Hydrogen-transfer reduction of carbonyl compounds promoted by nickel nanoparticles

Article abstract of DOI:10.1016/j.tet.2007.11.093

Nickel(0) nanoparticles, generated from nickel(II) chloride, lithium powder and a catalytic amount of 4,4-di-tert-butylbiphenyl (DTBB) in THF at room temperature, have been found to promote the reduction of a variety of ketones and aldehydes by transfer hydrogenation using isopropanol as the hydrogen donor. The nickel nanoparticles were characterised and could be re-utilised with a good performance in the absence of a base. A mechanistic study demonstrates that the reaction proceeds through a dihydride-type mechanism.

Full text of DOI:10.1016/j.tet.2007.11.093

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 1847e1852
www.elsevier.com/locate/tet
Hydrogen-transfer reduction of carbonyl compounds
promoted by nickel nanoparticles
*
Francisco Alonso , Paola Riente, Miguel Yus
*
Departamento de Qu´ımica Orga´nica, Facultad de Ciencias, and Instituto de S´ıntesis Orga´nica (ISO),
Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain
Received 4 October 2007; received in revised form 26 November 2007; accepted 28 November 2007
Available online 4 December 2007
This paper is dedicated to Professor Koichiro Oshima on occasion of his 60th birthday
Abstract
Nickel(0) nanoparticles, generated from nickel(II) chloride, lithium powder and a catalytic amount of 4,4-di-tert-butylbiphenyl (DTBB) in
THF at room temperature, have been found to promote the reduction of a variety of ketones and aldehydes by transfer hydrogenation using
isopropanol as the hydrogen donor. The nickel nanoparticles were characterised and could be re-utilised with a good performance in the absence
of a base. A mechanistic study demonstrates that the reaction proceeds through a dihydride-type mechanism.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Carbonyl compounds; Nickel nanoparticles; Hydrogen transfer
1. Introduction
The transfer hydrogenation of ketones has been mostly
accomplished using isopropanol as hydrogen donor under
homogeneous conditions in the presence of noble metal com-
plexes. The most efficient catalysts devised so far are centred
on second or third row metals in d6 or d8 electronic configura-
tion, Ru,4 Rh5 and Ir6 complexes being, apparently, more ac-
tive than comparable Pd, Pt, and Os derivatives. In fact,
ruthenium complexes have been by far the most studied cata-
lysts, especially for the asymmetric transfer hydrogenation of
aromatic ketones7 and from the mechanistic point of view.8
Less attention has been paid, however, to the hydrogen-trans-
fer reduction of non-aromatic carbonyl compounds, as well as
to processes under heterogeneous conditions or involving non-
noble metals. In fact, the use of heterogeneous catalysts offers
several advantages over the homogeneous systems, such as
easy recovery, recycling and enhanced stability.
A continuous interest has been shown towards the reduction
of carbonyl compounds to the corresponding alcohols as a fun-
damental and common functional-group transformation in
synthetic organic chemistry. Among the different methodolo-
gies to perform this reaction, four important general proce-
dures can be highlighted involving the use of (a) metal
hydrides, (b) dissolving metals, (c) catalytic hydrogenation,
and (d) transfer hydrogenation.1 Other methods, such as
electrochemical or enzymatic methods are of less general ap-
plication. Hydrogen-transfer reactions2 are advantageous with
respect to other reduction methods because of several reasons:
(a) the hydrogen donor is easy to handle (no gas containment
or pressure vessels are necessary), cheap and environmentally
friendly (e.g., isopropanol), (b) possible hazards are mini-
mised, (c) the mild reaction conditions applied can afford
enhanced selectivity, (d) catalytic asymmetric transfer hydro-
genation can be applied in the presence of chiral ligands.3
In this sense, nickel appears as a potential alternative to the
above expensive transition-metal complexes but little studied
in hydrogen-transfer reactions with isopropanol. For instance,
an activated form of nickel, prepared by thermal decomposition
of nickel isopropoxide in boiling isopropanol, catalysed the
transfer hydrogenation of aliphatic ketones and acetophenone
* Corresponding authors. Fax: þ34 965903549.
E-mail addresses: falonso@ua.es (F. Alonso), yus@ua.es (M. Yus).
