Preferential bond activation of sp3 C-H over sp2 C-H in α,β-unsaturated carboxylic acids by ruthenium complex

Article abstract of DOI:10.1246/cl.2001.1284

Reactions of Ru(1,5-cod)(1,3,5-cot) (1)/PMe₃ [cod = cyclooctadiene, cot = cyclooctatriene] with propenoic acids (CH₂=CH(R)COOH) give unsaturated ruthenalactones Ru[OC(O)C(R)=CH-κ²O,C](PMe₃]₄ [R = Me (

Full text of DOI:10.1246/cl.2001.1284

1284
Chemistry Letters 2001
Preferential Bond Activation of sp3 C–H over sp2 C–H in α,β-Unsaturated
Carboxylic Acids by Ruthenium Complex
Susumu Kanaya, Nobuyuki Komine, Masafumi Hirano, and Sanshiro Komiya*
Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology,
2-24-16 Nakacho, Koganei, Tokyo 184-8588
(Received September 13, 2001; CL-010907)
Reactions of Ru(1,5-cod)(1,3,5-cot) (1)/PMe3 [cod =
cyclooctadiene, cot = cyclooctatriene] with propenoic acids
(CH2=CH(R)COOH) give unsaturated ruthenalactones
Ru[OC(O)C(R)=CH-κ 2O,C](PMe3)4 [R = Me (2a), Et (2b), Pr
i
(2c), Pr (2d)]. In contrast, reactions of trans-2-methyl-2-
butenoic acid and 2-methylcinnamic acid (R'CH=C(Me)COOH)
give Ru[OC(O)C(CH2R')=CH-κ2O,C](PMe3)4 [R' = Me (2b), Ph
(2e)] as major products, suggesting the preferential activation of
the sp3 C–H over sp2 C–H bond on ruthenium(II) center.
Much attention has been currently paid for C–H bond activa-
tion by transition-metal complexes1–3 because of its potential
applications in environmentally benign organic synthesis. It is
generally accepted that transition metal complexes favor to
cleave sp2 C–H bond than sp3 C–H bond, though the former bond
dissociation energy is slightly larger (8–14 kJ/mol) than the lat-
ter.4 This trend is accounted for both kinetic and thermodynamic
reasons such as prior π-coordination of adjacent C=C bond to
metal, and stability of the resulting M–C(sp2) bond in the prod-
ucts.1b,3 However, the selectivity control in transition metal
mediated C–H activation is still limited and is rather unsettled.
We previously reported successive O–H and sp3 C–H bond
activation of ortho substituents of phenols by Ru(cod)(cot) (1)
in the presence of tertiary phosphines to give novel 5-mem-
bered oxaruthenacycle complexes.5 In these reactions, protona-
tion of 1 by phenols is a crucial step to provide an anchoring
phenoxo ligand, which would force the ortho-methyl C–H bond
close to the metal center.5,6 In a similar vein, unsaturated car-
boxylic acids are subjected to the reaction with 1/PMe3 to
cleave C–H bonds in these acids giving ruthenalactones.7,8 In
the reactions of 2-alkyl-2-alkenoic acids, both sp2 and sp3 C–H
bonds are susceptible to be cleaved as shown in the following
Chart. We now found preferential sp3 C–H bond activation
over sp2 bond in 2-alkyl-2-alkenoic acids by 1/PMe3 giving
unsaturated ruthenalactones.
The 31P{1H} NMR spectrum of 2a shows an AM2X pattern
and the H NMR spectrum shows one virtual triplet and two
1
doublets assignable to PMe3 ligands trans to each other and two
magnetically inequivalent PMe3, respectively, indicating the cis
configuration of 2a in an octahedral geometry. The resonances
1
at 7.86 (br) and 2.43 (s) ppm in the H NMR are assigned to
alkenyl and 2-methyl protons, respectively. In the IR spectrum
of 2a, a strong absorption band due to stretching vibration of
the carbonyl group appears at 1577 cm–1.8 Acidolysis of 2a by
HCl gave methacrylic acid in 86% yield. Selective deuteration
of alkenyl proton trans to the Me was observed on reaction of
2a with DCl, suggesting that the β-carbon is directly bonded to
ruthenium giving 5-membered ruthenacycle structure formed
by the regioselective C–H bond cleavage.
