Facile synthesis of (η5-Ph4C4COH)(CO)2RuCl and catalytic oxidation of alcohols with chloroform

Article abstract of DOI:10.1021/om020516m

A hydroxycyclopentadienyl ruthenium chloride, (η⁵-Ph₄C₄COH)(CO)₂RuCl (2), is formed quantitatively by heating the Shvo complex, {[(η⁵-Ph₄C₄CO)]₂H}Ru₂ (CO)₄(μ-H) (1), in chloroform containing a stoichiometric amount of ethanol. Alcohols are oxidized by the use of 1 or 2 as the catalyst in chloroform with sodium carbonate. Secondary alcohols and primary dials are oxidized selectively to give ketones and lactones, respectively, in excellent yields.

Full text of DOI:10.1021/om020516m

5674
Organometallics 2002, 21, 5674-5677
Notes
F a cile Syn th esis of (η5-P h 4C4COH)(CO)2Ru Cl a n d
Ca ta lytic Oxid a tion of Alcoh ols w ith Ch lor ofor m
Hyun Min J ung, J un Ho Choi, Soon Ok Lee, Yu Hwan Kim, J ung Hye Park, and
J aiwook Park*
Center for Integrated Molecular System, Department of Chemistry, Pohang University of
Science and Technology (POSTECH), San 31 Hyoja Dong, Pohang 790-784, Republic of Korea
Received J uly 1, 2002
Summary: A hydroxycyclopentadienyl ruthenium chlo-
ride, (η5-Ph4C4COH)(CO)2RuCl (2), is formed quantita-
tively by heating the Shvo complex, {[(η5-Ph4C4CO)]2H}-
Ru2(CO)4(µ-H) (1), in chloroform containing a stoichio-
metric amount of ethanol. Alcohols are oxidized by the
use of 1 or 2 as the catalyst in chloroform with sodium
carbonate. Secondary alcohols and primary diols are
oxidized selectively to give ketones and lactones, respec-
tively, in excellent yields.
The Shvo complex, {[(η5-Ph4C4CO)]2H}Ru2(CO)4(µ-H)
(1),1 has shown intriguing catalytic activities in many
hydrogen transfer reactions such as the Tishchenko type
disproportionation of aldehydes to esters,2 the water-
gas shift type reduction of aldehydes and ketones,3 and
the Oppenauer type oxidation of secondary alcohols.4
Furthermore, the Shvo complex has been introduced as
a racemization catalyst by Ba¨ckvall for the dynamic
kinetic resolution of secondary alcohols,5 and we have
used it in the related asymmetric transformations of
ketones and enol acetates into the corresponding chiral
acetates.6
F igu r e 1. ORTEP drawing (50% probability) of the struc-
ture of 2 at 233 K. Selected bond distances (Å): Ru1-C1
) 2.344(4), C1-O1 ) 1.316(5), Ru1-Cl ) 2.4090(12).
Sch em e 1. Tr a n sfor m a tion of th e Sh vo Com p lex 1
to Ru th en iu m Ch lor id e Com p lex 2
The transformation of 1 into 2 was observed unex-
pectedly when a solution of 1 in chloroform was heated.7
Ethanol contained as a stabilizer was found to be
essential for the clean and quantitative transformation
(Scheme 1). In contrast to previous reports of 2 as a
unstable product in the reaction of (η4-Ph4C4CO)Ru-
(CO)2(NMe3) with hydrogen chloride gas,8,9 it is, in fact,
a fairly stable solid, and crystals suitable for X-ray
diffraction analysis were obtained by recrystallization
from a solution of chloroform and hexane. The bond
lengths of 1.316(5) and 2.344(5) Å for C(1)-O(1) and
Ru(1)-C(1), respectively, are consistent with an η5-
coordination mode of the hydroxycyclopentadienyl lig-
and in the molecular structure of 2.10,11 Another fea-
ture of the structure is that the dihedral angle of
O(1)-C(1)-Ru(1)-Cl is only 9.8° (Figure 1).
(1) Shvo, Y.; Czarkie, D. J . Am. Chem. Soc. 1986, 108, 7400.
