Biomimetic methane generation and disulfide formation by catalysis with a nickel complex

Article abstract of DOI:10.1039/a705187e

The final metabolic process of methanogen, methane generation and simultaneous disulfide formation, is simulated by the reaction of thioanisole with a toluenethiyl radical in the presence of a nickel complex, which is generated by the photolysis of a toluenethiolato-nickel complex.

Full text of DOI:10.1039/a705187e

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Biomimetic methane generation and disulfide formation by catalysis with a
nickel complex
Masaru Tada* and Yoshihide Masuzawa
Department of Chemistry, Advanced Research Center for Science and Engineering and Materials Research Laboratory for
Bioscience and Photonics, Waseda University, Shinjuku, Tokyo 169, Japan
The final metabolic process of methanogen, methane genera-
tion and simultaneous disulfide formation, is simulated by
the reaction of thioanisole with a toluenethiyl radical in the
presence of a nickel complex, which is generated by the
photolysis of a toluenethiolato–nickel complex.
by Jaun5a and Berkessel.5b Thus the formation of a thiolato–
nickel complex [Ni]–S(HTP) and a sulfuranyl radical (co-
M)S Me– S(HTP)and a final methyl transfer from the sulfuranyl
radical to the nickel complex (F430) are the crucial features of
this mechanism.
Nickel(i) species have been reported to be active in an
enzyme system;6 therefore, we planned to produce the sulfur-
anyl radical and nickel(i) complex by the photolysis of the
thiolato–nickel complex formed by the reaction of a nickel(ii)
complex and a thiolate ion. The colour of the MeCN solution of
the nickel(ii) complex A changed from orange to blue–violet on
.
Methanogens1 are unique bacteria belonging to the Archae
bacteria, which constitute the third domain of terrestrial life in
addition to Eukaryotes and Prokaryotes.2 A methanogen
produces methane and water as metabolites to produce energy
from hydrogen and carbon dioxide3 [eqn. (1)]. The final step
methanogen
(1)
4H2 + CO2 ææææææÆCH4 + 2H2O
DG0 = -131 kJ mol-1
of this C1-unit transformation is the generation of a mixed
disulfide and methane (or methyl-F430) from methyl-coenzyme
M and 7-mercaptoheptanoylthreonine phosphate (HTP-SH)
(Scheme 1).3
Me
Me
N
N
Me
HN
HN
NH
NH
NiII
H
NiII
N
O
N
O
Me
• ClO4
• 2ClO4
B (4.6%)
A (24%)
–O3SCH2CH2SMe + HS-(HTP)
O2N
O
coenzyme-M
F430
COBut
–O3SCH2CH2SS(HTP) + Me-[Ni]
methyl-F430
N
H+
HN
NH
NH
HN
NH
Me-[Ni]
CH4 + [Ni]
NiII
NiII
Scheme 1
HN
HN
NH
• 2ClO4
• 2ClO4
We have no analogy for this in organic or inorganic
chemistry, and the main purpose of the present study is to
establish a biomimetic reaction analogous to the biological
methane production and gain insight into the biochemical
reaction mechanism. Reported here is the first successful
example of biomimetic production of methane and a mixed
disulfide by catalysis with a nickel(ii) complex, an F430 model.4
F430 is a nickel complex having a modified corrin nucleus which
is more flexible than the corrin nucleus in coenzyme B12. This
flexibility of F430 is considered to enable variation in coordina-
tion number during the catalysis.
C (1.7%)
D (1.3%)
addition of the toluene-p-thiolate anion, which was prepared in
situ from toluenethiol and NaH. A mixture of the toluene-
p-thiolate ion (1.5 3 1022 mol l21) and the nickel complex A
(5.0 3 1023 mol l21) was added to thioanisole (1.5 3 1022
mol l21) in MeCN, and the mixture was irradiated at 350 nm
under argon.†
Product analyses showed the formation of a mixed disulfide
3 in addition to ditolyl disulfide 4 and ditolyl sulfide 5 (Scheme
3). The time course of the formation of the mixed disulfide 3 and
ditolyl disulfide 4 is shown in Fig. 1, which shows that the initial
products 3 and 4 decompose on continuous irradiation.
Reference irradiation showed the decomposition of these
disulfides into the corresponding sulfides, and thus the initial
products are the disulfides 3 and 4.
On starting the experimental study, we adopted the working
hypothesis shown in Scheme 2, which was originally proposed
(co-M)-S-S(HTP)
Me
(co-M)-SMe
(co–M) S
•
(HTP)
S
Gas chromatography–mass spectral analysis (utilizing peaks
at m/z 12 and 15 to avoid contamination by oxygen) of the top
[NiI]430
[NiII]-S(HTP)
H+
(HTP)-SH
Me-[NiII]430
H+
CH4
TolS[Ni] + PhS-Me
hn, MeCN
[NiII]
+
+
+
+
Tol2S TolSMe
CH4 TolSSPh
(TolS)2
3
4
5
6
Scheme 2
Scheme 3
Chem. Commun., 1997
2161