In order to shed some light on the mechanism, the time-
course of this reaction was monitored by NMR. At the initial
stage of the reaction (12 h, at 50 °C), an A2X2 pattern assigned
to cis-Ru[OC(O)C(Me)=CH2]2(PMe3)4 (3a)10 was observed in
NMR (25% NMR yield) together with free 1,5-COD and 1,3-
COD (91% and 22% NMR yields, respectively). Then, signals
due to 3a gradually decreased, and instead an AM2X pattern of
2a increased with concomitant formation of two broad peaks
due to (aqua)bis(carboxylato)ruthenium(II) complex
Ru[OC(O)C(Me)=CH2]2(H2O)(PMe3)3 (4a) in 31P{1H} NMR
(24% NMR yield).11
Addition of PMe3 (10 equiv) increased the yield of 2a
without retardation of reaction (69% NMR yield), while addi-
tion of water (ca. 1 equiv.) in this system enhanced the forma-
tion of 4a (36% NMR yield). Thus, 2a is considered to be
obtained via an 18 e complex 3a without liberation of PMe3.
The reaction proceeds via divalent species and one of the
methacrylato ligands in 3a acted as a hydrogen acceptor
(Scheme 1).
Reaction of 1 with methacrylic acid at 70 °C in the pres-
ence of PMe3 resulted in the successive activation of O–H and
C–H bonds to give an unsaturated ruthenalactone cis-
Ru[OC(O)C(Me)=CH-κ2O,C](PMe3)4 (2a) (31% NMR yield)
accompanied by liberation of 1,5-COD and 1,3-COD (92% and
29% NMR yields)9 (Scheme 1).
Of particular interest is the reaction of 1 with (E)-2-methyl-
2-butenoic acid in the presence of PMe3. After 5 days at 70 °C
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1285
This study was financially supported by Industrial
Technology Research Grant Program from the New Energy and
Industrial Technology Development Organization (NEDO) of
Japan and Grant-in-Aid for Scientific Research from the Ministry
of Education, Culture, Sports, Science and Technology, Japan.
References and Notes
1
2
3
a) A. E. Shilov, “Activation of Saturated Hydrocarbons by Transition
Metal Complexes,” D. Reidel: Dordrecht, (1984). b) R. H. Crabtree,
Chem. Rev., 85, 245 (1985). c) E. P. Wasserman, C. B. Moore, and
R. G. Bergman, Science, 255, 315 (1992). d) “Homogeneous
Transition Metal Catalyzed Reactions,” Advances in Chemistry 230,
eds. by W. R. Moser and D. W. Slocum, American Chemical Society,
Washington, D.C. (1992). e) B. A. Arndtsen, R. G. Bergman, T. A.
Mobley, and T. H. Peterson, Acc. Chem. Res., 28, 154 (1995).
a) S. Komiya, T. Ito, M. Cowie, A. Yamamoto, and J. A. Ibers, J. Am.
Chem. Soc., 98, 3874 (1976). b) E. J. Moore, W. R. Pretzer, T. J.
O’Connell, J. Harris, L. LaBountry, L. Chou, and S. S. Grimmer, J.
Am. Chem. Soc., 114, 5888 (1992). c) S. Murai, F. Kakiuchi, S.
Sekine, Y. Tanaka, A. Kamatani, M. Sonoda, and N. Chatani, Nature,
366, 529 (1993). d) S. Murai, J. Synth. Org. Chem., Jpn., 52, 992
(1994). e) M. Sonoda, F. Kakiuchi, A. Kamatani, N. Chatani, and S.
