Acid-promoted hydrogen migration in (2-allylphenoxo)ruthenium(II) to form an η3-allyl complex

Article abstract of DOI:10.1021/om800318z

Treatment of RuCP[OC₆H₄(CH₂CH=CH ₂)-κ¹O: η²C,C′](PPh ₃) (3c) with a Bronsted acid (HX) such as 2-al-lylphenol results in facile migration of a benzylic proton to the aryloxide, giving the (η³-allyl)ruthenium(II) complex RuCp[CH₂-CHCH(C ₆H₄OH-2)-η³ C,C′,C″](PPh ₃) (Ac). Thermodynamic and kinetic studies suggest that 3c associates with acid to give 3c · HX, and further addition of HX to 3c · HX causes the C-H bond cleavage reaction to give 4c.

Full text of DOI:10.1021/om800318z

Organometallics 2008, 27, 3635–3638
3635
Acid-Promoted Hydrogen Migration in
(2-Allylphenoxo)ruthenium(II) To Form an η3-Allyl Complex
Masafumi Hirano,* Toshinori Kuga, Mariko Kitamura, Susumu Kanaya, Nobuyuki Komine,
and Sanshiro Komiya*
Department of Applied Chemistry, Graduate School of Engineering, Tokyo UniVersity of Agriculture and
Technology, 2-24-26 Nakacho, Koganei, Tokyo 184-8588, Japan
ReceiVed April 8, 2008
and RuCp(NR2)(dcype) were reported.6 Thus, we started to
prepare a series of RuCp(OAr)(PPh3)2 complexes. During the
course of this study, we found that Brφnsted acids promoted
an intramolecular C-H bond cleavage reaction of the (aryloxo)ru-
thenium(II) complex RuCp[OC6H4(CH2CHdCH2-2)-κ1O](PPh3)2
(2c), leading to RuCp[CH2CHCH(C6H4OH-2)-η3C,C′,C′′]-
(PPh3) (4c) under ambient conditions. Herein we report a new type
of acid-promoted C-H bond cleavage reaction.
Summary: Treatment of RuCp[OC6H4(CH2CHdCH2-2)-κ1O:
η2C,C′](PPh3) (3c) with a Brønsted acid (HX) such as 2-al-
lylphenol results in facile migration of a benzylic proton to the
aryloxide, giVing the (η3-allyl)ruthenium(II) complex RuCp[CH2-
CHCH(C6H4OH-2)-η3C,C′,C′′](PPh3) (4c). Thermodynamic and
kinetic studies suggest that 3c associates with acid to giVe
3c · HX, and further addition of HX to 3c · HX causes the C-H
bond cleaVage reaction to giVe 4c.
Metathetical reactions of chlorobis(triphenylphosphine)ru-
thenium(II) (1) with potassium aryloxide in THF under ambient
conditions gave the corresponding (aryloxo)(cyclopentadienyl)-
bis(triphenylphosphine)ruthenium(II) complexes Ru(OAr)Cp-
(PPh3)2 (Ar ) C6H5 (2a), C6H3Me2-2,4 (2b), 2-allylphenyl (2c),
2-propenylphenyl (2d), 2-propylphenyl (2e), 1-naphthyl (2f))
in moderate to high yields (eq 1).7 These complexes were
characterized by 1H and 31P{1H} NMR, IR, and elemental
analysis.8 It is notable that a pioneering work concerning the
reaction of RuCpClL2 with NaOMe or NaOEt was reported to
give RuCpHL2 by facile ꢀ-hydrogen elimination from the
putative (alkoxo)ruthenium(II).9
C-H bond cleavage reactions by ruthenium complexes
have been extensively studied, due to their potential utility
in organic synthesis.1 One of the important factors for C-H
