite the employment of carbenoids as nucleophiles in reactions with carbon-centered electrophiles, sp³-type alkyl halides remain elusive materials for selective one-carbon homologations. Our tactic levers on using carbonyls as starting materials and enables uniformly high yields and chemocontrol. The tactic is flexible and is not limited to carbenoids. Also, diverse carbanion-like species can act as nucleophiles, thus making it of general applicability.

Total Synthesis of the Proposed Structure for Chaunopyran A and Its Absolute Configuration

This paper describes a seven-step synthesis of the proposed structure for chaunopyran A produced by cocultivation of a Chaunopycnis sp. and Trichoderma hamatum. This synthesis included a coupling of a diene sulfone and a tetrahydropyranyl aldehyde as a key step. The sign of the specific rotation value of the synthetic sample was opposite that of the natural product, suggesting that the absolute configuration of the natural product should be revised.

Defunctionalisation catalysed by boron Lewis acids

Selective defunctionalisation of organic molecules to valuable intermediates is a fundamentally important transformation in organic synthesis. Despite the advances made in efficient and selective defunctionalisation using transition-metal catalysis, the cost, toxicity, and non-renewable properties limit its application in industrial manufacturing processes. In this regard, boron Lewis acid catalysis has emerged as a powerful tool for the cleavage of carbon-heteroatom bonds. The ground-breaking finding is that the strong boron Lewis acid B(C6F5)3 can activate Si-H bonds through η1 coordination, and this Lewis adduct is a key intermediate that enables various reduction processes. This system can be tuned by variation of the electronic and structural properties of the borane catalyst, and together with different hydride sources high chemoselectivity can be achieved. This Perspective provides a comprehensive summary of various defunctionalisation reactions such as deoxygenation, decarbonylation, desulfurisation, deamination, and dehalogenation, all of which catalysed by boron Lewis acids.

Diazaphosphinyl radical-catalyzed deoxygenation of α-carboxy ketones: a new protocol for chemo-selective C-O bond scission via mechanism regulation

C-O bond cleavage is often a key process in defunctionalization of organic compounds as well as in degradation of natural polymers. However, it seldom occurs regioselectively for different types of C-O bonds under metal-free mild conditions. Here we report a facile chemo-selective cleavage of the α-C-O bonds in α-carboxy ketones by commercially available pinacolborane under the catalysis of diazaphosphinane based on a mechanism switch strategy. This new reaction features high efficiency, low cost and good group-tolerance, and is also amenable to catalytic deprotection of desyl-protected carboxylic acids and amino acids. Mechanistic studies indicated an electron-transfer-initiated radical process, underlining two crucial steps: (1) the initiator azodiisobutyronitrile switches originally hydridic reduction to kinetically more accessible electron reduction; and (2) the catalytic phosphorus species upconverts weakly reducing pinacolborane into strongly reducing diazaphosphinane.

Modulating Electrostatic Interactions in Ion Pair Intermediates To Alter Site Selectivity in the C-O Deoxygenation of Sugars

Controlling which products one can access from the predefined biomass-derived sugars is challenging. Changing from CH₂ Cl₂ to the greener alternative toluene alters which C-O bonds in a sugar are cleaved by the tris(pentafluorophenyl)borane/HSiR₃ catalyst system. This increases the diversity of high-value products that can be obtained through one-step, high-yielding, catalytic transformations of the mono-, di-, and oligosaccharides. Computational methods helped identify this non-intuitive outcome in low dielectric solvents to non-isotropic electrostatic enhancements in the key ion pair intermediates, which influence the reaction coordinate in the reactivity-/selectivity-determining step. Molecular-level models for these effects have far-reaching consequences in stereoselective ion pair catalysis.

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.

Reduction of sugar derivatives to valuable chemicals: utilization of asymmetric carbons

Recent progress on non-furfural routes from sugar derivatives to valuable chemicals including chiral chemicals was reviewed.

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.

