Dehydrative allylation of alcohols and deallylation of allyl ethers catalyzed by [CpRu(CH3CN)3]PF6 and 2-pyridinecarboxylie aeid derivatives. Effect of π-accepting ability and COOH acidity of ligand on reactivity

Article abstract of DOI:10.1246/cl.2009.188

2-Quinolinecarboxylic acid efficiently promotes both the dehydrative allylation of alcohols and the deallylation of allyl ethers and esters in the presence of [CpRu(CH₃CN)₃JPF₆. Comparison of the relative reactivity of various 4-substituted 2-pyridinecarboxylic acids has revealed two linear relations with different ρ values in the Hammett plots. These phenomena can be rationalized by the balance between the π-accepting ability of the pyridine moiety and the acidity of the carboxylic acid of the 2-pyridinecarboxylic acid derivative. Copyright

Full text of DOI:10.1246/cl.2009.188

188
Chemistry Letters Vol.38, No.2 (2009)
Dehydrative Allylation of Alcohols and Deallylation of Allyl Ethers Catalyzed by
[CpRu(CH3CN)3]PF6 and 2-Pyridinecarboxylic Acid Derivatives. Effect of ꢀ-Accepting Ability
and COOH Acidity of Ligand on Reactivity
Shinji Tanaka, Hajime Saburi, Takuya Hirakawa, Tomoaki Seki, and Masato Kitamuraꢀ
Contribution from the Research Center for Materials Science and the Department of Chemistry,
Nagoya University, Chikusa-ku, Nagoya 464-8602
(Received September 4, 2008; CL-080843; E-mail: kitamura@os.rcms.nagoya-u.ac.jp)
2-Quinolinecarboxylic acid efficiently promotes both the
dehydrative allylation of alcohols and the deallylation of allyl
ethers and esters in the presence of [CpRu(CH3CN)3]PF6. Com-
parison of the relative reactivity of various 4-substituted 2-pyri-
dinecarboxylic acids has revealed two linear relations with dif-
ferent ꢀ values in the Hammett plots. These phenomena can
be rationalized by the balance between the ꢁ-accepting ability
of the pyridine moiety and the acidity of the carboxylic acid of
the 2-pyridinecarboxylic acid derivative.
OH
a:
b:
c:
d:
e:
X = H
X = OCH3
X = Cl
X = CF3
X = NO2
OH
O
N
X
O
3
2
1
2
1 mM [CpRu(CH3CN)3]PF6
1 mM 1a−e
HO
100 mM
+
+
3
H2O
CH2Cl2, reflux
100 mM
In the presence of a 1:1 mixture of 2-quinolinecarboxylic
acid and [CpRu(CH3CN)3]PF6, alcohols react with one equiv
of allyl alcohol in aprotic solvents such as dichloromethane, ace-
tone, and THF to give allyl ethers in high yield (Figure 1).1,2 The
reverse reaction—that is, deallylation of allyl ethers—is also
promoted by the same catalytic system in alcoholic solvents such
as methanol, ethanol, and 2-propanol, giving quantitatively the
corresponding alcohol.3–5 The allylation and deallylation reac-
tions are greatly affected by the structure of the 2-quinolinecar-
boxylic acid. When the quinoline ring is replaced with pyridine,
for example, the reactivity is decreased 5–10-fold. Condensation
of the benzene ring at C(5) and C(6) of 2-pyridinecarboxylic acid
should be advantageous for rate acceleration. To gain a deeper
understanding of this catalyst, we are investigating its structure–
reactivity relationship in a systematic manner.
Figure 2 shows the standard allylation and deallylation reac-
tion conditions. The parent ligand is 2-pyridinecarboxylic acid
(1a) and derivatives 1b–1e, which have the electronically differ-
ent substituents CH3O, Cl, CF3, and NO2 at the C(4) position of
1a, were used to compare the reactivity in the allylation of
2-phenylethanol (2) to give allyl phenethyl ether (3) ([2] =
100 mM, [1] = [{CpRu(CH3CN)3}PF6] = 1 mM, CH2Cl2,
1 mM [CpRu(CH3CN)3]PF6
1 mM 1a−e
CH3O
3
2
+
CH3OH, 30 °C
500 mM
Figure 2. Standard reaction and conditions for the rate compar-
ison.
Table 1. Relative reactivity in the [CpRu(CH3CN)3]PF6/4-
X-2-pyridinecarboxylic acid-catalyzed allylation of 2 with allyl
alcohol and deallylation of 3
Reactivity
pKa Allylationa Deallylationb
Entry Ligand (X)
ꢂp
1
2
3
4
5
1b (CH3O) ꢃ0:28
6.71
5.40
4.07
3.43
2.90
0.11
1
0.7
1
1.4
1.6
1.9
1a (H)
1c (Cl)
0.00
0.22
0.53
0.81
3.5
6.1
6.5
1d (CF3)
1e (NO2)
aCondition:
[2] = 100 mM;
[allyl
alcohol] = 100 mM;
[{CpRu(CH3CN)3}PF6] = 1 mM; [1] = 1 mM; solvent, CH2Cl2;
reflux. bCondition: [3] = 500 mM; [{CpRu(CH3CN)3}PF6] =
1 mM; [1] = 1 mM; solvent, CH3OH; temp., 30 ꢁC.
reflux) and in the deallylation of 3 to 2 ([3] = 500 mM,
[1] = [{CpRu(CH3CN)3}PF6] = 1 mM, CH3OH, 30 ꢁC).
First, we examined the relative reactivity of 2-pyridinecar-
boxylic acid (1a) and its derivatives 1b–1e in the allylation of
2 to 3 by using one equiv of allyl alcohol. The reaction was
interrupted at <20% conversion, and aliquots were analyzed
by GC (column: DB-WAX (0:25 mm ꢂ 15 m), initial temp.:
50 ꢁC, rate: 10 ꢁC/min, tR of 2: 6.0 min, tR of 3: 4.2 min). As
illustrated in Table 1, the presence of an electron-donating
CH3O group at the C(4) position of the pyridine ring decreased
the reactivity by a factor of 10. The ligand possessing the elec-
tron-withdrawing (EW) group Cl shows a reactivity ca. 3 times
higher than that of the parent ligand 1a (X ¼ H). The degree of
the enhancement became more significant with stronger EW
H2O
HO
no solvent or
aprotic solvent
PF6
Ru
N
O
ROH
RO
O
no additives
CH3OH
HO
Figure 1. Dehydrative allylation of alcohols and deallylation of
allyl ethers using [CpRu(CH3CN)3]PF6/2-pyridinecarboxylic
acid combined catalyst.
Copyright Ó 2009 The Chemical Society of Japan