bond cleavage reaction is prior coordination or binding
through an anchoring group to bring the unactivated C-H
bond in proximity to the ruthenium center. We previously
reported a series of reactions of Ru(η4-1,5-COD)(η6-1,3,5-
COT) with ortho-substituted phenols in the presence of PMe3,
where protonation of the 1,3,5-COT ligand immediately took
place to give the (η5-cyclooctadienyl)ruthenium(II) species
Ru(η5-C8H11)(SC6H3Me2-2,6)(PMe3)2 and [Ru(η5-C8H11)-
(PMe3)3]+[OC6H3Me2-2,6]-.2 They were key intermediates
for the C-H bond cleavage reactions, giving cis-Ru[YC6H3(2-
CH2)(6-Me)-κ2Y,C](PMe3)4 (Y ) S, O). The C-H bond
cleavage reaction in the (η5-cyclooctadienyl)ruthenium(II)
species prompted us to study isoelectronic (η5-cyclopenta-
dienyl)ruthenium(II) species. To our surprise, however,
mononuclear (aryloxo)ruthenium(II) complexes having a Cp
ligand were unprecedented, while limited numbers of related
complexessuchasRuCp*(OPh)(PMe3)2,3RuTp(OPh)(PMe3)2,4,5
The molecular structure of 2c has been unequivocally
determined by single-crystal X-ray diffraction, giving the
formulation as RuCp[OC6H4(CH2CHdCH2-2)-κ1O](PPh3)2 (2c)
with no apparent interaction of the allyl moiety with the
ruthenium center.10
When complex 2c was dissolved in benzene-d6, extensive
dissociation of a PPh3 ligand was observed, giving RuCp[OC6H4-
(6) Reviews of monomeric alkoxo and aryloxoruthenium complexes: (a)
Bergman, R. G. Polyhedron 1995, 14, 3227. (b) Bennett, M. A.; Khan, K.;
Wenger, E. In ComprehensiVe Organometallic Chemistry II; Abel, W.,
Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1995; Vol.
7, Chapter 8, p 490.
* To whom correspondence should be addressed. Tel and fax: +81 423
877 500. E-mail: hrc@cc.tuat.ac.jp (M.H.); komiya@cc.tuat.ac.jp.
(1) (a) Shilov, A. E. The ActiVation of Saturated Hydrocarbons by
Transition Metal Complexes; Reidel: Dordrecht, The Netherlands, 1984.
(b) Crabtree, R. H. Chem. ReV. 1985, 85, 245. (c) Wasserman, E. P.; Moore,
C. B.; Bergman, R. G. Science 1992, 255, 315. (d) Homogeneous Transition
Metal Catalyzed Reactions; Moser, W. R.; Slocum, D. W., Eds.;Advances
in Chemistry 230; American Chemical Society: Washington, D.C., 1992.
(e) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Påeterson, T. H. Acc.
Chem. Res. 1995, 28, 154.
(2) (a) Hirano, M.; Kurata, N.; Marumo, T.; Komiya, S. Organometallics
1998, 17, 501. (b) Hirano, M.; Kurata, N.; Komiya, S. J. Organomet. Chem.
2000, 607, 18. (c) Hirano, M.; Sakaguchi, Y.; Yajima, T.; Kurata, N.;
Komine, N.; Komiya, S. Organometallics 2005, 24, 4799. (d) Hirano, M.;
Sato, H.; Kurata, N.; Komine, N.; Komiya, S. Organometallics 2007, 26,
2005.
(7) Reaction of 1 with the potassium salt of 2,6-xylenol under the same
conditions gave a complex mixture involving RuHCp(PPh3)2. This reaction
will be reported elsewhere.
