Calcium-catalysed synthesis of amines through imine hydrosilylation: an experimental and theoretical study

A method to reduce aldimines through hydrosilylation is reported. The catalytic system involves calcium triflimide (Ca(NTf₂)₂) and potassium hexafluorophosphate (KPF₆) which have been shown to act in a synergistic manner. The expected amines are obtained in fair to very high yields (40-99%) under mild conditions (room temperature in most cases). To illustrate the potential of this method, a bioactive molecule with antifungal properties was prepared on the gram scale and in high yield in environmentally friendly 2-methyltetrahydrofuran. Moreover, it is shown in this example that the imine can be prepared in situ from the aldehyde and the amine without isolating the imine. The mechanism involved has been explored experimentally and through DFT calculations, and the results are in accordance with an electrophilic activation of the silane by the calcium catalyst.

Electrochemical deoxygenative reduction of ketones

Electrochemical reduction via paired electrolysis has been used to achieve deoxygenative reduction of ketones. As a result of the complexing of ketones with the triphenylphosphine radical cation generated by anodic oxidation, the reduction of carbonyl groups occurs readily. Through spontaneous β-scission of phosphoranyl radicals, C-O bonds are cleaved to form benzylic radical intermediates. These radical species are either able to abstract hydrogen from MeCN or undergo reduction at the cathode to give carbanions, upon workup forming reductive hydrogenation of ketones.

Calcium Bistriflimide-Mediated Sulfur(VI)-Fluoride Exchange (SuFEx): Mechanistic Insights toward Instigating Catalysis

We report a mechanistic investigation of calcium bistriflimide-mediated sulfur(VI)–fluoride exchange (SuFEx) between sulfonyl fluorides and amines. We determine the likely pre-activation resting state—a calcium bistriflimide complex with ligated amines—thus allowing for corroborated calculation of the SuFEx activation barrier at ∼21 kcal/mol, compared to 21.5 ± 0.14 kcal/mol derived via kinetics experiments. Transition state analysis revealed: (1) a two-point calcium-substrate contact that activates the sulfur(VI) center and stabilizes the leaving fluoride and (2) a 1,4-diazabicyclo[2.2.2]octane additive that provides Brønsted-base activation of the nucleophilic amine. Stable Ca–F complexes upon sulfonamide formation are likely contributors to inhibited catalytic turnover, and a proof-of-principle redesign provided evidence that sulfonamide formation is feasible with 10 mol % calcium bistriflimide.

We report a computational and experimental mechanistic study of sulfur(VI)-fluoride exchange mediated by a calcium salt. Ca²⁺ activates the substrate via two critical Lewis acid−base interactions. Stable fluoride-ligated Ca²⁺ complexes are formed upon sulfonamide formation, and computations indicate that heat and/or fluoride scavengers will facilitate regeneration of the active Ca²⁺ species. These calculations guided an experimental redesign, thus demonstrating improved catalytic efficiency using 10 mol % Ca²⁺.

Direct deoxygenation of active allylic alcohols via metal-free catalysis

Direct metal-free deoxygenation of highly active allylic alcohols catalyzed by a Brønsted acid was achieved, which avoids tedious reaction steps and eliminates metal contamination. By examining a series of Brønsted...

Cationic aluminum, gallium, and indium complexes in catalysis

The introduction of cationic charge allows cationic group 13 complexes to be excellent Lewis acid catalysts. Cationic aluminum, gallium, and indium complexes in catalysis are comprehensively reviewed based on the reaction type.

A Ba/Pd Catalytic System Enables Dehydrative Cross-Coupling and Excellent E-Selective Wittig Reactions

A Ba/Pd cooperative catalysis system was developed to enable the dehydrative cross-coupling of allylic alcohols with P-ylides to occur directly and promote a subsequent Wittig reaction in one pot. A variety of multisubstituted 1,4-dienes were isolated in good to excellent yields with broad P-ylides (stabilized by both ester and ketone carbonyl groups) and aldehyde (aliphatic and aromatic) substrates with excellent E selectivity.

Calcium-catalyzed reactions of element-H bonds

Investigation on organocalcium catalysis is just unfolding during the past decade. Beside conventional Ca salts with strong electron-withdrawing counter anions that may serve as Lewis acid catalysts, many Ca complexes have also been designed recently and found to be good catalysts in activation of element-H (EH) bonds like transition metal catalysts. These findings are interesting and may attract the interest of the chemists. Due to the great abundance, non-toxicity, and biocompatible features of Ca element, Ca-catalyzed reactions can be of great significance from the viewpoint of industry. This short review summarizes the recent advances on Ca-catalyzed reactions of EH bonds. We hope that it may provide a useful guide for interested readers from both the academy and industry.

