Amine-catalyzed Reactions Research Articles

Valorization of bio-renewable glycerol by catalytic amination reactions

Production of value-added chemicals from renewable feedstocks is an attractive platform to alleviate the shortage of petroleum resources and to minimize CO2 emissions.

Dual Nickel/Photoredox‐Catalyzed Synthesis of N‐formyl N,N’‐Diaryl Ethylenediamines via Multiple C−N/C−C Coupling of Nitroarenes with Trimethylamine

AbstractN,N’‐Diaryl ethylenediamines are common feedstocks in organic synthesis and important structural motifs in medicinal chemistry. Whereas the C−N formation reactions based on anilines and aryl halides are the most direct methods to access N,N’‐diaryl ethylenediamines, the photocatalytic synthetic version remains undeveloped. Herein we describe the dual nickel/photoredox‐catalyzed amination reaction using nitroarenes and trimethylamine as reaction substrates. This cascade transformation hinges on the multiple C−N and C−C coupling events based on the nitrogen radical and α‐aminomethyl radical derived from nitroarene and trimethylamine, respectively. A variety of N‐formyl N,N’‐diaryl ethylenediamines can be obtained under the reaction protocol. Subsequent hydrolysis offers the N,N’‐diaryl ethylenediamines for further derivatizations.

Iron- and Ruthenium-Catalyzed C–N Bond Formation Reactions. Reactive Metal Imido/Nitrene Intermediates

Transition-metal-catalyzed direct C–N bond formation via metal nitrenoid (metal imido/nitrene complexes) transfer reactions such as C–H bond amination/amidation and alkene aziridination is a versatile synthetic strategy for the preparation of nitrogen-containing compounds. In this area of development, iron catalysts are appealing in the context of cost, biocompatibility, and environmental sustainability, and iron-catalyzed direct amination reactions have become a complementary approach in the preparation of nitrogen-containing organic compounds with complexity. Reactive iron imido/nitrene species have been widely proposed as the reaction intermediates for direct reactions with C–H and C═C bonds. However, their short lifetimes in solution make the study of these species difficult. In comparison with iron, ruthenium nitrenoids are considerably more stable but yet are reactive enough for amination reactions, leading to reliable surrogates in understanding the reaction mechanism. In this Perspective, the developments in iron and ruthenium complexes supported by macrocyclic or polydentate ligands for the catalytic amination reactions that occur via metal imido/nitrene intermediates are summarized, as well as the mechanisms involved with highlights on the reactive intermediates. Current limitations, challenges, and potential opportunities for future developments are also discussed.

Amines as Catalysts: Dynamic Features and Kinetic Control of Catalytic Asymmetric Chemical Transformations to Form C-C Bonds and Complex Molecules.

Carbonyl transformations involving enolates and/or enamines have been used for various types of bond-forming reactions. In this account, catalysts and catalyst systems that have amino acids or primary, secondary, and/or tertiary amines as key catalytic functional groups that we have developed to accelerate chemical transformations, including regio-, diastereo- and enantioselective reactions, are discussed. Our chemical transformation strategies and methods that use amine derivatives as catalysts are also discussed. As amines can have different functions depending on protonation and on the species formed during the catalysis (such as enamines and iminium ions), dynamics and kinetic controls are the keys for understanding the catalysis. Further, strategies that harness dynamic steps and kinetic control in amine-catalyzed reactions have enabled the synthesis of complex molecules in stereocontrolled manners. Understanding the dynamic features and the kinetic controls of the catalysis will further the design of the catalysts and the development of chemical transformation strategies and methods.