0040-4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.11.093

1848
F. Alonso et al. / Tetrahedron 64 (2008) 1847e1852
in isopropanol at 95e100 ꢀC.9 The NiBr2eNaOHei-PrOH
system also showed to be effective in the reduction of a variety
of ketones, but longer reaction times were needed in order to
achieve higher conversions.10 A macrocyclic nickel(II) com-
plex,11 Ni-stabilised zirconia,12 and mesoporous NiMCM
molecular sieves13 were primarily applied to the reduction of
aromatic substrates, albeit the latter two could be re-used sev-
eral times without affecting their activity. In all these cases, the
addition of an external base was mandatory for the reaction to
take place. More recently, an excess of Raney nickel in
refluxing isopropanol containing a trace of HCl reduced a series
of aliphatic aldehydes to the corresponding primary alcohols.14
On the other hand, we have recently reported the fast syn-
thesis of nickel(0) nanoparticles from different nickel(II) chlo-
ride-containing systems by reduction with lithium powder and
a catalytic amount of an arene in THF at room temperature.15
The high reactivity and versatility of these nanoparticles were
demonstrated in the reduction of a wide variety of functional
groups,16 highly selective semihydrogenation of alkynes and
dienes,17 conjugate reduction of a,b-unsaturated carbonyl
compounds,18 and more recently, in the a-alkylation of
ketones with primary alcohols,19 in all the cases under mild
reaction conditions. Apparently, a hydrogen-transfer process
might be one of the key steps in the latter application (the re-
action mechanism is now under study), and was demonstrated
to be present, at least in part, in other applications involving an
alcohol. Therefore, we envisaged the possibility of applying
these nickel nanoparticles to the transfer hydrogenation of
carbonyl compounds.
the conditions shown in Table 1, only the NiNPs were able
to reduce acetophenone to 1-phenylethanol, whereas Raney
Ni led to the hydrogenolysis product, ethylbenzene.20 Two
blank experiments, in the absence of isopropanol (only THF
as solvent) and in the absence of nickel gave the unreacted
starting material. Low yield of the expected product was
obtained at room temperature or by using 10 and 20 mol %
NiNPs.
With this methodology in hand, we first studied the transfer
hydrogenation of aromatic carbonyl compounds (Table 2). A
series of differently substituted acetophenones were reduced,
in a fast manner, to the corresponding alcohols in modest-to-
good isolated yields (entries 1e6), the position and electronic
character of the substituent notably affecting the yield. For in-
stance, 4-trifluoromethylacetophenone was reduced in 57%
yield (entry 2), but 3-trifluoromethylacetophenone was re-
duced in only 27% isolated yield and 2-trifluoromethylaceto-
phenone was unaffected. As expected, the opposite effect
was observed for methoxy-substituted acetophenones, the 3-
substituted giving much better yield in shorter reaction time
than the 4-substituted acetophenone (entries 3 and 4). Less dif-
ference in yield was observed for methyl substituted acetophe-
nones (entries 5 and 6), whereas butyrophenone was reduced in
Table 2
Aromatic carbonyl compounds reduced by hydrogen-transfer with NiNPs-
i-PrOHa
Entry
Substrate
t (h)
Product
Yieldb (%)
82
O
OH
1
1
We want to present herein the transfer hydrogenation of
a representative series of aromatic and aliphatic carbonyl com-
pounds, promoted by nickel(0) nanoparticles in isopropanol in
the absence of any added base.
