Solvent-free, catalyst-free aza-Michael addition of cyclohexylamine to diethyl maleate: Reaction mechanism and kinetics

Reagentless Chemistry "On-Water": An Atom-Efficient and "Green" Route to Cyclic and Acyclic β-Amino Sulfones via aza-Michael Addition Using Microwave Irradiation

A reagentless, catalyst-free, and sustainable methodology was developed for facile access to cyclic and acyclic β-amino sulfones "on-water" using a microwave. A variety of aromatic and aliphatic amines undergo double aza-Michael addition on the surface of the water with water-insoluble divinyl sulfones upon microwave irradiation at 150 °C for 10 min to mostly afford solid cyclic β-amino sulfones as easily separable products in excellent yields by simple filtration avoiding any workup steps. Thus, all atoms of the substrates are reflected in the product making it a 100% atom-efficient method. Both electron-rich and electron-deficient amines participated well in the reaction as well as good functional group tolerance was observed. The competitive experiments expectedly revealed faster reaction kinetics for electron-rich amines. The methodology was extended to acyclic β-amino sulfones by interacting phenyl/ethyl vinyl sulfones with various amines in a similar manner. Expectedly, the method afforded very low environmental factors (in a range of 0.05-0.5) and a high Ecoscale score (up to 94). In an attempt toward sustainable development, this reagent-free, metal-free, organic solvent-free, cost-effective protocol is certainly a viable alternative to the available methods for β-amino sulfones.

Aza-Michael Addition in Explicit Solvent: A Relative Energy Gradient-Interacting Quantum Atoms Study

Aza-Michael additions are key reactions in organic synthesis. We investigate, from a theoretical and computational point of view, several examples ranging from weak to strong electrophiles in dimethylsulfoxide treated as explicit solvent. We use the REG-IQA method, which is a quantum topological energy decomposition (Interacting Quantum Atoms, IQA) coupled to a chemical-interpretation calculator (Relative Energy Gradient, REG). We focus on the rate-limiting addition step in order to unravel the different events taking place in this step, and understand the influence of solvent on the reaction, with an eye on predicting the Mayr electrophilicity. For the first time, a link is established between an REG-IQA analysis and experimental values.

Solvent-free and catalyst-free direct alkylation of alkenes

A convenient method for synthesizing trisubstituted alkenes through direct alkylation of alkenes was achieved under solvent-free and catalyst-free conditions. This reaction highlighted by a low E-factor and a high atom- and step-economy.

Catalyst-free, aza-Michael polymerization of hydrazides: polymerizability, kinetics, and mechanistic origin of an α-effect

Despite the powerful nature of the aza-Michael reaction for generating C-N linkages and bioactive moieties, the bis-Michael addition of 1° amines remains ineffective for the synthesis of functional, step-growth polymers due to the drastic reduction in reactivity of the resulting 2° amine mono-addition adduct. In this study, a wide range of commercial hydrazides are shown to effectively undergo the bis-Michael reaction with divinyl sulfone (DVS) and 1,6-hexanediol diacrylate (HDA) under catalyst-free, thermal conditions to afford moderate to high molecular weight polymers with M n = 3.8-34.5 kg mol-1. The hydrazide-Michael reactions exhibit two distinctive, conversion-dependent kinetic regimes that are 2nd-order overall, in contrast to the 3rd-order nature of amines previously reported. The mono-addition rate constant was found to be 37-fold greater than that of the bis-addition at 80 °C for the reaction between benzhydrazide and DVS. A significant majority (12 of 15) of the hydrazide derivatives used here show excellent bis-Michael reactivity and achieve >97% conversions after 5 days. This behavior is consistent with calculations that show minimal variance of electron density on the N-nucleophile among the derivatives studied. Reactivity differences between hydrazides and hexylamine are also explored. Overall, the difference in reactivity between hydrazides and amines is attributed to the adjacent nitrogen atom in hydrazides that acts as an efficient hydrogen-bond donor that facilitates intramolecular proton-transfer following the formation of the zwitterion intermediate. This effect not only activates the Michael acceptor but also coordinates with additional Michael acceptors to form an intermolecular reactant complex.