Chemistry Letters Vol.38, No.2 (2009)
189
equilibrium with a neutral [CpRuII(4-X-2-pyridinecarboxylato)]
species and a strong acid (HPF6). The carboxylic proton in the
cationic Ru complex can activate the hydroxy or alkoxy group
via hydrogen bonding, thereby increasing the electrophilicity
of the ꢃ-carbon of the allyl alcohol or allyl ether through charge
alternation. Combined with the highly nucleophilic RuII atom,
oxidative addition of the allyl C–O bond to RuII smoothly pro-
ceeds to give an ꢁ-allyl RuIV intermediate. This redox-mediated
donor–acceptor bifunctional catalyst ability7,8 is lost in a neutral
Ru carboxylato complex. If COOH in 1a is replaced with SO3H,
then deallylation is several times slowed. The highly acidic
SO3H group should more easily generate the monobasic salt
by liberating HPF6. The balance between the ꢁ-accepting ability
and the acidity of the ligand is thus important for gaining high
catalytic reactivity in these allylation and deallylation reactions.9
In summary, the relationship between reactivity and ligand
structure has been systematically investigated in the allylation
of alcohols by allylic alcohol and the deallylation of allyl ethers
catalyzed by a combined system of [CpRu(CH3CN)3]PF6 and 4-
X-2-pyridinecarboxylic acid. Both reactions proceed more rap-
idly with ligands possessing an EW group at C(4), but the pattern
of the increase in rate is not simple. Linear relationships are ob-
served for two ranges of ꢂp values: namely, ꢃ0:30–0.2 and 0.2–
0.8. This phenomenon can be rationalized by the balance be-
tween the ꢁ-accepting ability and the acidity of the ligand. A li-
gand with an EW group lowers the LUMO level to enhance the
reactivity, whereas one with increased acidity lowers the reactiv-
ity. Therefore, the ideal ligand should have high ꢁ-accepting
ability but maintain appropriate acidity. This observation would
explain the high efficiency of 2-quinolinecarboxylic acid.10
a
b
log(kH/kX)
1.0
log(kH/kX)
ρ = 3.04
CF3
0.4
ρ = 0.453
ρ = 0.63
NO2
NO2
ρ = 0.20
0.4
Cl
0.2
CF3
Cl
H
σp
1.0
−0.4
0
0.4
H
σp
−0.4
−0.4
0.4
0.8
CH3O
CH3O
−0.2
−1.0
Figure 3. Hammet plots for the allylation and deallylation pro-
moted by [CpRu(CH3CN)3]PF6/4-X-2-pyridinecarboxylic acid
combined catalytic system. a: the initial rate ratio/ꢂp relation
in allylation of 2 to 3. b: the initial rate ratio/ꢂp relation in deal-
lylation of 3 to 2. ꢀ = reaction constant.
substitutents. As the ꢂp value rose from 0.22 (Cl) to 0.53 (CF3),
for example, the reactivity doubled. With a ꢂp value of 0.81
(NO2), however, the reaction was not accelerated as much as
expected. A Hammett plot using standard ꢂp constants for the
substituents exhibited two linear free-energy relationships with
ꢀ values of +3.04 for 1a, 1b, and 1c, and +0.453 for 1c, 1d,
and 1e, respectively (Figure 3a).
A similar tendency was also observed in the deallylation
reaction, as shown in Figure 3b, although the rate dependence
on the substituents was less significant than in the allylation
