PASSflow reactor

Using a series of PASSflow reactors, with typical dimensions of 5 mm (i.d.) x 10 cm (long), Kirschning et al. (Kirschning and Gas 2003 Kirschning et al., 2001, 2006) demonstrated the ability to combine a series of immobilized reagents in order to perform a sequence of reaction steps. In one example, the group demonstrated an oxidation, followed by a silyl deprotection and reductive amination steps to afford a derivatized steroid 147, as illustrated in Scheme 38, in excellent yield and purity. 

O = Polymer powder, Raschig rings or PASSflow reactor... 

DMSO), a reaction temperature of 37 °C, and a residence time of 90 min, achieved by recirculating the reaction mixture through the PASSflow reactor at a flow rate of 1.0 ml min-1, the authors were able to attain 99.5% conversion of 116 to (R)-benzoin 159, determined by off-line GC analysis. Increasing the reactant concentration from 5.5 x 10 2 to 0.2 M, resulted in a reduction in benzoin 159 production of 5.9%, with longer reaction times required to attain high conversions with further increases in reactant concentration (typically 9h for 1.0 M 116). The authors subsequently demonstrated the cross-benzoin reaction between acetaldehyde... 

Fig. 5 Cross-sectional view through the PASSflow reactor (PTFE=polytetrafluoroethylene) and setup of the flow system...

More importantly, catalytic transformations such as transfer hydrogenations, Suzuki cross-coupling reactions, the Heck reaction, and the Sonogashira reaction were routinely performed in these PASSflow reactors under continuous flow conditions (Schemes 12-15) [41]. 

Scheme 15 Ligand-free palladium-catalyzed Suzuki-Miyaura reactions in the PASSflow reactor (first percentage refers to degree of transformation yields in parentheses refer to purified products)...

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