(8) As a typical example, the characterization of 2c is described. 2c: 1H
NMR (300 MHz, C6D6, 30.0 °C) δ 3.03 (d, 3JH-H ) 6.9 Hz, 2H, benzylic
3
CH2), 4.21 (s, 5H, Cp), 5.04 (d, JH-H ) 9.9 Hz, 1H, CH2d), 5.06 (d,
3JH-H ) 16.5 Hz, 1H, CH2d), 6.11 (ddt, 3JH-H ) 16.5, 3JH-H ) 9.9, 3JH-
H ) 6.9 Hz, 1H, dCH), 7.6 (m, 6H, PPh3), other aromatic resonances
obscured by other aromatic protons in 3c and free PPh3. 31P{1H} NMR
(122 MHz, C6D6): δ 40.4 (s). Complex 2c constitutes an equilibrium mixture
with the monophosphine complex 3c, having a π coordination of the alkenyl
group in benzene-d6 solution (see ref 11). IR (KBr, cm-1): 3051 (m), 2985
(w), 2869 (w), 1957 (vw), 1894 (vw), 1823 (vw), 1634 (w), 1584 (s), 1479
(s), 1469 (s), 1443 (s), 1281 (s), 1088 (m), 741 (s), 695 (s), 529 (s), 516(s).
Anal. Calcd for C50H44OP2Ru: C, 72.89; H, 5.38. Found: C, 72.93; H, 5.14.
(9) Bruce, M. I.; Humphrey, M. G.; Swincer, A. G.; Wallis, R. C. Aust.
J. Chem. 1984, 37, 1747.
(3) Bryndza, H. E.; Domaille, P. J.; Paciello, R. A.; Bercaw, J. E.
Organometallics 1989, 8, 379.
(4) Feng, Y.; Lail, M.; Foley, N. A.; Gunnoe, T. B.; Barakat, K. A.;
Cunddari, T. R.; Petersen, J. L. J. Am. Chem. Soc. 2006, 128, 7982.
(5) Gunnoe, T. B. Eur. J. Inorg. Chem. 2007, 1185, and references cited
therein.
j
(10) Crystallographic and physical data for 2c: triclinic, P1 (No. 2), a
) 13.898(3) Å, b ) 15.542(4) Å, c ) 10.008(2) Å, R ) 96.38(2)°, ꢀ )
110.60(2)°, γ ) 88.54(2)°, V ) 2010.6(9) Å3, Z ) 2. R (Rw) ) 0.0483
(0.0599) and GOF ) 1.101.
10.1021/om800318z CCC: $40.75
2008 American Chemical Society
Publication on Web 07/10/2008

3636 Organometallics, Vol. 27, No. 15, 2008
Communications
Unexpectedly, heating of the equilibrium mixture in toluene
at 100 °C slowly gave rise to the conversion to 4c in 78% yield
for 22 h (Scheme 1).
Scheme 1
(CH2CHdCH2-2)-κ1O:η2C,C′](PPh3) (3c)11 (eq 2). The ratio of
1
The H NMR spectrum of 4c in C6D6 shows typical allylic
signals at δ 1.01 (ddd, 1H, anti-CHH), 1.86 (dd, 1H, anti-CHAr),
2.97 (dd, 1H, syn-CHH), and 4.62 (tdd, 1H, central-CH).12 The
hydroxo group resonates at δ 8.6 as a broad peak, which
disappears on addition of excess D2O. The molecular structure
of 4c shows a syn arrangement of the 2-hydroxyphenyl group
and exo (prone) orientation of the allyl fragment.13 Such an
orientation is also consistent with Jia’s DFT calculations of
RuCp(η3-C3H5)(PH3), where the exo (prone) orientation is more
complexes 2c and 3c and free PPh3 in benzene-d6 at 25 °C was
estimated as 9:91:92 on the basis of the 31P{1H} NMR spectrum.
The 1H NMR spectrum of 3c shows five correlated resonances
at δ 1.86 (dd, 1H), 2.78 (dd, 1H), 3.14 (br dd, 1H), 3.20 (d,
1H), and 5.59 (m, 1H) due to the coordinated 2-allyl moiety,
where the signals at δ 1.86 and 3.14 are assignable to the
diasterotopic benzylic methylene protons. These results evidently
indicate coordination of the 2-allyl moiety to the ruthenium
center. In contrast, signals due to the 2-allyl group in 2c are
observed at δ 3.03 (d, 2H), 5.04 (d, 1H), 5.06 (d, 1H), and 6.11
(ddt, 1H), where the two benzylic methylene protons appear
equivalently at δ 3.03. When 8 equiv of free PPh3 was added
to the benzene solution of the mixture of 2c and 3c, the ratio
(2c/3c) changed to 37/63, suggesting that these compounds were
in equilibrium. The equilibrium constant K1 (K1 ) [3c][PPh3]/
[2c]) was estimated as 3.8 × 10-1 M at 303 K in C6D6.
