Isomerization of allylic alcohols to carbonyl compounds by aqueous-biphase rhodium catalysis

Article abstract of DOI:10.1039/b001478h

Isomerization of allylic and homoallylic alcohols is catalyzed by the zwitterionic Rh(I) complex (sulphos)Rh(cod) in water-n-octane to give the corresponding aldehyde or ketone in high yields and chemoselectivity. A π-allyl metal hydride mechanism is proposed on the basis of various independent experiments in both homogeneous and biphasic systems [sulphos = -O₃S(C₆H₄)CH₂C(CH₂ PPh₂)₃].

Full text of DOI:10.1039/b001478h

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Isomerization of allylic alcohols to carbonyl compounds by
aqueous-biphase rhodium catalysis
Claudio Bianchini,* Andrea Meli and Werner Oberhauser
Istituto per lo Studio della Stereochimica ed Energetica dei Composti di Coordinazione,
ISSECC-CNR, 50132 Firenze, Italy. E-mail: bianchin=Ð.cnr.it
L e t t e r
Received (in Montpellier, France) 18th February 2000, Accepted 21st April 2000
First published as an Advance Article on the web 14th June 2000
Isomerization of allylic and homoallylic alcohols is catalyzed by
the zwitterionic Rh(I) complex (sulphos)Rh(cod) in water–n-
octane to give the corresponding aldehyde or ketone in high
yields and chemoselectivity. A p-allyl metal hydride mechanism
is proposed on the basis of various independent experiments in
both homogeneous and biphasic systems [sulphos =
ÔO S(C H )CH C(CH PPh ) ].
(sulphos)Rh(CO) 6 (2), which was independently shown to
2
form upon reaction of 1 with propanal (and aldehydes in
general) and to be inactive for allylic alcohol isomerization. In
homogeneous phase, where the catalyst and the aldehyde
product are in the same Ñuid system (entry 1), the formation
of 2 was apparently much faster (NMR evidence) and the
catalyst could not be re-used for a second run (90% formation
of 2).
3
6
4
2
2
2 3
The isomerization of allylic alcohols to carbonyl compounds
is an important reaction that can be accomplished with homo-
geneous,1 heterogeneous2 and phase-transfer catalysts.3
Serious drawbacks have often been encountered in the appli-
cation of this reaction to obtain synthetically useful quantities
of either aldehydes or ketones, however. Most common draw-
backs are: (i) difficult separation of the catalyst from
product(s)/reagent(s);1,3 (ii) low chemoselectivity, generally
due to competitive acetalization and dehydration reactions;1,2
(iii) sensitivity to the degree of substitution on the double
bond;1 (iv) possible catalyst deactivation via decarbonylation
of aldehydes to give stable carbonyl metal compounds.1,4
The present paper constitutes a preliminary account of the
Ðrst example of e†ective aqueous-biphase isomerization of
allylic alcohols catalyzed by a water-soluble transition metal
complex in a chemoselective manner. A comparative, in situ
and reactor, study in the homogeneous phase has provided
valuable mechanistic information, particularly on the path-
ways to catalyst deactivation. Prior to our work, the isomer-
ization of geraniol to citronellal was attempted in
In both homogeneous and biphase conditions, 2-methyl-2-
propen-1-ol (entry 2) and 3-buten-2-ol (entry 3) were isomer-
ized to 2-methylpropanal and methyl ethyl ketone, respec-
tively. The biphasic reactions gave better results in general.
Increasing the steric hindrance at the C-3 carbon atom of the
allylic moiety as in 2-buten-1-ol (95 : 5 mixture of E and Z
isomers) (entry 4) or 3-methyl-2-buten-1-ol (entry 5) resulted
in a progressive decay of the catalytic performance.1 No cata-
lytic activity was observed when two methyl groups were
introduced in the C-1 position as in 1,1-dimethyl-2-propen-1-
ol (entry 6). The homoallylic alcohol 3-buten-1-ol was isomer-
ized by 1 to butanal in both phase systems (entry 7). Like the
2-buten-1-ol isomer, the conversion to aldehyde was higher in
the homogeneous phase, however.
