Chemoselective Hydrogen Transfer Reduction of Unsaturated Ketones to Allylic Alcohols with Solid Zr and Hf Catalysts

Article abstract of DOI:10.1002/1615-4169(200212)344:10<1120::AID-ADSC1120>3.0.CO;2-4

α,β-Unsaturated ketones were reduced to allylic alcohols with high chemo- and diastereoselectivity, using Zr and Hf compounds heterogenised on mesoporous molecular sieves.

Full text of DOI:10.1002/1615-4169(200212)344:10<1120::AID-ADSC1120>3.0.CO;2-4

FULL PAPERS
Chemoselective Hydrogen Transfer Reduction of Unsaturated
Ketones to Allylic Alcohols with Solid Zr and Hf Catalysts
Mario De bruyn, Dirk E. De Vos, Pierre A. Jacobs*
Center for Surface Chemistry and Catalysis, KULeuven, Kasteelpark Arenberg 23, 3001 Leuven, Belgium
Phone: ( ‡ 32)-16-321465, Fax ( ‡ 32)-16-321998, e-mail: pierre.jacobs@agr.kuleuven.ac.be
Received: May 16, 2002; Accepted: August 5, 2002
Dedicated to Roger Sheldon at the occasion of his 60th birthday.
Abstract: a,b-Unsaturated ketones were reduced to Keywords: allylic alcohols; hafnium; heterogeneous
allylic alcohols with high chemo- and diastereoselec- catalysis; a,b-unsaturated ketones; zirconium
tivity, using Zr and Hf compounds heterogenised on
mesoporous molecular sieves.
Introduction
In this paper we report on the selective reduction of a
variety of a,b-unsaturated ketones using heterogenised
Allylic alcohols are important intermediates in agro- zirconium and hafnium catalysts. Heterogeneous MPV
chemistry, pharmaceutical and fragrance chemistry. For catalysts of this type have previously only been applied
instance, a new and elegant synthesis of morphine starts to the reduction of ™simple∫ ketones, such as 4-t-
from an allyl-substituted cyclohexenol; the fragrance butylcyclohexanone.[17 19] As the hydride donor, either
(S)-a-damascone can be prepared starting from a cyclopentanol or 1-indanol are employed. Mesoporous
trimethyl-substituted cyclohexenol.[1] Next to olefin siliceous materials with mono- or tridimensional pore
autoxidation and oxygenation with singlet dioxy- structure, functionalised with Zr or Hf, are useful
gen,[2,3] chemoselective reduction of a,b-unsaturated catalysts. With truly catalytic amounts of the Zr or Hf
carbonyl compounds is an important synthetic entry to catalyst, high yields of the allylic alcohol are obtained
allylic alcohols.[4] The stoichiometric NaBH4/CeCl3 from unsaturated ketones with excellent chemo- and
system, and, most prominently, the ternary combina- diastereoselectivities.
tion of Ru(phosphine) with a diamine and a base proved
to be excellent for such reductions.[5,6] In parallel to these
homogeneous reagents or catalysts, considerable effort Results and Discussion
has been done to develop heterogeneous catalysts for
this type of reduction. Especially the combination of
group VIII metals (Pt, Rh, Ru) and some metal
Survey of the General Reaction Parameters
additives (Sn, Fe) is suited for the selective reduction
of unsaturated aldehydes to allylic alcohols.[7,8] How- In order to identify the best reaction conditions, the
ever, when applied to the selective reduction of a,b- Zr(O-n-Pr)n-MCM catalysts were tested with benzal-
unsaturated ketones, these catalysts often show high acetone (trans-4-phenyl-3-buten-2-one). This unsatu-
selectivity for the saturated ketone or the alcohol.[9] As rated ketone is highly reactive in MPV reduction,
an alternative to metal-catalysed hydrogenations, hy- probably due to the conjugation of the enone group
drogen transfer reactions of the Meerwein Ponndorf
with the aromatic nucleus. Initial reactions showed that
Verley (MPV) type form an attractive approach.[10 12] with the solid catalyst and cyclopentanol as a hydrogen
This reaction is intrinsically selective since only the donor, complete selectivity for the allylic alcohol was
carbonyl group coordinates with the Lewis acid reaction obtained. Addition of molecular sieves strongly in-
centre, while the double bond remains unactivated. creased the rate, probably by efficient trapping of water
MPV reactions are often performed with stoichiometric traces (compare Table 1 entries 1 and 2). Particularly
amounts of soluble metal compounds.[10] The use of 4 ä molecular sieve is suitable; with the Ca-containing
heterogeneous catalysts has received much less atten- 5 ä material acid-catalysed side reactions such as
tion,[13 16] while reactions with unsaturated ketones have etherifications or dehydrations may occur. An impor-
hardly been studied.[12]
tant reaction parameter is the reductant over enone
ratio. An optimum was observed at a cyclopentanol over
1120
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1615-4150/02/34410-1120 1125 $ 17.50-.50/0
Adv. Synth. Catal. 2002, 344, No. 10

Chemoselective Hydrogen Transfer Reduction of Unsaturated Ketones
FULL PAPERS
Table 1. Effect of reaction conditions on MPV reduction of
benzalacetone with solid Zr catalysts.