Thermodynamic parameters were estimated by a van’t Hoff plot
of K1 in the range 25.0-55.0 °C in C6D6 as follows: ∆H ) 29
( 2 kJ mol-1, ∆G298 ) 3 ( 4 kJ mol-1, ∆S ) 87 ( 5 J K-1
mol-1. These data concerning formation of 3c from 2c suggest
that the reaction is slightly endothermic and the positive entropy
change is consistent with liberation of a PPh3 ligand from 2c.
When the solution of a mixture of 2c and 3c in toluene-d8
was warmed above room temperature, the signals at δ 1.8 and
3.1, assignable to the diastereotopic benzylic methylene protons
of 3c, gradually broadened and collapsed into the baseline at
80 °C, while other signals due to 3c remained sharp. Further
warming to 100 °C gave one broad signal around δ 2.3-2.5.
The PPh3 resonance in the 31P{1H} NMR spectrum and the
stable than the endo (supine) one by 22.6 kJ mol-1 14
.
Treatment of the mixture of 2c and 3c with Brønsted acids
enhanced conversion to 4c in C6D6. For example, reaction of
the mixture with 5 equiv of PhOH (pKa ) 10.0) in C6D6 at 50
°C for 55 min gave 4c in 98% yield, with concomitant formation
of a trace amount of RuCp(OPh)(PPh3)2 and 2-allylphenol.
Similar treatment with 5 equiv of 2-allylphenol (pKa ) 10.23)15
for 70 min also produced 4c in 92% yield. With 5 equiv of
CF3CH2OH (pKa ) 12.4) for 3.5 h, 4c was produced in 91%
yield, accompanied by formation of RuCpH(PPh3)2 (12%) and
2-allylphenol (7%) under the same reaction conditions. It is
notable that when the reaction took place in the presence of 5
equiv of CF3CH2OD (90 atom % D), a deuterium atom was
not incorporated in the allylic moiety in 4c at all. With 5 equiv
of MeOH (pKa ) 15.5) and EtOH (pKa ) 16.0) for 1 day, 4c
was obtained in only 19% and 12% yields, respectively. A
Brønsted acid with a low pKa value seems to promote formation
of 4c more effectively. However, a stronger Brønsted acid such
as HCl (pKa ) -7) or HPF6 (pKa ) -20) caused protonolysis
to liberate 2-allylphenol and the isomer 2-propenylphenol
quantitatively.
1
To obtain insight into the mechanism, the H NMR spectra
of 2c/3c (9/91; 0.020 M) were monitored at various concentra-
tions of 2-allylphenol (0-0.20 M) in C6D6. The benzylic
1
signals of olefinic protons in the H NMR spectrum did not
show significant change in this temperature range. This dynamic
process is assignable to the exchange between two enantiomers
based on coordination of different prochiral faces of the 2-allyl
moiety to Ru (i.e., racemization), which arises from rapid
dissociation/site exchange/coordination of the 2-allyl moiety (eq
3). On the basis of the coalescence temperature (353 K) of the
1
3
3
(12) 4c: H NMR (300 MHz, C6D6) δ 1.01 (ddd, JH-P ) 16, JH–H
)
2
3
3
10, JH-H ) 1 Hz, 1H, anti-CHH), 1.86 (dd, JH-H ) 10, JH-P ) 2 Hz,
1H, CHAr), 2.97 (dd, 3JH-H ) 7, 2JH-H ) 2 Hz, 1H a syn-CHH), 4.12 (s,
5H, Cp), 4.62 (tdd, 3JH-H ) 10, 3JH-H ) 7, 3JH-P ) 1 Hz, 1H, central-CH),
6.80 (td, JH-H ) 7, JH-H ) 1 Hz, 1H, 4-C6H4), 7.01 (m, 10H, m- and
p-PPh3), 7.16 (overlapped with signal due to residual signal of deuterated
benzene, C6H4), 7.40 (d, JH-H ) 8 Hz, 1H, 6-C6H4), 7.71 (t, JH-H ) 10
Hz, 6H, o-PPh3), 8.62 (br s, 1H, OH); 31P{1H} NMR (122 MHz, C6D6) δ
63.6 (s). Anal. Calcd for C32H29OPRu: C, 68.44; H, 5.20. Found: C, 68.62;
H, 5.98.
