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Y. Na et al. / Tetrahedron Letters 45 (2004) 7863–7865
Table 1. Relationship between catalytic activity of Ru complexes and
their current ratio icat/i in the hydrolytic oxidation reaction of
(A)
6 u A
diphenylmethylsilane
Ru (2 mol %)
Ph2MeSiH
+
H2O
Ph2MeSiOH
CH3CN
25 oC, 10 min
Entry
Ru-catalyst
Yielda (%)
icat/ib
(B)
(C)
1
2[RuCl
[RuCl2(p-cymene)]2
2(benzene)]2
RuCl2(PPh3)3
99
94
6.5
4.2
6 u A
3
4
5
421.9
20
13
[CpRu(NCCH3)3]PF6
[(g5-Ph4C5O)(CO)2Ru]2H2
1.2
1.1
a Isolated yield.
6 u A
b The anodic current ratio in CV for Ru-complex (1.0mM)/Ph2MeSiH
(25mM)/H2O (50mM)/TBAP (0.1M) and Ru-complex (1.0mM)/
TBAP (0.1M) in CH3CN, which was measured at 1.24V (vs Fc+jFc)
and the scan rate was 0.1V/s.
1.0
0.5
0.0
E / V vs Fc+ | Fc
Figure 1. CVs of 1.0mM of 1 in CH3CN (A) and CH2Cl2 (B), and
1.0mM of 2 in CH3CN (C) containing 0.00, 6.25, 12.5, and 25.0mM
Ph2SiHCH3 from top to bottom, respectively, and 0.10M TBAB using
a Pt disk (dia 1.6mm) electrode and a scan rate of 0.10V/s (twice molar
amounts of H2O with respect to those of Ph2SiHCH3 are also included
in each case).
Interestingly, in the hydrolytic oxidation reaction, bi-
nuclear species, that is, [RuCl2(p-cymene)]2 or [RuCl2-
(benzene)]2 shows higher activity in CH3CN than mono-
nuclear complexes, that iþs, RuCl2(PPh3)3 or a cationic
catalyst CpRuðNCCH3Þ PFÀ6 .7 The higher activity
3
observed from the dimeric catalysts can be attributed
to the in situ generation of effective mononuclear species
bearing relatively a weak Ru–S bond that facilitates a
subsequent exchange with reactants such as silanes.
However, it should be mentioned that this conclusion
can be applied only to metal complexes of similar elec-
tronic and steric properties. For example, despite the
binuclear dimeric structure, [(g5-Ph4C5O)(CO)2Ru]2H2
(Shvo complex)8 gives a poor catalytic activity under
the hydrolytic reaction conditions. This may be attrib-
uted to the completely distinct electronic nature of the
carbonyl ruthenium complex, compared to those of
[RuCl2(arene)]2 types.
presence of reactants as in the real reaction systems with
1/5 concentration scale (Fig. 1).6
When the hydrolytic oxidation of organosilanes to silan-
ols was performed using water as an oxidant, it was ob-
served that 1/CH3CN system turned out to be the most
effective in efficiency and selectivity.3c In electrochemical
experiments, we adjusted reactantsÕ concentration scale
by 1/5 to avoid fouling of measurements due to hydro-
gen evolution. After the addition of diphenylmethyl-
silane to the solution of 1/CH3CN, the oxidation
current was increased at a potential more positive than
0.93V depending on the amounts of diphenylmethyl-
silane (Fig. 1A), indicating an electrocatalytic system
(Scheme 1). However, such a large increase of the oxida-
tion current was not observed in 1/CH2Cl2 system (Fig.
With 1 as a catalyst (0.02equiv), oxidation was com-
pleted in 10min at room temperature to give diphenyl-
methylsilanol in high yield. The current ratio of 1 in
CH3CN was 6.5 in the presence of the silane. Under
the same conditions except the concentration scale,
chemical yields and icat/i of other binuclear and mono-
nuclear ruthenium complexes were obtained. To our
surprise, there exists a close correlationship between
two parameters of those Ru complexes investigated.
While catalysts of good activity give high icat/i ratio,
those of poor efficiency exhibit low values of the ratio.
Although more accurate and quantitative correlation
factors have to be set up, the present observation that
a noticeable relationship exists between catalytic activity
and measurable electrochemical parameter would be ex-
tended similarly to predict catalytic efficiency in other
systems.
1B) or CpRuðNCCH3ÞþPF6À (2)/CH3CN system (Fig.
3
1C). The difference in current between 1/CH3CN and
1/CH2Cl2 system seems to be stemmed from the Lewis
basicity of two solvents, that is, the degree of formation
of the labile bond of Ru–S. To analyze the results we
used the oxidation current ratio of Ru-complex with
and without Ph2MeSiH at 1.24V, icat/i, as the electro-
chemical measurement of effectiveness of catalysts in
CV experiments. The result of the electrochemical exper-
iments is summarized in Table 1 along with the chemical
yield of the hydrolytic oxidation of organosilanes.
electrode
Ru cat (Red)
Ru cat (Ox)
+ H
Ph MeSiOH
2
2
In summary, we have shown that electrochemical
parameters of ruthenium complexes are effectively corre-
lated with their catalytic activities and structural
changes of the species under the reaction conditions.
Although there are more factors taken into considera-
tion in transition metal-catalyzed reactions such as coor-
-
e
+ H O
Ph MeSiH
2
2
Scheme 1.