A well-defined monomeric aluminum complex as an efficient and general catalyst in the Meerwein-Ponndorf-Verley reduction

Article abstract of DOI:10.1002/chem.201404994

The metal-catalyzed Meerwein-Ponndorf-Verley (MPV) reduction allows for the mild and sustainable reduction of aldehydes and ketones but has not found widespread application in organic synthesis due to the high catalyst loading often required to obtain satisfactory yields of the reduced product. We report here on the synthesis and structure of a sterically extremely overloaded siloxide-supported aluminum isopropoxide capable of catalytically reducing a wide range of aldehydes and ketones (52 examples) in excellent yields under mild conditions and with low catalyst loadings. The unseen activity of the developed catalyst system in MPV reductions is due to its unique monomeric nature and the neutral donor isopropanol weakly coordinating to the aluminum center. The present work implies that monomeric aluminum alkoxide catalysts may be attractive alternatives to transition-metalbased systems for the selective reduction of aldehydes and ketones to primary and secondary alcohols.

Full text of DOI:10.1002/chem.201404994

DOI: 10.1002/chem.201404994
Communication
&
Synthetic Methods
A Well-Defined Monomeric Aluminum Complex as an Efficient and
General Catalyst in the Meerwein–Ponndorf–Verley Reduction
Brian McNerney,[a] Bruce Whittlesey,[a] David B. Cordes,[b] and Clemens Krempner*[a]
Dedicated to Prof. Gerhard Roewer on the occasion of his 75th birthday
as an inexpensive, safe and nontoxic reducing agent.[5] The
MPV reduction proceeds via hydride transfer from a secondary
alcohol to a carbonyl compound mediated by coordination to
a Lewis acidic metal center,[6] most often aluminum[7] but also
other metals have been employed[8] (Scheme 1).
Abstract: The metal-catalyzed Meerwein–Ponndorf–Verley
(MPV) reduction allows for the mild and sustainable reduc-
tion of aldehydes and ketones but has not found wide-
spread application in organic synthesis due to the high
catalyst loading often required to obtain satisfactory
yields of the reduced product. We report here on the syn-
thesis and structure of a sterically extremely overloaded
siloxide-supported aluminum isopropoxide capable of cat-
alytically reducing a wide range of aldehydes and ketones
(52 examples) in excellent yields under mild conditions
and with low catalyst loadings. The unseen activity of the
developed catalyst system in MPV reductions is due to its
unique monomeric nature and the neutral donor isopro-
panol weakly coordinating to the aluminum center. The
present work implies that monomeric aluminum alkoxide
catalysts may be attractive alternatives to transition-metal-
based systems for the selective reduction of aldehydes
and ketones to primary and secondary alcohols.
Scheme 1. Commonly accepted mechanism of the MPV reaction.
From the prospective of sustainability aluminum is an attrac-
tive catalyst component as it is the most abundant metal on
earth (only challenged by iron), environmentally benign and of
relatively low toxicity. Many of its inorganic and organometallic
compounds are commercially available at low cost. Despite
these advantages, the homogeneous aluminum alkoxide medi-
ated MPV reduction has not found widespread utility in syn-
thetic organic chemistry and natural product synthesis,[9] pri-
marily due to the high catalyst loadings required to obtain sat-
isfactory yields of the reduction product and the rather narrow
substrate scope.[7,8,10] The low activity of simple aluminum alk-
oxides and aryloxides can largely be attributed to the forma-
tion of AlÀO bridged aggregated structures. These very stable
aggregates seem to persist even in solution resulting in coordi-
natively saturated aluminum centers of reduced Lewis acidity.
We envisioned that a sterically encumbered bidentate disil-
oxide ligand, which we developed recently,[11] would prevent
the resulting cyclic aluminum complex from being aggregated
and further enhance the electrophilicity of aluminum center
owing to the electron-withdrawing properties of the siloxide
groups.[12] Herein, we will demonstrate for the first time that
this conceptually new approach, leads to a well-defined and
thermally robust aluminum isopropoxide that is monomeric
and thereby an exceptionally active catalyst in the MPV reduc-
tion of ketones and aldehydes.
