Following the chemical literature over the past few years, it is pretty obvious that the number of publications dealing with continuous flow applications is increasing at a very rapid rate. At the same time, there is a growing interest in using flow chemistry in both discovery and process research laboratories within the pharmaceutical, agro, fine-chemical, petrochemical, and fragrance industries. Despite this increased awareness and interest, there is no dedicated journal available which focuses specifically on the inherent problems associated with flow chemistry applications.
Optimizing current chemical processes alone does not yield the improvements required in the fine chemical and pharmaceutical industries. At least partially, a switch from batch to continuous manufacturing is needed. Cost-, time-, and atom-efficient routes frequently demand the application of high temperatures, pressures, and concentrations, and/or the use of highly reactive reagents. These chemistries often cannot be employed in conventional reactors. Costly and long alternative synthetic routes are chosen instead. The application of continuous-flow microreactors allows to access “harsh” or “hazardous” reaction conditions and, furthermore, enables entirely new transformations.
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.
A safe and scalable procedure for the synthesis of 2-oxopropanethioamide, an intermediate in the synthesis of a potent β-secretase (BACE-1) inhibitor, from the reaction of acetyl cyanide with hydrogen sulfide gas under continuous-flow conditions has been developed. The toxic gas could be accurately dosed using a mass-flow controller or a peristaltic pump. The reaction proceeded smoothly at room temperature in the presence of a small amount of triethylamine as basic catalyst. After a residence time of 15 min, excellent yield (96%) and purity (>99%) were obtained for the target compound. The high reaction selectivity permitted a simple workup procedure consisting of evaporation of all volatiles.
Herein, we present the development of a continuous-flow process for a difluoromethylation with difluorocarbene as the reactive reagent. The difluoromethylated product is a key intermediate during the synthesis of eflornithine, a pharmaceutical that is on the World Health Organization’s Model List of Essential Medicines. The developed procedure uses inexpensive and commercially available chlorodifluoromethane (CHF2Cl, Freon 22) as difluorocarbene source. Deprotonation of CHF2Cl with NaOH in a biphasic mixture of organic solvent-water generates the carbene. A fast subsequent reaction of the difluorocarbene with the substrate generates the desired product with excellent selectivity.
The crucial structural motive in viral protease inhibitors such as atazanavir and darunavir is a chiral aminoalcohol structure. The structure is generally introduced during the synthesis of the protease inhibitor via an α-chloroketone intermediate. The α-chloroketone can be synthesized in a multistep sequence from naturally occurring l-phenylalanine. Herein, we report a onepot synthesis of an α-chloroketone starting from N-Boc-l-phenylalanine in a novel type of “tube-in-flask” semi-batch diazomethane generator. Activation of the amino acid to the mixed anhydride was carried out in the flask, while diazomethane was generated from in situ formed N-nitroso-N-methylurea within a gas-permeable tubing contained inside the flask. The diazomethane diffused through the gas-selective membrane into the flask, and reacted with the anhydride to the diazoketone (Arndt—Eistert reaction). The addition of aqueous hydrogen chloride provided the α-chloroketone and destroyed any excess of diazomethane. The desired product was isolated by extraction in excellent purity and yield (90%–96%).
Hydrazoic acid (HN3) was used in a safe and reliable way for the synthesis of 5-substitued-1H-tetrazoles and for the preparation of N-(2-azidoethyl)acylamides in a continuous flow format. Hydrazoic acid was generated in situ either from an aqueous feed of sodium azide upon mixing with acetic acid, or from neat trimethylsilyl azide upon mixing with methanol. For both processes, subsequent reaction of the in situ generated hydrazoic acid with either organic nitriles (tetrazole formation) or 2-oxazolines (ring opening to β-azido-carboxamides) was performed in a coil reactor in an elevated temperature/pressure regime. Despite the explosive properties of HN3, the reactions could be performed safely at very high temperatures to yield the desired products in short reaction times and in excellent product yields. The scalability of both protocols was demonstrated for selected examples. Employing a commercially available benchtop flow reactor, productivities of 18.9 g/h of 5-phenyltetrazole and 23.0 g/h of N-(1-azido-2-methylpropan- 2-yl)acetamide were achieved.