3
4
benzylic methylene protons, the activation free energy ∆Gq
3
3
353
for this process was estimated as 69 kJ mol-1
.
(11) 3c: 1H NMR (300 MHz, C6D6, 18.0 °C) δ 1.84 (dd, 2JH-H ) 12.0,
(13) Crystallographic and physical data for 4c: orthorhombic, P212121
(No. 61), a ) 16.843(4) Å, b ) 19.073(5) Å, c ) 15.619(5) Å, V ) 5011(1)
Å3, Z ) 8. R (Rw) ) 0.0305 (0.0485) and GOF ) 1.244.
(14) Bi, S.; Ariafard, A.; Jia, G.; Lin, Z. Organometallics 2005, 24,
680.
(15) Stradins, J.; Hasanli, B. J. Electroanal. Chem. Interfacial Electro-
chem. 1993, 353, 57.
3
3
3JH-H ) 11.7 Hz, 1H, benzylic CHH), 2.78 (dd, JH-H ) 13.8, JH-P
)
10.8 Hz, 1H, CH2d), 3.14 (br dd, 2JH-H ) 12, 3JH-H ) 4 Hz, 1H, benzylic
3
CHH), 3.20 (d, JH-H ) 7.8 Hz, 1H, CH2d), 3.96 (s, 5H, Cp), 5.59 (m,
1H, )CH-), 6.81 (td, 3JH-H ) 7.5, 4JH-H ) 1.2 Hz, 1H, 4-C6H4), 6.9 (br
3
m, 1H, C6H4), 7.0 (m, 9H, m- and p-PPh3), 7.26 (t, JH-H ) 8 Hz, 2H,
C6H4), 7.8 (m, 6H, o-PPh3); 31P{1H} NMR (122 MHz, C6D6) δ 48.97 (s).

Communications
Organometallics, Vol. 27, No. 15, 2008 3637
Scheme 2
Figure 1. Relation between the concentration of 2-allylphenol and
kobs for the formation of 4c in benzene. Conditions: initial
concentration of 2c/3c 0.0012 M, temperature 30 °C.
In the presence of excess PPh3, the absorption maximum
shifted to 425 nm. This blue shift is likely due to the shift of
the equilibrium to the 2c side. Addition of a large excess of
2-allylphenol (250 equiv) to this solution caused a decrease of
the absorption band, and the reaction rate was also obeyed first-
order rate kinetics. The observed rate constant kobs gradually
decreased with an increase in added PPh3 concentration (0-900
equiv: 0-1.08 M). In the presence of 1.08 M (900 equiv) of
PPh3, the ratio 2c/3c is estimated to be 74/26.
methylene resonance in 3c characteristically shifted downfield
upon increasing the concentration of 2-allylphenol.16 Meantime,
the hydroxy resonance in 2-allylphenol shifted upfield upon
increasing the concentration of 2-allylphenol.17 This phenom-
enon suggests a rapid associative equilibrium between 3c and
2-allylphenol (K3 ) [3c · HOAr]/([3c][HOAr]) ) 12 ( 3 M-1
at 30 °C). The most probable interaction between 3c and
2-allylphenol is hydrogen bonding between the aryloxo oxygen
in 3c and the hydroxo proton in 2-allylphenol, as observed in
alkoxo and aryloxo ligands in late-transition-metal complexes
(eq 4).18,19 The downfield shift of the methylene signals in
3c · HOAr suggests increase of the acidity by association of acid.