Ph
Ph
O
OH
OH
2
3
1
57
34
F3C
F3C
O
2. Results and discussion
24
MeO
MeO
The nickel(0) nanoparticles (NiNPs) were generated from
anhydrous nickel(II) chloride, lithium powder and a catalytic
amount of DTBB (4,40-di-tert-butylbiphenyl, 5 mol %) in
THF at room temperature. A preliminary study was carried
out using acetophenone as a model substrate in the presence
of different catalysts (Table 1). It is noteworthy that, under
O
OH
4
5
6
1
1
1
73
72
51
OMe
OMe
O
OH
Table 1
Transfer hydrogenation of acetophenone with different nickel catalysts
O
OH
O
OH
catalyst (1 mmol)
i-PrOH, 76 °C, 24 h
O
O
OH
OH
7
8
1
1
2
73
86
42
Ph
Ph
Ph
Ph
Entry
Catalyst
Product
Yielda (%)
1
2
3
4
5
6
a
NiNPs
Raney Ni
NieAl
1-Phenylethanol
94
95
Ph
Ph
OH
Ethylbenzene
Acetophenone
Acetophenone
Acetophenone
Acetophenone
O
100
100
100
100
9
Ph
Ni/TiO2
Ni/SiO2eAl2O3
NiO
Ph
H
a
Substrate (1 mmol), NiNPs (1 mmol) in THF (2 mL), i-PrOH (4 mL),
76 ꢀC.
b
GLC yield.
Isolated yield after column chromatography (hexane/EtOAc).

F. Alonso et al. / Tetrahedron 64 (2008) 1847e1852
1849
Table 3
Aliphatic carbonyl compounds reduced by hydrogen-transfer with NiNPs-
good yield (entry 7). Surprisingly, chloro- and bromo-
substituted acetophenones remained untouched under the above
conditions. The reduction of chalcone gave the corresponding
saturated alcohol in high yield (entry 8). Benzaldehyde was re-
duced to benzyl alcohol in modest yield (entry 9) due to partial
hydrogenolysis leading to toluene.
i-PrOHa
Entry
Substrate
t (h)
Product
Yieldb (%)
73
O
OH
1
1
3
3
3
3
We studied next the transfer hydrogenation of aliphatic
ketones (Table 3). Dialkyl ketones such as 6-undecanone
and 4-phenyl-2-butanone gave the corresponding alcohols in
good yields and short reaction times (entries 1 and 2). How-
ever, the more sterically demanding dicyclohexyl ketone
required much longer reaction time to give dicyclohexyl
methanol in moderate yield (entry 3). A variety of cyclic ke-
tones were also readily reduced, some of them prone to yield
diastereomeric alcohol products. 2-Adamantanone furnished
2-adamantanol in excellent isolated yield (entry 4). In the
case of 4-tert-butylcyclohexanone, the thermodynamically
more stable equatorial alcohol, trans-4-tert-butylcyclohexanol
(72% yield), was the major product (eq/ax 85:15) (entry 5).
The minor axial alcohol cis-4-tert-butylcyclohexanol could
be isolated by column chromatography in 13% yield. The
reduction of trans-decalone provided an ax/eq 90:10 ratio of
the corresponding epimeric alcohols (entry 6), whereas (ꢁ)-
norcamphor led mainly to the more stable of the two possible
norborneol stereoisomers: exo/endo 85:15 (entry 7). Interest-
ingly, the steroids oestrone and androsterone were transformed in
high yields into oestradiol and androstanediol, respectively,
in both cases as a 1:1 mixture of diastereoisomers (entries 8
and 9).
The reduction of a,b-unsaturated ketones furnished the cor-
responding saturated alcohols in modest to good yields (entries
10e12). Regarding the stereochemistry of the reaction prod-
ucts, isophorone was reduced to the more stable 1,5-diequato-
rial 3,3,5-trimethylcyclohexanol with high diastereoselectivity.
The reduction of (þ)-pulegone also rendered the most stable
of the four possible stereoisomers, (ꢂ)-menthol, as the major
product, accompanied by minor amounts of neomenthol and
neoisomenthol (entry 12, footnote h). The aliphatic aldehyde
n-decanal was reduced to n-decanol in modest yield due to
the competitive condensation reaction (entry 13).
O
OH
2
3
4
1
48
1
83
53
92
O
OH
O
OH
O
OH
5
1
72c
O
HO
H
H
H
H
6
7
1
1
64d
58e
OH
H
O
O
O
OH
OH
H
8
9
1
1
84f
H
H
H
H
H
HO
HO
HO
HO
H
H
89f
H
H
H
H
H
O
OH
10
11
2
1
70
O
OH
39g
The formation of nickel nanoparticles was confirmed by
carrying out a blank experiment consisting of a standard reac-
tion in the absence of the substrate (i.e., with nickel(II) chlo-
ride, lithium, DTBB, THF, and isopropanol at 76 ꢀC).