Investigating the impact of micropore volume of aminosilica functionalized SBA-15 on catalytic activity for amine-catalyzed reactions

Mesoporous materials such as SBA-15 are used in many applications since the materials are robust and have tunable properties. Whereas these materials are thought to be ideal, the materials are complex with the common synthesis methods producing materials that contain micropores. This work will investigate the impact of micropores on catalytic activity of aminosilica materials. For comparison, materials are formed using a standard method that produces materials with micropores (REG-SBA-15) and using a modified method that produces materials with negligible micropore volume (NMP-SBA-15). After grafting the calcined support with an aminosilane (either (N-methyl amino propyl) trimethoxy silane (denoted as 2°Am) or (N,N-diethyl-3-amino propyl) trimethoxy silane (3°Am)), catalytic testing experiments reveal that NMP-SBA-15 functionalized materials are more active than REG-SBA-15 functionalized materials for a range of reactions. The reactions include (a) glucose isomerization where amines interacting with surface silanols limit catalytic activity and (b) aldol and Knoevenagel chemistry where amine-silanol interactions increase the reaction rate. For aldol chemistry, the observed difference in rate is further investigated using site quantification experiments. Interestingly, the results reveal that three types of sites exist for each material, including sites that are (1) highly active, (2) intermediate active, and (3) inactive. NMR experiments indicate that amines immobilized on REG-SBA-15 supports are less mobile than amines on NMP-SBA-15. For REG-SBA-15, the materials are found to have aminosilanes that have limited mobility and the largest fraction of inactive sites, which suggests that the inactive catalytic sites are in the micropores. Overall, the design of highly active catalytic materials can be achieved using materials that have limited micropore volumes.

Pd(0)-catalyzed amination in the synthesis of chiral derivatives of BINAM and their evaluation as fluorescent enantioselective detectors

Abstract A mini-review covers recent successes in the synthesis of (S)-1,1′-binaphthyl-2,2′-diamine (BINAM) using Pd(0)-catalyzed amination reactions. As a result, versatile compounds with C2-chiral backbone were synthesized, among them are derivatives bearing additional chiral amino and fluorophore groups like dansyl amide, 7-methoxycoumarin, 6-aminoquinoline, different macrocyclic compounds with oxadiamine and polyamine linkers were obtained as well. BINAM derivatives of various structures were evaluated as fluorescent enantioselective detectors for a series of model amino alcohols. Many of them were shown to be efficient in sensing certain enantiomers of the amino alcohols by selective changes in the emission in the presence of these analytes. Small changes in the structure of the BINAM derivatives lead to serious difference in the recognition ability of the compounds under investigation.

Advances in the Asymmetric Total Synthesis of Natural Products Using Chiral Secondary Amine Catalyzed Reactions of α,β-Unsaturated Aldehydes.

Chirality is one of the most important attributes for its presence in a vast majority of bioactive natural products and pharmaceuticals. Asymmetric organocatalysis methods have emerged as a powerful methodology for the construction of highly enantioenriched structural skeletons of the target molecules. Due to their extensive application of organocatalysis in the total synthesis of bioactive molecules and some of them have been used in the industrial synthesis of drugs have attracted increasing interests from chemists. Among the chiral organocatalysts, chiral secondary amines (MacMillan’s catalyst and Jorgensen’s catalyst) have been especially considered attractive strategies because of their impressive efficiency. Herein, we outline advances in the asymmetric total synthesis of natural products and relevant drugs by using the strategy of chiral secondary amine catalyzed reactions of α,β-unsaturated aldehydes in the last eighteen years.

Synthesis of some alkylaminothiophene derivatives from 3,4-dibromothiophene and their theoretical calculations

In this study, the copper catalyzed amination reactions of 3,4-dibromothiophene with some primary, cyclic and acyclic secondary amines were investigated to prepare novel electron rich-thiophenes which are expected to be used as novel N-containing donor type monomer candidates for conductive polymers. In order to obtain better yields, this SNAr type amination reaction was optimized by studying the reaction conditions, such as the copper catalyst, the type of copper source, the presence of a base and the type of the solvent on the model reaction between 3,4-dibromothiophene and n-butylamine. A variety of 3,4-(N,N′-dialkylamino)thiophenes and 3-(N-alkylamino)thiophenes were synthesized in moderate yields under the optimized reaction conditions. In addition, two new heterocyclothiophene derivatives were successfully prepared from the cyclization reaction of 3,4-bis(N-butylamino)thiophene. The characterization of the isolated alkylaminothiophenes was performed by FTIR, 1H and 13C NMR, GCMS and elemental analysis. The theoretical calculations for all alkylaminothiophenes were executed by using the DFT/B3LYP/6-311+G(2d,p) approach.