parameters are the presence and nature of the support
and the molecular sieve, as well as the amount and the
nature of the reducing alcohol. A discussion of the
influence of the nature of the metal and of the precursor
follows.
Entry
Conditions[a]
t [h]
Conversion
[%][b]
1
2
3
4
5
6
7
8
9
no 4 ä mol. sieve
22
22
22
22
22
22
3
26
91
42
55
29
79
10
22
30
with 4 ä mol. sieve
2 equiv. cyclopentanol
5 equiv. cyclopentanol
10 equiv. cyclopentanol
10 g heptane solvent
Zr(O-n-Pr)4
Scope of the Reaction
The extension of the scope of the reaction from
benzalacetone to other groups of enones is shown in
Table 2. Clearly aliphatic as well as alicyclic unsaturated
enones can be reduced smoothly to allylic alcohols.
Even if the enone group is not activated, but simply part
of an aliphatic chain, as in a-ionone, an 89% yield is
eventually obtained (entry 6). Remarkably not only the
Zr(O-n-Pr)n-MCM-41
Zr(O-n-Pr)n-MCM-48
3
3
[a]
General conditions for substrate, catalyst, reductant,
solvent, unless specified otherwise: 2 mmol benzalace-
tone, 0.1 mmol Zr catalyst as 50 mg Zr(O-n-Pr)n-MCM-41,
10 mmol (5 equiv.) cyclopentanol, 100 mg 5 ä molecular
sieve (entries 3 9) or 4 ä (entry 2), 6 g heptane, 75 8C.
Chemoselectivity was in all cases superior to 98%, hence
the enone conversion practically equals the allylic alcohol
yield.
ˆ
C C double bond in the enone group of a-ionone, but
also the endocyclic C C bond are left totally intact. In
ˆ
[b]
the initial phase of the reaction, where kinetics
dominate thermodynamics, the relative reactivity order
can be written as follows: isophorone < 3,5-dimethyl-2-
cyclohexen-1-one < a-ionone ꢀ 3-methyl-2-cyclohex-
en-1-one ꢀ R-carvone < b-ionone < 2-cyclohexen-1-
benzalacetone ratio of about 5 (entries 3 5). This one ꢁ benzalacetone.
optimum can be explained by competitive adsorption
Within the group of cyclohexenones, it is clear that an
of the reactants on the surface; a too high alcohol increasing number of alkyl substituents leads to a
concentration probably impedes sufficient adsorption decreased reactivity, partially due to electronic effects.
of the enone. Solvent composition is important as well; In, e.g., 3-methyl-2-cyclohexen-1-one, the presence of
ˆ
thus the enone conversion is faster if the volume of the an alkyl group in the b position to the C O group
heptane solvent is increased (entries 6 vs. 4). This agrees decreases the susceptibility of the carbonyl group to
with the idea that coordination of alcohol and ketone on hydrogen transfer. On the other hand, steric effects are
MPV catalysts is easiest in apolar media.[10]
probably important as well, as demonstrated by the low
The presence and the nature of the siliceous support reaction rate of the trimethyl-substituted isophorone.
have a remarkable effect on the activity of the catalyst
(Table 1, entries 7 9). In experiments with identical
total Zr concentrations, a homogeneous catalyst was Effect of Metal Precursor and Nature of the Metal
compared with heterogeneous Zr catalysts based on
MCM-41 or MCM-48. With Zr(O-n-Pr)n-MCM-41 and Catalysts prepared from different Zr sources, viz. Zr(O-
Zr(O-n-Pr)n-MCM-48, the reaction is between 2 and n-Pr)4 and ZrCp2Cl2 were compared in several enone
3 times faster than with the homogeneous Zr com- reductions. If a strong reductant such as indanol is used
pound. The rather low catalytic activity of dissolved in a facile reaction with, e.g., benzalacetone, differences
Zr(O-n-Pr)4 is probably due to self-association of the between both catalysts are minor (Table 3, entry 2a).
