Mechanism of Boron-Catalyzed N-Alkylation of Amines with Carboxylic Acids

Aryl Borane as a Catalyst for Dehydrative Amide Synthesis

Tris(pentafluorophenyl)borane B(C6F5)3·H2O is reported as a catalyst for dehydrative amidation of carboxylic acids and amines. This protocol is applicable across a wide range of >35 substrates, including aromatic and aliphatic amines and acids, resulting in amides in ≤92% yields. The scalability of the reaction up to 10 mmol, along with the synthesis of drugs such as ibuprofen amide, moclobemide, and phenacetin, demonstrates the industrial potential of our protocol.

Cu-UiO-66 Catalyzed Synthesis of Imines via Acceptorless Dehydrogenative Coupling of Alcohols and Amines

Herein, the Cu-UiO-66 catalyst was developed for acceptorless dehydrogenative coupling (ADC) between alcohols and amines to produce imines. The Cu-UiO-66 catalyst was synthesized by installing Cu2+ onto Zr-oxo clusters in UiO-66, and the catalyst efficiently catalyzes the ADC reaction under mild and environmentally friendly conditions with excellent selectivity. Mechanistic studies reveal that the O2⋅- radicals and porosity of formed in Cu-UiO-66 participate cooperatively during the catalytic cycle. Meanwhile, the only by-product of the system is environmentally benign water. Cycling tests and hot filtration tests showed that the Cu-UiO-66 catalyst exhibited excellent stability and catalytic activity during the reaction. Importantly, the Cu-UiO-66 catalyst might provide a promising strategy for the ADC reaction between alcohols and amines to produce imines.

Modulating the Potency of BRD4 PROTACs at the Systems Level with Amine-Acid Coupling Reactions

Protein degradation using proteolysis targeting chimeras (PROTACs) represents a promising therapeutic strategy. PROTACs are heterobifunctional molecules that consist of a target-binding moiety and an E3 ligase binding moiety, connected by a linker. These fragments are frequently united via amide bonds. While straightforward to synthesize, amides may impart suboptimal drug properties to the overall molecule. From a systems level perspective, we envisioned that the potency of PROTACs could be modulated through selection of reaction conditions─wherein different catalysts produce distinct linkers from the same two building blocks. We present a suite of BRD4 PROTAC degraders prepared via four new amine-acid coupling reactions alongside the classic amide coupling. Our findings reveal that variations in reaction conditions affect the physicochemical properties of PROTACs, resulting in a spectrum of properties. Notably, several new PROTACs demonstrated enhanced BRD4 degradation efficacy compared to those employing amide linkers, emphasizing the potential of systems chemistry as a therapeutic optimization strategy.

Deoxygenative Functionalizations of Aldehydes, Ketones and Carboxylic Acids

Conversion of carbonyl compounds, including aldehydes, ketones and carboxylic acids, into functionalized alkanes via deoxygenation would be highly desirable from a sustainability perspective and very enabling in chemical synthesis. This review covers the recent methodology development in carbonyl and carboxyl deoxygenative functionalizations, highlighting some typical and significant contributions in this field. These advances will be categorized based on types of bond formation, and in each part, selected examples will be discussed from their generalized mechanistic perspectives. Four summarized reactivity modes of aldehydes and ketones during the deoxygenation, namely, bis-electrophile, carbenoid, bis-nucleophile and alkyl radical, are presented, while the carboxylic acids are deoxygenated mainly via activated carbonyl or acetal intermediates.

Catalytic Synthesis of Oligosiloxanes Mediated by an Air Stable Catalyst, (C6F5)3B(OH2)

The utility of (C₆F₅)₃B(OH₂) as catalyst for the simple and environmentally benign synthesis of oligosiloxanes directly from hydrosilanes, is reported. This protocol offers several advantages compared to other methods of synthesizing siloxanes, such as mild reaction conditions, low catalyst loading, and a short reaction time with high yields and purity. The considerable H₂O-tolerance of (C₆F₅)₃B(OH₂) promoted a catalytic route to disiloxanes which showed >99% conversion of three tertiary silanes, Et₃SiH, PhMe₂SiH, and Ph₃SiH. Preliminary data on the synthesis of unsymmetrical disiloxanes (Si-O-Si') suggests that by modifying the reaction conditions and/or using a 1:1 combination of silane to silanol the cross-product can be favored. Intramolecular reactions of disilyl compounds with catalytic (C₆F₅)₃B(OH₂) led to the formation of novel bridged siloxanes, containing a Si-O-Si linkage within a cyclic structure, as the major product. Moreover, the reaction conditions enabled recovery and recycling of the catalyst. The catalyst was re-used 5 times and demonstrated excellent conversion for each substrate at 1.0 mol% catalyst loading. This seemingly simple reaction has a rather complicated mechanism. With the hydrosilane (R₃SiH) as the sole starting material, the fate of the reaction largely depends on the creation of silanol (R₃SiOH) from R₃SiH as these two undergo dehydrocoupling to yield a disiloxane product. Generation of the silanol is based on a modified Piers-Rubinsztajn reaction. Once the silanol has been produced, the mechanism involves a series of competitive reactions with multiple catalytically relevant species involving water, silane, and silanol interacting with the Lewis acid and the favored reaction cycle depends on the concentration of various species in solution.