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Ph
Me
30
[NiI]
STol
(a)
Me
S
[NiI]
Ph
S
•
TolS
20
10
(b)
[NiI]
(CH2)nPh
S
Me
•
Ph
•
TolS
SPh
10
20
S-Tol
t / h
n = 1,3
(c)
Fig. 1 The time course of the formationn of disulfides (5) 3 and (8) 4
(d)
Fig. 2 Possible reaction modes of the sulfuranyl radical
gas of the reaction vessel showed the evident formation of
methane, although the yield could not be determined due to the
solubility of methane in MeCN. Irradiation of the same mixture
containing an equivalent amount of compound 7 gave com-
pound 8‡ in 12% yield. The formation of 8 and ditolyl disulfide
4 shows the intervention of the toluenethiyl radical (Scheme
4).
phenylalkane. Thus the transition state (c) of the nickel attack
must be more hindered than the transition state (a) in which the
least bulky methyl group is the reaction centre, and the present
reaction seems to be under severe steric control. On the other
hand, steric control is less important for mode (b) because the
reaction centre is a sulfuranyl radical.
Finally, we detected a trace amount of methyl tolyl sulfide 6
from the present model reaction, and the methyl group on the
sulfuranyl radical (a) seems to swing to some extent through a
transition state (d). We are studying the effect of this methyl
migration on the present intramolecular methyl transfer of
biomimetic type.
Me
Me
Bz-O
N
O•
Me
Me
We thank Waseda University and the Ministry of Education,
Sports, and Culture of Japan for their financial support through
the Annual Project Program (95A-252) and a 1997 Grant-in-aid
(09640649) respectively.
hn
Me
Me
Me
S(O)Tol
Me
+
Bz-O
N
(TolS)2
Footnotes and References
* E-mail: mtada@mn.waseda.ac.jp
† An MeCN solution (10 cm3) of the reaction mixture was irradiated by a
Rayonett Photoreactor equipped with 350 nm lamps for the specified
periods. The products were identified by comparison with authentic
material. The time course of the product yields was determined by gas
chromatography using internal standards.
‡ We prefer the sulfinylamide structure 8 formed by the rearrangement of
the direct radical coupling product because the base peak in the mass
spectrum is seen at m/z 139, showing the existence of TolS(O), but only a
weak peak corresponding to (TolS) was seen.
§ Addition of piperidine to those neutral complexes did not produce a major
spectral change, but the UV absorption maxima of complex A (385.0 and
409.5 nm in EtOH) were replaced by maxima at 340.0, 454.5 and 578
(broad) nm or by a broad absorption band at ca. 560 nm upon addition of
piperidine or toluenethiolate ion.
Scheme 4
Other nickel(ii) complexes B–D showed some catalytic
activity, although with less efficiency, and the yields of 3 after
6 h irradiation are shown in parentheses beneath their structures.
Neutral complexes such as bis(dimethylglyoximato)nickel(ii)
and tetraphenylporphyrin–nickel(ii) showed no activity.§ For-
mation of the mixed disulfide was not observed when either the
nickel(ii) complex or irradiation was absent. This reaction
catalysed by the nickel(ii) complex A is summarized by Scheme
5 and the net reaction is written as eqn. (2).
TolS2 + MeSPh + H+ ? TolSSPh + CH4
(2)
1 W. E. Balch, G. E. Fox, L. J. Woose and R. S. Wolfe, Microbiol. Rev.,
1979, 43, 260; R. H. Crabtree, Chem. Rev., 1995, 95, 987.
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5088.
3 A. A. DiMarco, T. A. Bobik and R. S. Wolfe, Annu. Rev. Biochem., 1990,
59, 355; R. S. Wolfe, Annu. Rev. Microbiol., 1991, 45, 1; B. Jaun, Metal
ions in biological systems, ed. H. Siegel and A. Siegel, Marcel Dekker,
New York, 1993, vol. 29, ch. 9, p. 287.
There are two possible action modes of the nickel complex as
shown in Fig. 2; (a) an SH2 type substitution on the methyl
group or (b) a ligand coupling on the hypervalent state sulfur.7
We prefer mode (a) because the use of w-phenylalkyl
phenylsulfide instead of thioanisole yields no mixed sulfide and
4 R. P. Gunsalus and R. S. Wolfe, FEMS Microbiol. Lett., 1978, 3, 191;
A. Pfaltz, D. A. Livingston, B. Jaun, D. Diekert, R. K. Thauer and
A. Eschenmoser, Helv. Chim. Acta, 1985, 68, 133.
Me
MeSPh
TolS• + [NiI]
Tol SS Ph
•
5 (a) B. Jaun, Helv., 1990, 73, 2209; (b) A. Berkessel, Bioorg. Chem., 1991,
19, 101.
hn
6 B. Jaun and A. Pfaltz, J. Chem. Soc., Chem. Commun., 1988, 293;
S. P. J. Albracht, D. Ankel-Fuchs, R. Bocher, J. Ellermann, J. Moll,
J. W. Van Der Zwaan and R. K. Thauer, Biochem. Biophys. Acta, 1988,
955, 86.
TolS-[Ni]
TolS–
+ [Ni]-Me
H+
TolSSPh
7 S. Oae, Rev. Heteroatom Chem., Myu, Tokyo, 1988, vol. 1, p. 304; 1991,
vol. 4, p. 195.
[NiII] + CH4
Scheme 5
Received in Cambridge, UK, 21st July 1997; 7/05187E
2162
Chem. Commun., 1997