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.