homogeneous alkoxides or even to formation of oxide However, if a less potent reductant or a less reactive
particles in solution. These phenomena considerably substrate is used, the zirconocene-based catalyst shows
lower the Lewis acidity of the metal centres and the better performance than the material based on the
number of available free coordination sites. In contrast, alkoxide. For instance, in the reduction of isophorone
the surface of the MCM materials provides many with 1-indanol, Zr(Cp)n-MCM-41 is more active than
docking points for the Zr alkoxides, resulting in a Zr(O-n-Pr)n-MCM-41 (Table 3, entry 1). Similarly, the
more dispersed and reactive material.[16] The tridimen- former catalyst gives better product yields in benzal-
sional pore architecture of MCM-48 seems to facilitate acetone reduction with cyclopentanol (Table 3, en-
the access to the large internal surface area. Together try 2b). The high activity of the zirconocene-based
with the observation that the reaction practically stops catalyst is likely related to the high dispersion degree
when the Zr-loaded MCM-41 is removed from the of Zr, and to its slightly higher metal content. Anchoring
reaction suspension, the acceleration of the Zr catalysis of metallocene chlorides to a siliceous surface is known
by the MCM supports is a clear indication that surface- to result in mononuclear species,[21] while clusters are
associated, heterogeneous Zr species are responsible for easily formed when large amounts of metal alkoxides
the catalytic activity. Summarising, critical reaction are supported on SiO2 type materials.[22]
Adv. Synth. Catal. 2002, 344, 1120 1125
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Mario De bruyn et al.
FULL PAPERS
Table 2. MPV reduction of a variety of a,b-unsaturated ketones over a heterogenised Zr(O-n-Pr)n-MCM-41 catalyst.[a]
Entry
1
Substrate
Reductant
t [h]
Yield,
[%]
Chemoselectivity[b]
[%] (de, %)
benzalacetone
1-indanol
4
94
98
2
3
2-cyclohexen-1-one
1-indanol
1-indanol
4
5
89
62
94
R-carvone
97 (92 cis)
4
5
3-methyl-2-cyclohexen-1-one
1-indanol
1-indanol
5
5
68
52
95
3,5-dimethyl-2-cyclohexen-1-one
95 (83 cis)
6
7
a-ionone
b-ionone
1-indanol
1-indanol
9
9
89
80
93
94
8
isophorone
1-indanol
29
48
23
20
97
9
R-carvone
cyclopentanol
98 (78 cis)
[a]
Reaction conditions: 2 mmol substrate, 10 mmol 1-indanol, 6 g heptane, 50 mg catalyst (Si/Zrˆ 5), 100 mg molecular sieve
4 ä, 75 8C.
[b]
Chemoselectivity to allylic alcohol.
ˆ
ˆ
Table 3. Comparison between zirconocene [Zr(Cp)n-MCM-41] ( A)and Zr-alkoxide [Zr(O-n-Pr)n-MCM-41] ( B) based
catalysts in the reduction of enones to allylic alcohols.[a]
Entry
Substrate
Reductant
t [h]
Yield [%]
Yield [%]
with A
with B
1
isophorone
1-indanol
24
4
20
90
32
33
94
53
2a
benzalacetone
1-indanol
2b
cyclopentanol
3
[a]
Reaction conditions: 1 mmol substrate, 5 mmol 1-indanol or cyclopentanol, 6 g heptane, 25 mg catalyst, 100 mg molecular
sieve 4 ä, 75 8C; yields are given for allylic alcohols.
