Chemoselective transfer hydrodechlorination of aryl chlorides catalyzed by Cp*Rh complexes

Article abstract of DOI:10.1039/b209855e

An effective Cp*Rh catalyzed transfer hydrodechlorination of aryl chlorides was achieved with high tolerance towards a variety of functional groups using 2-butanol as a hydrogen source.

Full text of DOI:10.1039/b209855e

Chemoselective transfer hydrodechlorination of aryl chlorides catalyzed
by Cp*Rh complexes
a
b
b
Ken-ichi Fujita,* Maki Owaki and Ryohei Yamaguchi*
a
Department of Natural Resources, Graduate School of Global Environmental Studies, Kyoto University,
Kyoto 606-8501, Japan. E-mail: fujitak@kagaku.mbox.media.kyoto-u.ac.jp; Fax: +81-75-753-6634;
Tel: +81-75-753-6827
b
Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan
Received (in Cambridge, UK) 9th October 2002, Accepted 24th October 2002
First published as an Advance Article on the web 8th November 2002
An effective Cp*Rh catalyzed transfer hydrodechlorination
of aryl chlorides was achieved with high tolerance towards a
variety of functional groups using 2-butanol as a hydrogen
source.
Table 2 Dechlorination of various aryl chlorides catalyzed by Cp*Rh
complexesa
Catalyst
mol%
Rh)
(
b
Entry Aryl chloride
Base (eq.)
Yield (%)
The dechlorination of aryl chlorides is an extremely important
chemical transformation from the preparative and environ-
1
2
3
4
5
6
7
8
9
0
1
4-Chlorotoluene
3-Chlorotoluene
2-Chlorotoluene
2,6-Dichlorotoluene
3,4-Dichlorotoluene
2,3-Dichlorotoluene
1-Chloronaphthalene
2-Chloronaphthalene
4-Chlorobiphenyl
2 (2.5) KOH (2.0)
1 (5.0) KOH (2.0)
1 (5.0) KOH (2.0)
1 (5.0) KOH (3.8)
1 (5.0) KOH (4.0)
2 (5.0) KOH (3.8)
2 (1.0) KOH (2.0)
2 (1.0) KOH (2.0)
2 (1.0) KOH (2.0)
(96)
(92)
(93)
(96)
(76)
(47)
98
1
mental point of view. Recently, much attention has focused on
the transition metal catalyzed dechlorination of aryl chlorides.
A large number of dechlorination systems have been developed,
but these systems are mainly applicable to the dechlorination of
aryl chlorides without a functional group. These dechlorination
2
3
systems involve the use of metal hydrides, metal alkoxide,
96
100
4
5
6
hydrosilanes, molecular hydrogen, Grignard reagents, and
sodium formate7 as the hydrogen source. Dechlorination
systems with alcohols as the hydrogen source have been
relatively unexplored8 in spite of their usefulness in other
hydrogen transfer reactions as a mild hydrogen donor. In this
paper we report a new system for transfer hydrodechlorination
of a variety of aryl chlorides catalyzed by Cp*Rh complexes
with 2-butanol as a hydrogen source.
1
1
Methyl 4-chlorobenzoate 2 (5.0) Cy
2
NMe (1.0) 95
NMe (1.0) 95
4-Chlorobenzamide
4-Chlorobenzoic acid
4-Chloroacetophenone
4-Chloroanisole
1-Amino-4-chloro-
naphthalene
2 (5.0) Cy
2
12
13
14
2
2 (5.0) Cy NMe (2.0) 96
2 (5.0) Cs
2
CO
3
(1.0)
15 + 85c
(93)
79
2 (5.0) KOH (2.0)
2 (2.0) KOH (2.0)
1
5
1
6
4-Chloro-1-naphthol
2 (1.0) KOH (3.1)
88
Initially, we investigated the rhodium-catalyzed dechlorina-
tion of 4-chlorotoluene under various conditions. The reactions
were carried out in the presence of a catalytic amount of a
rhodium complex together with base in 2-butanol under reflux
for 17 h to afford toluene (eqn. (1)). The results are
a
The reaction was carried out under reflux with aryl chloride (2.0 mmol),
3
b
catalyst and base in 2-butanol (6 cm ) for 17 h. Isolated yield. The values
in parentheses are GC yields. Yield of acetophenone (15%) and
c
1
-phenylethanol (85%).
2 2
(1) and Cp*Rh(OAc) ·H O (2) showed high catalytic activity to
give toluene in yields of 92 and 96%, respectively (entries 1, 2).
On the other hand, rhodium complexes without Cp* ligands
were less active (entries 3, 4). Next, the effect of various bases
(1)
was examined with [Cp*RhCl
activity was closely related to the base. Among inorganic bases,
KOH and Cs CO were highly effective for the present reaction
2 2
] (1) as a catalyst. The catalytic
summarized in Table 1. At first we examined the catalytic
activities of several rhodium complexes. The complexes with
2
3
5
h -pentamethylcyclopentadienyl (Cp*) ligands, [Cp*RhCl
2
]
2
Table 1 Dechlorination of 4-chlorotoluene by various catalytic systemsa
b
Entry
Catalyst (mol% Rh)
Base (eq.)
Yield (%)
1
2
3
4
5
6
7
8
9
[Cp*RhCl
Cp*Rh(OAc)
RhCl(PPh
[RhCl(CO)
None
1 (5.0)
1 (5.0)
1 (5.0)
1 (5.0)
1 (5.0)
1 (5.0)
1 (5.0)
1 (5.0)
2
]
2
(1) (2.5)
·H O (2) (2.5)
(2.5)
2 2
] (2.5)
KOH (2.0)
KOH (2.0)
KOH (2.0)
KOH (2.0)
KOH (2.0)
NaOH (2.0)
92
96
63
15
0
50
61
85
66
84
64
60
2
2
2
3 3
)
K
Cs
Et
Cy
2
2
CO
CO
N (2.0)
NMe (2.0)
3
(1.0)
2
3
(1.0)
3
1
1
1
1
a
0
1
2
3
KOH (1.0)
KOH (2.0)
KOH (2.0)
c
d
The reaction was carried out under reflux with 4-chlorotoluene (2.0
3
b
mmol), catalyst, and base in 2-butanol (6 cm ) for 17 h. Determined by
GC. The reaction was carried out in 2-propanol. The reaction was
performed in ethanol.
c
d
Scheme 1 Possible mechanism for the dechlorination of aryl chlorides
catalyzed by Cp*Rh complexes.
2
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CHEM. COMMUN., 2002, 2964–2965
This journal is © The Royal Society of Chemistry 2002