Synthesis of polymacrocyclic compounds via Pd-catalyzed amination and evaluation of their derivatives as metal detectors

Abstract A mini-review is dedicated to the application of Pd(0)-catalyzed amination reactions for the synthesis of polymacrocyclic compounds of various architectures. Such molecules comprise diazacrown ether moieties or structural fragments of tri- and tetraazamacrocycles, polyamine or polyoxadiamine chains of different length and aromatic spacers. Polymacrocyclic compounds thus formed possess an increased number of coordination sites and are suitable for testing as potential receptors of metal cations. Some of these macrocyclic compounds were decorated with the exocyclic dansyl fluorophore group or were coupled with porphyrin units to form polymacrocyclic conjugates. Fluorescent spectra of these compounds were studied in the presence of various metal perchlorates and several macrocycles were found to act as chemosensors or molecular probes for such cations like Cu(II), Al(III), Cr(III).

Experimental and theoretical study on the kinetics and mechanism of the amine-catalyzed reaction of oxiranes with carboxylic acids

In this work, a systematic study on the kinetics and mechanism of ring-opening reaction of oxirane by carboxylic acid initiated by a tertiary amine is presented. Kinetic parameters of β-hydroxypropyl ester formation including reaction orders, rate constants, and activation energies were established at the temperature range 323–353 K. The experimental values of Δ≠H° and Δ≠S° are characteristic for the SN2-like processes. In the initial reaction system, the acid, oxirane and amine exist mainly as hydrogen-bonded complex acid-oxirane, free oxirane and free base. H-bonding was analyzed using IR-spectroscopy. The reaction pathways were examined by the density functional theory (DFT) method at the B3LYP/6-31+G** level. Optimized equilibrium configurations of transition states and corresponding activation parameters were established. In accordance to both experimental and theoretical approaches, it is reasonable to suggest that amine-catalyzed ring-opening reaction of oxirane by carboxylic acid is a series of parallel consecutive stages: (1) quaternization of tertiary amine by activated oxirane; (2) carboxylate anion participation in ring-opening of both nonactivated and activated oxirane. The kinetic model, which adequately describing all observations, is proposed.

Rational design of nucleophilic amine sites via computational probing of steric and electronic effects

Accessibility of the nucleophilic site in organocatalysts is essential to ensure adequate catalytic activity. Gas-phase trimethylborane (TMB) Lewis basicity and Brønsted proton basicity of several amine based organocatalysts have been calculated using the CBS-QB3 model chemistry. This TMB basicity scale can, as opposed to the proton basicity scale, account for steric effects encountered in the initial nucleophilic attack of the nitrogen free electron pair on a substrate. Since such a step is the first one in several amine catalyzed reactions, severe steric hindrance of the nucleophilic center would render the catalyst ineffective. Comparing the TMB basicity and proton basicity with the experimentally observed catalytic activity of both homogeneous and heterogeneously supported amine sites found in literature for the aldol reaction of acetone with 4-nitrobenzaldehyde showed that, due to the inclusion of these steric effects, the TMB basicity scale is a much better predictor of catalytic activity than the proton basicity. According to this computational Lewis basicity scale, potential steric hindrance in alternative nitrogen containing active sites was probed. This resulted in 3-propylpyrrolidine being proposed among the most promising monofunctional amine groups and 1-(methylamino)propan-2-ol among the most promising bifunctional amine-hydroxyl groups for heterogeneous aldol reaction catalysts.

