Electrochemical methyl-transfer reaction to alkylthiol catalyzed by hydrophobic vitamin B12

Article abstract of DOI:10.1246/cl.2009.26

The catalytic methyl-transfer reaction from methyl tosylate to 1-octanethiol was carried out in the presence of heptamethyl cobyrinate perchlorate, hydrophobic vitamin B₁₂, under electrochemical conditions at -1.0 V vs. Ag/AgCl using a carbon-f

Full text of DOI:10.1246/cl.2009.26

2
6
Chemistry Letters Vol.38, No.1 (2009)
Electrochemical Methyl-transfer Reaction to Alkylthiol Catalyzed by Hydrophobic Vitamin B12
ꢀ
Ling Pan, Hisashi Shimakoshi, and Yoshio Hisaeda
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
744 Motooka, Nishi-ku, Fukuoka 819-0395
(Received October 17, 2008; CL-080999; E-mail: yhisatcm@mail.cstm.kyushu-u.ac.jp)
The catalytic methyl-transfer reaction from methyl tosylate
to 1-octanethiol was carried out in the presence of heptamethyl
cobyrinate perchlorate, hydrophobic vitamin B12, under electro-
chemical conditions at ꢁ1:0 V vs. Ag/AgCl using a carbon-felt
cathode and a zinc plate anode as the sacrificial electrode in an
undivided cell. This catalytic reaction proceeded via the forma-
tion and dissociation of the cobalt–carbon bond in the hydropho-
bic vitamin B12.
Co(II)
1
e- H+
Co(I)
O
S
CH3
C H17SCH3
8
O
H+
+
O
O
S
CH3
O
Co(III)
C8H17SH
O
2
Cobalamin-dependent methionine synthase catalyzes the
methyl-transfer reaction from methyltetrahydrofolate to homo-
Figure 1. Methyl-transfer reaction catalyzed by 1.
1
cysteine in most mammals and bacteria. The methyl transfer
from methyltetrahydrofolate to cob(I)alamin and the demethyla-
tion of the resulting methylcobalamin are generally considered
to be a double-displacement mechanism similar to SN2 reac-
reaction of 1-octanethiol with the methylated hydrophobic vita-
min B12 2, which was synthesized from 1 by a reported method.6
It was reported that 1-hexanethiol was methylated by 2 in the
2
3a
tions. Although some model studies of the nonenzymatic meth-
3
presence of pyridine and ZnCl2 in refluxing methanol. When
yl transfer have been reported, the entire catalytic cycle with a
we carried out the methylation of 1-octanethiol under similar re-
action conditions, it could be methylated with a yield of 68%
based on the complex 2. Furthermore, the methylation proceeded
at a yield more than 55% without pyridine.
reasonable yield has been difficult to achieve up to now.
On the other hand, hydrophobic vitamin B12, heptamethyl
cobyrinate perchlorate, [Cob(II)7C1ester]ClO4 (1) (Chart 1),
which has ester groups in place of the peripheral amide moieties
As the next step, we applied the catalytic reaction using an
electrochemical method. In order to examine the reactivity of the
complex 1, the redox behavior of 1 was investigated in DMF
by cyclic voltammetry in the presence of TsOCH3 as shown
4
of the naturally occurring cobalamin, was found to be an excel-
lent model compound for the functional simulation of cobala-
5
min-dependent enzymes. Herein, the hydrophobic vitamin
B12-catalyzed the methyl-transfer reaction from methyl tosylate
II
I
in Figure 2. The Co /Co couple was observed at ꢁ0:5 V vs.
Ag/AgCl, and an irreversible reduction peak was observed at
ca. ꢁ1:3 V vs. Ag/AgCl after the addition of TsOCH3. This
potential was consistent with that for the one-electron reduction
of the complex 2. This redox behavior indicates that the hydro-
(
reported as shown in Figure 1. The controlled-potential electrol-
TsOCH3) to 1-octanethiol under electrochemical conditions is
I
ysis insured the continuous Co species, accepting the methyl
group from TsOCH3 and donating it to 1-octanethiol. The com-
plex 1 was chosen as the initial catalyst for conveniently forming
I
phobic vitamin B12 is reduced to the Co species at ꢁ0:5 V vs.
I
the supernucleophilic Co species. The turnover behavior was
observed in this study for the first time under nonenzymatic
conditions.
Ag/AgCl and then reacts with TsOCH3 to form the methylated
complex 2. The cobalt–carbon bond in the complex 2 is cleaved
I
to form the methyl radical and Co species at ꢁ1:3 V vs. Ag/
5
c
The formation and heterolytic cleavage of the Co–CH3 bond
in complex 2 is considered to be an indispensable process in this
catalytic cycle. For the heterolytic cleavage of the Co–CH3 bond
under thermodynamic conditions, it was monitored by the direct
AgCl.