The recent years have witnessed tremendous research efforts
in sustainable catalysis primarily aimed at the use of readily
available feedstocks, the development of metal-catalyzed
atom-economic reactions, the reduction of chemical waste and
most importantly the replacement of precious metals with
more abundant and less toxic main group or first-row transi-
tion metals. Classic examples of this development are iron-cat-
alyzed hydrogenations,[1] transfer hydrogenations[2] and hydro-
silylations[3] of aldehydes and ketones to primary and second-
ary alcohols, respectively, which are important synthetic inter-
mediates in the pharmaceutical and fine chemical industry.[4]
Less explored in this regard is the classical and highly sus-
tainable metal-catalyzed Meerwein–Ponndorf–Verley (MPV) re-
duction of ketones and aldehydes, which exhibits exceptional
chemoselectivity, is operationally simple and uses isopropanol
[a] Dr. B. McNerney, Prof. Dr. B. Whittlesey, Prof. Dr. C. Krempner
Department of Chemistry and Biochemistry, Texas Tech University
Box 1061, Lubbock, Texas, 79409-1061 (USA)
E-mail: clemens.krempner@ttu.edu
The straightforward synthesis of the aluminum methyl com-
plex 2 and the aluminum isopropoxide 3, is illustrated in
Scheme 2. Compound 3 can conveniently be prepared in
a one-pot procedure starting from the sterically encumbered
1,4-disilane-diol 1[11] in yields of 70–80%. Isopropoxide 3 is
[b] Dr. D. B. Cordes
School of Chemistry, St. Andrew University
Purdie Building Fife KY16 9ST, St. Andrews (UK)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201404994.
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a thermally stable crystalline material that is moisture-sensitive
but air-stable. Note that the alcoholysis of the AlÀCH3 bond of
2 occurs only very slowly to produce the isopropoxide 3. The
half life of 2 in dry isopropanol is about 24 h at 258C and 2 h
at 508C demonstrating the very effective steric protection of
the aluminum center of 2. Diffusion experiments (1H DOSY
NMR spectroscopy) of monomeric 2 and 3 at 258C in C6D6
reveal both compounds to have very similar diffusion coeffi-
cients clearly indicating a monomeric structure for 3 in solu-
tion.
Scheme 2. Synthesis of aluminum disiloxide 3.
The results of an X-ray data analysis of 3 further confirm its
monomeric nature and the tremendous steric protection of
the tetracoordinated aluminum center (Figure 1). Single crys-
tals of two monomeric forms, 3a and 3b, were grown from
isopropanol. Both contain in the solid state one molecule of
isopropanol weakly coordinating to the aluminum center [3a:
Al1ÀO3 182.4 pm; 3b: Al1ÀO4 182.1 pm]. Form 3a contains
two additional highly disordered isopropanol molecules locat-
ed in the periphery, while in 3b the two isopropanol molecules
are in proximity to the aluminum center, connected via an
array of hydrogen bonds. Note that intermolecular donor ad-
ducts of neutral, monomeric aluminum isopropoxides are ex-
tremely rare,[13] and there are no examples of monomeric iso-
propanol adducts.
Figure 1. Solid-state structures of 3a (top, disordered iPrOH molecules are
not shown) and 3b (bottom). All H atoms are omitted for clarity (black=car-
bon, dark gray=silicon). Selected distances [pm] and angles [8]: 3a: Si1ÀO1
162.5(2), Si8ÀO2 163.3(2), Al1ÀO1 171.0(2), Al1ÀO2 171.0(2), Al1ÀO3 182.4(2),
Al1ÀO4 174.5(2), O3ÀC31 145.5(4), O4ÀC34 143.1(4), O1-Al1-O2 115.4(1), O4-
Al1-O3 104.4(1), Si1-O1-Al1 144.2(1), Si8-O2-Al1 140.1(1); 3b: Si1ÀO1 163.6(2),
Si8ÀO2 163.5(2), Al1ÀO1 170.8(2), Al1ÀO2 170.6(2), Al1ÀO3 176.6(2), Al1ÀO4
182.1(2), O2-Al1-O1 113.1(1), O3-Al1-O4 97.4(1), Si1-O1-Al1 142.0(1), Si8-O2-
Al1 143.9(1).