Murai, Chem. Lett. 1996, 109.
a) C. L. Hill, “Activation and Functionalization of Alkanes,” Wiley,
New York, 1989. b) W. D. Jones and F. J. Feher, Acc. Chem. Res.,
22, 91 (1989). c) W. D. Jones and F. J. Feher, J. Am. Chem. Soc.,
107, 620 (1984). d) J. Halpern, Inorg. Chim. Acta, 100, 41 (1985).
A. L. Castelhano and D. Griller, J. Am. Chem. Soc., 104, 3655 (1982).
a) M. Hirano, N. Kurata, T. Marumo and S. Komiya,
Organometallics, 17, 501 (1998). b) M. Hirano, N. Kurata, T.
Marumo, and S. Komiya, J. Organomet. Chem., 607, 18 (2000).
a) G. Vitulli, P. Pertici, and C. Bigelli, Gazz. Chim. Ital., 115, 79
(1985). b) F. Bouachir, B. Chaudret, and I. Tkatchenko, J. Chem.
Soc., Chem. Commun, 1986, 94. c) K. Osakada, G. A. Grohmann,
and A. Yamamoto, Organometallics, 9, 2092 (1990).
4
5
6
7
in benzene, an analogous but unexpected product cis-
Ru[OC(O)C(Et)=CH-κ2O,C](PMe3)4 (2b) was obtained as a
major product in 80% yield with concomitant formation of
Ru[OC(O)C(Me)=C(Me)-κ2O,C](PMe3)4 (6b) in 10% yield
(Scheme 2).12 Similar treatment with (E)-2-methylcinnamic
acid also gave analogous products 2e and 6e in 70% and 30%
NMR yields, respectively.13 31P{1H} NMR of 2b and 2e
Analogous reactions of 1/PMe3 and 1/PPh3 with 3-butenoic acid are
reported to give Ru(η5-C8H11)[OC(O)CH2CH=CH2-κO, η2-C3,C4]-
(PMe3) [ref 6c] and Ru[OC(O)C3H4-κO, η3-C2,C2,C3](PPh3)2 [ref 8],
respectively.
8
9
K. Sano, T. Yamamoto, and A. Yamamoto, Z. Naturforsch. 40b, 210
(1985).
2a: Anal. Calcd for C16H40O2P4Ru: C, 39.26; H, 8.24%. Found: C,
39.00; H, 8.03%.
1
showed a similar AM2X pattern to 2a and H NMR indicated
10 3a: 1H NMR (300 MHz, C6D6): δ 1.08 (distorted vt, J = 4.5 Hz, 18H),
1.33 (vt, J = 1.5 Hz, 18H), 2.25 (s, 6H), 5.32 (s, 2H), 6.25 (d, J = 1.6
Hz, 2H). 31P{1H} NMR (122 MHz, C6D6): δ –2.6 (t, J = 32 Hz, 2P),
14.5 (t, J = 32 Hz, 2P).
the existence of ethyl and benzyl moieties, respectively.
Acidolyses of a mixture of 2b and 6b (7:3) with HCl give 2-
ethylpropenoic acid and 2-methyl-2-butenoic acid in 60% and
12% yield, respectively. It should be noted that isomerization
of these substrates was not catalyzed by 1/PMe3 at 70 °C. Thus,
formation of 2b and 2e was consistently interpreted by the pref-
erential sp3 C–H bond cleavage of the 2-methyl group over sp2
C–H on Ru(II) as follows. First of all, these acids react with
1/PMe3 to give cis-bis(carboxylato)ruthenim(II) complex 3.
The sp3 C–H bond of the 2-methyl-2-alkenoato ligand is initial-
ly cleaved to give cis-Ru[OC(O)C(=CHR')CH2-κ 2O,C](PMe3)4
[R' = Me (5b), Ph (5e)], which spontaneously isomerizes to
thermodynamically stable unsaturated ruthenalactone 2b or 2e
by 1,3-shift of the hydrogen atom.
