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

Abstract

The stereoselective control of chemical reactions has been achieved by applying electrical fields in a micro reactor generating controlled concentration gradients of the reagent streams. The chemistry based upon well-established Wittig synthesis was carried out in a micro reactor device fabricated in borosilicate glass using photolithographic and wet etching techniques. The selectivity of the cis (Z) to trans (E) isomeric ratio in the product synthesised was controlled by varying the applied voltages to the reagent reservoirs within the micro reactor. This subsequently altered the relative reagent 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 reaction length, concentration, solvent and stoichiometry (i.e., 1.0∶1.5∶1.0 reagent ratios) gave a Z/E in the range 2.8–3.0. However, when the stoichiometric 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, the E isomer is the more favourable form. Thus indicating that localised concentration gradients generated by careful flow control due to the diffusion limited non-turbulent mixing regime within a micro reactor, leads to the observed stereo selectivity for the cis and trans isomers.

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Introduction

The controlled production of stereoselective products from organic reactions represents a significant development for the production of pharmaceutical drugs and the synthesis of polyunsaturated natural products. Such control, at the touch of a switch, has now been achieved so minimising the need for separation and purification of stereoisomers post-reaction. The micro reactor systems used exploit electroosmotic (EOF) and electrophoretic mobility to create a unique reaction environment in which localised diffusional mixing occurs in the presence of controlled electric fields.^1–5 To demonstrate Z/E control we have used well-established Wittig chemistry in which 2-nitrobenzyltriphenylphosphonium bromide (0.01 M) is reacted with methyl 4-formylbenzoate (0.01 M) in dry methanol at a 1 to 1 stoichiometry to illustrate the experimental protocol. In the traditional batch synthesis, the kinetically favoured cis stereoisomer is produced with a typical Z/E ratio of 3.0. The reaction is normally performed at room temperature (25 °C) in the presence of an electrophilic phosphorous reagent (PPh₃) diluted by a polar, aprotic solvent.

Micro reactor—experimental

The micro reactor (Fig. 1) was fabricated in borosilicate glass using photolithography and wet etching⁶ techniques to produce appropriate channel geometries (200 μm wide and 100 μm deep). Microporous silica frits⁷ positioned in the subsidiary manifold channels and were used to minimise hydrodynamic pressure improving the precision of electroosmotic pumping and electrophoretic mobility¹ of the solutions used. The network of interconnecting channels was subsequently annealed to a 17 mm top plate (680 °C) using a microwave furnace. The top plate included a series of 3 mm id pre-drilled holes (four reservoirs in total), which acted as reagent reservoirs and supported the gold electrodes. The final outer dimensions of the micro reactor were 20 mm × 20 mm square and 25 mm in depth.

Fig. 1 Schematic diagram of the micro reactor used for the investigation of Z/E ratios in Wittig chemistry.

Using a prototype power supply developed by Kingfield Electronics (Sheffield, UK) see Fig. 2, the Wittig reaction was investigated at a number of external applied voltages generating a variety of EOF rates. Using the Kingfield power supply, the micro reactor was positioned on the top plate of the unit into which a solution of 2-nitrobenzyltriphenylphosphonium bromide (0.01 M, 50 μL) in dry methanol (MeOH) was added to reservoir A. Sodium methoxide (0.015 M, 50 μL in MeOH) was added into reservoir B and methyl 4-formylbenzoate (0.01 M, 50 μL, in MeOH) was added to reservoir C. Finally, 40 μL of dry MeOH was added to reservoir D that was the collection reservoir and common ground for the system. Using multivariate experimental design,⁸ a number of chemical reactions were performed that investigated a series of voltages within the range 494 V to 754 V for a period of 20 min. Previous experimental design data suggested that the voltage region outlined above minimised the relative standard deviation between data points resulting in the models precision and potential prediction capability being high. In total, the multivariate experimental method designed produced 27 voltage combinations, which investigated the region 494 V to 754 V sufficiently. After each reaction combination was finished, the reagent volume in reservoir D was recorded and the product yield was determined using off-chip reversed phase high performance liquid chromatographic (HPLC) analysis (conditions: Phenomenex C18(2) 3 μm column, 75 × 4.60 mm, mobile phase composition: 0.% trifluroracetic acid in water and 0.1% trifluoroacetic acid in acetonitrile (90% aqueous to 10% aqueous over 6 min) at 40 °C, flow rate 3 μl min⁻¹).⁹ The compound yield was calculated using the knowledge of the input weight of the phosphonium salt, the volume of the solution in reservoirs A and D before and after the reaction, the measurement of the concentration of phosphonium salt in reservoirs A and D at the end of the reaction and the concentration of the products in reservoir D.

Fig. 2 The Kingfield electronics power supply.