Table 1. Aluminum-catalyzed MPV reduction of cyclohexanone.[a]
Next, 3 and the commercially available simple alkoxides
Al(OiPr)3, Al(OsecBu)3, Al(OtBu)3 and AlMe3 were tested as cata-
lysts in the MPV reduction. Cyclohexanone as a model sub-
strate and 5 equiv of dry iPrOH as the reducing agent (Table 1)
were used under equilibrium conditions (sealed scintillation
vial in a glove box). Catalyst 3 (0.5 mol%) catalyzed the quanti-
tative reduction of cyclohexanone within 4–6 h, while 1 mol%
of Al(OiPr)3, Al(OsecBu)3, Al(OtBu)3 and AlMe3, respectively, re-
duced only about 30–80% of cyclohexanone after 24 h. The
observation that 2 only slowly converts into 3 (t1/2 ~24 h) at
room temperature and that by far not all of catalyst 3 was dis-
solved in iPrOH encouraged us to further reduce the catalyst
loading under otherwise similar conditions. The results re-
vealed exceptional activity of 3 even at loadings of 0.05 mol%;
TOF’s of up to 103 were achieved at 608C (entry 10).
Entry
Catalyst
Loading
[mol%]
Yield
[%][b]
t
[h]
TON
TOF
Yield
[%][c]
[hÀ1
]
1
2
3
4
5
6
7
8
9
10
Al(OiPr)3
Al(OsecBu)3
Al(OtBu)3
AlMe3
1
1
1
1
10
32
32
35
22
96
90
98
25
94
10
5
10
32
32
35
44
192
900
980
500
1880
1
6
6
8
10
20
48
50
80
35
>99
>99
>99
>99
>99
5
4.5
4.5
4.5
4.5
2
AlMe3
0.5
0.5
0.1
0.1[d]
0.05
0.05[d]
3
3
3
3
3
43
200
490
250
940
2
2
[a] The experiments were carried out in a sealed 20 mL scintillation vial
(plastic screw cap equipped with a Teflon liner); conditions: room tem-
perature, 5 equiv iPrOH, no co-solvent; [b] determined by NMR spectros-
copy; [c] final yields determined by NMR spectroscopy after 24 h;
[d] 608C.
To demonstrate the efficiency and scope of the MPV reduc-
tion with 3 as the catalyst a range of aromatic and aliphatic al-
dehydes were reduced in the presence of excess isopropanol
(Table 2). Excellent catalyst activities were noted for most of
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the aldehydes and almost all of them could fully be converted
to the corresponding primary alcohols. Astonishingly, unde-
sired side reactions such as Tishchenko and aldol reactions
were not seen with these reduction conditions, which is re-
markable given the susceptibility of aliphatic and aromatic al-
dehydes to readily undergo ester formation via Tishcenko reac-
tion.
could almost completely be reduced with 3 as the catalyst
system (Table 3). Again, the electron-withdrawing 4-nitro and
4-CF3 substituted acetophenones (entries 26, 27) reacted faster
than the electron-rich 2-OMe and 4-OMe substituted acetophe-
nones as well as acetyl-thiophene and -furan (entries 32, 41,
45, 46). Astonishingly, even the notoriously difficult 2,6-dime-
thoxy-acetophenone (entry 42)[8g] could also quantitatively be
reduced. The bulky a,a,a-trifluoro- and a,a,a-trimethyl-aceto-
phenones (entry 37 and 38) were more challenging and re-
quired extensive heating to achieve moderate to good yields
of the corresponding alcohols.