Droplets of the solution containing the nickel nanoparticles
at different reaction times (1, 4, and 7 h) were analysed by
transmission electron microscopy (TEM), showing similar re-
sults. A typical TEM micrograph and size distribution graphic
are depicted in Figures 1 and 2, respectively. Spherical, mono-
disperse, and highly uniform nanoparticles were obtained ac-
cording with the narrow range of particle size observed
(0.75e2.88 nm, ca. 1.75ꢁ1.00 nm). Interestingly, most of
the nickel nanoparticles had diameters <2 nm (ca. 75%).
This result is quite different from that obtained when the nano-
particles were prepared at room temperature in only THF
(25% of the nanoparticles <2 nm) but similar to that in the
presence of ethanol at room temperature.15 Therefore, it can
12
1
1
62h
40i
O
OH
O
13
n-C9H19
OH
n-C9H19
H
a
Substrate (1 mmol), NiNPs (1 mmol) in THF (2 mL), i-PrOH (4 mL), 76 ꢀC.
Isolated yield after column chromatography (hexane/EtOAc).
Isolated yield of the trans stereoisomer. Diastereomeric ratio: trans/cis
b
c
85:15. The cis stereoisomer was isolated in 13% yield.
d
Diastereomeric ratio: cis/trans 90:10.
e
Diastereomeric ratio: exo/endo 85:15.
Diastereomeric ratio: 1:1.
f
g
Diastereomeric ratio: cis/trans 92:8.
Diastereomeric ratio: (ꢂ)-menthol/neoisomenthol/neomenthol 77:9:14.
h
i
GLC yield.

1850
F. Alonso et al. / Tetrahedron 64 (2008) 1847e1852
OH
H*
H
O
O
O
- acetone
- acetone
H*
[M-H*]
OH*
H
(50%)
(50%)
H
O
+
H*
OH
H*
[H-M-H*]
Scheme 1. The hydridic route for the transition-metal catalysed transfer hydro-
genation with isopropanol.
catalytic cycle. Certainly, many other homogeneous catalyst
systems using an alcohol as the hydrogen donor appear to
need a base (KOH) for their reactivity. More recently, it is
generally accepted that transition-metal catalysts operate
through a hydridic route with two possible pathways (Scheme
1).8c In the monohydride mechanism, the hydride and the pro-
ton keep their identity, only the CeH hydrogen of the donor
forming the hydride on the metal, while the OeH hydrogen
adds to the carbonyl oxygen. In the dihydride mechanism,
the two hydrogens of the donor become equivalent after being
transferred to the metal to give the dihydride.
In the case of our transfer hydrogenation system with
NiNPs the scenario is quite different, since the reaction is con-
sidered to take place under heterogeneous conditions. None-
theless, we carried out a series of experiments with different
deuterium labelled isopropanols that could provide any
evidence about the reaction mechanism (Scheme 2). In experi-
ment (1), isopropanol-OD was used, leading to a relatively low
incorporation of D at the benzylic position and similar incor-
poration into the methyl group. Experiment (2) was carried out
in order to see any effect of adding 2 mmol excess of lithium,
which might generate molecular deuterium, under the same
conditions as in experiment (1). In this case, the deuterium in-
corporation was relatively low at both the benzylic position
Figure 1. TEM micrograph of the nickel nanoparticles.
45
40
35
30
25
20
15
10
5
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
diameter (nm)
Figure 2. Size distribution of nickel nanoparticles determined by TEM. The
sizes were determined for 220 nanoparticles selected at random.
be concluded that the presence of isopropanol at 76 ꢀC has
a beneficial effect in the generation of the nickel nanoparticles
as regards their size and uniformity.
