The generation of concentration gradients using electroosmotic flow in micro reactors allowing stereoselective chemical synthesis

The stereoselective control of chemical reactions has been achieved by appl ying electrical fields in a micro reactor generating controlled concentrati on gradients of the reagent streams. The chemistry based upon well-establis hed Wittig synthesis was carried out in ct micro reactor device fabricated in borosilicate glass using photolithographic and wet etching techniques. T he selectivity of the cis (Z) to trans (E) isomeric ratio in the product sy nthesised was controlled by varying the applied voltages to the reagent res ervoirs within the micro reactor. This subsequently altered the relative re agent concentrations within the device resulting in Z/E ratios in the range 0.57-5.21. By comparison, a traditional batch method based on the same rea ction length, concentration, solvent and stoichiometry (i.e., 1.0:1.5:1.0 r eagent ratios) gave a Z/E in the range 2.8-3.0. However, when the stoichiom etric ratios were varied up to ten times as much, the Z/E ratios varied in accordance to the micro reactor i.e., when the aldehyde is in excess, the Z isomer predominates whereas when the aldehyde is in low concentrations, th e E isomer is the more favourable form. Thus indicating that localised conc entration gradients generated by careful flow control due to the diffusion limited non-turbulent mixing regime within a micro reactor, leads to the ob served stereo selectivity for the cis and trans isomers.