I
II
Co /Co
A
CO2CH3
CO2CH3
ClO4-
H3CO2C
H3CO2C
CO2CH3
CoII/CoI
Reduction of
Co-CH3
X
Co
Y
4 µ A
C3H7
C2H5
Br
C3H7
N
N
N
N
N
N
B
Co
N
O
N
2 5
C H
Br
H
-
2
-1.5
-1
-0.5
0
O
V vs. Ag/AgCl
CO2CH3
CO2CH3
3
Figure 2. Cyclic voltammograms of [Cob(II)7C1ester]ClO4 (1)
in DMF containing 0.1 M Bu4NClO4 at room temperature: A, 1
1:0 ꢂ 10 M); B, after addition of TsOCH3 (3:3 ꢂ 10 M) to
1: X = Y = None, [Cob(II)7C1ester]ClO4
2: X = CH3, Y = H2O, [(CH3)(H2O)Cob(III)7C1ester]ClO4
ꢁ
3
ꢁ2
(
the solution of 1.
Chart 1.
Copyright ꢀ 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.1 (2009)
27
Table 1. Methyl-transfer reaction from TsOCH3 to 1-
a
A
B
1.2
1
octanethiol
Cb
Product (C8H17SCH3)
Entry
Catalyst
ꢁ
F mol 1
c
Yield/%d
0.8
/
/mmol
Abs.
0
.6
1
1
1
12.8
5.3
18.6
2.2
16.7
158.9
104 ꢄ 3
16 ꢄ 2
18 ꢄ 2
22 ꢄ 2
98 ꢄ 5
12 ꢄ 2
400
62
70
—
375
46
e
2
3
0.4
f
1
None
3
0
.2
0
4
5
6
g
300
400
500
600
700
1
Wavelength/ nm
a
Controlled-potential electrolyses were carried out in DMF at ꢁ1:0 V
ꢃ
vs. Ag/AgCl at 50 C under N2 atmosphere in the dark with a car-
bon-felt cathode and a zinc plate anode in the undivided cell. Initial
Figure 3. Electronic spectra observed during electrolysis with 1
as the catalyst: A, Controlled potential at ꢁ1:0 V vs. Ag/AgCl
for 2 h in DMF; B: After irradiation with visible light under aero-
bic conditions.
ꢁ
3
ꢁ2
concentration: [catalyst], 1:8 ꢂ 10 M; TsOCH3, 1:3 ꢂ 10 M; 1-oc-
ꢁ
2
b
tanethiol: 1:2 ꢂ 10 M; Bu4NClO4, 0.1 M. Electrical charge passed
c
per mol of the catalyst. The quantity of the product is the average of
d
Zn2 coming from the sacrificial zinc plate anode is impor-
tant for this methyl-transfer reaction and is expected to work as
a Lewis acid to activate the SH group to become a better nucleo-
þ
at least two repeated experiments. Yield is based on the initial mole
e
f
of hydrophobic vitamin B12 used. At room temperature. Under irradi-
g
ation of 500-W visible light. At the potential of ꢁ1:4 V vs. Ag/AgCl.
ꢁ
9
phile, i.e., the S anion. When Pt mesh was used as the anode,
no methylated product could be detected. The catalytic reaction
can proceed by the scheme shown in Figure 1 using electronic
spectroscopy and product analyses under various conditions.
In conclusion, a methyl-transfer cycle from TsOCH3 to 1-
octanethiol catalyzed by the hydrophobic vitamin B12 (1) was
developed under specific electrochemical conditions. The turn-
over behavior was observed for the first time in a nonenzymatic
model system.
We applied the catalytic methylation by electrochemical
means. The controlled-potential electrolyses were carried out
in DMF in the presence of 1-octanethiol, TsOCH3, and the com-
plex 1 in an undivided cell under various reaction conditions.
The following results were obtained on the basis of the product
analyses listed in Table 1. (i) The methyl-transfer reaction could
ꢃ
be effectively achieved with a turnover number of 4 at 50 C and
1:0 V vs. Ag/AgCl in the dark as shown in Entry 1. On the oth-
ꢁ
er hand, the methyl-transfer reaction did not effectively proceed
at room temperature or without complex 1 (Entries 2 and 4). (ii)
When the electrolysis was carried out under irradiation by visi-
ble light as shown in Entry 3, which facilitated the formation of
the CH3 radicals, the catalytic cycle was scarcely observed. (iii)
The reaction at ꢁ1:4 V vs. Ag/AgCl also did not proceed with
good results as shown in Entry 6. This suggests that the CH3 rad-
ical generated at this potential is not utilized for the methylation
of 1-octanethiol. (iv) When we used a simple model complex 3
as a catalyst, the methyl-transfer reaction proceeded as shown
in Entry 5. However, complex 3 decomposed during the elec-
trolysis.