11 4a: 1H NMR (300 MHz, C6D6): δ 1.05 (br s, 27H), 2.09 (s, 6H), 5.25
(t, J = 1.8 Hz, 2H), 6.22 (d, J = 2.7 Hz, 2H), 10.3 (br s, 2H). 31P{1H}
NMR (122 MHz, C6D6): δ 25.9 (br d, J = 38 Hz, 2P), 28.0 (br t, J =
38 Hz, 1P).
12 Recrystallization of these products from THF/hexane gave a mixture
of 2b and 6b in 7:3 molar ratio. 2b: 1H NMR (300 MHz, C6D6): δ
0.90 (d, J = 2.9 Hz, 9H), 0.97 (vt, J = 1.1 Hz, 18H), 1.08 (d, J = 2.7
Hz, 9H), 1.50 (t, J = 3.0 Hz, 3H), 2.83 (br q, J = 3.0 Hz, 2H), 7.89 (br,
1H). 31P{1H} NMR (122 MHz, C6D6): δ –11.7 (td, J = 26, 16 Hz, 1P),
0.16 (dd, J = 35, 26 Hz, 2P), 11.5 (td, J = 35, 16 Hz, 1P). 6b: 1H
NMR (300 MHz, C6D6): δ 2.24–2.27 (m, 6H) signals due to PMe3
ligands are overlapped with those of 2b. 31P{1H} NMR (122 MHz,
C6D6): δ –14.6 (td, J = 24, 15 Hz, 1P), –0.2 (dd, J = 34, 24 Hz, 2P),
6.0 (td, J = 34, 15 Hz, 1P).
Although 2a is considered to be apparently formed via
cleavage of only sp2 C–H bond of the methacrylato ligand, ini-
tial sp3 C–H activation of the Me group followed by isomeriza-
tion can also explain the reaction pathway. However, when
other 2-alkyl-2-alkenoic acids such as 2-ethylpropenoic acid, 2-
propylpropenoic acid and 2-isopropylpropenoic acid were used
as reactants, only ruthenalactones 2b (23%), 2c (23%) and 2d
(47%)14 were obtained, in which only sp2 C–H bonds were
cleaved. Therefore, steric factor at the α- and β-carbon atoms
seems to be important to control the selectivity between sp2 and
sp3 C–H bond activation at Ru(II).
13 Recrystallization of these products from THF/hexane exclusively
gave 2e. 2e: 1H NMR (300 MHz, C6D6): δ 0.87 (vt, J = 2.7 Hz, 18H),
0.90 (d, J = 7.2 Hz, 9H), 1.07 (d, J = 6.3 Hz, 9H), 3.99 (s, 2H),
7.02–7.68 (m, 5H), 7.89 (br, 1H). 31P{1H} NMR (122 MHz, C6D6): δ
–11.6 (td, J = 26, 16 Hz, 1P), –0.1 (dd, J = 35, 26 Hz, 2P), 11.4 (td, J
= 35, 16 Hz, 1P). 6e: 31P{1H} NMR (122 MHz, C6D6): δ –15.6 (td, J
= 24, 15 Hz, 1P), 6.3 (td, J = 30, 15 Hz, 1P), one of phosphorus signal
is overlapped with the major signals.
1
14 2c as representative data: H NMR (300 MHz, C6D6): δ 0.88 (d, J =
7.5 Hz, 9H), 0.96 (vt, J = 2.7 Hz, 18H), 1.08 (d, J = 6.6 Hz, 9H), 1.22
(t, J = 7.5 Hz, 3H), 1.99 (qt, J = 7.5 Hz, 2H), 2.74 (br t, J = 7.2 Hz,
2H), 7.92 (br, 1H). 31P{1H} NMR (122 MHz, C6D6): δ –11.7 (br td, J
= 25, 16 Hz, 1P), 0.3 (dd, J = 35, 25 Hz, 2P), 11.3 (td, J = 35, 16 Hz,
1P). IR (KBr, cm–1): 1580 (vs, νC=O), 945 (vs, νC–O).