Cascade aza-Michael Addition-Cyclizations; Toward Renewable and Multifunctional Carboxylic Acids for Melt-Polycondensation

Although the aza-Michael addition reaction on various unsaturated (di-)carboxylic acids and esters of, for example, itaconic acid, is well-known, the consecutive cyclization reaction has not received much attention in literature. The products of this aza-Michael cascade reaction, being mono- or di-carboxylic acid or ester functionalized N-alkyl-pyrrolidone structures, prove interesting for melt-polycondensation reactions as they exhibit excellent stability at elevated temperatures. In other words, this reaction is a toolbox for the generation of renewable monomers and, in turn, polymers with tunable physiological properties. Therefore, this work provides an overview of the state-of-the-art of the cascade aza-Michael addition-cyclization reactions on biobased unsaturated acids and esters, and their use in polymerization reactions. Furthermore, we extend this overview with the cascade aza-Michael addition-cyclization reaction of trans-trimethyl aconitate with di-amines to form a tetra-functional N-alkyl-bis-(pyrrolidone dimethylcarboxylate), which exhibits excellent thermal stability and could effectively be used as monomer in polycondensation reactions. Importantly, the aza-Michael addition reaction between primary amines and trans-trimethyl aconitate can be considered a click-reaction; it proceeds quantitatively within minutes under ambient conditions and follows the principles of green chemistry.

Chemoselective aza-Michael addition of indoles to 2-aroyl-1,3-diarylenones

A total of 20 examples of 2-[(1 H-indol-1-yl)(aryl)methyl]-1,3-diphenylpropane-1,3-diones were efficiently synthesized by the aza-Michael addition at the N1 position of indoles with 2-aroyl-1,3-diarylenones at room temperature in the presence of potassium hydroxide. The salient features of this protocol are no transition-metal catalysts, mild conditions, high chemoselectivity, high atom economy, high yields, and a simple work-up procedure.

Tandem grinding reactions involving aldol condensation and Michael addition in sequence for synthesis of 3,4,5-trisubstituted isoxazoles

A one-pot, base-catalyzed, tandem grinding process involving carrying out aldol condensation and Michael addition in sequence to produce 3,4,5-trisubstituted isoxazoles from 3,5-dimethyl-4-nitroisoxazole, aromatic aldehydes and activated methylene compounds has been developed. In the presence of 10 mol% of pyrrolidine, aldol condensations of 3,5-dimethyl-4-nitroisoxazole with various aromatic aldehydes were performed with 3-10 minutes of grinding to provide 5-styryl-3-methyl-4-nitroisoxazoles in good to quantitative yields without further purification. Then, Michael additions between 5-styryl-3-methyl-4-nitroisoxazoles and activated methylene compounds (including ethyl 2-nitroacetate and alkyl 2-cyanoacetates) were carried out in the presence of 10 mol% of Et3N in the same mortar with 3-5 minutes of continuous grinding to produce 3,4,5-trisubstituted isoxazoles in good to excellent yields.

Improving the Post-polymerization Modification of Bio-Based Itaconate Unsaturated Polyesters: Catalyzing Aza-Michael Additions With Reusable Iodine on Acidic Alumina

Bio-based platform molecules such as itaconic, fumaric, and muconic acid offer much promise in the formation of sustainable unsaturated polyester resins upon reaction with suitable diols and polyols. The C=C bonds present in these polyester chains allows for post-polymerization modification and such moieties are conventionally utilized in curing processes during the manufacture of coatings. The C=C modification sites can also act as points to add useful pendants which can alter the polymers final properties such as glass transition temperature, biodegradability, hardness, polarity, and strength. A commonly observed modification is the addition of secondary amines via an aza-Michael addition. Conventional procedures for the addition of amines onto itaconate polyesters require reaction times of several days as a result of undesired side reactions, in particular, the formation of the less reactive mesaconate regioisomer. The slow reversion of the mesaconate back to itaconate, followed by subsequent amine addition, is the primary reason for such extended reaction times. Herein we report our efforts toward finding a suitable catalyst for the aza-Michael addition of diethylamine onto a model substrate, dimethyl itaconate, with the aim of being able to add amine onto the itaconate units without excessive regioisomerization to the inactive mesaconate. A catalyst screen showed that iodine on acidic alumina results in an effective, heterogeneous, reusable catalyst for the investigated aza-Michael addition. Extending the study further, itaconate polyester was prepared by Candida Antartica Lipase B (CaL-B) via enzymatic polytranesterification and subsequently modified with diethylamine using the iodine on acidic alumina catalyst, dramatically reducing the required length of reaction (>70% addition after 4 h). The approach represents a multidisciplinary example whereby biocatalytic polymerization is combined with chemocatalytic modification of the resultant polyester for the formation of useful bio-based polyesters.