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Abstract
This paper describes the next stage in our development of self-optimising reactors. We demonstrate that the same reaction can be optimised for a series of different criteria including yield, space–time yield, E factor and a weighted yield function (the product of space–time yield and yield). In different experiments, we achieved 97.6% yield, space–time yield of 42.9 kg/L/h and E factors of 1.4 and 3.3 (including CO2) and the weighted yield, which gave a promising balance between yield, E factor and space–time yield.
Abstract
O-Alkylation of phenol compounds was performed in a continuous flow apparatus under biphasic liquid/liquid conditions and promoted by tetrabutylammonium bromide (TBAB) as a phase-transfer catalyst. The segmented flow that is generated within the flow system is able to afford the desired ethers in high yield and in very short times.
Abstract
Cyanohydrins are synthetically versatile chiral building blocks in organic synthesis. They can be conveniently synthesized in enantiomerically pure form via chemoenzymatic hydrogen cyanide addition onto the corresponding aldehyde using hydroxynitrile lyase. Recently, we reported that such transformations can be efficiently carried out in a continuous flow manner using microreactors. Since racemization of enantiopure cyanohydrins occurs readily under slightly basic conditions, they should be protected before the follow-up reactions, preferably under acidic conditions. In this contribution, we demonstrate that the methoxyisopropyl protection of mandelonitrile can be conveniently optimized in an automated microscale continuous flow system and subsequently scaled up under the same conditions by applying a larger flow reactor.
This perspective article discusses the basic concept of time control by space based on flow and micro, some examples that realized extremely fast reactions which were difficult to achieve by conventional flask chemistry, and the future of this fascinating chemistry.
The continuous-flow Meerwein arylation is demonstrated for a set of few aryl donors (anilines and m-aminoacetophenone) and specific radical acceptors. Homogeneous catalyst (CuBr in HBr and CuCl in HCl) was used to facilitate the reaction. The effect of parameters, viz., temperature, catalyst concentration, residence time, and concentration of the radical acceptor on the yield of the arylated product, was studied. The yield of the aryl derivative obtained by continuous-flow syntheses was always better than the respective experiments in batch mode. Flow synthesis allows easy variation in these parameters and thus allows going close to the maximum possible yields in a system where the relative rates of different reactions create a complex situation. Temperature plays a crucial role by affecting the rates as well as by governing the system homogeneity. The nitrogen bubbles generated in the reaction helped to avoid any channel blockage.
Abstract
The use of glycerol derived from biodiesel industry is an important development to add value to this actual waste. Several products can be obtained from glycerol, but acetins are very interesting molecules with a wide range of applications in pharmaceutical, cosmetics, food, and fuel industry. Herein we report our results on biocatalyzed batch and continuous-flow process for valorization of glycerol derived from biodiesel industry towards acetin production. Excellent results can be obtained with different selectivities depending on the nature of glycerol used and reaction conditions being able to produce monoacetin, diacetin, or triacetin depending on the reaction condition.
Abstract
A solvothermal continuous-flow method for the scalable and shape tunable synthesis of rod-like/spherical TiO2 nanocrystals (NCs) has been developed. The as-prepared colloidal NCs show photocatalytic activity in an addition–cyclization cascade under continuous-flow conditions.
Abstract
The first example of a sequential heterocycle formation/multicomponent reaction using an automated continuous flow microreactor assembly is reported. Consecutive Hantzsch thiazole synthesis, deketalization, and Biginelli multicomponent reaction provides rapid and efficient access to highly functionalized, pharmacologically significant 5-(thiazol-2-yl)-3,4-dihydropyrimidin-2(1H)-ones without isolation of intermediates. These complex small molecules are generated in reaction times less than 15 min and in high yields (39–46%) over three continuous chemical steps.
Abstract
An extended cost study consisting of 14 process scenarios was carried out to envisage the cost impact of microprocessing and microwaves separately or in combination for two liquid-phase model reactions in fine-chemicals synthesis: (1) Ullmann C–O cross-coupling reaction and (2) the aspirin synthesis. The former, a Cu-catalyzed substitution reaction, was based on an experimental investigation, whereas the latter, a noncatalyzed aromatic esterification reaction, was based on literature data. The cost of 4-phenoxypyridine production, as a pharmaceutical intermediate in the synthesis of vancomycin or vancocin, was compared with that of the synthesis of aspirin, a key example of large-scale fine-chemical production plants. The operating costs in the Ullmann synthesis were found to be related to material-based process (reactant excess, pretreatment, and catalyst synthesis), whereas those in the aspirin synthesis appeared to be related to downstream-based process (workup, waste treatment). The impact of an integrated microwave heating and microprocessing system on profitability was demonstrated with respect to operational cost and chemical productivity. Different modes of microwave heating and catalyst supply were studied and compared with conventional oil-bath-heated systems in batch and continuous processes. The overall costs including profitability breakthrough for a competitive market price of product were obtained from various combinations of heating and processing. In case of the Ullmann synthesis, the CAPEX (capital expenditure) was negligible compared to the OPEX (operational expenditure), whereas in the aspirin synthesis, the CAPEX was found around 40%, both at a production scales of 1–10 kg/day using proposed upscale methods. The source of the catalyst strongly determined the profitability of a continuously operated Ullmann process due to its effect on the chemical performance. Higher energy efficiencies could be attained using single-mode microwave irradiation; however, the energy contribution to the overall cost was found to be negligible. Different scenarios provided a cost-feasible and profitable process; nevertheless, an integrated microwave heating and microflow processing led to a cost-efficient system using a micropacked-bed reactor in comparison to wall-coated microreactor, showing a profit margin of 20%.
Abstract
The 3rd Frontiers in Organic Synthesis Technology” (FROST3) focusing on the “Progress in Flow Chemistry”, organized jointly by the Flow Chemistry Society and Akadémiai Kiadó Zrt. (a Wolters Kluwer company – publisher of the Journal of Flow Chemistry) was held in Budapest on the 11th–13th October 2011. 58 people attended from all over the world, and 11 internationally renowned speakers presented their latest results. Since the last conference held in 2009 we have witnessed a rapid development of flow chemistry technologies, both in the applications and the reactor design.