A New Solvent System for Recycling Catalysts for Chelation-Assisted Hydroacylation of Olefins with Primary Alcohols

Article abstract of DOI:10.1021/ja038071w

A new method to separate and reuse the catalyst system consisting of a rhodium complex and 2-aminopyridines for chelation-assisted hydroacylation was achieved using phenol and 4,4′-dipyridyl as the reaction medium, and 4-diphenylphosphinobenzoic acid as a ligand. Copyright

Full text of DOI:10.1021/ja038071w

Published on Web 12/18/2003
A New Solvent System for Recycling Catalysts for Chelation-Assisted
Hydroacylation of Olefins with Primary Alcohols
Duck-Ho Chang, Dae-Yon Lee, Boo-Sun Hong, Jun-Hack Choi, and Chul-Ho Jun*
Department of Chemistry and Center for BioactiVe Molecular Hybrid, Yonsei UniVersity, Seoul 120-749, Korea
Received August 22, 2003; E-mail: junch@yonsei.ac.kr
C-H bond activation by transition metal catalysts is one of the
current interests in organic synthesis.1 Particularly, hydroacylation
through aldehydic C-H bond activation provides an efficient
synthetic method for preparing ketones.2 In the course of our studies
on chelation-assisted hydroacylation,3 a one-pot protocol to obtain
ketones from primary alcohols and olefins was devised using a
cocatalyst system of a rhodium complex and 2-aminopyridine.4
Recently, we demonstrated the reuse of a rhodium catalyst in this
protocol by using polystyrene-based phosphine.4c However, the
Figure 1. A phase separation of the reaction mixture consisting of benzyl
alcohol, 1-decene, [(C8H14)2RhCl]2, 4-PBA, 2-amino-4-picoline, phenol, and
4,4′-dipyridyl. (a) Two phases immersed into one phase upon heating (120
°C), and then (b) separated into two phases at room temperature.
polymer-based system exhibited inferior reactivity as compared with
the homogeneous catalysis due to heterogeneity. Moreover, 2-ami-
nopyridines, used as a chelation auxiliary,3c should be added at each
cycle. Therefore, we have investigated a biphasic system which
fulfills the following two requirements: complete homogeneity
during the reaction and the easy separation of 2-aminopyridines as
Table 1. Recycling of the Catalysts for Hydroacylation of 2a with
1a
well as the transition metal complex after the reaction. There have
been various approaches to separate and reuse homogeneous
catalysts,5 such as aqueous biphasic systems,6 fluorous biphasic
systems,7 and soluble polymer-based ligands.8 However, these
isolated yield of 6a (%)
methods need to immobilize both 2-aminopyridines and a transition
entry phosphine (4)
additive
1
2
3
4
5
6
7
metal complex to polymer or tagging corresponding functional
groups. Attempts to use ionic liquids as a reaction medium9 failed,
presumably due to heterogeneity derived from limited solubility
of olefin.10 Herein described is a new approach to recycle the
catalysts for chelation-assisted hydroacylation of olefins with
primary alcohols, using a hydrogen-bonding solvent, in which a
rhodium complex and 2-aminopyridine are confined after the
reaction and thus separated from products for reuse.
1
2
4-PBA (4a)
PPh3 (4b)
88 96 92 95 91 93 96
PhCO2H (20 mol %) 93 34 28 12
94 56 61 43 29
3a (4a)
a The reaction was performed in the absence of phenol.
be established at high temperature, these two phases turned into
one phase upon heating as shown in Figure 1a. Thus, a homoge-
neous catalysis could be achieved during the reaction, and the
reaction mixture separated into two phases after the reaction (Figure
1b). In this biphasic system, it was found that 2-aminopyridine and
the rhodium complex stayed in the polar phase (phenol and 4,4′-
dipyridyl),12 while the product of hydroacylation, ketone, largely
stayed in the nonpolar hydrocarbon. Therefore, the catalysts could
be separated from ketone for recycling after the reaction (Scheme
1b).
In our experiment, a mixed solvent consisting of phenol and 4,4′-