Step-by-step real time monitoring of a catalytic amination reaction

The multiple reaction monitoring mode of a triple quadrupole mass spectrometer is used to examine the Buchwald-Hartwig amination reaction at 0.1% catalyst loading in real-time using sequential addition of reagents to probe the individual steps in the cycle. This is a powerful new method for probing reactions under realistic conditions.

Efficient synthetic methodology for the construction of dihydronaphthalene and benzosuberene molecular frameworks

Benzosuberene analogues (1 and 2) and dihydronaphthalene analogues (3 and 4) function as potent inhibitors of tubulin polymerization, demonstrate pronounced cytotoxicity (low nM to pM range) against human cancer cell lines, and are promising vascular disrupting agents (VDAs). As such, these compounds represent lead anticancer agents with potential translatability towards the clinic. Methodology previously established by us (and others) facilitated synthetic access to a variety of structural and functional group modifications necessary to explore structure activity relationship considerations directed towards the development of these (and related) molecules as potential therapeutic agents. During the course of these studies it became apparent that the availability of synthetic methodology to facilitate direct conversion of the phenolic-based compounds to their corresponding aniline congeners would be beneficial. Accordingly, modified synthetic routes toward these target phenols (benzosuberene 1 and dihydronaphthalene 3) were developed in order to improve scalability and overall yield [45-57% (1) and 32% (3)]. Moreover, benzosuberene-based phenolic analogue 1 and separately dihydronaphthalene-based phenolic analogue 3 were successfully converted into their corresponding aniline analogues 2 and 4 in good yield (>60% over three steps) using a palladium catalyzed amination reaction.

Organocatalyzed Decarboxylative Trifluoromethylation of β-Ketoacids

A new protocol has been developed for the synthesis of α-trifluoromethyl ketones via organocatalyzed decarboxylative trifluoromethylation of β-ketoacids with Togni’s reagent. Among various simple amine catalysts, primary amines and secondary amines were identified to be more effective than tertiary amines, with piperazine being the most effective. Mechanistic investigations suggested that the primary or secondary amine-catalyzed reactions proceed mainly through trifluoromethylation of an enamine intermediate, which is more effective than the tertiary amine-catalyzed pathway that involves an enol intermediate. By using piperazine as the optimal organocatalyst, various β-ketoacids, including the sterically hindered α,α-disubstituted ones, were converted into the corresponding α-trifluoromethyl ketones in good yields. This research not only provides a useful strategy for the efficient synthesis of a wide range of α-trifluoromethyl ketones under mild conditions, but also constitutes one of the few studies on decarboxylative alkylation of β-ketoacids, which can intrigue further exploitation on organocatalyzed asymmetric decarboxylative alkylation reactions.

Synthesis of Trimacrocyclic Compounds Comprising Diazacrown Ether Moieties via Pd(0)-Catalyzed Amination Reactions

Synthesis of Trimacrocyclic Compounds Comprising Diazacrown Ether Moieties via Pd(0)-Catalyzed Amination Reactions

NIXANTPHOS: a highly active ligand for palladium catalyzed Buchwald-Hartwig amination of unactivated aryl chlorides.

Xantphos is one of the two most common ligands used in palladium catalyzed Buchwald-Hartwig amination reactions, because of its broad scope and high probability of success. It does not, however, work well with unactivated aryl chlorides. Herein NIXANTPHOS is compared to Xantphos and an array of mono- and bidentate phosphines. NIXANTPHOS outperforms Xantphos and all other bidentate ligands examined. Under the optimal reaction conditions, unactivated aryl chlorides afford the expected products in good to excellent yield with as low as 0.05 mol% (500 ppm) palladium loading.

Stereocontrol in Diphenylprolinol Silyl Ether Catalyzed Michael Additions: Steric Shielding or Curtin-Hammett Scenario?

