Solvent-free Reactions

Highly (regio)selective hydroformylation of olefins using self-assembling phosphines

New phosphines with self-assembling 6-pyridinone moities were prepared, characterized, and examined in the hydroformylation of diverse olefins. Testing various known and novel ligands in the presence of [Rh(acac)(CO)2] under industrially relevant conditions, the hydroformylation of 1-octene proceeds best with 6,6'-(phenylphosphanediyl)bis(pyridin-2(1H)-one) (DPONP). Control experiments and modelling studies indicate dimerization of this ligand at higher temperatures (>100 °C). The optimal catalyst system is able to conserve high product linearity (>90%) for a broad range of olefins at industrially-employed temperatures at low ligand loading.

Recent Advances in the Enzymatic Synthesis of Polyester

Polyester is a kind of polymer composed of ester bond-linked polybasic acids and polyol. This type of polymer has a wide range of applications in various industries, such as automotive, furniture, coatings, packaging, and biomedical. The traditional process of synthesizing polyester mainly uses metal catalyst polymerization under high-temperature. This condition may have problems with metal residue and undesired side reactions. As an alternative, enzyme-catalyzed polymerization is evolving rapidly due to the metal-free residue, satisfactory biocompatibility, and mild reaction conditions. This article presented the reaction modes of enzyme-catalyzed ring-opening polymerization and enzyme-catalyzed polycondensation and their combinations, respectively. In addition, the article also summarized how lipase-catalyzed the polymerization of polyester, which includes (i) the distinctive features of lipase, (ii) the lipase-catalyzed polymerization and its mechanism, and (iii) the lipase stability under organic solvent and high-temperature conditions. In addition, this article also focused on the advantages and disadvantages of enzyme-catalyzed polyester synthesis under different solvent systems, including organic solvent systems, solvent-free systems, and green solvent systems. The challenges of enzyme optimization and process equipment innovation for further industrialization of enzyme-catalyzed polyester synthesis were also discussed in this article.

Mechanochemistry: New Tools to Navigate the Uncharted Territory of "Impossible" Reactions

Mechanochemical transformations have made chemists enter unknown territories, forcing a different chemistry perspective. While questioning or revisiting familiar concepts belonging to solution chemistry, mechanochemistry has broken new ground, especially in the panorama of organic synthesis. Not only does it foster new "thinking outside the box", but it also has opened new reaction paths, allowing to overcome the weaknesses of traditional chemistry exactly where the use of well-established solution-based methodologies rules out progress. In this Review, the reader is introduced to an intriguing research subject not yet fully explored and waiting for improved understanding. Indeed, the study is mainly focused on organic transformations that, although impossible in solution, become possible under mechanochemical processing conditions, simultaneously entailing innovation and expanding the chemical space.

Sodium Butylated Hydroxytoluene: A Functional Group Tolerant, Eco-Friendly Base for Solvent-Free, Pd-Catalysed Amination

NaBHT (sodium 2,6-di-tert-butyl-4-methylphenolate), a strong, but hindered and lipophilic base, has been effectively paired with similarly lipophilic, high-reactivity Pd-NHC (N-heterocyclic carbene) catalysts to produce an ideal combination for performing solvent-free (melt) cross-coupling amination. The mild nucleophilicity of NaBHT, coupled with the anti-oxidant properties of its conjugate acid byproduct, BHT means the process seems to have no functional group incompatibilities. Highly effective coupling of base-sensitive and redox-active functional groups was observed in all cases with only 0.1-0.2 mol percent catalyst. Comparisons using the standard base for this reaction, KOtBu, led to poor couplings or complete degradation in most applications - only NaBHT works.

Phase Behavior That Enables Solvent-Free Carbonate-Promoted Furoate Carboxylation

Solvent-free chemistry has been used to streamline synthesis, reduce waste, and access novel reactivity, but the physical nature of the reaction medium in the absence of solvent is often poorly understood. Here we reveal the phase behavior that enables the solvent-free carboxylation reaction in which carbonate, furan-2-carboxylate (furoate), and CO₂ react to form furan-2,5-dicarboxylate (FDCA^2-). This transformation has no solution-phase analogue and can be applied to convert lignocellulose into performance-advantaged plastics. Using operando powder X-ray diffraction and thermal analysis to elucidate the temperature- and conversion-dependent phase composition, we find that the reaction medium is a heterogeneous mixture of a ternary eutectic molten phase, solid Cs₂CO₃, and solid Cs₂FDCA. During the reaction, the amounts of molten phase and solid Cs₂CO₃ diminish as solid Cs₂FDCA accumulates. These insights are critical for increasing the scale of furoate carboxylation and provide a framework for guiding the development of other solvent-free transformations.

