Telescoped diazo transfer and rhodium-catalysed S–H insertion in continuous flow

Process Development of Heterogeneous Rh Catalyzed Carbene Transfer Reactions Under Continuous Flow Conditions

A very simple Rh-based catalyst operates under heterogeneous flow conditions for the carbene transfer of methyl diazoacetate (MDA) with several substrates. Two different methods for heterogenizing the catalyst in a column reactor have been applied. Different X-H (X=O, S, Si, CH2 ) were successfully functionalized by the carbene and cyclopropenation was performed under very mild continuous flow conditions. Following these promising results, catalyst recycling experiments using both methodologies were conducted in which up to 5 catalytic cycles have been achieved for the carbene O-H insertion reaction and interestingly, a sequential transformation of different substrates with up to 10 consecutive runs per reactor were achieved with no loss in the catalytic activity, thus allowing the production of families of compounds.

Continuous Flow Technology as an Enabler for Innovative Transformations Exploiting Carbenes, Nitrenes, and Benzynes

Miniaturization offered by microreactors provides for superb reaction control as well as excellent heat and mass transfer. By performing chemical reactions in microreactors or tubular systems under continuous flow conditions, increased safety can be harnessed which allows exploitation of these technologies for the generation and immediate consumption of high-energy intermediates. This Synopsis demonstrates the use of flow technology to effectively exploit benzynes, carbenes, and nitrenes in synthetic chemistry programs.

Generation of Tosyl Azide in Continuous Flow Using an Azide Resin, and Telescoping with Diazo Transfer and Rhodium Acetate-Catalyzed O-H Insertion

Generation of tosyl azide 12 in acetonitrile in flow under water-free conditions using an azide resin and its use in diazo transfer to a series of aryl acetates are described. Successful telescoping with a rhodium acetate-catalyzed O-H insertion has been achieved, thereby transforming the aryl acetate 8 to α-hydroxy ester 10, a key intermediate in the synthesis of clopidogrel 11, without requiring isolation or handling of either tosyl azide 12 or α-aryl-α-diazoacetate 9, or indeed having significant amounts of either present at any point. Significantly, the solution of α-diazo ester 9 was sufficiently clean to progress directly to the rhodium acetate-catalyzed step without any detrimental impact on the efficiency of the O-H insertion. In addition, the rhodium acetate-catalyzed O-H insertion process is cleaner in flow than under traditional batch conditions. Use of the azide resin offers clear safety advantages and, in addition, this approach complements earlier protocols for the generation of tosyl azide 12 in flow; this protocol is especially useful with less acidic substrates.