Metal-Free Catalytic Reductive Cleavage of Enol Ethers

In contrast to the well-known reductive cleavage of the alkyl-O bond, the cleavage of the alkenyl-O bond is much more challenging especially using metal-free approaches. Unexpectedly, alkenyl-O bonds were reductively cleaved when enol ethers were reacted with Et₃SiH and a catalytic amount of B(C₆F₅)₃. Supposedly, this reaction is the result of a B(C₆F₅)₃-catalyzed tandem hydrosilylation reaction and a silicon-assisted β-elimination. A mechanism for this cleavage reaction is proposed based on experiments and density functional theory (DFT) calculations.

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.

Chemoselective deoxygenation of ether-substituted alcohols and carbonyl compounds by B(C6F5)3-catalyzed reduction with (HMe2SiCH2)2

B(C6F5)3-catalyzed deoxygenation of ether-substituted alcohols and carbonyl compounds has been developed using (HMe2SiCH2)2 as the reductant. This unique reagent shows distinct superiority over traditional one silicon-centered hydrosilanes, giving the corresponding alkanes in high yields with good tolerance of ethers, aryl halides and alkenes. The control experiments suggest that (HMe2SiCH2)2 might facilitate the approach in an intramolecular Si/O activation manner.

Non-Metal-Catalyzed Heterodehydrocoupling of Phosphines and Hydrosilanes: Mechanistic Studies of B(C6F5)3-Mediated Formation of P-Si Bonds

Non-metal-catalyzed heterodehydrocoupling of primary and secondary phosphines (R¹R²PH, R² = H or R¹) with hydrosilanes (R³R⁴R⁵SiH, R⁴, R⁵ = H or R³) to produce synthetically useful silylphosphines (R¹R²P-SiR³R⁴R⁵) has been achieved using B(C₆F₅)₃ as the catalyst (10 mol %, 100 °C). Kinetic studies demonstrated that the reaction is first-order in hydrosilane and B(C₆F₅)₃ but zero-order in phosphine. Control experiments, DFT calculations, and DOSY NMR studies suggest that a R¹R²HP·B(C₆F₅)₃ adduct is initially formed and undergoes partial dissociation to form an "encounter complex". The latter mediates frustrated Lewis pair type Si-H bond activation of the silane substrates. We also found that B(C₆F₅)₃ catalyzes the homodehydrocoupling of primary phosphines to form cyclic phosphine rings and the first example of a non-metal-catalyzed hydrosilylation of P-P bonds to produce silylphosphines (R¹R²P-SiR³R⁴R⁵). Moreover, the introduction of PhCN to the reactions involving secondary phosphines with hydrosilanes allowed the heterodehydrocoupling reaction to proceed efficiently under much milder conditions (1.0 mol % B(C₆F₅)₃ at 25 °C). Mechanistic studies, as well as DFT calculations, revealed that PhCN plays a key mechanistic role in facilitating the dehydrocoupling reactions rather than simply functioning as H₂-acceptor.

Late-stage chemoselective functional-group manipulation of bioactive natural products with super-electrophilic silylium ions

The selective (and controllable) modification of complex molecules with disparate functional groups (for example, natural products) is a long-standing challenge that has been addressed using catalysts tuned to perform singular transformations (for example, C-H hydroxylation). A method whereby reactions with diverse functional groups within a single natural product are feasible depending on which catalyst or reagent is chosen would widen the possible structures one could obtain. Fluoroarylborane catalysts can heterolytically split Si-H bonds to yield an oxophilic silylium (R₃Si⁺) equivalent along with a reducing (H^-) equivalent. Together, these reactive intermediates enable the reduction of multiple functional groups. Exogenous phosphine Lewis bases further modify the catalyst speciation and attenuate aggressive silylium ions for the selective modification of complex natural products. Manipulation of the catalyst, silane reagent and the reaction conditions provides experimental control over which site is modified (and how). Applying this catalytic method to complex bioactive compounds (natural products or drugs) provides a powerful tool for studying structure-activity relationships.