1. Introduction
Controlling reaction rate and selectivity is a long-standing goal and
challenge in chemistry.1-3 Conventional ways to
control chemical reactions include heating,4 stirring,
using solvents5 or catalysts.6Recently, inspired by the pioneering work of Shaik and
co-workers,7-9 attention has turned to the
manipulation of activation energy barriers and the stabilization of
reactive species in chemical reactions by oriented external electric
fields (OEEFs).10,11 The influence of OEEFs upon
chemical reaction performance has raised a number of scientifically
interesting questions, and increasing evidence has shown that OEEFs
could be employed as smart reagents and catalysts in
chemistry.10,12-15 It was proposed by Meir et
al.8 that organic reactions can be theoretically
accelerated when an external electric field is oriented along the
“reaction axis” which enhances the electron flow, leading to a lower
reaction barrier and the stabilization of transition states. This
simulated prediction was experimentally demonstrated by Aragones et al.
in an OEEF promoted Diels-Alder reaction at the single molecule
scale.16 Huang et al. also proved this idea by
selective catalysis of a Diel-Alder addition reaction with an external
electric field oriented along the reaction
coordinate.17 More importantly, Wang et al. predicted
that the reaction enantioselectivity may be controlled by aligning the
electric field along the dipole moment of the
reactants.2 While it is potentially significant for
asymmetric synthesis, there is no direct experimental evidence so far to
support it.
Despite the promising prospects, there are two major challenges to be
met in order to utilize OEEFs for controlling reaction rates and
selectivity. One requirement is to orient the electric field along the
reaction axis,10 the other is the high electric field
strength (107 to 109V/m).17,18 At molecular level, these challenges can be
addressed by using scanning tunneling microscope break junction (STMBJ)
techniques16,19,20 or mechanically controllable break
junction (MCBJ) techniques.17,21,22 Nevertheless, when
conducting experiments on a bulk scale, the question of how to align the
electric field along the reaction axis arises when thousands of reactant
molecules are randomly oriented in the reaction system. Furthermore,
applying thousands of volts to a lab-scale (or even larger scales)
reactor could pose safety issues. Therefore, the significance of OEEFs
on controlling reaction rates and selectivity experimentally in bulk
quantity reactions for organic synthesis remains understudied. However,
apart from lowering the reaction barrier with huge energy input, a
simple way to manipulate the reaction rates by external electric fields
is to control the diffusion of reactive species in the reaction
system,23 and alter the local concentration of
reactants.24 This has been validated in our previous
study that the reaction rate can be either promoted or inhibited by
simply flipping the orientation of the external electric
field.25
To further explore the possibilities of controlling reaction rate and
even selectivity with external electric field on bulk scales, catalytic
transfer hydrogenation of acetophenone was studied in a biphasic liquid
system. Previous studies showed that the phase transfer hydrogenation of
acetophenone with sodium formate as hydrogen source is a mass transfer
limited reaction happening at the organic-aqueous
interface.25,26 When an external electric field was
applied, the charged reactive species could be either transferred to the
interface to promote the reaction or constrained in the corresponding
liquid phases decelerating the reaction by the electrostatic
forces.25 This raises the possibility of manipulating
the reaction rates by controlling the diffusion of reactive species in
the presence of varying electric field potential for interfacial
reaction/catalysis in a biphasic liquid system.
In this contribution, we have demonstrated the ability of external
electric fields to control reaction rates in the catalytic phase
transfer hydrogenation of acetophenone and its influence on reaction
enantioselectivity. Mathematical modeling and simulation results further
support the controllable migration of reactive species under external
electric fields. While evincing fundamental principles, the results also
show the possible decomposition effects of the external electric field
on the catalyst when applied in the negative orientation, resulting in
lower reaction conversion and enantioselectivity. In general, this work
shows the importance of comprehensively understanding the role of
external electric field in organic synthesis and catalysis on bulk
scale.