O
OH
2Li-NiCl2-DTBB, THF
(CH3)2CHOD, 76 °C
20% D
(1)
(2)
18% D
We were also interested in studying the reaction mechanism
of the above processes. The hydrogen-transfer reduction
mechanism of carbonyl compounds with isopropanol has been
a subject of intense debate, especially in homogeneous cataly-
sis,8 which depends on every particular catalytic system. A
MeerweinePonndorfeVerley-type mechanism21 involves a di-
rect hydrogen transfer through a six-membered transition state
composed of the hydrogen acceptor, metal, and hydrogen
donor, without the participation of metal hydride intermedi-
ates. The mechanism of hydrogen transfer from isopropanol
to a ketone using the catalyst RhCl(PPh3)3 was extensively
investigated.22 An essential component for this reaction is
potassium hydroxide, which was believed to be effective in re-
moving a proton from the reacting complex during part of the
O
OH
4Li-NiCl2-DTBB, THF
(CH3)2CHOD, 76 °C
10% D
12% D
99% D
OD
O
O
97% D
2Li-NiCl2-DTBB, THF
(CD3)2CDOD, 76 °C
(3)
CD3
D
96% D
OH
2Li-NiCl2-DTBB, THF
(CD3)2CDOH, 76 °C
10% D
(4)
45% D
Scheme 2. Labelling experiments in the transfer hydrogenation of
acetophenone.

F. Alonso et al. / Tetrahedron 64 (2008) 1847e1852
1851
O
O
used for the transfer hydrogenation with isopropanol (either
homogeneous or heterogeneous), the reduction with the NiNPs
can proceed in the absence of a base. Consequently, the reac-
tion system is simpler and avoids some undesired secondary
reactions such as condensation reactions.
Li, DTBB, THF
(CH3)2CHOD, 76 °C
30% D
Scheme 3.
and methyl group. In experiment (3), isopropanol-d8 led to an
almost quantitative incorporation of deuterium in all the
reactive positions. Experiment (4) was carried out using iso-
propanol-2-d1 providing 45% incorporation of deuterium at
the benzylic position. It is worthy of note that deuterium incor-
poration at the OH was detected only in experiment (3). Up to
15% deuterium incorporation at the OH was detected for the
reaction crudes in experiments (1)e(3). However, some
ODeOH exchange during the purification by column chroma-
tography over silica could account for these results.
3. Conclusion
Nickel(0) nanoparticles, prepared from nickel(II) chloride,
lithium powder, and a catalytic amount of DTBB, have shown
to promote readily the transfer hydrogenation with isopropanol
of a variety of aromatic and aliphatic ketones and aldehydes in
the absence of any added base at 76 ꢀC. Modest-to-high yields
of the corresponding alcohols were obtained, depending on the
functional groups and/or the structure of the substrate. The re-
ducing system showed to be diastereoselective for most of the
cyclic ketones studied and superior to other forms of nickel
under the same reaction conditions. Moreover, the nickel
nanoparticles could be re-utilised several times, maintaining
a high activity in a very simple reaction medium composed
of the nickel nanoparticles, isopropanol and the substrate,
without any added base. According to some deuteration experi-
ments, the reaction seems to proceed through a dihydride-type
mechanism. Efforts to stabilise the nickel nanoparticles and
use them catalytically are under way.
From the above experiments it can be inferred that a hydro-
gen-transfer process from isopropanol is occurring, instead of
alternative catalytic hydrogenation- or dissolving metal-type
processes. Under the standard reaction conditions (in which
no excess of lithium is used), small amounts of lithium
isopropoxide can be generated, acting as a base and leading
to some enolisation prior to the transfer hydrogenation. This
process was demonstrated by reacting acetophenone with
some lithium, DTBB and isopropanol-OD in THF at 76 ꢀC
(Scheme 3). The fact that the OeD deuterium is partially
transferred to the carbonyl carbon atom [experiments (1) and
(2), Scheme 2], is more in agreement with a dihydride-type
mechanism. However, a fast HeD exchange between the ace-
tophenone CH3 group and the isopropanol-OD, together with
a possible primary kinetic isotope effect, could account for
a deuterium incorporation (18%) somewhat far from the theo-
retically expected (50%) at that position. Experiment (4) was
more definitive in confirming a dihydride-type mechanism,
since w50% theoretical deuterium incorporation was achieved
at the benzylic position. It must be clarified that dihydride spe-
cies refer in this case to those resulting from the transfer of the
two hydrogen atoms of the donor to the surface of the metal.