As expected, the more electron-rich 4-methoxy-, 2-methoxy-
and 2,3-dimethoxy substituted benzaldehydes (entries 16–18)
reacted more slowly than 4-nitro- and 4-cyano-benzaldehydes
(entries 12, 13). Other functional groups were also tolerated as
demonstrated for the quantitative reductions of thiophene-,
furan- and 3-pyridine-carboxaldehydes (entries 14, 19–21). The
fact that 2-pyridine-carbaldehyde was not reduced at all ap-
pears to be due to its excellent chelating properties, leading to
the deactivation of the catalyst by irreversible binding to the
aluminum center. Astonishingly, challenging a,b-unsaturated
aldehydes such as cinnamaldehyde, myrtenal and citral (en-
tries 23–25) can selectively be reduced, albeit under more forc-
ing conditions and with slightly higher catalyst loadings.
Under classical MPV conditions, reductions of most aromatic
ketones to secondary alcohols are more difficult to achieve,
thus often requiring elevated temperatures to readily equili-
brate and a large excess of reducing agent to shift the equilib-
rium in favor of the products. Increasing the iPrOH to catalyst
ratio, on the other hand, may suppress substrate binding and
hence lowers the reaction rate. Therefore, we were pleased to
see that a variety of differently substituted acetophenones
The ability to conveniently reduce cyclic and acyclic aliphatic
ketones to secondary alcohols under mild conditions is of high
importance in natural product synthesis. Therefore, a range of
cyclic ketones of various size and cyclohexanone derivatives
that contain functional groups were screened as these struc-
tural motifs are incorporated in many drugs and natural prod-
ucts (Table 4). S-, O- and N-containing cyclohexanones can
smoothly and quantitatively be reduced (entries 47–51). Vari-
ous aliphatic ketones of different steric profile could also readi-
ly be reduced (entries 52–57). Note that the sterically hindered
pinacolone (entry 55) converted quantitatively to the corre-
sponding alcohol, while at least 50% of the extremely bulky
diisopropylketone (entry 56) converted to the corresponding
alcohol clearly demonstrating that even sterically challenging
substrates can be reduced with the MPV methodology.
Furthermore, 5a-cholestane-3-one, a synthetic intermediate
in the chemistry of steroids, was tested and also found to be
Table 2. Aluminum siloxide catalyzed MPV reduction of aromatic and aliphatic aldehydes to primary alcohols.[a]
Entry
Product
3
iPrOH
[equiv]
t
[h]
T
[8C]
Yield
[%][b]
Entry
Product
3
iPrOH
[equiv]
t
[h]
T
[8C]
Yield
[%][b]
[mol%]
[mol%]
11
12
13
14
0.2
0.2
0.2
0.5
5
5
5
5
24
24
24
24
50
40
40
40
>99
>99
>97
>99
19
20
21
22
0.5
0.5
0.7
0.5
25
25
25
25
6
6
50
50
60
25
>99
>99
>99
>99
24
24
15
16
0.5
0.5
5
24
6
50
50
>99
>99
23
24
1
3
5
24
24
80
80
>99
>99
25
0.5
17
18
0.5
0.5
25
25
6
6
50
50
>99
>99
25
0.5
5
24
80
90
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner), no co-solvent; [b] determined by
NMR spectroscopy.