As a general feature, neither hydrates of aldehydes1 nor
dehydration products1,2 were observed in the biphasic reac-
tions, whereas some homogeneous reactions gave dehydration
products in the form of free and coordinated butadiene or iso-
prene. Actually, the complexes (sulphos)Rh(g4-butadiene) (3)
and (sulphos)Rh(g4-isoprene) (4) were either detected by in situ
NMR experiments or isolated as termination products of the
homogeneous reactions involving 2-buten-1-ol and 3-buten-1-
ol or 3-methyl-2-buten-1-ol, respectively.¤ The transformation
of homoallyl alcohol most likely involves its preliminary isom-
erization to the allylic isomer 2-buten-1-ol. Indeed, 1 is an effi-
cient oleÐn isomerization catalyst as shown by the conversion
of the cod ligand to free cycloocta-1,3-diene7 (1H NMR,
GC-MS) as well as the isomerization of allyl ethyl ether to
cis-propenyl ethyl ether (entry 8).
tolueneÈwater using Ni(cod) Èdppbts [tetrasulfonated 1,4-bis-
2
(diphenylphosphino)butane]. Very low conversion (1%) and
selectivity (3%) were obtained, however.5
The zwitterionic rhodium complex (sulphos)Rh(cod) (1) is
an efficient catalyst for the hydrogenation and hydro-
formylation of oleÐns in waterÈn-octane [sulphos \
[O S(C H )CH C(CH PPh ) ].6 We show here that the
3
6
4
2
2
2 3
same catalytic system can successfully be employed for the
chemoselective transformation of both allylic and homoallylic
alcohols into the corresponding aldehyde or ketone. Selected
results of aqueous-biphase and homogeneous reactions (in 1,2-
dichloroethane) are reported in Table 1.
The two established pathways for transition metal-catalyzed
isomerization of allylic alcohols are the p-allyl metal
In biphasic conditions, 100 equiv. of allylic alcohol was con-
verted to propanal within 1 h at 100 ¡C (entry 1). By simple
phase-separation, the aldehyde product ([99% in n-octane)
was separated from the catalyst, which was recycled three
times to give the following turn-over-frequencies [mol product
(mol catalyst)~1 h~1]: 490, 430 and 390. Appreciable deacti-
vation (ca. 10%) occurred after each run due to partial incorp-
oration of active rhodium into the dicarbonyl complex
Scheme 1 Proposed mechanism for the isomerization reaction of
allyl alcohol to propanal catalyzed by 1.
DOI: 10.1039/b001478h
New J. Chem., 2001, 25, 11È12
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This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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Table 1 Homogeneous and liquid-biphase isomerization of allyl alcohols and related compounds catalyzed by 1a
Entry
Substrate
Products
Homogeneous yield(%)
Liquid-biphase yield(%)
1
2
95
38
100
73
3
4
5
98
62
4
100
30
\1
4
1
\1
\1
4
9
\1
\1
6
7
none
89
33
8
14
\1
22
31
a Reaction conditions: autoclave, 100 mL; catalyst, 0.021 mmol; substrate, 2.1 mmol; 1,2-dichloroethane (homogeneous) or 1 : 1 waterÈn-octane
(liquid-biphase), 30 mL; 100 ¡C; 1 h.
hydride1c,3 and the metal hydride addition-elimination mecha-
Notes and references
nisms.1a,b A third variation is the ““internal redoxÏÏ mechanism
¤ Satisfactory elemental analyses were obtained for compounds 3 and
suggested by Trost and Kulawiec that involves the coordi-
nation of the allylic alcohol as bidentate ligand.1b,f
The results from the catalytic reactions and from various
independent experiments are consistent wih the p-allyl metal
hydride mechanism shown in Scheme 1, which is quite similar
to that proposed by Bergens and Bosnich for
4.9 Selected spectroscopic data for 3: 31PM1HN NMR (CD Cl , 81.01
2
2
MHz): 25 ¡C, d 12 (br, P ); [30 ¡C, d 14.4 [br d, J(P Rh) \ 100 Hz,
A
M
P ], 4.5 [dt, J(P P) \ 22.1 Hz, J(P Rh) \ 153.2 Hz, P ]; [70 ¡C, d
M
A
A
A
22 (br, P ), 8 (br, P ), 3.8 [br d, J(P Rh) \ 150 Hz, P ]. 1H NMR
Q
M
A
A
(CD Cl , 200.13 MHz, 25 ¡C): g4-butadiene hydrogens d 5.61 (m, 2H,
2
2
CH Hs), 2.0È1.8 (m, 4H, CH ). For 4: 31PM1HN NMR (CD Cl , 81.01
2
2 2
MHz): 25 ¡C, d 11 (br); [30 ¡C, d 13.4 (2nd order m, P -P ), d 1.2
A
B
[Rh(diphosphine)(solvent) ]` catalyst precursors.1c Common-
[ddd, J(P P) \ 23.8, 15.8 Hz, J(P Rh) \ 151.7 Hz, P ]. 1H NMR
2
M
M
M
alities between the latter Rh systems and 1 are: (i) the inter-
(CD Cl , 200.13 MHz, 25 ¡C): g4-isoprene hydrogens d 5.58 [t, 1H,
2
2
J(HH) \ 6.9 Hz, H Hs], 2.15 (m, 2H, H ] H ), 1.81 (s, 3H, Me),
1s 4s
ception of enol intermediates [Z- and E-CH(CH )2CHOH]c
3
3
1.52 (m, 2H, H ] H ).