(
entries 1, 6–8). An organic base, Cy
2
NMe was also effective
group also occurred to give 1-phenylethanol (85%) as well as
acetophenone (15%).
(entry 10), while Et N showed rather low activity (entry 9). The
3
best result was obtained with two equivalents of KOH. The
yield of toluene was reduced when an equimolar amount of
KOH was employed (entry 11). When the reaction was carried
out in 2-propanol or ethanol, the yield of toluene decreased
Although the mechanism for the present dechlorination
reaction is not completely clear as of yet, a possible mechanism
is shown in Scheme 1. The first step of the reaction would
involve the formation of Cp*Rh alkoxide (A) by the reaction of
Cp*Rh precursor with 2-butanol and base. Then the inter-
mediate A would transform into Cp*Rh hydride (B) via b-
hydrogen elimination from alkoxo ligand. Aryl chlorides would
react with B to give dechlorinated arene and Cp*Rh chloride
(
entries 12, 13), indicating that 2-butanol would be the best
hydrogen donor for the present transfer hydrodechlorination
system.
Under the optimal conditions determined above, dechlorina-
tion reactions of a variety of aryl chlorides were carried out
(C). The intermediate C would react with 2-butanol and base to
regenerate the intermediate A.
(
eqn. (2)). The results are summarized in Table 2. All the
In summary, we have shown a new catalytic system for
transfer hydrodechlorination of a large variety of aryl chlorides
with use of Cp*Rh catalysts and 2-butanol as a hydrogen
source.
isomers of chlorotoluene were dechlorinated giving excellent
(2)
Notes and references
yields of toluene (entries 1–3). The double dechlorination of
dichlorotoluenes proceeded by use of 3.8–4.0 equivalents of
KOH (entries 4–6). Chloronaphthalenes and 4-chlorobiphenyl
were dechlorinated quite readily, requiring a smaller amount of
catalyst (1.0 mol% Rh) (entries 7–9). Various electron-
withdrawing (ester, amide, carboxylic acid and ketone) and
electron-donating groups (alkoxy, amino and hydroxy) in aryl
chlorides were well-tolerated for present catalytic dechlorina-
tion to give the corresponding aromatic products in excellent
yields (entries 10–16). In the reactions of aryl chlorides with
ester and amide substituents (entries 10 and 11), employment of
1 V. V. Grushin and H. Alper, Chem. Rev., 1994, 94, 1047.
2 F. Massicot, R. Schneider, Y. Fort, S. Illy-Cherrey and O. Tillement,
Tetrahedron, 2000, 56, 4765.
3
(a) C. Desmarets, S. Kuhl, R. Schneider and Y. Fort, Organometallics,
002, 21, 1554; (b) M. S. Viciu, G. A. Grasa and S. P. Nolan,
2
Organometallics, 2001, 20, 3607.
4
(a) M. A. Esteruelas, J. Herrero, F. M. López, M. Martín and L. A. Oro,
Organometallics, 1999, 18, 1110; (b) R. Boukherroub, C. Chatgilialoglu
and G. Manuel, Organometallics, 1996, 15, 1508.
5 D. T. Ferrughelli and I. T. Horváth, Chem. Commun., 1992, 806.
6
R. Hara, K. Sato, W.-H. Sun and T. Takahashi, Chem. Commun., 1999,
45.
8
7
M. A. Atienza, M. A. Esteruelas, M. Fernández, J. Herrero and M.
Oliván, New J. Chem., 2001, 25, 775.
Cy
Dechlorination of 4-chloroacetophenone proceeded effectively
with use of Cs CO as a base, but reduction of the carbonyl
2
NMe as a base was appropriate to prevent hydrolysis.
8 M. E. Cucullu, S. P. Nolan, T. R. Belderrain and R. H. Grubbs,
Organometallics, 1999, 18, 1299.
2
3
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