(2) Menashe, N.; Shvo, Y. Organometallics 1991, 10, 3885.
(3) Shvo, Y.; Czarkie, D. J . Organomet. Chem. 1986, 315, C25.
(4) (a) Almeida, M. L. S.; Beller, M.; Wang, G-.Z.; Ba¨ckvall, J -.E.
Chem. Eur. J . 1996, 2, 1533. (b) Almeida, M. L. S.; Kocˇovsky´, P.;
Ba¨ckvall, J .-E. J . Org. Chem. 1996, 61, 6587.
(5) (a) Larsson, A. L. E.; Persson, B. A.; Ba¨ckvall, J .-E. Angew.
Chem., Int. Ed. 1997, 36, 121. (b) Persson, B. A.; Larsson, A. L. E.;
Ray, M. L.; Ba¨ckvall, J .-E. J . Am. Chem. Soc. 1999, 121, 1645.
(6) (a) J ung, H. M.; Koh,J . H.; Kim, M-.J .; Park, J . Org. Lett. 2000,
2, 2487. (b) J ung, H. M.; Koh, J . H.; Kim, M-.J .; Park, J . Org. Lett.
2000, 2, 409. (c) Kim, M-.J .; Choi, Y. K.; Choi, M. Y.; Kim, M. J .; Park,
J . J . Org. Chem. 2001, 66, 4736.
(9) Casey has made another hydroxycyclopentadienyl ruthenium
chloride, [2,5-Ph2-3,4-Tol2(η5-C4COH)]Ru(CO)2Cl, by bubbling anhy-
drous HCl through a CH2Cl2 solution of {[2,5-Ph2-3,4-Tol2(η5-C4CO)]-
Ru(CO)2}2: Casey, C. P.; Vos, T. E.; Singer, S. W.; Guzei, I. A.
Organometallics 2002, 21, 5038.
(10) In contrast, the corresponding bond lengths are 1.22 and 2.53
Å, respectively, in the structure of (η4-C4Ph4CO)Ru(CO)3, which
exhibits a typical η4 coordination: Blum, Y.; Shvo, Y.; Chodosh, D. F.
Inorg. Chim. Acta 1985, 97, L25.
(11) The iodide analogue of 2 is reported to be formed in the reaction
of the Shvo complex with iodomethane or iodine: Schneider, B.;
Goldberg, I.; Reshef, D.; Stein, Z.; Shvo, Y. J . Organomet. Chem. 1999,
588, 92. The iodide analogue also acts as an oxidation catalyst under
the conditions of the entry 4 in Table 1, but its reactivity is much lower
than that of 2 (44% yield after 48 h).
(7) An aminocyclopentadienyl ruthenium chloride is formed by
heating the corresponding iminocyclopentadiene and Ru3(CO)12 in
chloroform: Choi, J . H.; Kim, Y. H.; Nam, S. H.; Shin, S. T.; Kim, M.-
J .; Park, J . Angew. Chem., Int. Ed. 2002, 41, 2373.
(8) Our 1H and 13C NMR data for 2 are different from those reported
previously: Bailey, N. A.; J assai, V. S.; Vefghi, R.; White, C. J . Chem.
Soc., Dalton Trans. 1987, 2815.
10.1021/om020516m CCC: $22.00 © 2002 American Chemical Society
Publication on Web 11/06/2002

Notes
Organometallics, Vol. 21, No. 25, 2002 5675
Sch em e 2. P r op osed P a th w a y for th e Ca ta lytic Oxid a tion of Alcoh ols w ith Ch lor ofor m
Ta ble 1. Ca ta lytic Oxid a tion of 1-P h en yleth a n ol to
Acetop h en on e in Ch lor ofor m w ith 2
with decreasing reaction temperature but is reasonable
still at 60 °C (entries 8-10).