In order to evaluate metal leaching and catalyst the filtrate is negligible (33% after 90 minutes), while
reusability, a test was performed for the benzalacetone addition of fresh reactants to the recuperated catalyst
reduction with indanol in heptane, using Zr(Cp)n- shows that the latter still has a high reactivity (from 0 to
MCM-41 as the catalyst. At 30 minutes and an enone 66% conversion in 90 minutes). Thus, the reaction is
conversion of 32%, solution and catalyst were separated truly heterogeneously catalysed by the solid material.
by centrifugation. Further conversion of the enone in
1122
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Chemoselective Hydrogen Transfer Reduction of Unsaturated Ketones
FULL PAPERS
Figure 2. Yields of allylic alcohols vs. time, for the reduction
of enones with 5 mol % of Zr(Cp)n (open symbols) and
Hf(Cp)n (filled symbols) catalysts grafted on MCM-41 and
with 1-indanol as hydride donor, starting from following
Figure 1. Yields of allylic alcohols vs. time, for the reduction
of enones with 5 mol % of Zr(Cp)n (open symbols) and
Hf(Cp)n (filled symbols) catalysts grafted on MCM-41 and
with 1-indanol as hydride donor, starting from following
&
&
~
enones: carvone ( , ), 3,5-dimethyl-2-cyclohexen-1-one, ( ,
~
* *
) and isophorone ( , ). Conditions as in Table 2.
&
&
^
^
~ ~
enones: benzalacetone ( , ), a-ionol ( , ), b-ionol ( , ),
*
*
and mesityl oxide ( , ). Conditions as in Table 2.
higher for the Zr catalyst than for the Hf catalyst. For
instance in the reduction of 3,5-dimethyl-2-cyclohexen-
As shown for different substrates in Figures 1 and 2, 1-one, the initial de in the allylic alcohol product for Zr
the rates can also be substantially increased by sub- and Hf amounts to 89% and 83%, respectively. This is
stitution of Zr for Hf. Catalysts based on the metal- clearly due to the smaller size of the Zr ion, which causes
locenes ZrCp2Cl2 and HfCp2Cl2 were compared. De- more steric crowding in the coordination sphere, hence
pending on the substrate, the rate increase in the initial yielding higher de values. This effect is less pronounced
phase of the reaction is on the average 1.5- to 3-fold, in the reduction of carvone, for which the de is only 2%
except for sterically hindered substrates such as iso- smaller with Hf than with Zr (90 vs. 92%). Clearly, the
phorone. With indanol as a reductant, high yields are larger size of the isopropenyl group in carvone results in
obtained within reasonable time with relatively small a more effective stereocontrol of the ketone reduc-
amounts of catalyst (5 mol %). Recommended standard tion.
MPV protocols with homogeneous catalysts employ
As shown in Figure 3, the diastereomeric excess
regularly between 20 and 100 mol % of metal cata- values change in time. In the initial, kinetically con-
lyst.[10]
trolled phase of the reaction, the de is high and almost
constant and the cis-allylic alcohols prevail. When
towards the end of the reaction thermodynamics start
to control the product distribution, the de decreases as
more of the trans-products are formed, such as trans-3,5-
Diastereoselective and Enantioselective Reactions
With complex enones such as terpenic substrates, the dimethyl-2-cyclohexen-1-ol and trans-carveol. The epi-
reactions not only display high chemoselectivity, but merisation seems to be a catalytic process; it is clearly
also proceed diastereoselectively. For instance, R- faster with the heterogeneous Hf catalyst than with the
carvone and 3,5-dimethyl-2-cyclohexen-1-one after 4 h Zr material. This trend is essentially the same as for the
gave rise to a diastereomeric excess (de) of 92 and 83%, actual enone reduction, and may be related to a higher
respectively (Table 2, entries 3 and 5). Interestingly, the ligand exchange rate on the larger Hf metal centre. The
diastereomeric excess proved to be dependent on the lowering of the de is probably due to back-reaction of
nature of the reductant: a higher de value was obtained the initially formed carveol, with predominant cis-
with the sterically more demanding 1-indanol than with configuration, with either carvone or indanone. In
cyclopentanol (Table 2, entries 3 and 9). This indicates control reactions of pure cis-carveol (91% de) with 1-
that for the heterogeneous MPV reaction, reductant and indanone or carvone over the Zr(Cp)n-MCM-41 cata-
enone must simultaneously coordinate to the reactive lyst, a substantial lowering of the carveol de was
centre, in analogy to what happens in homogeneous observed, proving that the loss of de is related to the
catalysis. Similarly, initial de values are significantly reversibility of the reaction.
Adv. Synth. Catal. 2002, 344, 1120 1125
1123

Mario De bruyn et al.
