The ruthenium-catalyzed reductive decarboxylation of esters: Catalytic reactions involving the cleavage of Acyl-Oxygen bonds of esters [2]

Full text of DOI:10.1021/ja0103501

J. Am. Chem. Soc. 2001, 123, 4849-4850
4849
Scheme 1. Two Modes of the Cleavage of C-O Bonds of
Esters by Transition Metal Complexes
The Ruthenium-Catalyzed Reductive
Decarboxylation of Esters: Catalytic Reactions
Involving the Cleavage of Acyl-Oxygen Bonds of
Esters
Naoto Chatani, Hiroto Tatamidani, Yutaka Ie,
Fumitoshi Kakiuchi, and Shinji Murai*
Department of Applied Chemistry, Faculty of Engineering
Scheme 2. Working Hypothesis
Osaka UniVersity, Suita, Osaka 565-0871, Japan
ReceiVed February 8, 2001
The cleavage of a carbon-oxygen bond promoted by transition
metal complexes has become an important process in organic
synthesis.1,2 The majority of these reactions reported thus far
involve the cleavage of an alkyl-O bond (alkyl fission) in esters
(Scheme 1). A typical example is the Pd-catalyzed reactions of
allyl acetates with nucleophiles, in which allyl-O bonds are
cleaved.3 The catalytic cleavage of benzyl-O or vinyl-O bonds
is also well-known.1 In contrast, the cleavage of an acyl-O bond
(acyl fission) in esters, catalyzed by transition metal complexes,
is very rare, and only a few stoichiometric reactions have been
reported to date. Yamamoto reported on the stoichiometric
reaction of phenyl acetate with Ni(cod)2 in the presence of
bipyridine. In this reaction, a methylnickel phenoxide complex
is formed via the oxidative addition of the acetyl-O bond of
phenyl acetate to the nickel complex followed by decarbonyla-
tion.4 Other complexes, such as Mo(N2)2(dppe)2,5 HRh(PPh3)4,6
and Ru(CO)2(PPh3)3,7 have also been found to promote the
cleavage of acyl-O bonds of esters, with the ester carbonyl group
being ultimately converted to a CO ligand, which is attached to
the metal. It is noteworthy in this respect that a related acyl
complex was isolated from the reaction of aryl acetate with a
Rh(I) complex by Grotjahn.8 Recently, Yamamoto and co-workers
reported that electronicallyl activated esters, such as aryl trifluo-
roacetates, oxidatively add to Pd(0) under mild conditions to give
the corresponding (aryloxo)(trifluoroacetyl)palladium complexes.9
More recently, they also reported on the Pd-catalyzed reaction
of aryl trifluoroacetates with arylboronic acids, leading to aryl
trifluoromethyl ketones.10 To our knowledge, this represents the
first example of a catalytic reaction, which involves the cleavage
of an acyl-O bond of an ester by transition metals.11,12 As a result
of recent studies in this laboratory,13-15 we have discussed a new
catalytic transformation of esters, which involves the cleavage
of an acyl C-O bond in esters, and we report herein on the Ru3-
(CO)12-catalyzed reductive decarboxylation of esters. Ammonium
formate was used as the reducing reagent.
Our strategy is based on the methodology that heteroatom-
directing groups promote the site-selective cleavage of unreactive
bonds, as shown in Scheme 2. We have previously reported on
catalytic reactions which involve the site-selective cleavage of
C-H,13 C-C,14 and C-F15 bonds by taking advantage of the
coordination of the directing group to transition metals. These
observations prompted us to examine the possibility that this
approach might be used in the cleavage of C-O bonds in esters.
The coordination of the pyridine nitrogen in ester I to ruthenium
renders the metal more nucleophilic. The cleavage of a C-O bond
takes place via a tetrahedral intermediate III to give an acyl
ruthenium complex IV, which reacts with pronucleophiles to give
a product and 2-pyridinemethanol, along with the regenerated
ruthenium. 2-Pyridylmethyl 2-naphthalenecarboxylate (1) was
chosen as the test substrate and Ru3(CO)12 as the catalyst. Among
the wide variety of pronucleophiles examined, we found that
ammonium formate, HCOONH4, can be used as a nucleophile,
resulting in a clean reduction reaction, but the expected aldehydes
were not obtained. The reaction of 1 (0.2 mmol) and HCOONH4
(0.6 mmol) in the presence of Ru3(CO)12 (0.01 mmol) in toluene
(0.6 mL) at 160 °C for 40 h gave naphthalene (2) in 55% GC
yield, along with 17% of unreacted 1. The corresponding benzyl
ester did not react with HCOONH4, indicating that the presence
of the nitrogen in the substrate is essential for the reaction to
proceed. The 1H NMR spectrum indicated that 2-pyridinemethanol
is formed in a yield comparable to that of 2. Of the solvent
examined, dioxane is the solvent of choice, although the type of
solvent had no significant effect on the yield of 2 (dioxane 95%,
PrCN 71%, CH3CONMe2 84%, toluene 55%). The use of other
(1) For recent reviews on the cleavage of ester bonds, see: Lin, Y.-S.;
Yamamoto, A. In ActiVation of UnreactiVe Bonds and Organic Synthesis;
Murai, S., Ed.; Springer: Berlin, 1999; pp 161-192. Yamamoto, A. AdV.