dipyridyl, which was known to form a hydrogen-bonding complex,11
was used as a reaction medium. As depicted in Scheme 1a, it forms
Scheme 1. (a) A Biphasic System Using Hydrogen-Bonding
Solvent, and (b) a Schematic Diagram of the Recycling of the
Catalysts for Chelation-Assisted Hydroacylation with Primary
Alcohol
This solvent system consisting of phenol and 4,4′-dipyridyl was
applied to the reaction of benzyl alcohol (1a) and 1-hexene (2a) in
the presence of [(C8H14)2RhCl]2 (3, 5 mol %), phosphine (4), and
2-amino-4-picoline (5, 100 mol %) at 150 °C for 6 h. After the
homogeneous reaction, it was cooled to room temperature to form
two phases. Heptanophenone (6a) was separated from the catalysts
by decanting the upper layer and washing the lower layer with
n-pentane three times. The remaining lower layer containing
catalysts was recycled for the next reaction (Table 1).
When 4-diphenylphosphinobenzoic acid (4a, 4-PBA) was used
as an external ligand, the reactivity was retained for repeated uses,
giving 88-96% isolated yields (av. 93%) of heptanophenone 6a
(entry 1). In this system, rhodium catalyst was strongly immobilized
in the polar phase, which was confirmed by ICP-MS analyses. Only
a trace amount (0.088 and 0.16 µg, respectively) of Rh was found
in the separated nonpolar phases of cycles 1 and 2, corresponding
two immiscible phases with hydrocarbon such as alkane or olefin
at room temperature. Because a hydrogen-bonding network cannot
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10.1021/ja038071w CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Table 2. Recycling of the Catalyst for Hydroacylation of 2 with 1
Scheme 2
isolated yield of product (%)
entry
R1 (1)
R2 (2)
ketone (6)
1
2
3
4
5
6
7
8
1
2
3
4
5
6
H (1a)
(1a)
(1a)
CF3 (1b) (2a)
MeO (1c) (2a)
(1c)
t-Bu (2b)
C6H13 (2c)
C8H17 (2d)
6b
6c
6d
6e
6f
90 87 93 96 97 95 92 93
85 77 97 95 98 90 83 92
89 91 86 90 88 94 82 89
87 92 97 95 89 91 88 87
92 98 92 96 92 94 91 97
94 91 92 96 90 95 91 87
application of this protocol to other catalytic reactions and develop-
ing more general recycling catalytic systems, because this system
cannot be applied to the reaction in which the product is soluble in
the polar phase.
(2b)
6g
Acknowledgment. This work was supported by the National
Research Laboratory (NRL) (2000-N-NL-01-C-271) Program ad-
ministered by the Ministry of Science and Technology and CBMH.
to a leaching of 0.005% and 0.01% of initial Rh (1.63 mg). A
chelation auxiliary 5 also stayed mainly in the polar phase even
though a very small amount of 5 was found in the nonpolar phase
occasionally (2-5% based on the initial amount), which resulted
in no significant decrease in the yield of 6a in the next run.13 On
the other hand, when the reaction was performed in the presence
of triphenylphosphine (4b) and benzoic acid,14 instead of 4a, the
yield of 6a dropped after the second run (entry 2).
A high affinity of rhodium catalyst to the hydrogen-bonding
solvent in the presence of 4a as a ligand might be ascribed to the
existence of acid functionality in 4a. When the reaction was carried
out in the absence of phenol, a solid/liquid phase separation took
place by precipitation after the reaction, while a homogeneous
catalysis was achieved during the reaction.15 We assumed that the
precipitation might be attributed to a hydrogen-bonding assembly
consisting of an Rh(I) complex bearing 4a and 4,4′-dipyridyl as in
7.16 The solid phase could be separated by decanting the liquid,
but the reactivity was not retained to result in a gradual decrease
in the yield of 6a, which might be due to the large leaching of
2-amino-4-picoline (entry 3).17
Supporting Information Available: Experimental details and the
result of the reaction using ionic liquid (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
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