58
B.-L. Chen et al. / Electrochemistry Communications 42 (2014) 55–59
consistent with the result that no product is obtained in the absence of
catalyst at a potential of −1.54 V (entry 2, Table 1).
the electrolysis is carried out in a more concentrated solution (N50 mM,
Table 1, entries 16–19). Considering the ee and yield of PhCH(CH3)
COOH, the optimal substrate concentration is 50 mM. Besides, due
to the presence of the non-faradaic current, the faradaic efficiency
cannot attain 100%. However, increasing the number of faradays
per mole of substrate above 3.0 does not improve the CO2 fixation
significantly.
However, the yields of PhCH(CH3)COOH are relatively low (Table 1).
The likely reason for this is the reductions of the intermediates involved
in the process leading to the decrease of PhCH(CH3)COOH yield. For
example, in the electrocarboxylation of PhCH(CH3)Cl in the presence
of [CoII-(R,R)(salen)] (entry 1, Table 1) along with unreacted
PhCH(CH3)Cl (32%), 2,3-diphenylbutane (39%), phenylethane (9%),
and styrene (3%) are also detected by GC–MS. In theory, the intermedi-
ate [PhCH(CH3)]• (Eq. (5)) may undergo further reactions, such as dis-
proportionation (Eq. (7)), radical–radical coupling (Eq. (11)), hydrogen-
atom abstraction (Eq. (12)), and reduction to [PhCH(CH3)]− (Eq. (8)),
which is then protonated by any available proton source in the reaction
medium (Eqs. (13) and (14)). Coupling of alkyl radical and protonation
of the alkyl carbanion are well documented in the literature [21]. In this
report, only the yields of PhCH(CH3)COOH is reported; other products
stemming from subsequent reaction of [PhCH(CH3)]• or [PhCH(CH3)]−
with reactants other than CO2, although quite often observed in relatively
high yields, are omitted since they are not relevant to the discussion on
asymmetric electrocarboxylation.
It is known that an increase in temperature leads to a decrease in the
solubility of reactant CO2 in DMF [22]; however, high temperatures also
affect both the thermodynamics and the kinetics of the reaction. Increas-
ing the temperature from 0 °C to 50 °C reduces the yield, whereas the ee
increases from 3% to 83%. Further increase in temperature from 50 °C to
80 °C decreases the ee to 13% (Table 1, entries 22, 24–29). At a lower tem-
perature, it is likely that the reaction rate of Eq. (9) is slower than that of
Eq. 5, which can account for the observed low ee at 0 °C (Table 1, entry
24). Besides, a faster rise in the rate of Eq. (9) when compared to that of
Eq. (5) with the increase in temperature can account for the gradual
increase in the ee with the increase in temperature from 0 °C to 50 °C
(Table 1, entries 22, 24–26). By contrast, at higher temperatures
(N50 °C), the concentration of CO2 becomes so low that, together with
the instability of the Co–R bond, [CoII-(R,R)(salen)(PhCH(CH3))]− can
readily decompose to [PhCH(CH3)]• (Eq. (5)), thereby decreasing the
ee (Table 1, entries 22, 27–29). Therefore, the optimum temperature
is 50 °C (Table 1, entry 22). Efforts are currently underway to investi-
gate the thermodynamic and the kinetic aspects of the asymmetric
electrocarboxylation.
•
2½PhCHðCH3Þꢁ →PhCHðCH3ÞCHðCH3ÞPh
ð11Þ
ð12Þ
ð13Þ
ð14Þ
½PhCHðCH3Þꢁ þ SH→PhCH2CH3 þ S•
•
4. Conclusion
½PhCHðCH3Þꢁ þ SH→PhCH2CH3 þ S–
–
In summary, optically active 2-phenylpropionic acid has been suc-
cessfully synthesized through asymmetric catalytic carboxylation of 1-
phenylethyl chloride via electrogenerated [CoI-(R,R)(salen)]−. The elec-
trochemical behavior of CoII-(R,R)(salen) shows that the asymmetric
carboxylation reaction is an electrocatalytic process. Further advances
in this electrocarboxylation system can provide a new method for the
synthesis of optically active carboxylic acids from racemic substrates
and CO2.
½PhCHðCH3Þꢁ þ H2O→PhCH2CH3 þ OH–
–
3.2. Influence of various experimental parameters
With the aim of optimizing the yield and ee of the PhCH(CH3)COOH,
we turned our attention on the various experimental parameters
influencing the reaction. These include applied potential, catalyst-to-
substrate ratio, substrate concentration, charge passed, and tempera-
ture. The results of these experiments are reported in Table 1.
Initially, the influence of variation in applied potential is examined.
As is evident from Table 1 (entries 1, 5–8), a more negative potential
favors an increase in PhCH(CH3)COOH yield. No substantial change in
ee is observed when the potential is changed from −1.49 to −1.59 V,
whereas on further increase in the applied potential from −1.59 to
−1.74 V, the ee of PhCH(CH3)COOH decreases. Considering the yield
and ee, −1.59 V is the optimal working potential.
Next, as the catalyst strongly influences the reaction (entries 1–4,
Table 1), the effect of the catalyst-to-substrate ratio is evaluated. As
shown in Table 1 (entries 6, 9–13), the ee of PhCH(CH3)COOH is indepen-
dent of the catalyst-to-substrate ratio. On the other hand, the yield
increases from 9% to 27% as the catalyst-to-substrate ratio increases
from 1 to 15 mol%, whereas subsequent increase in the catalyst quantity
to 25 mol% has no significant effect on the product yield (entries 11–13,
Table 1). A lower catalyst-to-substrate ratio generates lower quantities
of [CoIII-(R,R)(salen)(PhCH(CH3))], hence lowering the probability of
contact between CO2 and [CoII-(R,R)(salen)(PhCH(CH3))]− (Eq. (9)),
which results in lower yield. Once the catalyst-to-substrate ratio reaches
15%, it is likely that the reaction rate is limited, which results in a nearly
constant yield of the desired product.
Acknowledgments
Research support from the National Natural Science Foundation
of China (20973065, 21173085, 21203066, 21373090) is gratefully
acknowledged.
References
[3] L. Moreschini, A. D. Vesco, G. Carraro, Alkali metal- and alkaline earth metal salts of
carboxylic acids of benzyl halides and their alkyl and/or aryl derivatives, Italian patent,
IT 894274 1971-10-15.
Entries 11 and 14–19 (Table 1) present the results obtained with dif-
ferent initial substrate concentrations. The yield of desired product in-
creases from 27% to 34% as substrate concentration increases from 10 to
100 mM. On the other hand, the ee remains unchanged, except when