From these experimental results, the reaction mechanism for
the present acid-promoted isomerization of (2-allylphenoxo)
ruthenium(II) to the (η3-(2-hydroxyphenyl)allyl)ruthenium(II)
complex is proposed to be as shown in Scheme 2.
First of all, reversible dissociation of the PPh3 ligand
accompanied by coordination of the CdC bond is established
to give a mixture of bis- and mono(triphenylphosphine)
complexes 2c and 3c. Without addition of PPh3, this equilibrium
(K1 ) 0.38 M) lies far to the 3c side under these conditions
([2c]/[3c] ) 1/99). There is also an equilibrium (K3 ) 12 ( 3
M-1) between 3c and 3c · HX in the presence of acid (HX stands
for an acid). It is notable that the rate for constitution of this
equilibrium is considered to be much faster than the following
C-H bond cleavage reaction, because a merged resonance
between 3c and 3c · HX was observed by the 1H NMR spectrum
before producing 4c, when HX was added into the solution
containing 3c.
The benzene solution of 2c/3c (0.0012 M) showed an
absorption maximum at 467 nm. Addition of a large excess of
2-allylphenol (250 equiv) as a Brønsted acid to the solution
caused a gradual decrease of the absorption band by the
formation of 4c. The time courses of the reaction under various
conditions were monitored by UV-vis spectroscopy. The
reaction obeyed good pseudo-first-order kinetics. The estimated
rate constant (kobs) is proportional to the concentration of
2-allylphenol (Figure 1).
If 4c is produced directly from 3c · HX, the kobs value should
be proportional to the concentration of [3c · HX] but not to the
concentration of acid [HX]. When the acid concentration was
increased sixfold, the estimated concentration of [3c · HX] would
be doubled on the basis of the equilibrium constant K3 ([3c]/
[3c · HX] changed from 58/42 to 19/81). In fact, however, a
sixfold increase of acid concentration (0.0613 to 0.360 M)
caused an approximately sevenfold increase of the kobs value
(1.0 × 10-4 to 7.3 × 10-4 s-1), as shown in Figure 1. This
fact shows increase of the kobs Value is in direct proportion to
acid concentration [HX] but not to the concentration of
[3c · HX]. Such proportionality can be explained only by the
association of the second acid (HX) to [3c · HX] to give 4c, as
shown in Scheme 2.20
(16) Resonance of the methylene group in 3c (2c/3c 0.02 M) in benzene-
d6 at 20 °C: δ 1.98 (0.02 M of 2-allylphenol), δ 2.1 (0.04 M of
2-allylphenol), δ 2.18 (0.06 M of 2-allylphenol), δ 2.28 (0.10 M of
2-allylphenol), δ 2.43 (0.20 M of 2-allylphenol) (cf. methylene resonence
in 3c: δ 1.84).
(17) Resonance of the hydroxy group in added 2-allylphenol in benzene-
d6 at 20 °C: δ 5.52 at 0.02 M, δ 5.05 at 0.10 M, δ 4.73 at 0.20 M in the
presence of 2c/3c (0.02 M) (cf. free 2-allylphenol: δ 4.43).
(18) Bickford, C. C.; Johnson, T. J.; Davidson, E. R.; Caulton, K. G.
Inorg. Chem. 1994, 33, 1080.
(19) (a) Yong-Joo, K.; Jae-Young, L.; Osakada, K. J. Organomet. Chem.
1998, 558, 41. (b) Komiya, S.; Iwata, M.; Sone, T.; Fukuoka, A. J. Chem.
Soc., Chem. Commun. 1992, 1109. (c) Simpson, R. D.; Bergman, R. G.
