238
A. E. MUCIENTES ET AL.
2
A0 þ A1½OHꢀꢂ þ A2½OHꢀꢂ
which then decomposes slowly to produce a reduced
form of catalyst, RuO3ðOHÞ3ꢀ, and a protonated ketone
v0 ¼
ð3Þ
2
1 þ B1½OHꢀꢂ þ B2½OHꢀꢂ
as follows:
The v0–[OHꢀ] data were fitted to Eqn (3) by means of a
non-linear regression program. The best average error
was obtained for Ru(VI) when A2 ¼ 0 and for Ru(III)
when A0 ¼ 0.
---- þ
k2
3ꢀ
0
C21ꢀꢀ! RR C---- OH þ RuO3ðOHÞ
ð5Þ
Such an intermediate would have the following structure:
The possible formation of free radicals as intermedi-
ates was investigated by adding radical scavengers to the
reaction mixture. The addition of either 0.01 M acryloni-
trile or 1.6 ꢁ 10ꢀ4 M 2,4,6-tri-tert-butylphenol (a stronger
radical scavenger) did not have any effect on the reaction
rate when Ru(VI) was used as catalyst. For Ru(III),
however, the presence of 0.01 M acrylonitrile reduced v0
by 15% and the addition of 1.6 ꢁ 10ꢀ4 M 2,4,6-tri-tert-
butylphenol decreased v0 by 40%. Moreover, when 0.7 M
acrylonitrile was added to the reaction mixture, poly-
meric species were observed after a few minutes.
The oxidation of cyclobutanol was carried out because
the nature of its oxidation products depends on the
reaction mechanism. One-electron oxidation produces
acyclic four-carbon compounds, which appear to be
derived from the primary free radical .CH2CH2CH2CHO,
whereas two-electron oxidation produces cyclobutanone
directly.12,13 The following kinetic conditions were em-
ployed in this experiment: [OHꢀ] ¼ 0.1 M, I ¼ 0.5 M and
T ¼ 30 ꢄC. Under these conditions the oxidation of 0.08 M
cyclobutanol by 2.0 ꢁ 10ꢀ3 M hexacyanoferrate(III) using
2.5 ꢁ 10ꢀ6 M catalyst produced butanal as the major
product in the case of Ru(III) and cyclobutanone in the
case of Ru(VI).
----
Step 5 involves a hydride transfer from the ꢀ-C H
bond of the alcohol to the oxo ligand of ruthenium, a
process that is favoured by the prior coordination of the
organic substrate to the metal through the oxygen of the
hydroxy group.15 The occurrence of this hydride transfer
is supported by the following experimental results: (a) a
moderate kinetic isotope effect, which indicates cleavage
----
of a C H bond, and the absence of free radicals in the
reaction mixture, (b) oxidation of cyclobutanol produces
cyclobutanone as the sole product and (c) the negative
value of the Hammett reaction constant found for the
oxidation of benzyl alcohol.16
The following rapid reaction would yield the corre-
sponding ketone:
---- þ
RR C---- OH þ OH ꢀ! R CO R þ H2O
0
ꢀ
0
---- ----
ð6Þ
Although the organic substrate is a diol, only the 2-
hydroxy group will be oxidized, as observed experimen-
tally, because tertiary alcohols (0.1 M tert-butanol) were
found not to react under kinetic conditions.11 The pre-
sence of a hydrogen on the ꢀ-carbon of the alcohol is
therefore necessary for the reaction to progress.14
The dependence of v0 on ½FeðCNÞ3ꢀꢂ0 can be explained
6
if it is accepted that the oxidation of the reduced form of
catalyst occurs. In this way, the role of the co-oxidant,
FeðCNÞ36ꢀ, is solely the regeneration of the catalyst
through steps (7) and (8):
----
----
The observed oxidation rate of CD3 CDOD CD3
----
----
k3
was compared with that of CH3 CHOH CH3 in order
to verify the existence of a kinetic isotope effect. A
substantial primary kinetic isotope effect was indeed
observed [(v0,H=v0,D) ¼ 5.9] for both catalysts under the
following kinetic conditions: [catalysts] ¼ 2.0 ꢁ 10ꢀ6 M,
2ꢀ
RuO3 ðOHÞ3ꢀ þ FeðCNÞ63ꢀ ꢀ! RuO3 ðOHÞ
4ꢀ
þ FeðCNÞ6 ð7Þ
3ꢀ
½FeðCNÞ6 ꢂ ¼ 1:2 ꢁ 10ꢀ3 M, [diol] ¼ 0.5 M, [OHꢀ] ¼
RuO3 ðOHÞ2ꢀ þ FeðCNÞ6 ꢀ! RuO3 ðOHÞꢀ
3ꢀ
0.2 M, I ¼ 0.5 M and T ¼ 30 ꢄC.
4ꢀ
þ FeðCNÞ6
ð8Þ
ð9Þ
DISCUSSION
RuO3 ðOHÞꢀ þ OHꢀ ꢀ! RuO42ꢀ þ H2O
For Ru(VI) the dependence of the initial rate on
0
----
----
[R CHOH R ]0 [where R ¼ (CH3)2COHCH2 and
R0 ¼ CH3] suggests the formation of an intermediate
complex, C21ꢀ, between RuO24ꢀ and the organic substrate:
Step (7) is supported by the previously discovered fact
that the oxidation of alcohols by catalytic quantities of
ruthenate proceeds at a similar rate to the reoxidation of
3ꢀ
the reduced form of the catalyst by FeðCNÞ6 .11 Step (7)
k1
0
----
2ꢀ
R CHOH R þ RuO4 Ð C21ꢀ
ð4Þ
is fast relative to oxidation of the substrate at high
----
½FeðCNÞ36ꢀꢂ0 and, under these circumstances, v0 does
J. Phys. Org. Chem. 2004; 17: 236–240
k
ꢀ1
Copyright # 2004 John Wiley & Sons, Ltd.