reaction. The two linear lines had high correlation coefficients
(R ¼ 0:996 and 0.967); however, the correlation was less when
all of the measurements were used in a least-squares approxi-
mation.
This two-line behavior observed in both allylation and deal-
lylation can be rationalized by considering two factors: one is the
ꢁ-accepting ability of the pyridine part, and the other is the acid-
ity of the COOH part. Although the detailed mechanism is not
clear at present, we believe that the ꢁ-allyl intermediate is in
the resting state and that the nucleophilic attack of the alcohol
on the ꢁ-allyl ligand determines the overall reaction rate. As
the rate-determining step proceeds via reduction of the central
Ru atom from RuIV to RuII, a ligand with higher ꢁ-accepting
ability should stabilize the transition state. Consistent with this
view, ꢁ-expanded 2-quinolinecarboxylic acid, in which the low-
est unoccupied molecular orbital (LUMO) level is 0.6 eV lower
than that of 1a (2.12 vs. 1.52),6 is 5–10-fold more reactive.1,2 In-
troduction of an EW group at C(4) of 1a also lowers the LUMO
level, and the reactivity increases. At the same time, however,
a strong EW group with a high ꢂp value raises the acidity of
COOH. As shown in Figure 4, the dibasic mono salt,
[CpRuII(4-X-2-pyridinecarboxylic acid)]PF6, is essentially in
This work was supported by Grant-in-Aid for Scientific
Research on Priority Areas (No. 19028024, ‘‘Chemistry of Con-
certo Catalysis’’) from Ministry of Education, Culture, Sports,
Science and Technology, Japan.
Dedicated to Professor Ryoji Noyori on the occasion of his
70th birthday.
References and Notes
1
H. Saburi, S. Tanaka, M. Kitamura, Angew. Chem., Int. Ed. 2005, 44,
1730.
2
3
S. Tanaka, H. Saburi, M. Kitamura, Adv. Synth. Catal. 2006, 348, 375.
S. Tanaka, H. Saburi, Y. Ishibashi, M. Kitamura, Org. Lett. 2004, 6,
1873.
4
5
6
S. Tanaka, H. Saburi, T. Murase, M. Yoshimura, M. Kitamura, J. Org.
Chem. 2006, 71, 4682.
S. Tanaka, H. Saburi, T. Murase, Y. Ishibashi, M. Kitamura, J. Orga-
nomet. Chem. 2007, 692, 295.
Calculated by SPARTAN 02’ for Macintosh software from Wavefunc-
tion Inc. at 6-31Gꢀꢀ level.
strong acid salt
7
8
R. Noyori, M. Kitamura, Angew. Chem., Int. Ed. Engl. 1991, 30, 49.
M. Kitamura, S. Suga, K. Kawai, R. Noyori, J. Am. Chem. Soc. 1986,
108, 6071.
−
+PF6
−
H+PF6
strong acid
X
N
Ru
X
N
Ru
O
9
Less basicity of the oxygen atom in highly acidic groups may weaken
the hydrogen bond to the alcoholic hydrogen atom, leading to less
reactivity. A theoretical calculation of the transition state is now under
investigation.
weak acid salt
O−H+
O
O
weak acid
10 For the steric effect of anionic ligand on the reactivity in allylation
using CpꢀRu(ꢁ-allyl) complexes, see: A. B. Zaitsev, S. Gruber, P. A.
bifunctionality
no bifunctionality
Figure 4. Equilibrium between dibasic monosalt and monoba-
sic salt/acid.
Pluss, P. S. Pregosin, L. F. Veiros, M. Worle, J. Am. Chem. Soc. 2008,
¨
¨
130, 11604.
Published on the web (Advance View) February 5, 2009; doi:10.1246/cl.2009.188