Organometallics 1992, 11, 4306. (d) Osakada, K.; Ohshiro, K.; Yamamoto,
A. Organometallics 1991, 10, 404. (e) Osakada, K.; Kim, Y.; Tanaka, M.;
Ishiguro, S.; Yamamoto, A. Inorg. Chem. 1991, 30, 197. (f) Kim, Y.;
Osakada, K.; Takenaka, A.; Yamamoto, A. J. Am. Chem. Soc. 1990, 112,
1096. (g) Osakada, K.; Kim, Y.; Yamamoto, A. J. Organomet. Chem. 1990,
382, 303. (h) Kegley, S. E.; Schaverien, C. J.; Freudenberger, J. H.;
Bergman, R. G.; Nolan, S. P.; Hoff, C. D. J. Am. Chem. Soc. 1987, 109,
6563.
On the basis of these considerations, formation rate equation
of 4c is derived as shown in eq 5. From the best calculated
curves based on the curve-fitting iteration of experimental data,
the association constants of acid to 3c (K3) and 2c (K4) and the
formation rate constants of 4c from 3c · HX (kP1) and 2c · HX

3638 Organometallics, Vol. 27, No. 15, 2008
Communications
(kP2) in eq 5 were estimated as follows: K3 ) 32 ( 17 M-1, K4
) 2 ( 2 M-1, kP1 ) 2.1 ( 1.0 × 10-3 M-1 s-1, and kP2 ≈ 0
mechanism. The present kinetic data also suggest that formation
of 4c occurs from 3c · HX, not from 2c · HX. Probably,
coordination of the CdC bond to the ruthenium(II) center would
provide a more stable transition state for the C-H bond cleavage
reaction. Other mechanisms such as (a) protonation of the
ruthenium center followed by abstraction of a benzylic proton
by the counteranion and (b) oxidative addition of the benzylic
C-H bond followed by reductive elimination between the
resulting hydride and aryloxo groups are less likely. Because
process a gives a much more polar transition state and process
b normally requires coordinative unsaturation, these mechanisms
cannot explain the present results. It is notable that several
pioneering works were recently reported for the C-H bond
activation concerned with the metal-oxygen bond in (hydroxo)-
ruthenium(II),22 (hydroxo)iridium(III),23 (methoxo)iridium(III),24
(trifluoroethoxo)iridium(III),25and(acetato)iridium(III)26complexes.
In summary, present work shows the acid-promoted hydrogen
migration reaction. It is notable that the addition of acid did
not cause simple protonolysis of aryloxoruthenium(II) compound
giving a phenol derivative but the acid is employed to form
six-membered transition state leading to the formal hydrogen
migration from benzylic proton to aryloxo oxygen under ambient
conditions. The present result proposes a new C-H bond
cleavage mechanism of (aryloxo)ruthenium(II) complexes.
M-1 s-1
.
d([Ru]total - [4c])
-
)
dt
(kP1K1K3 + kP2K4[PPh3])[HX]2([Ru]total-[4c])
(K1K3 + K4[PPh3])[HX] + [PPh3] + K1
(5)
To obtain further information on the present C-H bond
cleavage reaction, the following experiments were performed
in the absence of added PPh3. No retardation was observed for
the formation of 4c in the presence of galvinoxyl as a radical
scavenger. When a polar solvent such as acetone or THF was
employed in this reaction, the formation rate of 4c was
reduced.21 Without addition of PPh3, the equilibrium between
2c and 3c lies far to the 3c side ([2c]/[3c] ) 1/99). An Eyring
plot for the formation of 4c in benzene showed the following
kinetic parameters: ∆Gq298 ) 91 ( 10 kJ mol-1, ∆Hq ) 59 (
5 kJ mol-1, and ∆Sq ) -108 ( 17 J mol-1 K-1
.
These features of the present C-H bond cleavage reaction
can be summarized as follows: (a) the C-H bond cleavage
reaction proceeds from 3c · HX, (b) the formation of 3c · HX is
not an intrinsic factor for the C-H bond cleavage reaction and
further addition of acid to 3c · HX is required to give 4c, (c) no
exchange reaction occurred among the benzylic methylene
protons and acid during the formation of 4c, (d) the reaction is
not a radical process, (e) polar solvents retard the formation of
4c, and (f) this reaction shows a large negative entropy of
activation. By taking these facts into account, we can propose
the most likely process for the present C-H bond cleavage
reaction via a concerted mechanism involving a six-membered
cyclic transition state (Chart 1).