Organomet. Chem. 1992, 34, 111.
(2) For a recent review paper on the cleavage of ether bonds, see: van der
Boom, M. E.; Liou, S.-Y.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. J.
Am. Chem. Soc. 1998, 120, 6531.
(3) Tsuji, J. Palldium Reagents and Catalysts; Wiley: Chichester, U.K.,
1995; pp 290-422. Harrington, P. J. In ComprehensiVe Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon:
Oxford, U.K., 1995; Vol. 12, pp 797-904.
(4) Yamamoto, T.; Ishizu, J.; Kohara, T.; Komiya, S.; Yamamoto, A. J.
Am. Chem. Soc. 1980, 102, 3758.
(5) Tatsumi, T.; Tominaga, H.; Hidai, M.; Uchida, Y. J. Organomet. Chem.
1981, 218, 177.
(6) Yamamoto, T.; Miyashita, S.; Naito, Y.; Komiya, S.; Ito, T.; Yamamoto,
A. Organometallics 1982, 1, 808.
(7) Hiraki, K.; Kira, S.; Kawano, H. Bull. Chem. Soc. Jpn. 1997, 70, 1583.
(8) Grotjahn, D. B.; Joubran, C. Organometallics 1995, 14, 5171.
(9) Nagayama, K.; Shimizu, I.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1999,
72, 799.
(10) Kakino, R.; Shimizu, I.; Yamamoto, A. Bull. Chem. Soc. Jpn. 2001,
74, 371.
(11) Catalytic reactions involving the cleavage of C-O bonds of acid
anhydrides or â-lactones are known. For recent papers, see: Nagayama, K.;
Kawataka, F.; Sakamoto, M.; Shimizu, I.; Yamamoto, A. Chem. Lett. 1995,
367. Kokubo, K.; Miura, M.; Nomura, M. Organometallics 1995, 14, 4521.
Stephan, M. S.; Teunissen, A. J. J. M.; Verzijl, G. K. M.; de Vries, J. G.
Angew. Chem., Int. Ed. 1998, 37, 662. Nagayama, K.; Shimizu, I.; Yamamoto,
A. Chem. Lett. 1998, 1143. Carmichael, A. J.; Earle, M. J.; Holbrey, J. D.;
McCormac, P. B.; Seddon, K. R. Org. Lett. 1999, 1, 997.
(12) For recent papers on catalytic reaction involving the cleavage of
C(O)-S bonds of esters, see: Liebeskind, L. S.; Srogl, J. J. Am. Chem. Soc.
2000, 122, 11260. Shimizu, T.; Seki, M. Tetrahedron Lett. 2001, 42, 429.
(13) Chatani, N.; Fukuyama, T.; Tatamidani, H.; Kakiuchi, F.; Murai, S.
J. Org. Chem. 2000, 65, 4039 and reference cited therein.
(14) Chatani, N.; Ie, Y.; Kakiuchi, F.; Murai, S. J. Am. Chem. Soc. 1999,
121, 8645.
(15) Ishii, Y.; Chatani, N.; Yorimitsu, S.; Murai, S. Chem. Lett. 1998, 157.
10.1021/ja0103501 CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/26/2001

4850 J. Am. Chem. Soc., Vol. 123, No. 20, 2001
Communications to the Editor
Table 1. The Ru3(CO)12-Catalyzed Reaction of Esters with
HCOONH4
hydrogenation reagents, such as HCOONa, i-PrOH, and HSiMe2-
Ph, was not effective.
a
A variety of functional groups can be tolerated in the reaction
as shown in eq 2. Both electron-donating and -withdrawing groups
did not appreciably affect the reaction. However, the reaction of
a 4-nitro ester, such as 4f, resulted in the reduction of the nitro
group to give 2-pyridylmethyl 4-aminobenzoate in 27% yield with
no nitrobenzene being obtained. Although an ester group (COOMe)
at the 4-position on the phenyl ring was tolerated, the use of a
cyano-substituted ester, such as 4k, resulted in a low yield of
benzonitrile 5k.16 Steric bulkiness around the carbonyl group, as
in 4b, led to a low conversion.