FULL PAPERS
beneficial conditions for obtaining ee×s up to 60%
(Figure 4). This is to our knowledge the first example of
a heterogeneously performed enantioselective transfer
reaction using an MPV type of catalyst. Moreover, even
in homogeneous MPV catalysis few reports exist of
effective chiral transfer reactions; often the substrate
scope is limited, or ee values are rather low.[23,24]
Conclusion
Zr and Hf catalysts were prepared by immobilisation of
alkoxide or metallocene precursor complexes on or-
dered siliceous matrices such as MCM-41. In optimised
reaction conditions, and with suitable alcohols as
reductants, these materials catalyse the chemoselective
reduction of most unsaturated ketones to the corre-
sponding allylic alcohols. The materials based on Hf are
more active catalysts than those containing Zr. More-
over, the metallocene preparation procedure yields
more active materials than the metal-alkoxide routes. In
addition to the high chemoselectivity, a high diastereo-
selectivity is observed for reactions with natural chiral
molecules such as carvone. Finally, it is demonstrated
that chirality can be efficiently transferred from a chiral
reductant to a substrate enone, resulting in ee×s up to
60% for the allylic alcohol.
Figure 3. MPV reduction of 3,5-dimethyl-2-cyclohexen-1-one
with as catalysts Zr(Cp)n-MCM-41 (open symbols) and
Hf(Cp)n-MCM-41(filled symbols), and with 1-indanol as
hydride donor: yield of cis-3,5-dimethyl-2-cyclohexen-1-ol
~
~
^ ^
( , ), yield of trans-3,5-dimethyl-2-cyclohexen-1-ol ( , ),
&
&
and diastereomeric excess ( , ). Conditions as in Table 2.
Experimental Section
Preparation of Mesoporous Supports
MCM-41 was prepared as follows. 19.43 g of a 20 wt. %
tetraethylammonium hydroxide (TEAOH) solution, 16.16 g
cetyltrimethylammonium chloride (CTMACl) and 20 mL of
water were consecutively added to 19.27 g of LUDOX AS-40
(Dupont) under vigorous stirring. After 15 minutes an addi-
tional amount of 32.33 g CTMACl and 20 mL H2O were added.
The resulting mixture was stirred for another 15 minutes. The
mixture was transferred into a stainless steel autoclave. The
rotating autoclave was heated at 110 8C for 24 h. MCM-48 was
prepared according to the procedure reported by Kevan et
al.[20]
Figure 4. Influence of the temperature on the allylic alcohol
yield (filled symbols) and the enantiomeric excess (open
symbols) in the reduction of 2-cyclohexen-1-one with a 3-fold
excess of (S)-1-indanol as hydride donor and with 5 mol %
~
~
^ ^
Hf(Cp)n-MCM-41 as the catalyst: 60 8C ( , ), 80 8C ( , ),
&
&
100 8C ( , ). Other conditions as in Table 2.
Grafting of Alkoxides
After calcination, 1 g of siliceous MCM material was predried
at 200 8C, and contacted with a solution of 0.936 g Zr(O-n-Pr)4
(as a 70 wt % solution in 1-propanol) in 10 g of hexane for 3 h
at room temperature. The solid was filtered under N2, washed
with an excess of hexane and finally dried under a helium flow.
Elemental analysis indicated a typical Si/Zr ratio of 6. XRD
measurements, as well as SEM, show that the structure of the
MCM-41 or MCM-48 materials remains unaltered. The
material containing the grafted alkoxides is denoted as Zr(O-
n-Pr)n-MCM (Scheme 1, a); Zr(O-n-Pr)n-MCM-41 typically
Finally, the Hf and Zr MPV catalysts were not only
applied to diastereoselective but also to enantioselec-
tive reactions. In the reduction of 2-cyclohexen-1-one,
enantiomerically pure (S)-1-indanol was used as a
reductant instead of the racemic alcohol. The results
show that it is indeed possible to transfer chirality onto
the allylic alcohol in an effective way. The ee value
depends on the used excess of (S)-1-indanol as well as on
the temperature. A high concentration of the chiral
hydride donor and low temperature are the most contains 0.81 mmol Zr per g.
1124
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Chemoselective Hydrogen Transfer Reduction of Unsaturated Ketones
FULL PAPERS
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Acknowledgements
The financial support of the Belgian Federal government is
gratefully acknowledged (IUAP Supramolecular Catalysis).
MDb is indebted to IWT for a fellowship.
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Adv. Synth. Catal. 2002, 344, 1120 1125
1125