290
K.-i. Fujita et al. / Journal of Organometallic Chemistry 649 (2002) 289–292
Table 2
2). The reaction satisfactorily proceeded with use of
even 0.020 mmol of K2CO3 (entry 3). When the reac-
tion was carried out in a larger amount of acetone (30
ml), the yield increased up to 87% (entry 4). It was
apparent that the reaction did not proceed at all with-
out the catalyst (entry 5). Another iridium catalyst,
[IrCl(cod)]2 [cod=1,5-cyclooctadiene], and a rhodium
catalyst, [Cp*RhCl2]2 were examined, but they showed
lower activity than that of [Cp*IrCl2]2 (entries 6 and 7).
The ruthenium catalyst RuCl2(PPh3)3, which has been
reported as an excellent catalyst for the oxidation of
secondary alcohols to ketones by Ba¨ckvall and co
workers [2c,2d], showed little activity for the present
reaction (entry 8).
Oxidation of primary alcohols to aldehydes catalyzed by a pen-
tamethylcyclopentadienyliridium complex a
Results of oxidation of various primary alcohols by
the present catalytic system are summarized in Table 2
The present oxidation reaction could be applied to
substrates with various functional groups. In each reac-
tion, no formation of carboxylic acid or condensation
product of the aldehyde was observed. Benzyl alcohols
with a substituent at the aromatic ring were trans-
formed into the corresponding aldehydes (entries 2–8).
The yield of aldehyde was much higher in the reaction
of a substrate with an electron-donating group at the
para-position (entries 2 and 3), while an electron-with-
drawing group lowered the yield (entries 7 and 8). The
yield was relatively low in the reaction of ortho-substi-
tuted substrate (entry 4), while the meta-substituent
showed little effect (entry 5). p-Hydroxybenzyl alcohol
could be also oxidized to give p-hydroxybenzaldehyde
with use of excess amounts of a base (K2CO3) (entry 6).
The present catalytic system was also applicable to
oxidation of non-aromatic primary alcohols (entries 9
and 10), but these reactions should be carried out at
reflux temperature to obtain moderate yields.
Results of oxidation of secondary alcohols by the
present catalytic system are summarized in Table 3. In
contrast to primary alcohols, secondary alcohols could
be much easily oxidized to give the corresponding
ketones with use of smaller amounts of the catalyst
[Cp*IrCl2]2 (0.50 mol%/Ir) and acetone (2 ml). 1-
Phenylethanol, 2-octanol and cyclopentanol were trans-
formed into the corresponding ketones in excellent
yields (entries 1–3). Cyclohexanol was oxidized in
rather lower yield (79%, entry 4) than the other sec-
ondary alcohols, showing a similar tendency to that
observed in the ruthenium-catalyzed reaction [2c,2d].
When the oxidation of 1-phenylethanol was per-
formed in acetone-d6, formation of acetophenone con-
taining no deuterium and (CD3)CHOH was observed
by NMR analysis, indicating the apparent hydrogen
transfer from 1-phenylethanol to acetone-d6.
a The reaction was performed at room temperature for 6 h with
primary alcohol (1.0 mmol), [Cp*IrCl2]2 (2.0 mol%/Ir) and K2CO3
(0.10 mmol) in acetone (30 ml).
b Determined by GC based on the starting alcohol.
c The value in parentheses is isolated yield.
d Reaction was performed with 1.1 mmol of K2CO3.
e Determined by 1H-NMR.
f Reaction was performed at reflux temperature.
2. Results and discussion
Oxidation of benzyl alcohol to benzaldehyde was
investigated with a variety of catalytic systems (Table
1). The reactions were carried out with benzyl alcohol
in acetone at room temperature. In each reaction, ben-
zaldehyde was formed as a single product; no forma-
tion of benzoic acid or other products was observed.
The pentamethylcyclopentadienyliridium complex,
[Cp*IrCl2]2 exhibited some catalytic activity, affording
benzaldehyde in a yield of 13% (entry 1). Addition of
K2CO3 (0.10 mmol) as a base improved the catalytic
activity to give benzaldehyde in a yield of 71% (entry
Although the mechanism for the present reaction,
involving the catalytic active species being mononuclear
or binuclear, is not completely clear as of yet, a possible
mechanism is shown in Scheme 1. This is based on the