Phase Transfer Catalyzed Wittig Reaction in the Microtube Reactor under Liquid–Liquid Slug-Flow Pattern

Wittig and Wittig-Horner Reactions under Sonication Conditions

Carbonyl olefinations are among the most important organic syntheses that form C=C bonds, as they usually have high yields and in addition offer excellent stereoselectivity. Due to these advantages, carbonyl olefinations have important pharmaceutical and industrial applications. These reactions contain an additional step of an α-functionalized carbanion to an aldehyde or ketone to produce alkenes, but syntheses performed using metal carbene complexes are also known. The Wittig reaction is an example of carbonyl olefination, one of the best ways to synthesize alkenes. This involves the chemical reaction between an aldehyde or ketone with a so-called Wittig reagent, for instance phosphonium ylide. Triphenylphosphine-derived ylides and trialkylphosphine-derived ylides are the most common phosphorous compounds used as Wittig reagents. The Wittig reaction is commonly involved in the synthesis of novel anti-cancer and anti-viral compounds. In recent decades, the use of ultrasound on the Wittig reaction (and on different modified Wittig syntheses, such as the Wittig-Horner reaction or the aza-Wittig method) has been studied as a green synthesis. In addition to the advantage of green synthesis, the use of ultrasounds in general also improved the yield and reduced the reaction time. All of these chemical syntheses conducted under ultrasound will be described further in the present review.

Laboratory Scale Continuous Flow Systems for the Enantioselective Phase Transfer Catalytic Synthesis of Quaternary Amino Acids

The use of stereoselective phase-transfer catalysis as a reliable method for the enantioselective synthesis of optically active α-amino acid derivatives using achiral Schiff base esters has been well-developed in batch in the last 40 years. Recently, continuous flow technology has become of great interest in the academy and industry, since it offers safer process operating conditions and higher efficiency compared to a traditional batch processing. Herein, we wish to report the first example of enantioselective phase transfer benzylation of alanine Schiff base ester, under continuous flow conditions. Two different methodologies were investigated: a liquid-solid phase transfer catalytic benzylation using a packed-bed reactor and a liquid-liquid phase transfer catalytic benzylation in continuous stirred-tank reactors. Liquid-liquid phase transfer process in flow showed slightly better productivity than the batch process, while solid-liquid phase transfer benzylation proved much more advantageous in terms of productivity and space-time yield. Furthermore, continuous flow system allowed the isolation of benzylated product without any work up, with a significant simplification of the process. In both cases, phase transfer asymmetric benzylation promoted by Maruoka catalyst demonstrated high enantioselectivity of target quaternary amino ester in flow, up to 93% ee.

Production of performic acid through a capillary microreactor by heterogeneous catalyst

Microreactors are small in size with significant heat and mass transfer. Performic acid (PFA) is an important organic compound. It has broad applications in food, oil and chemical industries because of its oxidizing properties. In the present work PFA is produced in a continuous flow Teflon spiral capillary microreactor. The PFA is produced with and without a heterogeneous catalyst. The formic acid (FA) and hydrogen peroxide (HP) are the reactants to produce the PFA. It is a reversible reaction. The aim of the present work to monitor the consequence of hydrogen peroxide concentration, temperature and heterogeneous catalyst (Amberlite) for conversion of the FA. The experimental results showed that the formation of the PFA is effected with increase in hydrogen peroxide concentration, percentage of catalyst and temperature. The PFA formed within short residence time by the use of solid catalyst. The heterogeneous catalysts are better in decreasing corrosion and segregation of the catalyst compared to homogeneous catalysts. The best conditions for the PFA synthesis reaction were noted that 10 min residence time, 30 w/v% of HP, 6 wt% of catalyst concentration based on formic acid and 30 °C. Hence, the maximum concentration of the PFA was recorded 2.8 mol/L (XFA = 39.4%)

Biphasic nucleophilic aromatic substitution using a microreactor under droplet formation conditions

Biphasic nucleophilic aromatic substitution of 4-fluoronitrobenzene proceeded efficiently using a packed bed reactor.

Two-Phase Dibromocyclopropanation of Unsaturated Alcohols Using Flow Chemistry

Dibromocyclopropanations are conventionally done by addition of dibromocarbene to alkenes under phase-transfer conditions in batch reactions using a strong base (50% NaOH (aq)), vigorous stirring and long reaction times. We have shown that cyclopropanation of unsaturated alcohols can be done under ambient conditions using continuous flow chemistry with 40% (w/w) NaOH (aq) as the base. The reactions were generally rapid; the yields were comparable to yields reported in the literature for the conventional batch reaction

Continuous flow synthesis of indoles by Pd-catalyzed deoxygenation of 2-nitrostilbenes with carbon monoxide

The palladium-catalyzed cyclization of o-vinylnitrobenzenes employing carbon monoxide as terminal reductant is investigated. The reaction proceeds with 1 to 2 mol% of Pd(OAc)₂ and generates carbon dioxide as the only stoichiometric side-product.

Preparation of W/O nanoemulsion using tandem acoustic emulsification and its novel utilization as a medium for phase-transfer catalytic reaction

We have successfully demonstrated that W/O nanoemulsion prepared by the tandem acoustic emulsification is extremely useful medium for enhancing the rate of phase-transfer catalytic reactions.

Ultrasound and microstructures--a promising combination?

Short diffusion paths and high specific interfacial areas in microstructured devices can increase mass transfer rates and thus accelerate multiphase reactions. This effect can be intensified by the application of ultrasound. Herein, we report on the design and testing of a novel versatile setup for a continuous ultrasound-supported multiphase process in microstructured devices on a preparative scale. The ultrasonic energy is introduced indirectly into the microstructured device through pressurized water as transfer medium. First, we monitored the influence of ultrasound on the slug flow of a liquid/liquid two-phase system in a channel with a high-speed camera. To quantify the influence of ultrasound, the hydrolysis of p-nitrophenyl acetate was utilized as a model reaction. Microstructured devices with varying channel diameter, shape, and material were applied with and without ultrasonication at flow rates in the mL min(-1) range. The continuous procedures were then compared and evaluated by performing a simplified life cycle assessment.