It was anticipated that aldehydes, which are formed via
reduction of esters by HCOONH4, would be produced initially
and then undergo decarbonylation to give arenes under the
reaction conditions used here. To examine this possibility, some
controlled experiments were performed. The reaction of 2-naph-
thalenecarbaldehyde (3) under the reaction conditions identical
to those in eq 2 gave naphthalene in 23% GC yield, along with
a 7% yield of 2-naphthalenemethanol (GC yield was determined
as silyl ether) and a 61% yield of 2-methylnaphthalene, with the
latter two products being obtained by reduction of 3 by HCOONH4.
Treatment of a mixture of 3 and 2-pyridylmethyl 3,4,5-tri-
methoxybenzoate (4d) with HCOONH4 under identical conditions
gave a 91% recovered yield of 4d and 2-naphthalenemethanol
(45% GC yield). The expected 1,2,3-trimethoxybenzene (5d) was
obtained in trace amounts, and naphthalene and 2-methylnaph-
thalene were not detected by GC. These results show that the
present reductive decarboxylation does not involve an aldehyde
intermediate.
The reaction is not limited to a benzene ring system, but it is
applicable to heteroaromatic as well as aliphatic systems, as shown
in Table 1. The reaction of ferrocenecarboxylic ester 6 gave
ferrocene (7) in 56% yield, along with 28% of unreacted 6 (entry
1). The reaction of 8 under the same conditions as those described
in eq 2 gave indole (9) in 23% yield. However, changing solvent
to toluene increased the product yield to 70% (entry 2). It was
found that aliphatic esters 10 and 12 give ethylbenzene efficiently,
and that styrene, which could have been formed by â-hydride
elimination, was not present in the reaction mixture (entries 3
and 4). The reaction was not limited to esters, in which a
2-pyridylmethyl group is attached as the directing group at the
alcohol moiety in the substrates (entries 5-7). These results show
the apparent potential utility of this transformation in terms of
its applications to organic synthesis.
a Reaction conditions: ester (0.2 mmol), HCOONH4 (0.6 mmol),
Ru3(CO)12 (0.01 mmol), dioxane (0.6 mL) at 160 °C for 20 h. R )
2-pyridinylmethyl. b GC yield. c Toluene was used as a solvent.
The present reaction demonstrates a new and rare example of
a catalytic reaction that involves the cleavage of acyl C-O bonds
in esters. As it impacts organic synthesis, the present reaction
represents a new type of reductive decarboxylation in which esters
can be converted to hydrocarbons,18 similar to decarbonylation
of aldehydes, a reaction which is frequently employed in organic
synthesis.19 In addition, a wide variety of functional groups can
be tolerated in the reaction. Further investigation of the catalytic
conversion of esters into carbonyl compounds using pronucleo-
philes is currently under way.
Acknowledgment. This work was supported, in part, by grants from
Monbusho (The Ministry of Education, Science, Sports, and Culture) and
the Japan Society for the Promotion of Science. N.C. wishes to thank to
the Tokuyama Foundation for financial support.
Supporting Information Available: Experimental details (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
JA0103501
We propose that the reaction proceeds via a mechanism which
is similar to that shown in Scheme 1. The reason why the
decarbonylation step from IV is unexpectedly fast is unclear at
present.17
(17) When the reaction was carried out even at 1 atm of CO, no reaction
took place.
(18) Crich, D. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming,
I., Eds.; Pergamon: Oxford, 1991; Vol. 7, pp 720-721.
(16) The reaction of a cyano group by HCOONH4 would have taken place
through the reaction. In fact, we found that the reaction of 2-naphthalenecarb-
nitrile under identical reaction conditions gives 2-methylnaphthalene in 27%
yield and unidentified products.
(19) Colquhoun, H. M.; Thompson, D. J.; Twigg, M. V. Carbonylation:
Direct Synthesis of Carbonyl Compounds; Plenum Press: New York and
London, 1991. For a recent paper, see: Beck, C. M.; Rathmill, S. E.; Park,
Y. J.; Chen, J.; Crabtree, R. H. Organometallics 1999, 18, 5311.