Natural Phosphate and Potassium Fluoride Doped Natural Phosphate as New Catalysts for the Vilsmeir–Haack type Reaction

The catalytic activity of the inexpensive natural- or potassium fluoride doped phosphates as bases was evaluated in a Vilsmeier–Haack type reaction. High efficiency, in term of chemical yields and reaction rates was found in the preparation of 2-chlor-3-cyanopyridines starting from 2-propylidene-malonitriles.

Solvent-Free L-Proline Catalysed Condensation of Ethyl Cyanoacetate with Aldehydes

Solvent-free Knoevenagel condensation of ethyl cyanoacetate with aromatic aldehydes, catalysed by environmentally friendly silica gel supported L-proline under microwave irradiation is studied.

Optical, Electrochemical and Thermal Studies of Conjugated Polymers Synthesized by Eutectic Melt Reaction

This paper reports on the synthesis of a novel donor-acceptor conjugated polymers, P1 and P2 by solvent free eutectic melt polymerization reaction. Triisopropylsilylethynyl(TIPS) substituted benzo[1,2-b:4,5-b']dithiophene(BDT) is used as donor, thienithiophene(TT) and thienopyrroledione(TPD) are utilized as acceptors for demonstrating eutectic polymerization. The most important fact in the solvent-free reaction between solid reactants actually proceeds through bulk liquid phases. Such liquid phases are possible due to the formation of eutectics between the reactants and product(s) and any evolution of heat. Naphthalene is explored in this reaction for forming eutectics with the reactants, resulting in desired polymers. Thermal stability, optical and electrochemical properties of these polymers were determined. Optical band gaps of the polymers were found to be 1.58 and 1.65 eV. Electrochemical studies by cyclic voltametry experiment revealed HOMO and LUMO energy levels to be -5.22, -5.60 eV, and -3.76, -4.16 eV, respectively. The polymers were thermally stable up to 285-400 °C. Thermal, optical and electrochemical studies indicated these materials to be promising candidates in organic electronic applications.

Iron-catalyzed unprecedented formation of benzo[d]imidazo[2,1-b]thiazoles under solvent-free conditions

The unprecedented formation of benzo[d]imidazo[2,1-b]thiazole during iron-catalyzed coupling of 2-aminobenzothiazole, aldehydes with nitroalkane in air has been observed.

Brønsted acid ionic liquid as a solvent-conserving catalyst for organic reactions

A sulfonyl-containing ammonium-based Brønsted acid ionic liquid was prepared and used as a liquid heterogeneous catalyst for organic reactions. The unique macroscopic phase heterogeneity of the IL in the reaction system not only ensures an excellent catalytic activity of the IL catalyst but also avoids the use of organic reaction solvents. The catalyst system is applicable for a wide range of reactions.

Solvent and catalyst free route to 3-indolyl glycoconjugates: synthesis of sugar tethered isoxazolines and isoxazoles from 3-indolyl nitroalkanes

An expedient, solvent and catalyst free strategy for the synthesis of sugar based bis(indolyl)methanes and indolyl nitroalkanes has been developed. Further, the nitroalkanes were converted to novel isoxazolines and isoxazoles by nitrile oxides cycloaddition.

Efficient Synthesis of Alkane-Bridged N,N’-Diaryl Bisimidazolium Chlorides under Solvent-Free Conditions

A series of N, N’-diaryl bisimidazolium dichlorides with different alkyl bridged length ( n = 2–6, 10) was synthesised from N-arylimidazoles with the less expensive, but less reactive dialkyl chlorides effectively in solvent-free conditions at ambient pressure. Compared with conventional methods, this protocol features a short reaction time, good yields and an eco-friendly process.

An efficient and recyclable ionic liquid-supported proline catalyzed knoevenagel condensation

The Knoevenagel condensation reaction of aldehydes with malononitrile was described in this study, which was catalyzed by an efficient and recyclable ionic liquid-supported proline. The method represented an attractive alternative to the classical synthesis strategies and exhibited the advantage of performing homogeneous chemistry on a large scale additionally avoided large excesses of reagents. The products were obtained in good yields and reasonable purities without the need for further chromatographic purification. Moreover, the catalyst could be reused for at least four times.