Dichloromethane and the Shvo complex (1) were
2
Na2CO3
(equiv)
concn
(M)
yield
(%)b
T (°C)a
t (h)
1
observed in the H and 13C NMR spectra of the reaction
entry
(mol %)
mixture.13 On the basis of this observation, a plausible
pathway for the catalytic oxidation is proposed in
Scheme 2: the coordinatively unsaturated species 3 is
generated from 2 by the removal of hydrogen chloride
with the aid of base. An alcohol is dehydrogenated to
the corresponding carbonyl compound by 3, which
simultaneously transforms to ruthenium hydride 4. The
oxidizing species 3 is regenerated from 4 in the reduc-
tion of dichlorocarbene intermediate 6 to give the
dichloromethyl complex 7.14,15 Dichloromethane is liber-
ated from 7, producing 3. The carbene complex 6 is
formed through the reaction of 3 with chloroform
followed by the dehydrochlorination of the resulting
intermediate 5 with the aid of base. Meanwhile, 3 and
4 are in equilibrium with 1 in the reaction mixture.16
In fact, 1 was recovered in more than 95% yield and
showed a catalytic activity similar to that of 2 for the
oxidation of alcohols.
The scope of our catalyst system is illustrated by the
highly selective oxidation of nonactivated aliphatic
alcohols as well as activated benzylic alcohols in excel-
lent yields with the use of 1 or 2 (Table 2): all the
secondary alcohols are transformed into the correspond-
ing ketones selectively and almost quantitatively. The
benzylic alcohol with an electron-donating substituent
at the benzene ring reacts faster than that with an
electron-withdrawing substituent (entries 1-3). An
1
2
3
4
5
6
7
8
4
2
4
2
2
0.2
2
2
2
0
0.10
0.10
0.10
0.10
0.05
1.0
90
90
90
90
90
90
90
60
40
25
12
24
1.5
5
5.0
58.2
99.7
99.5
98.5
99.9
99.5
98.5
6.8
0.6
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
5
12
18
24
5
0.25c
0.10
0.10
0.10
9
10
2
5
5.3
a
b
Heating-bath temperature. Determined by gas chromatog-
raphy. c Toluene was employed as solvent with 3 equiv of CHCl3.
Fomally, 2 is an adduct of hydrogen chloride and a
16-electron species, (η4- Ph4C4CO)Ru(CO)2, which is
known as an active hydrogen acceptor in the catalytic
reactions involving the Shvo complex.12 Thus, we an-
ticipated that a proper base would abstract hydrogen
chloride from 2 and generate the hydrogen acceptor that
can oxidize alcohols. In fact, acetophenone was obtained
from 1-phenylethanol in 91% yield in the initial attempt
using 3 equiv of triethylamine and 4 mol % of 2 in
boiling chloroform within 24 h. More interestingly,
replacement of triethylamine with sodium carbonate led
to the completion of the oxidation within 5 h to give
acetophenone quantitatively despite decreasing the
amount of 2 to 2.0 mol %. Table 1 shows the effects of
reaction conditions on the oxidation of 1-phenyletha-
nol: The use of base is crucial for the catalytic oxidation.
Only 5% of the substrate is oxidized in 12 h without
base (entry 1), and more than 1 equiv of Na2CO3 is
needed to complete the oxidation (entry 2). The reaction
rate increases when the amount of the catalyst is
increased but is still effective with 0.2 mol % of 2 (entry
6). The concentration of the substrate little affects the
efficiency of the catalytic oxidation in the range of
0.05-1.0 M (entries 5 and 6). The use of chloroform as
a solvent is optional and can be minimized by employing
another solvent (entry 7). The reaction rate slows down
(13) CDHCl2 was formed in the reaction of 1-phenylethanol with
CDCl3 in 89% of acetophenone. The amount of CDHCl2 was estimated
by measuring the intensity of the peaks at 5.28 ppm (t, J HD ) 0.96
Hz) in the 1H NMR spectrum. The characteristic peak for the hydride
of 1 was observed at -18.3 ppm.
(14) The suggestion of dichlorocarbene complex 6 as an intermediate
is supported by the previous examples that show generation of metal
dichlorocarbene complexes from the activation of chloroform as isolable
compounds as well as reactive intermediates: (a) Park, J . H.; Koh, J .