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Table 3. Aluminum siloxide catalyzed MPV reduction of aromatic ketones to secondary alcohols.[a]
Entry
Product
3
iPrOH
[equiv]
t
[h]
T
[8C]
Yield
[%][b]
Entry
Product
3
iPrOH
[equiv]
t
[h]
T
[8C]
Yield
[%][b]
[mol%]
[mol%]
26
27
0.5
0.5
25
25
24
24
50
50
98
98
37
38
0.5
0.5
25
25
48
24
80
80
61
62
28
29
30
0.5
0.5
0.5
25
25
25
24
24
24
50
50
50
93
96
98
39
40
41
0.5
0.7
0.5
25
25
25
24
48
24
80
80
80
93
94
98
31
0.5
25
24
50
93
42
43
0.5
0.5
25
25
24
24
80
50
98
95
32
33
0.5
0.7
25
25
48
24
80
60
85
>99
44
45
46
0.5
0.5
0.5
25
25
25
24
21
24
80
50
80
93
90
83
34
35
36
0.7
0.5
0.5
25
25
25
24
48
24
60
50
50
93
95
85
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner), no co-solvent; [b] determined by
NMR spectroscopy.
reduced to the respective a,b-cholesterol (ratio a/b=67:33) in
almost quantitative yields under very mild conditions and with
low catalyst loadings (Scheme 3). In contrast, under flow condi-
tions with ZrO2 as catalyst at 1308C only 60% of the product
was obtained.[8g] Encouraged by these results, the reduction of
oxo-lanosterol was attempted as well. Although by far more
sterically demanding than 5a-cholestane-3-one, complete re-
duction could be achieved at slightly higher temperature (ratio
a/b=25:75) to yield a,b-lanosterol, an important precursor in
the enzymatic biosynthesis of cholesterol.[14]
To further demonstrate the applicability of the newly devel-
oped catalyst system, 5a-cholestane-3-one, 2,3-dimethoxy-ben-
zaldehyde, 2,6-dimethoxy-acetophenone, 1-boc-4-piperidone,
Scheme 3. Catalytic MPV reduction of 5a-cholestan-3-one to cholestan-3-ol
citral and decanal were quantitatively reduced on a 1 g scale.
The formed alcohols were isolated simply by vacuum distilla-
tion with isolated yields ranging from to 86 to 97%; tedious
purification by column chromatography was not necessary.
In conclusion, the unprecedented activity and selectivity of
a sterically encumbered and monomeric aluminum siloxide
(top) and 3-oxo-lanosterol to a/b-lanosterol (bottom).
catalyst in the MPV reduction of a wide variety of aliphatic and
aromatic aldehydes and ketones under mild conditions was
demonstrated (52 examples). Reductions occurred with low
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Table 4. Aluminum siloxide catalyzed MPV reduction of aliphatic ketones to secondary alcohols.[a]
Entry
Product
3
iPrOH
[equiv]
t
[h]
T
[8C]
Yield
[%][b]
Entry
Product
3
iPrOH
[equiv]
t
[h]
T
[8C]
Yield
[%][b]
[mol%]
[mol%]
47
48
49
0.5
0.5
0.2
25
25
5
17
28
24
50
50
50
>99
>99
>99
54
55
56
0.5
0.5
1.0
25
25
5
24
48
24
60
50
80
>99
94
53
50
51
0.5
0.5
25
25
48
6
50
50
98
57
58
0.5
0.5
25
25
21
24
50
50
89
94
>99
52
53
0.2
0.5
5
24
24
50
50
>99
59
0.5
25
24
50
97
25
90
[a] The experiments were carried out in a sealed 20 mL scintillation vial (plastic screw cap equipped with a Teflon liner), no co-solvent; [b] determined by
NMR spectroscopy.
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readily replaced by the incoming carbonyl substrate, facilitat-
ing the rate determining hydride transfer. Finally, the present
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Acknowledgements
The authors would like to acknowledge Texas Tech University
for support and the NSF for funding the acquisition of a Jeol
Eclipse 400 MHz NMR spectrometer (CRIF MU grant CHE-
1048553). We would like to thank Prof. Dr. W. David Nes (Texas
Tech University) for kindly providing us with a sample of 3-
oxo-lanosterol.
Keywords: aldehydes · aluminum · ketones · MPV reduction ·
siloxide
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Received: August 25, 2014
Published online on October 5, 2014
Chem. Eur. J. 2014, 20, 14959 – 14964
www.chemeurj.org
14964
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