A selective removal of the secondary hydroxy group from ortho-dithioacetal-substituted diarylmethanols

We present a successful deoxygenation reaction of ortho-1,3-dithianylaryl(aryl)methanols which enables a selective removal of the secondary hydroxy group in presence of the 1,3-dithianyl moiety under reductive conditions. This reaction proceeds well with ZnI₂/Na(CN)BH₃ in dichloroethane or benzene for both unsubstituted and substituted aryls (by electron-rich groups). This is leading to formyl-protected diarylmethanes with potential application in the synthesis of new pharmaceuticals and optoelectronic materials. This synthetic approach gives an access to a wide variety of functionalized ortho-1,3-dithianylaryl(aryl)methanes in 26–95% yields and is recommended for the substrates containing sulfur atoms, for which transition metal-induced reactions fail.

Acid-catalysed intramolecular addition of β-ketoesters to 1,3-dienes

1,3-Dienyl β-keto esters are cyclised into bicyclolactones using the Bi(OTf)₃/TfOH catalytic system. This reaction represents a rare case of simultaneous C-C and C-O bond formation at positions 1 and 3 of a 1,3-diene. Application to the synthesis of ramulosin is presented.

Direct Substitution of Arylalkynyl Carbinols Provides Access to Diverse Terminal Acetylene Building Blocks

To develop next generation antifolates for the treatment of trimethoprim-resistant bacteria, synthetic methods were needed to prepare a diverse array of 3-aryl-propynes with various substitutions at the propargyl position. A direct route was sought whereby nucleophilic addition of acetylene to aryl carboxaldehydes would be followed by reduction or substitution of the resulting propargyl alcohol. The direct reduction, methylation, and dimethylation of these readily available alcohols provide efficient access to this uncommon functional array. In addition, an unusual silane exchange reaction was observed in the reduction of the propargylic alcohols.

Chemo- and regioselective reductive deoxygenation of 1-en-4-yn-ols into 1,4-enynes through FeF3 and TfOH co-catalysis

We report chemo- and regioselective direct reductive deoxygenation of 1-en-4-yn-3-ols into 1,4-enynes through FeF₃ and TfOH cooperative catalysis under NBSH-mediated conditions.

Catalytic applications of [IPr·GaX2][SbF6] and related species

The broad applicability of cationic NHC complexes of gallium in molecular catalysis is revealed.

Mild deoxygenation of aromatic ketones and aldehydes over Pd/C using polymethylhydrosiloxane as the reducing agent

Herein, a practical and mild method for the deoxygenation of a wide range of benzylic aldehydes and ketones is described, which utilizes heterogeneous Pd/C as the catalyst together with the green hydride source, polymethylhydrosiloxane. The developed catalytic protocol is scalable and robust, as exemplified by the deoxygenation of ethyl vanillin, which was performed on a 30 mmol scale in an open-to-air setup using only 0.085 mol % Pd/C catalyst to furnish the corresponding deoxygenated product in 93 % yield within 3 hours at room temperature. Furthermore, the Pd/C catalyst was shown to be recyclable up to 6 times without any observable decrease in efficiency and it exhibited low metal leaching under the reaction conditions.

Gallium(III)- and calcium(II)-catalyzed Meyer–Schuster rearrangements followed by intramolecular aldol condensation or endo-Michael addition

The first gallium- and calcium-catalyzed Meyer–Schuster rearrangements are described. The incipient conjugated ketones can undergo calcium-catalyzed aldol condensation (a) or endo-Michael addition (b) with β-keto esters or amides.

Chemoselective calcium-catalysed direct amidation of carboxylic esters

Cheap, non-toxic and environmentally benign CaI₂ catalyses direct amide bond formation between unactivated carboxylic esters and primary amines in excellent yields.

The first calcium-catalysed Nazarov cyclisation

The first calcium-catalysed Nazarov cyclisation is described.

Calcium-based Lewis acid catalysts

Recently, Lewis acidic calcium salts bearing weakly coordinating anions such as Ca(NTf₂)₂, Ca(OTf)₂, CaF₂ and Ca[OCH(CF₃)₂]₂ have been discovered as catalysts for the transformation of alcohols, olefins and carbonyl compounds. High stability towards air and moisture, selectivity and high reactivity under mild reaction conditions render these catalysts a sustainable and mild alternative to transition metals, rare-earth metals or strong Brønsted acids.