Rhodium catalyzed allylic isomerization in water

Article abstract of DOI:10.1081/SCC-120016328

A water-soluble rhodium bis-phosphine complex was used for the aqueous phase isomerization of three allylic alcohols. For the isomerization of cinnamyl alcohol, the catalyst was recycled with slight loss of activity and the optimum phosphine:rhodium ratio was found to be 6:1.

Full text of DOI:10.1081/SCC-120016328

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Rhodium Catalyzed Allylic Isomerization in Water
a
b
D. Andrew Knight & Terence L. Schull
a
Department of Chemistry , Loyola University , New Orleans, Louisiana, USA
Center for Biomolecular Science and Engineering , Code 6950 , Naval Research
b
Laboratories , Washington, DC, USA
Published online: 17 Aug 2006.
To cite this article: D. Andrew Knight & Terence L. Schull (2003) Rhodium Catalyzed Allylic Isomerization in Water, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 33:5, 827-831, DOI:
10.1081/SCC-120016328
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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 5, pp. 827–831, 2003
Rhodium Catalyzed Allylic Isomerization in Water
1
,
2
D. Andrew Knight * and Terence L. Schull
1
Department of Chemistry, Loyola University,
New Orleans, Louisiana, USA
2
Center for Biomolecular Science and Engineering,
Code 6950, Naval Research Laboratories,
Washington, DC, USA
ABSTRACT
A water-soluble rhodium bis-phosphine complex was used for the
aqueous phase isomerization of three allylic alcohols. For the iso-
merization of cinnamyl alcohol, the catalyst was recycled with slight
loss of activity and the optimum phosphine:rhodium ratio was found
to be 6:1.
Key Words: Water-soluble phosphine; Aqueous phase catalysis;
Allylic isomerization.
*
Correspondence: D. Andrew Knight, Department of Chemistry, Loyola
University, 6363 St. Charles Ave., New Orleans, Louisiana 70118, USA;
E-mail: daknight@loyno.edu.
8
27
DOI: 10.1081/SCC-120016328
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
www.dekker.com

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8
28
Knight and Schull
The isomerization of allylic alcohols is an important organic reaction
used for the synthesis of aldehydes, ketones and rearranged allylic
alcohols, and may be catalyzed under single phase homogeneous
conditions using a variety of transition metal complexes. Particularly
[
1,2]
effective are rhodium and ruthenium complexes in organic solvents,
[
3,4]
ruthenium complexes under aqueous conditions,
and rhodium
5]
[
complexes with a biphasic water/heptane solvent system. Grubbs has
reported the isomerization of a number of allylic alcohols in water
[
4]
using Ru(H O) (tos) (tos ¼ p-toluenesulfonate) as a catalyst, and the
2
6
2
phase-transfer catalytic allylic isomerization reaction has been known
[6]
for some time. In addition, the enantioselective rhodium catalyzed
isomerization of allylic alcohols has recently been reported with
a high degree of enantioselectivity when using a rhodium-phosphaferro-
[7]
cene complex. Many of these methods suffer from problems related to
catalyst recycle or indeed, lack of chemoselectivity. The problem of
catalyst recycle and reuse can often be circumvented by using a biphasic
aqueous/organic reaction medium in which a water-soluble catalyst
resides in the aqueous layer and the reactant/product mixture remains
in the organic layer. Simple separation of catalyst from product via
decantation is then possible. We, and others, have been investigating
the use of water-soluble phosphines as stabilizing ligands for transition
[8]
metals. It has been demonstrated that under biphasic or simple aqueous
conditions, catalyst recycle, with minimal or no loss of catalytic
activity, is possible for a large number of important homogeneous
catalytic reactions such as hydrogenation, hydroformylation and
carbonylation. We now wish to report the aqueous phase isomerization
of cinnamyl alcohol to dihydrocinnamaldehyde (Sch. 1) using a rhodium
[
9]
p-TPPMP
(TPPMP ¼ para-triphenylphosphine monophosphonate)
catalyst, with very high chemoselectivity and catalyst recycle with
slight loss of activity. The allylic isomerization of alternative substrates
is also described.
Cinnamyl alcohol was chosen as our test substrate for the rhodium
catalyzed allylic isomerization reaction, and the catalyst was prepared by
reacting [Rh(cod) ]BF (cod ¼ cyclooctadiene) with the highly water-
2
4
Scheme 1. Aqueous catalytic isomerization of cinnamyl alcohol.

