178
Table 1. Electrolysis of 3 mediated by B12 model complex 2a
Ph
CO2Et
EtO2C
CO2Et Ph
CO2Et
Ph
CO2Et
+
+
CO2Et
CO2Et
Ph
CO2Et
Br
3
A
B
C
Product ratio/%e
Potential Charge
Conversion
Entry
¹1 c
/Vb
/F mol
/%d
A
B
C
1
¹0.90
¹1.20
¹1.20
¹1.20
0.4
1.5
0.3
1.7
3
10
10
36
98
36
83 11
86
2
1
trace
62
6
2
3f
4g
8
6
aControlled-potential electrolyses were carried out in DMF using a
three-electrode cell with a Pt mesh cathode and a Pt mesh anode
under a N2 atmosphere. Initial concentration: [2] = 5.0 © 10¹4 M,
Figure 2. Cyclic voltammograms of 2 (4.5 mM) in the presence
of an excess of 3 (10 mM) and in DMF containing n-Bu4NPF6
(5.0 © 10¹2 M); sweep rate: 100 mV s¹1; scan ranges: (solid line)
¹1.2 to 1.0 V vs. Ag-AgCl, (broken line) 0 to 1.0 V vs. Ag-AgCl.
b
[3] = 5.0 © 10¹2 M, [n-Bu4NPF6] = 0.25 M. Employed potential
(V) vs. Ag-AgCl. cElectrical charge passed per mol of 3.
e
dConversion was estimated by the recovery of 3. Products were
analyzed by GC-MS. fThe cell was divided into two internal
compartments with a single sheet of a microporous polypropylene
It has been demonstrated that the Co(III) dimethylated
complex is inactive for the electrochemical reduction, but active
for the electrochemical oxidation resulting in the fragmentation
to the Co(III) monomethylated complex as shown in eq 4.4,5
Thus, the irreversible oxidation peak at +0.86 V vs. Ag-AgCl in
Figure 2 is ascribed to the one-electron oxidation of the Co(III)
dialkylated complex, which results from the one-electron
reduction of the Co(III) monoalkylated complex followed by
disproportionation (eq 3). This assignment is consistent with the
fact that the irreversible oxidation peak was not observed in the
range of 0 to 1.0 V vs. Ag-AgCl as shown in Figure 2 (broken
line), which suggests that the Co(III) dialkylated complex was
not formed in this scan range. The anodic electrochemistry of the
Co(III) dialkylated complex is a key process in the electrolysis
of 3 as discussed below.
g
membrane. Use of a Zn anode instead of a Pt mesh anode.
into two internal compartments with a polypropylene membrane
at the same employed potential, A and B were mainly obtained
as shown in Entry 3. This indicates that the membrane prevents
the diffusion of the Co(III) dialkylated complex formed at the
cathode to the anode, resulting in the decreased selectivity for C.
The comparison of the product ratios of C between Entries 2 and
3 suggests that the coupled redox processes are necessary for the
1,2-migration of the carboxylic ester group. In addition, the use
of a zinc plate anode instead of a Pt mesh anode also reduced
the selectivity for C as shown in Entry 4. This suggests that
the oxidation of the zinc plate as a sacrificial anode is superior
to that of the Co(III) dialkylated complex, resulting in the
decreased selectivity for C and the higher conversion. Therefore,
the oxidation of the Co(III) dialkylated complex is crucial for the
1,2-migration of the carboxylic ester group.
CH3
CH3
+
+
CH3
CoIII
- e-
CoIII
CH3
Co
+
Product ð4Þ
CH3
To confirm the participation of the Co(III) dialkylated
complex as an oxidizable intermediate during the 1,2-migration
of the carboxylic ester group, ESI-MS was performed under the
same conditions as Entry 2 in Table 1. The Co(III) monoalky-
lated and dialkylated complexes were detected as shown in
Figure 3, consistent with the former voltammetric study. It
should be emphasized that this mass spectrometric data are the
first evidence to directly detect the Co(III) dialkylated inter-
mediate during the 1,2-migration of the carboxylic ester group.
On the other hand, the participation of the Co(III) monoalkylated
complex in the 1,2-migration of the carboxylic ester group is
ruled out because the Co(III) monoalkylated complex is inactive
for the electrochemical oxidation.4
The phenyl-migrated product B was also obtained as a
carbon-skeleton rearrangement product under all the conditions
as shown in Table 1. We reported the catalysis of 3 mediated by
heptamethyl cobyrinate 1 under electrochemical and photo-
chemical conditions.3,7 These studies demonstrated that the
phenyl-migrated product B is formed via a radical intermediate,
which results from the one-electron reduction of the Co(III)
monoalkylated complex (eq 5.1).3,7 Thus, in the present study of
the imine/oxime-type complex 2, it is reasonable to expect that
the product B is also formed via a radical intermediate. The
Based on the investigation by cyclic voltammetry, the
controlled-potential electrolysis of 3 in the presence of a
catalytic amount of 2 was carried out in DMF under various
conditions.8 The products were analyzed by GC-MS as summa-
rized in Table 1. The electrolysis did not efficiently proceed at
¹0.90 V vs. Ag-AgCl, and the simple reduced product A was
almost exclusively obtained as shown in Entry 1. This product
probably results from the relatively slow thermolysis of the
Co-C bond of the monoalkylated complex, which is formed by
the reaction between the Co(I) species and 3. On the other hand,
the electrolysis more efficiently proceeded at ¹1.20 V than at
¹0.90 V vs. Ag-AgCl due to the effective electrochemical
reduction of the Co(III) monoalkylated complex as shown in
Entry 2. Interestingly, under the present conditions, the carbox-
ylic ester migrated product C was obtained as the major product.
It should be noted that this electrochemical 1,2-migration of the
carboxylic ester group serves as an analogy for the carbon-
skeleton rearrangement of methylmalonate to succinate mediated
by MMCM (eq 1).
The participation of an oxidizable intermediate during the
1,2-migration of the carboxylic ester group was confirmed by the
following experiments. When the electrolysis cell was divided
Chem. Lett. 2011, 40, 177-179
© 2011 The Chemical Society of Japan
Tahara, Keishiro