H.; Park, J . Organometallics 2001, 20, 1892. (b) Grushin, V. V.; Alper,
H. Organometallics 1993, 12, 3846 and references therein.
(15) In a separate experiment, a radical scavenger 2,4,6-trimeth-
ylphenol did not inhibit the catalytic oxidation of 1-phenylethanol.
(16) (a) Mays, M. J .; Morris, M. J .; Raithby, P. R.; Shvo, Y.; Czarkie,
D. Organometallics 1989, 8, 1162. (b) Casey, C. P.; Singer, S. W.;
Powell, D. R.; Hayashi, R. K.; Kavana, M. J . Am. Chem. Soc. 2001,
123, 1090.
(12) Csjernyik, G.; EÄ ll, A. H.; Fadini, L.; Pugin, B.; Ba¨ckvall, J .-E.
J . Org. Chem. 2002, 67, 1657 and references therein.

5676 Organometallics, Vol. 21, No. 25, 2002
Notes
Ta ble 2. Ca ta lytic Oxid a tion of Va r iou s Alcoh olsa
Ta ble 3. Cr ysta llogr a p h ic Da ta for 2
formula
mol wt
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C31H21ClO3Ru
578.00
P21/n
11.7387(3)
8.1410(2)
27.1190(7)
90
101.5400(10)
90
2539.23(11)
4
1.512
Z
d
calcd, g cm-3
radiation (λ, Å)
0.710 73
1168
0.754
233(2)
ω
F(000)
µ, mm-1
T, K
scan mode
no. of measd rflns
no. of indep rflns
refinement mothod
goodness of fit on F2
final R indices (I > 2σ(I))
R indices (all data)
10162
4057
full-matrix least-squares on F2
1.171
R1 ) 0.0467, wR2 ) 0.1243
R1 ) 0.0621, wR2 ) 0.1379
rings are produced selectively from the corresponding
primary diols in high yields (entries 12-15). Meanwhile,
a complex mixture is obtained in the reaction of pen-
tane-1,4-diol as an example of unsymmetrical diols.
5-Methyldihydrofuran-2-one and 5-hydroxypentan-2-one
1
are observed as the major products by H NMR (entry
16).
In summary, we have found the clean transformation
of the Shvo complex (1) into the hydroxycyclopentadi-
enyl ruthenium chloride 2. A simple and efficient
method for the catalytic oxidation of alcohols has been
demonstrated with the use of 1 or 2 as the catalyst,
chloroform as the stoichiometric oxidant, and sodium
carbonate as the acceptor of hydrogen chloride.
Exp er im en ta l Section
a
Perfomed with an alcohol (1.0 mmol), 2 (2.0 mol %), Na2CO3
All manipulations except workup and purification were
carried out under an atmosphere of argon with the use of
standard Schlenk techniques. The Shvo complex was prepared
according to the literature procedures.2 If not otherwise stated,
all NMR spectra were recorded on a Bruker AM 300 or DPX
300 instrument. Chemical shifts are given in δ (ppm) downfield
from tetramethylsilane as an internal standard. IR spectra
were taken for KBr pellets. Mass spectral analysis was
(1.5 equiv), and degassed CHCl3 (8 mL) in a closed vessel heated
b
to 90 °C. The 1H NMR data of the products are identical with
those reported previously. c The isolated yield of the product. The
d
percent conversion of the substrate is given in parentheses. The
percent conversions of 1-(p-methoxyphenyl)ethanol, 1-phenyletha-
nol, and 1-(p-chlorophenyl)ethanol are 97, 75, and 65, respectively,
after 4 h. e The Shvo complex (1.0 mol %) was used instead of 2.
g
f 16 mL of CHCl3 and 4 mol % of 2 were used. Benzyl benzoate
h
was isolated in 8% yield. The yield of octanal was estimated by
recorded on
a J EOL J MS-AM505WA instrument and is
i
j
1H NMR. 16 mL of CHCl3 was used. 5-Methyldihydrofuran-2-
one, 5-hydroxypentan-2-one, 4-oxopentyl 4-oxopentanoate, and
4-oxopentanal were observed in the mole ratio of 42:37:16:5 by
1H NMR.
reported in units of mass to charge (m/e). Melting points were
measured on a Thomas-Hoover capillary melting point ap-
paratus and are uncorrected. Elemental analyses were per-
formed by the Center for Biofunctional Molecules at Pohang
University of Science and Technology.
olefinic group remote from the hydroxy group remains
intact under the oxidation conditions (entry 7). However,
a mixture of 2-pentanone and pent-3-en-2-one is ob-
tained from the reaction of pent-4-en-2-ol through the
migration of the carbon-carbon double bond (entry 8).