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Isomerization in Water
829
[
10]
soluble ligand p-TPPMP (1) in water. Cinnamyl alcohol was added to
this aqueous catalytic solution giving a catalyst/substrate ratio of 1:100,
and the reaction mixture was refluxed for 16 h. GC-MS analysis of the
reaction mixture after exhaustive extraction into diethyl ether indicated
the formation of dihydrocinnamaldehyde and unreacted starting material.
[
11]
No other products resulting from possible double bond migration were
detected using GC-MS, indicating the reaction proceeds in a chemoselec-
tive manner. Standard work-up procedures were used to isolate the alde-
hyde in 41% yields and the identity of dihydrocinnamaldehyde was
1
confirmed using H NMR and mass spectrometry.
An excess of p-TPPMP was found to be necessary for maintaining
activity of the catalyst, presumably for stabilization of the rhodium
center. Several phosphine/rhodium ratios were screened in order to
optimize the yield of dihydrocinnamaldehyde and it was found necessary
to maintain a ratio of at least 6:1. Increasing the ratio beyond this value
had no significant effect on yield, or selectivity for aldehyde. We were
interested to know whether the aqueous phase containing the rhodium
complex 2 could be recycled after separation from the organic phase, and
preliminary experiments indicated that very little loss of activity occurs
on recycling the catalyst. Thus a second and third isomerization reaction
after aqueous phase separation and recycle resulted in a 41% and 35%
yield of dihydrocinnamaldehyde respectively under the same conditions
as described above. Unreacted cinnamyl alcohol was also identified and
recovered from these reaction mixtures.
In order to investigate the scope of the reaction, the isomerizations of
a second simple allyl alcohol and a secondary allyl alcohol were studied.
Thus, using identical reaction conditions to those described above,
the rhodium catalyzed allylic isomerization of (a) crotyl alcohol and
(
b) 2-cyclohexene-1-ol gave the expected products butanal and cyclo-
hexanone in 58% and 62% isolated yields respectively. No other
products were observed by GC-MS. The nature of the rhodium
phosphine species in water was investigated. Addition of two equivalents
of water-soluble phosphine ligand 1 to an aqueous solution of
[Rh(cod) ]BF results in the formation of a new rhodium phosphine
species [Rh(cod)( p-TPPMP)2] (2) as evidenced by the
2 4
þ
31
P NMR
Figure 1. Isomerization catalyst 2.

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8
30
Knight and Schull
spectrum in water (Fig. 1). The spectrum contains a doublet centered at
6.2 ppm with characteristic coupling constants for a cis-arrangementof
2
phosphine ligands around the metal center (JRhP ¼ 146 Hz) and a
1
singlet due to the uncoordinated phosphonate group. The H NMR
spectrum of 2 recorded in D O is also consistent with the structure
2
shown in Fig. 1. Addition of excess phosphine ligand (two and four
additional equivalents respectively) to the solution showed only
3
1
unreacted p-TPPMP and 2 in the P NMR spectrum indicating that
substitution of the second cyclooctadiene ligand by p-TPPMP does not
take place.
EXPERIMENTAL
General
All reactions were conducted under dry, pre-purified N2 using
standard Schlenk-line and catheter-tubing techniques unless otherwise
stated. GC/MS analyses were performed on a Hewlett-Packard 6890
equipped with an HP-5 (5%)-diphenyl-(95%)-dimethylpolysiloxane
capillary column (30 m, 0.25 mm). The mass spectra under electron
impact conditions (EI) were recorded at 70 eV ionizing potential. All
1
13
31
H, C, and P NMR spectra were recorded on a Bruker AC-300
1
spectrometer and referenced to internal tetramethylsilane ( H), CDCl
3
1
3
31
C) and external H PO ( P).
3 4
(
Solvents and reagents were obtained as follows: diethyl ether, ethyl
acetate, hexane, petroleum ether (Fisher), cinnamyl alcohol, crotyl
alcohol, 2-cyclohexene-1-ol (Aldrich) D O (Cambridge Isotopes)
2
used as received. p-TPPMP was prepared according to the literature
Silica gel (Scientific Products, 230–400 mesh), and
[10]
procedure.
magnesium sulfate (Fisher) were used as received.
Catalysis
The following procedure is representative: A 100 mL Schlenk flask
was charged with cinnamyl alcohol (0.54 g, 4.0 mmol), [Rh(cod) ]BF
2
4
(0.016 g, 0.039 mmol), p-TPPMP (0.132 g, 0.240 mmol), degassed
water (20 mL) and a stirrer bar. The solution was stirred and refluxed
for 16 h under N , and the aqueous phase extracted with ether
2
(
filtered and the solvent removed in vacuo. Column chromatography on
2 ꢀ 20 mL). The organic phase was dried over anhydrous MgSO ,
4

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Isomerization in Water
831
silica gel (ethyl acetate/petroleum ether) and removal of solvent gave
dihydrocinnamaldehyde (0.21 g, 41% yield).
ACKNOWLEDGMENTS
We would like to thank the donors of the Petroleum Research Fund
PRF 31395-G3) administered by the American Chemical Society,
(
Monsanto Company and the Office of Naval Research under the Naval
Research Core Program for partial support of this work, and the ASEE
for a Summer Faculty Research Fellowship (to D. A. K.). We would also
like to thank Dr. Todd Johnson for the synthesis of [Rh(cod)Cl]2.
REFERENCES
¨
1. Slugovc, C.; Ruba, E.; Schmid, R.; Kirchner, K. Organometallics
1999, 18, 4230.
2
3
. Trost, B.M.; Kulawiec, R.J. Tetrahedron Lett. 1991, 32, 3039.
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4
5
. McGrath, D.V.; Grubbs, R.H. Organometallics 1994, 13, 224.
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. Alper, H.; Hashem, K. J. Org. Chem. 1980, 45, 2269.
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. For an overview see: (a) Li, C.-J.; Chan, T.-H. Organic Reactions in
Aqueous Media; Wiley: New York, 1997; Chapter 5; (b) Cornils, B.;
Herrmann, W.A. Aqueous-Phase Organometallic Catalysis: Concepts
and Applications; Wiley: New York, 1998.
9. Throughout the text p-TPPMP refers to the disodium salt of
para-triphenylphosphine monophosphonic acid.
1
0. Schull, T.L.; Fettinger, J.C.; Knight, D.A. Inorg. Chem. 1996,
5, 6717.
1. [Rh(phosphine) (solvent) ] systems catalyze the double bond
3
þ
2
1
2
migration of cinnamyl alcohol in organic solvents see: Bergens, S.H.;
Bosnich, B. J. Am. Chem. Soc. 1991, 113, 958.
Received in the USA February 22, 2002

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