1
along the isomerization of allylic alcohol to propanal; (ii) the
1a
4a
(a) D. V. McGrath and R. H. Grubbs, Organometallics, 1994, 13,
224; (b) B. M. Trost and R. J. Kulawiec, J. Am. Chem. Soc., 1993,
115, 2027; (c) S. H. Bergens and B. Bosnich, J. Am. Chem. Soc.,
1991, 113, 958; (d) C. Bianchini, E. Farnetti, M. Graziani, M.
Peruzzini and A. Polo, Organometallics, 1993, 12, 3753; (e) F. P.
Pruchnik, P. Smolenski and K. WajdaÈHermanowicz, J.
Organomet. Chem., 1998, 570, 63; ( f ) C. Slugovc, E. Ruba, R.
Schmid and K. Kirchner, Organometallics, 1999, 18, 4230.
C. HoangÈVan and O. Zegaoui, Appl. Catal. A, 1997, 164, 91.
H. Alper and K. Hachem, J. Org. Chem., 1980, 45, 2269.
(a) J. P. Collman and L. S. Hegedus, Principles and Applications
of Organotransition Metal Chemistry, University Science Books,
Mill Valley, CA, 1980, ch. 9, p. 496; (b) C. M. Beck, S. E. Rath-
mill, Y. J. Park, J. Chen, R. H. Crabtree, L. M. LiableÈSands and
A. L. Rheingold, Organometallics, 1999, 18, 5311; (c) This work.
H. Bricout, E. MonÑier, J.-F. Carpentier and A. Mortreux, Eur. J.
Inorg. Chem., 1998, 1739.
selective incorporation of deuterium in the C-2 position of
propanal when allylic alcohol is isomerized in D O; (iii) the
2
failure to isomerize 1,1-dimethyl-2-propen-1-ol. Unlike the
cationic Rh-diphosphine systems, the zwitterionic Rh-sulphos
fragment also converts homoallylic alcohol into butanal due
to its peculiar ability to isomerize oleÐns.7
In conclusion, we have shown here that aqueous-biphase
catalysis can be successfully employed for the isomerization of
both allylic and homoallylic alcohols to carbonyl compounds.
The main advantage of the present system over homogeneous
systems is the facile catalyst recycling by simple phase separa-
tion with no loss of rhodium metal.
Further modiÐcation of sulphos-based catalysts (di†erent
metals, introduction of stereocenters), as well as extension of
the range of substrates and prospects for asymmetric
induction8 remain exciting avenues to explore for the clean
and selective transformation of allylic residues into aldehyde
or ketone functional groups by aqueous-biphase catalysis.
2
3
4
5
6
(a) C. Bianchini, P. Frediani and V. Sernau, Organometallics,
1995, 14, 5458; (b) C. Bianchini, A. Meli, V. Patinec, V. Sernau
and F. Vizza, J. Am. Chem. Soc., 1997, 119, 4945.
7
Complex 1 catalyzes the isomerization of ca. 90 equiv. of
cycloocta-1,5-diene to cycloocta-1,3-diene in waterÈMeOHÈn-
octane (1 : 1 : 2; v : v : v) at 100 ¡C in 1 h.
C. Botteghi and G. Giacomelli, Gazz. Chim. Ital., 1976, 106, 1131.
Compounds 3 and 4 were isolated by column chromatography as
yellowÈorange solids from the catalytic solutions.
Acknowledgements
MURST and CNR (legge 95/95) are gratefully acknowledged
for Ðnancial support.
8
9
12
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