In order to examine the reaction mechanism, the reaction
was followed by electronic spectroscopy. The electronic spec-
trum observed during the electrolysis is shown in Figure 3A.
This spectrum had absorption maxima at 307, 375, and
We wish to thank Mr. H. Horiuchi for preparing the special
electrode cells. The present work was partially supported by a
Grant-in-Aid for Scientific Research on Priority Areas ‘‘Chemis-
try of Concerto Catalysis’’ and Global COE Program ‘‘Science
for Future Molecular Systems’’ and from the Ministry of Educa-
tion, Culture, Sports, Science and Technology (MEXT) of Japan.
References
7
1
a) R. G. Matthews, Acc. Chem. Res. 2001, 34, 681. b) Vitamin B12
and B12-Proteins, ed. by Kr a¨ utler, D. Arigoni, B. T. Golding,
Wiley-VCH, Germany, 1998. c) Chemistry and Biochemistry of
B12, ed. by R. Banerjee, John Wiley, New York, 1999.
2
3
L. D. Zydowsky, T. M. Zydowsky, E. S. Hass, J. W. Brown, J. N.
Reeve, H. G. Floss, J. Am. Chem. Soc. 1987, 109, 7922.
a) C. Wedemeyer-Exl, T. Darbre, R. Keese, Helv. Chim. Acta
1
999, 82, 1173. b) W. Galezowski, Inorg. Chem. 2005, 44,
495 nm, and the spectrum was changed to Figure 3B after irradi-
ation by visible light. These absorption spectra and the photo-
1530. c) M. Machuqueiro, T. Darbre, J. Inorg. Biochem. 2003,
94, 193. d) J. M. Pratt, P. R. Norris, M. S. A. Hamza, R. Bolton,
J. Chem. Soc., Chem. Commun. 1994, 1333.
Y. Murakami, Y. Hisaeda, A. Kajihara, Bull. Chem. Soc. Jpn.
1983, 56, 3642.
a) Y. Murakami, Y. Hisaeda, J. Kikuchi, T. Ohno, M. Suzuki,
8
chemical behavior were similar to those for the complex 2. This
I
4
5
result indicates that the complex 1 was reduced to form the Co
species at this potential and reacted with TsOCH3 to form com-
plex 2 with a cobalt–carbon bond.
The formally heterolytic cleavage of CH3–Co in the com-
Y. Matsuda, T. Matsuura, J. Chem. Soc., Perkin Trans. 2 1988,
1
237. b) Y. Murakami, Y. Hisaeda, T. Ohno, J. Chem. Soc., Perkin
I
þ
plex 2 to Co species and CH3 species was confirmed by a spec-
troscopic experiment. When an excess amount of 1-octanethiol
was added to the DMF solution of complex 2 under anaerobic
conditions, the electronic spectrum dramatically changed to that
with an absorption maximum at 393 nm, which is characteristic
Trans. 2 1991, 405. c) Y. Hisaeda, T. Nishioka, Y. Inoue, K.
Asada, T. Hayashi, Coord. Chem. Rev. 2000, 198, 21. d) Y.
Hisaeda, T. Masuko, E. Hanashima, T. Hayashi, Sci. Technol.
Adv. Mater. 2006, 7, 655.
Y. Murakami, Y. Hisaeda, Bull. Chem. Soc. Jpn. 1985, 58, 2652.
Y. Murakami, Y. Hisaeda, S.-D. Fan, Y. Matsuda, Bull. Chem.
Soc. Jpn. 1989, 62, 2219.
H. Shimakoshi, A. Nakazato, T. Hayashi, Y. Tachi, Y. Naruta, Y.
Hisaeda, J. Electroanal. Chem. 2001, 507, 170.
K. Peariso, C. W. Goulding, S. Huang, R. G. Matthews, J. E.
Penner-Hahn, J. Am. Chem. Soc. 1998, 120, 8410.
6
7
I
of the Co species. The electrolysis results suggest that the meth-
yl-transfer reaction did not proceed under methyl radical form-
ing conditions as shown in Entries 3 and 6 in Table 1, which
are also supported by the heterolytic cleavage in this methyl-
transfer reaction.
8
9
Published on the web (Advance View) November 29, 2008; doi:10.1246/cl.2009.26