(-)-Menthol is successfully transformed to (-)-men-
thone in high yield (entry 9) while maintaining the
stereochemistry at the 2-position. The catalytic system
is less effective for the transformation of primary
alcohols to aldehydes because of the competitive forma-
tion of esters (entries 10 and 11).2 Octyl octanoate (51%
yield) is the major product in the dehydrogenation of
1-octanol, although benzaldehyde is obtained from ben-
zyl alcohol in good yield by decreasing the concentration.
Notably, however, lactones of five- to seven-membered
(η5-P h 4C4COH)Ru (CO)2Cl (2). In a 50 mL flask equipped
with a grease-free high-vacuum stopcock, 1 (200 mg, 0.184
mmol) was dissolved in degassed HPLC grade chloroform (9.0
mL), containing ethanol as the stabilizer, under argon. After
the flask was closed, the solution was stirred at 90 °C for 7 h.
A pale yellow solid (213 mg, 100% yield) was obtained after
the solvent was removed under high vacuum. Mp: 152 °C dec.
1H NMR (CDCl3): δ 7.44-6.81 (m, 20H), 5.50 (br s, 1H). 13C
NMR (CDCl3): δ 197.7, 139.8, 132.2, 131.9, 129.5, 129.12,
129.07, 128.4, 128.1, 100.3, 89.6. IR (KBr, cm-1): ν(CO) 2033
(s), 1983 (s). MS (FAB, m/z): 578 (M+). Anal. Calcd for
C
31H21O3ClRu: C, 64.42; H, 3.66. Found: C, 64.13; H, 3.83.
X-r a y Cr ysta llogr a p h y of (η5-P h 4C4COH)Ru (CO)2Cl. A
suitable crystal coated with Paratone was mounted on a
Siemens SMART diffractometer equipped with a graphite-
monochromated Mo KR (λ ) 0.710 73 Å) radiation source and

Notes
Organometallics, Vol. 21, No. 25, 2002 5677
a CCD detector. Data collection was performed with a detector
distance of 6 cm. The raw data collected were processed to
produce conventional intensity data by the program SAINT.
The intensity data were corrected for Lorentz and polarization
effects. The structure was solved by a combination of Patterson
and difference Fourier methods provided by the program
package SHELXTL. All the non-hydrogen atoms were refined
anisotropically. Hydrogen atom positions were calculated and
included in the final cycle of refinement. Crystallographic data
of 2 are summarized in Table 3.
was concentrated and chromatographed on a silica gel column
to give acetophenone (118 mg, 98% yield).
Ack n ow led gm en t. We are grateful for the financial
support by MOST through the National Research
Laboratory of Chirotechnology and by KOSEF through
the Center for Integrated Molecular System. H.M.J . is
grateful for his postdoctoral fellowship through the
Brain Korea 21 Project in 2001 by the Korean Ministry
of Education.
Rep r esen ta tive P r oced u r e for th e Oxid a tion of Alco-
h ols. In a 100 mL flask equipped with a grease-free high-
vacuum stopcock, 2 (12 mg, 0.021 mmol), Na2CO3 (159 mg,
1.50 mmol), and 1-phenylethanol (121 µL, 1.00 mmol) were
mixed with degassed HPLC grade CHCl3 (8 mL). After the
flask was closed, the mixture was stirred at 90 °C for 8 h. The
reaction mixture was filtered through Celite, and the filtrate
Su p p or tin g In for m a tion Ava ila ble: Structural diagrams
with full atom labeling and tables of bond distances, angles,
anisotropic parameters, and atomic coordinates for 2. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM020516M