The enantioselectivity of amine-catalyzed reactions of aldehydes with electrophiles is often explained by simple steric arguments emphasizing the role of the bulky group of the catalyst that prevents the approach of the electrophile from the more hindered side. This standard steric shielding model has recently been challenged by the discovery of stable downstream intermediates, which appear to be involved in the rate-determining step of the catalytic cycle. The alternative model, referred to as the Curtin-Hammett scenario of stereocontrol, assumes that the enantioselectivity is related to the stability and reactivity of downstream intermediates. In our present computational study, we examine the two key processes of the catalytic Michael reaction between propanal and β-nitrostyrene that are relevant to the proposed stereoselectivity models, namely the C-C bond formation and the protonation steps. The free energy profiles obtained for the pathways leading to the enantiomeric products suggest that the rate- and stereodetermining steps are not identical as implied by the previous models. The stereoselectivity can be primarily controlled by C-C bond formation even though the reaction rate is dictated by the protonation step. This kinetic scheme is consistent with all observations of experimental mechanistic studies including those of mass spectrometric back reaction screening experiments, which reveal a mismatch between the stereoselectivity of the back and the forward reactions.

Synthesis of N- and O-Containing Macrobicycles with Central Biphenyl Moiety via Pd(0)-Catalyzed Amination Reactions

Background: Simple, efficient and enough versatile approaches to macrobicycles and macrotricycles, which are of great interest for the coordination of metal cations, were proposed in early 1990s using nucleophilic substitution reactions. Our research group was the first to begin a wide-range synthesis of N- and Ocontaining macrocycles using Pd-catalyzed amination reactions. Objective: The goal of the present research is to develop the catalytic synthesis of nitrogen- and oxygen containing macrobicycles organized around the central biphenyl moiety. Method: The method employed is the Pd(0)-catalyzed amination reactions which led to the formation of the first macrocycle comprising 3,3'-disubstituted biphenyl followed by the modification with two bromobenzyl moieties and the second Pd(0)-catalyzed macrocyclization which afforded target macrobicycles. Results: Macrocycles with the endocyclic 3,3'-disubstituted biphenyl moiety and diazadioxa, diazatrioxa, and tetraamine linkers were synthesized in 26-44% yields using Pd(0)-catalyzed amination reactions. They were modified with two 3-bromobenzyl substituents and introduced in the second Pd(0)-catalyzed macrocyclization to give macrobicycles in 8-27% yields. The result was shown to be dependent on the structure of the reacting compounds. The reaction with diazacrown ether provided a macrotricyclic derivative. Conclusion: An approach to a variety of N- and O-containing macrobicyclic compounds comprising a central biphenyl moiety and two oxadiamine (polyamine) chains via two consecutive Pd(0)-catalyzed amination reactions was elaborated.

Cinchona Alkaloid Catalyzed Enantioselective [4 + 2] Annulation of Allenic Esters and in Situ Generated ortho-Quinone Methides: Asymmetric Synthesis of Functionalized Chromans.

A novel enantioselective [4 + 2] annulation of the allenoates having a unique positive ortho-effect with in situ generated ortho-quinone methides has been developed under the catalysis of Cinchona alkaloid. This chiral amine-catalyzed reaction provides an alternative route to asymmetric catalytic construction of synthetically interesting, highly functionalized chiral chromans in good to excellent enantioselectivities (up to 97% ee).

Monofunctional primary amine: A new class of organocatalyst for asymmetric Aldol reaction

A new class of organocatalysts involving a primary amine as the only functional group is developed for catalytic asymmetric aldol reaction of cyclohexanone/cyclopentanone with various aryl aldehydes in the presence of benzoic acid as an additive at −10∘C. In an unexpected observation, the primary amine catalyzed reactions gave excellent yield and good to excellent stereoselectivity, while secondary amines were found to have little or no reactivity under similar reaction conditions. A new class of aminocatalysts with a primary amine as the only catalytically active functional group was developed for catalytic asymmetric aldol reaction of cyclohexanone/cyclopentanone with various aryl aldehydes in the presence of benzoic acid as an additive at -10oC under neat conditions.