Batch reaction—experimental

For comparison purposes, a series of batch reactions were performed varying the concentration of the aldehyde. This was achieved by stirring the following reagents for 20 min at room temperature, 2-nitrobenzyltriphenylphosphonium bromide (0.01 M in 10 ml of MeOH) with sodium methoxide (0.015 M in MeOH) and the aldehyde, methyl 4-formylbenzoate at various concentrations. The concentration values investigated were 0.005 M, 0.01 M, 0.03 M, 0.05 M and 0.1 M. The yields were determined using the same HPLC method detailed above.

Results and discussion

In Fig. 3 we illustrated how applying a voltage of +494 V at reservoir A, increases the amount of ylide produced in the reaction channel. This is simply due to the negative potential difference (−166 V) between reservoirs A and B (with respect to A), allowing the sodium methoxide (reservoir B) to ‘flow back’ towards reservoir A. The opposite trend applies when the voltage is increased at reservoir A (+694 V), giving a potential difference between reservoir A and B of +34 V, which reduces the amount of ylide intermediate formed due to the increased flow rate of 2-nitrotriphenylphosphonium bromide. Monitoring the current during the reaction indicated that the current in channel C was extremely small (0.01 μA) and could be classed as the system noise. However, a large current in the region of 50 to 70 μA was recorded between reservoir B and D in the main channel and this maybe due to the formation of sodium bromide (step 1 by-product), which will influence locally the solution mobility and the stereoselectivity of the reaction product.

Fig. 3 The schematic representation of the spatial production of the ylide intermediate as a function of varying the applied voltage at reservoir A.

The lowest and highest Z/E ratios obtained in this particular data set were 0.57 (reservoir A: 495 V, reservoir B: 660 V and reservoir C: 678 V) and 5.21 (reservoir A: 550 V, reservoir B: 300 V and reservoir C: 500 V) respectively, which when compared with the Z/E ratio (≈3.0) obtained for a similar batch reaction (the same reaction length, concentration, stoichiometry and solvent system), illustrate the significant variation and control possible on the products produced. In the course of this work a number of additional aldehydes, detailed previously⁹ have been observed to also display stereoselectivity using the same basic Wittig chemistry and investigations are continuing in this area to establish the effective scope of the methodology described.

The origin of the Z/E selectivity can be rationalised in terms of the localised concentrations that occur within the capillary channel system of the micro reactor where diffusion limited non-turbulent mixing conditions predominate. This effect, which is supported by batch based experiments, illustrates the potential control and selectivity possible when using micro reactors for synthetic chemistry.¹ Thus when the solution velocity through the microchannels is high and hence the residency time or local concentration of the reagents in the channels is low, the cis isomer is kinetically favoured and hence the ylide reacts with the excess aldehyde to give the cis-oxaphosphotane. The cis-oxaphosphotane then quickly passes through the reaction channel before eliminating triphenylphosphine oxide to yield the cis-stilbene in excess. However, the formation of the cis isomer is not absolute and even at high field strengths some trans isomer is produced. As the residency time is increased, by reducing the applied electric field, the relative concentration of the ylide to aldehyde increases and the equilibrium position moves to the more thermodynamically favoured trans-oxaphosphotane, which is produced in excess. Again, the equilibrium between the two isomers ensures that there is never absolute selectivity of the trans-isomer, see Fig. 4. Experimental evidence to support the proposition that localised concentration gradients are being generated and controlled with in a micro reactor is given in Fig. 5. This demonstrates that the variation in the Z/E ratios is dependant on the concentration of reactants. When the aldehyde is in excess, the Z isomer is the predominant product whereas if the phosphonium salt is in excess, the E isomer is the predominant product. This concentration effect is currently being investigated further within the author’s laboratory looking at the fundamental kinetics of the reaction.

Fig. 4 The stereoselective mechanism for the Wittig chemistry within a micro reactor device.

Fig. 5 Plot to show the effect of aldehyde concentration on the Z/E ratio.

In conclusion, the micro reactor investigated using Wittig chemistry has demonstrated that stereoselective control of products can be induced using applied electrical fields in a micro reactor device. This was achieved by varying the voltage at reservoirs A, B and C, which allowed the precise control of the reagent concentration within the reactor channels. The micro reactor system described is currently being used in conjunction with statistical modelling techniques to predict the Z/E ratio with reference to the applied voltages at reservoirs A, B and C. The micro reactor developed offers considerable potential for use in a number of chemical fields such as synthetic development and optimisation, combinatorial synthesis and drug discovery enabling rapid synthesis of a number of compounds with versatile selective control.

Acknowledgement

This work was financially supported by SmithKline Beecham Pharmaceuticals and EPSRC.

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