Finally, we also devised the possibility of re-utilisation of
the NiNPs. Thus, once the reaction was stopped, the NiNPs
were decanted and the supernatant removed, followed by the
addition of more isopropanol and the substrate. Table 4 shows
the recycling results after 1 h obtained for the reduction of
acetophenone. The NiNPs could be re-used during four consecu-
tive runs without any apparent loss of activity. It is notewor-
thy that although the yield dropped significantly from the fifth
run, the amount of ethylbenzene formed (the major product ob-
tained when using Raney Ni) was in all cases <5%. Therefore,
we would expect better yields from the fifth run for longer re-
action times than 1 h. These recycling experiments also dem-
onstrate that, in contrast with most of the catalytic systems
4. Experimental
4.1. General
Flash column chromatography was performed using silica
gel 60 of 40e60 mm. THF was directly used without any
purification (Acros, 99.9%). Anhydrous nickel(II) chloride
(Aldrich), lithium powder (MEDALCHEMY S. L.), isopropa-
nol (Panreac, Acros), isopropanol-OD (Isotel), isopropanol-d8
(Aldrich), and isopropanol-2-d1 (Aldrich) were commercially
available. All the starting materials were commercially avail-
able of the best grade (Aldrich, Acros, Alfa Aesar) and were
used without further purification. TEM images were recorded
using a JEOLJEM2010 microscope, equipped with a lantha-
num hexaboride filament, operated at an acceleration voltage
of 200 kV. A drop of the nickel nanoparticle suspension was
added to a holey carbon-coated 300 mesh copper grid allowing
the solvent to evaporate before being introduced into the
microscope.
4.2. General procedure for the NiNPs-promoted transfer
hydrogenation of carbonyl compounds
Nickel chloride (130 mg, 1 mmol) was added over a suspen-
sion of lithium (14 mg, 2 mmol) and DTBB (13 mg,
0.05 mmol) in dry THF (2 mL) at room temperature under
argon. The reaction mixture, which was initially dark blue,
changed to black, indicating that nickel(0) nanoparticles were
formed. After 10 min, i-PrOH (4 mL) and the corresponding
carbonyl compound (1 mmol) were consecutively added. The
reaction mixture was warmed up to 76 ꢀC and monitored by
Table 4
Re-utilisation of NiNPs in the hydrogen-transfer reduction of acetophenone
Run
1
2
3
4
5
6
7
Yielda (%)
94
93
92
95
65
69
59
a
GLC yield after 1 h.

1852
F. Alonso et al. / Tetrahedron 64 (2008) 1847e1852
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methoxyphenyl)ethanol, 1-phenylbutanol, benzyl alcohol,
4-phenyl-2-butanol, dicyclohexylmethanol, 2-adamantanol,
(ꢁ)-exo-2-norborneol,23 b-oestradiol,24 cyclohexanol, and
(ꢂ)-menthol25 were characterised by comparison of their
physical and spectroscopic data with those of commercially
available samples (Aldrich). 1-(3-Methoxyphenyl)ethanol,26
1-(4-tolyl)ethanol,27 1-(3-tolyl)ethanol,28 1,3-diphenyl-1-
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(1R*,4aR*,8aS*)-decahydro-1-naphthol,32
5a-androstane-
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3a,17a-diol and 5a-androstane-3a,17b-diol,33 and cis-3,3,5-
trimethylcyclohexanol,34 were characterised by comparison
of their physical and spectroscopic data with those described
in the literature.
Acknowledgements
This work was generously supported by the Spanish Minis-
´
terio de Educacion y Ciencia (MEC; grant no. CTQ2004-
01261; Consolider Ingenio 2010-CSD2007-00006) and the
Generalitat Valenciana (GV; grants no. GRUPOS03/135 and
GV05/005). P.R. thanks the MEC for a predoctoral grant.
22. Sharf, V. Z.; Freidlin, L. Kh.; Krutii, V. N. Izv. Akad. Nauk SSSR, Ser.
Khim. 1977, 735e739; Chem. Abstr. 1977, 87, 38552.
23. Diastereomeric ratio (1H NMR): dH (CHO exo-norborneol) 3.76 ppm; dH
(CHO endo-norborneol) 4.24 ppm.
24. Diastereomeric ratio (1H NMR): dH (OH b-oestradiol) 4.50 ppm; dH (OH
a-oestradiol) 4.34 ppm.