Acknowledgment. We gratefully acknowledge support
from the Ministry of Education, Education, Culture, Sports,
Science and Technology of Japan.
Supporting Information Available: Text, tables, and figures
giving full experimental details involving the preparations and
characterizations of 2a-f, 3c, 3c · HX, and 4c, the thermal conver-
sion of 2c/3c to 4c, treatment of 2c/3c with Brønsted acid, a van’t
Hoff plot for the equilibrium between 2c and 3c, the fluxional
behavior in 3c, the association of 3c with 2-allylphenol (Scatchard
plot), the effect of the concentration of added 2-allylphenol on the
observed rate constant, the effect of the concentration of PPh3 on
the observed rate constant, the estimation of equilibrium constants
and rate constants, an Eyring plot for the formation of 4c, and
crystallographic analyses of 2c and 4c and CIF files giving X-ray
crystal data. This material is available free of charge via the Internet
at http://pubs.acs.org.
Chart 1
OM800318Z
The kinetic data suggest prerequisite addition of acid to
3c · HX for the C-H bond cleavage reaction, and the large
negative entropy of activation also supports this associative
(22) (a) Gunnoe, T. B. Eur. J. Inorg. Chem. 2007, 1185. (b) Feng, Y.;
Gunnoe, T. B.; Grimes, T. V.; Cundari, T. R. Organometallics 2006, 25,
5456. (c) Feng, Y.; Lail, M.; Foley, N. A.; Gunnoe, T. B.; Barakat, K. A.;
Cundari, T. R.; Petersen, J. L. J. Am. Chem. Soc. 2006, 128, 7982. (d)
Feng, Y.; Lail, M.; Barakat, K. A.; Cundari, T. R.; Gunnoe, T. B.; Petersen,
J. L. J. Am. Chem. Soc. 2005, 127, 14174.
(23) (a) Oxgaard, J.; Tenn, W. J., III; Nielsen, R. J.; Periana, R. A.;
Goddard, W. A., III Organometallics 2007, 26, 1565. (b) Tenn, W. J., III;
Young, K. J. H.; Oxgaard, J.; Nielsen, R. J.; Goddard, W. A., III; Periana,
R. A. Organometallics 2006, 25, 5173.
(24) Tenn, W. J., III; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard,
W. A., III; Periana, R. A. J. Am. Chem. Soc. 2005, 127, 14172.
(25) Koike, T.; Ikariya, T. Organometallics 2005, 24, 724 In this
reference, putative (alkoxo)- and (aryloxo)ruthenium(II) complexes were
reported to cause an intramolecular C-H bond cleavage reaction.
(26) Davies, D. L.; Donald, S. M. A.; Al-Duaij, O.; Macgregor, S. A.;
Po¨lleth, M. J. Am. Chem. Soc. 2006, 128, 4210.
(20) Although 2c · HX could not be observed, it is reasonable to propose
analogous hydrogen bonding under acidic conditions to give 4c. However,
we believe that this process is not a major process, because preliminary
results show that treatment of the DPPE analogue of 2c,
RuCp(OC6H4CH2CHdCH2)(dppe), with 2-allylphenol did not produce the
corresponding η3-allyl complex at all under these conditions. Moreover,
the estimated value for kP2 is almost 0 M-1 s-1 (see the Supporting
Information). However, since we cannot exclude the formation process via
2c · HX completely, such a possibility is also depicted in Scheme 2.
(21) The observed rate constants are as follows: 6.2 × 10-4 s-1
(benzene), 1.6 × 10-4 s-1 (acetone), and 2.2 × 10-5 s-1 (THF). Conditions:
initial concentration of 2c/3c 0.0012 M, 2-allylphenol 250 equiv, temperature
30 °C.