Scheme 1. Proposed mechanism for the phase transfer hydrogenation of
acetophenone with aqueous formate as hydrogen source and Ru-TsDPEN
catalyst.
The stoichiometry and rate expression consistent with the proposed
mechanism are summarized in Equations (3) - (9), assuming that the
coordination of formate and decarboxylation are reversible and
equilibrated prior to hydrogen transfer,26 whereK 1 is the equilibrium constant, andk 1, k 2,k 3 are the respective reaction rate constants.ACP and ACPH- represent acetophenone and
its corresponding alkoxide, while 1-PE refers to 1-phenylethanol
and [Ru ] represents the total concentration of catalyst. As
CO2 rapidly reacts with hydroxide during the reaction,
the concentration of CO2 is expected to be small,
therefore the rate expression can be further simplified as a second
order kinetics over the concentration of catalyst and substrate
(Equation (9)). This is consistent with the study by Wu et
al.26 and with the above observation showing that the
reaction rates were limited by the concentration of Ru catalyst and the
alkoxide at the interface region. Both of these concentrations were
affected by the electromigration under the influence of external
electric fields. These observations suggest an important impact of
electric fields on kinetics of interfacial reactions involving reactive
ions.
\(2\ +\ \text{HCOO}^{\mathrm{-}}3\ +\ \text{CO}_{2}\) (3)
\(3\ +\ \text{ACP}2\ +\ \text{ACPH}^{\mathrm{-}}\) (4)
\(\text{ACPH}^{\mathrm{-}}\ +\ H_{2}O1-PE\ +\ \text{OH}^{\mathrm{-}}\)(5)
\(\text{CO}_{2}\ +\ \text{OH}^{\mathrm{-}}\text{HCO}_{3}^{\mathrm{-}}\)(6)
Overall:\(\text{HCOO}^{\mathrm{-}}\ +\ \text{ACP}\ +\ H_{2}O\rightarrow 1-PE\ +\ \text{HCO}_{3}^{\mathrm{-}}\)(7)
Rate expression:\(\text{\ \ \ \ \ \ r}\ =\ \frac{K_{1}k_{1}[Ru][ACP][\text{HCOO}^{\mathrm{-}}]}{[\text{CO}_{2}]\ +{\ K}_{1}[\text{HCOO}^{\mathrm{-}}]}\)(8)
When\([\text{CO}_{2}]\ll K_{1}[\text{HCOO}^{-}]\),\(r\ =\ k_{1}[Ru][ACP]\) (9)
4.4 Electric field polarity and product enantioselectivity
As shown in Table 2, the enantioselectivity of (S)-1-phenylethanol
remained constant with respect to electric field potential applied in
the positive orientation. Since the orientation of OEEF was proposed as
being important for controlling reaction selectivity, a reversed
external electric field was then applied to the reaction system. The
results are presented in Table 3. In these cases, the reaction was
inhibited when the electric potential was applied in the negative
orientation, which further confirms the hypothesis of controllable mass
transfer and reaction rates by external electric fields, in accord with
past findings. The yield of 1-phenylethanol increased with the increased
negative electric field strength, which was unexpected as a larger
inhibiting electrostatic force would be expected to result in a lower
reaction rate and conversion. This increase is likely due to the
significant increase on the current flow (Figure 4) in the reaction
system. Increase in current flow is consistent with the local
temperature increase due to joule heating, which would compete with the
inhibiting effects of higher negative electrostatic force and possibly
promote the reaction.
Table 3. Summary for experiments with various negative external electric
fields applied.