References and notes
25. Diastereomeric ratio (1H NMR): dH (CHO menthol) 3.40 ppm; dH (CHO
isomenthol) 3.80 ppm; (CHO neomenthol) 4.10 ppm.
1. For reviews, see: (a) Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 8, Chapters 1.1e1.8;
´
(b) Hudlicky, M. Reductions in Organic Chemistry, 2nd ed.; ACS:
Washington, DC, 1996.
26. Ookawa, A.; Kitade, M.; Soai, K. Heterocycles 1988, 27, 213e216.
27. Dictionary of Organic Compounds, 5th ed.; Buckingham, J., Ed.;
Chapman and Hall: New York, NY, 1987; p 408; 1st suppl.
28. Ref. 27, p 407.
2. For reviews, see: (a) Johnstone, R. A. W.; Wilby, A. H. Chem. Rev. 1985,
85, 129e170; (b) Kellogg, R. M. Comprehensive Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon: Oxford, 1991; Vol. 8, Chapter
29. Alonso, F.; Yus, M. Tetrahedron 1998, 54, 1921e1928.
30. Dictionary of Organic Compounds, 5th ed.; Buckingham, J., Ed.;
Chapman and Hall: New York, NY, 1982; Vol. 5, p 5662.
1.3; (c) Backvall, J. E. J. Organomet. Chem. 2002, 652, 105e111; (d)
¨
´
Hydrogen Transfer Reactions; Hynes, J. T., Klinman, J. P., Limbach,
H. H., Schowen, R. L., Eds.; Wiley-VCH: Weinheim, 2007.
3. For reviews, see: (a) Ohkuma, T.; Noyori, R. Comprehensive Asymmetric
Catalysis; Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer:
Berlin, 1999; Vol. I, pp 227e246; (b) Gladiali, S.; Mestroni, G. Transition
Metals for Organic Synthesis; Beller, M., Bolm, C., Eds.; Wiley-VCH:
Weinheim, 2004; Vol. 2, Chapter 1.3.
31. Abraham, R. J.; Byrne, J. J.; Griffiths, L.; Perez, M. Magn. Reson. Chem.
2006, 44, 491e509; Diastereomeric ratio (1H NMR): dH (CHO trans-4-
tert-butylcyclohexanol) 3.52 ppm; dH (CHO cis-4-tert-butylcyclohexanol)
4.03 ppm.
32. Di Maio, G.; Mascia, M. G.; Vecchi, E. Tetrahedron 2002, 58, 3313e3318;
Diastereomeric ratio (1H NMR): dH [CHeqO (1R*,4aR*,8aS*)-decahydro-1-
naphthol] 3.74 ppm; dH [(CHaxO (1S*,4aR*,8aS*)-decahydro-1-naphthol]
3.15 ppm.
4. For a recent example, see for instance: Enthaler, S.; Jackstell, R.;
Hagemann, B.; Junge, K.; Erre, G.; Beller, M. J. Organomet. Chem.
2006, 691, 4652e4659.
5. See for instance: Nindakova, L. O.; Shainyan, B. A.; Belogonova, L. N.
Russ. J. Org. Chem. 2003, 39, 1484e1488.
33. Dictionary of Organic Compounds, 5th ed.; Buckingham, J., Ed.;
Chapman and Hall: New York, NY, 1982; Vol. 1, p 353; Diastereomeric
ratio (1H NMR): dH (CHO 5a-androstane-3a,17b-diol) 3.62 ppm; dH
(CHO 5a-androstane-3a,17a-diol) 3.72 ppm.
6. See for instance: Wu, X.; Liu, J.; Li, X.; Zanotti-Gerosa, A.; Hancock,
F.; Vinci, D.; Ruan, J.; Xiao, J. Angew. Chem., Int. Ed. 2006, 45, 6718e
6722.
34. Eliel, E.; Haubenstock, H. J. Org. Chem. 1961, 26, 3504e3506; Diaste-
reomeric ratio (1H NMR): dH (CHO cis-3,3,5-trimethylcyclohexanol)
3.77 ppm; dH (CHO trans-3,3,5-trimethylcyclohexanol) 4.21 ppm.