Chemoselective Hydrosilylation of the α,β-Site Double Bond in α,β- and α,β,γ,δ-Unsaturated Ketones Catalyzed by Macrosteric Borane Promoted by Hexafluoro-2-propanol

The B(C₆F₅)₃-catalyzed chemoselective hydrosilylation of α,β- and α,β,γ,δ-unsaturated ketones into the corresponding non-symmetric ketones in mild reaction conditions is developed. Nearly 55 substrates including those bearing reducible functional groups such as alkynyl, alkenyl, cyano, and aromatic heterocycles are chemoselectively hydrosilylated in good to excellent yields. Isotope-labeling studies revealed that hexafluoro-2-propanol also served as a hydrogen source in the process.

Density Functional Theory Mechanistic Study of Boron-Catalyzed N-Alkylation of Amines with Formic Acid: Formic Acid Activation by Silylation Reaction

New methodology for the alkylation of amines is an intriguing issue in both academia and industry. Recently, several groups reported the metal-free B(C₆ F₅ )₃ -catalyzed N-alkylation of amines, but the mechanistic details of these important reactions are unclear. Herein, a computational study was performed to elucidate the mechanism of the N-alkylation of amines with formic acid catalyzed by the Lewis acid B(C₆ F₅ )₃ in the presence of hydrosilane. We found that the reaction started with the activation of formic acid through a novel model. Then, the high electrophilicity of the C center of the formic acid unit and the nucleophilic character of the amine resulted in a C-N coupling reaction. Finally, two sequential silyl-group and H^- transfer steps occurred to generate the final product. Upon comparing the reaction barrier and the hydrogenation of indole, our mechanism is more favorable than that proposed by the group of Yu and Fu.

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.

Efficient and Selective N-Methylation of Nitroarenes under Mild Reaction Conditions

Herein, we report a straightforward protocol for the preparation of N,N-dimethylated amines from readily available nitro starting materials using formic acid as a renewable C₁ source and silanes as reducing agents. This tandem process is efficiently accomplished in the presence of a cubane-type Mo₃ PtS₄ catalyst. For the preparation of the novel [Mo₃ Pt(PPh₃ )S₄ Cl₃ (dmen)₃ ]⁺ (3⁺ ) (dmen: N,N'-dimethylethylenediamine) compound we have followed a [3+1] building block strategy starting from the trinuclear [Mo₃ S₄ Cl₃ (dmen)₃ ]⁺ (1⁺ ) and Pt(PPh₃ )₄ (2) complexes. The heterobimetallic 3⁺ cation preserves the main structural features of its 1⁺ cluster precursor. Interestingly, this catalytic protocol operates at room temperature with high chemoselectivity when the 3⁺ catalyst co-exists with its trinuclear 1⁺ precursor. N-heterocyclic arenes, double bonds, ketones, cyanides and ester functional groups are well retained after N-methylation of the corresponding functionalized nitroarenes. In addition, benzylic-type as well as aliphatic nitro compounds can also be methylated following this protocol.

Expanding Water/Base Tolerant Frustrated Lewis Pair Chemistry to Alkylamines Enables Broad Scope Reductive Aminations

Lower Lewis acidity boranes demonstrate greater tolerance to combinations of water/strong Brønsted bases than B(C6 F5 )3 , this enables Si-H bond activation by a frustrated Lewis pair (FLP) mechanism to proceed in the presence of H2 O/alkylamines. Specifically, BPh3 has improved water tolerance in the presence of alkylamines as the Brønsted acidic adduct H2 O-BPh3 does not undergo irreversible deprotonation with aliphatic amines in contrast to H2 O-B(C6 F5 )3 . Therefore BPh3 is a catalyst for the reductive amination of aldehydes and ketones with alkylamines using silanes as reductants. A range of amines inaccessible using B(C6 F5 )3 as catalyst, were accessible by reductive amination catalysed by BPh3 via an operationally simple methodology requiring no purification of BPh3 or reagents/solvent. BPh3 has a complementary reductive amination scope to B(C6 F5 )3 with the former not an effective catalyst for the reductive amination of arylamines, while the latter is not an effective catalyst for the reductive amination of alkylamines. This disparity is due to the different pKa values of the water-borane adducts and the greater susceptibility of BPh3 species towards protodeboronation. An understanding of the deactivation processes occurring using B(C6 F5 )3 and BPh3 as reductive amination catalysts led to the identification of a third triarylborane, B(3,5-Cl2 C6 H3 )3 , that has a broader substrate scope being able to catalyse the reductive amination of both aryl and alkyl amines with carbonyls.