Aluminumoxyhydride: Improved synthesis and application as a selective reducing agent

Article abstract of DOI:10.1021/ic061276w

Aluminumoxyhydride (HAlO) has been obtained by the reaction of aluminum hydride with the siloxane (Me₂HSi)₂O or the stannoxane (Bu₃Sn)₂O as an amorphous colorless insoluble powder. The highest-purity product resulted from the reaction of H₃Al· NMe₃ with (Me₂HSi)₂O. However, HAlO suspensions in tetrahydrofuran (THF) of sufficient quality for synthetic applications can be prepared from commercially available reagents with only minor precautions. A LiAlH₄ solution in THF was treated successively with Me ₃SiCl and (Me₂HSi)₂O, followed by heating at 60°C for 20 h. The resulting suspensions are 0.4-0.5 M in active hydride content and selectively reduce aldehydes and ketones to the respective alcohols in the presence of any other common nonprotic functional group.

Full text of DOI:10.1021/ic061276w

Inorg. Chem. 2006, 45, 8807−8811
Aluminumoxyhydride: Improved Synthesis and Application as a
Selective Reducing Agent
Brij B. Tewari, Sukesh Shekar,† Longchuan Huang,† Carolyn E. Gorrell,‡ Timothy P. Murphy,‡
†
Kevin Warren, Nasri Nesnas,† and Rudolf J. Wehmschulte*
‡
,†
Department of Chemistry, Florida Institute of Technology, 150 West UniVersity BouleVard,
Melbourne, Florida 32901, and Department of Chemistry and Biochemistry,
UniVersity of Oklahoma, 620 Parrington OVal, Room 208, Norman, Oklahoma 73019
Received July 10, 2006
2 2
Aluminumoxyhydride (HAlO) has been obtained by the reaction of aluminum hydride with the siloxane (Me HSi) O
or the stannoxane (Bu
from the reaction of H Al
quality for synthetic applications can be prepared from commercially available reagents with only minor precautions.
A LiAlH solution in THF was treated successively with Me SiCl and (Me HSi) O, followed by heating at 60 C for
0 h. The resulting suspensions are 0.4 0.5 M in active hydride content and selectively reduce aldehydes and
ketones to the respective alcohols in the presence of any other common nonprotic functional group.
3 2
Sn) O as an amorphous colorless insoluble powder. The highest-purity product resulted
3
‚
NMe
3
with (Me HSi) O. However, HAlO suspensions in tetrahydrofuran (THF) of sufficient
2
2
4
3
2
2
°
2
−
Introduction
most nonprotic functional groups, we systematically inves-
tigated its reactivity. Here, we report the results of this study,
discuss a variety of synthetic approaches to HAlO, and pres-
ent a simple procedure for the synthesis of HAlO using read-
ily available precursor chemicals.
During the past 2 decades nonhydrolytic sol-gel proce-
dures have become an important addition to the toolbox for
the generation of metal oxides.1 Some of the advantages
include low reaction temperatures, homogeneous reaction
mixtures, high surface area, and often very low hydroxyl
contents. In this context, we have investigated dehydrosily-
,2
Experimental Section
lation as a route to high-surface-area aluminas.3,4 Interest-
General Procedures. All work was performed under anaerobic
and anhydrous conditions by using either modified Schlenk tech-
niques or an Innovative Technologies or a Vacuum Atmospheres
drybox. Solvents were freshly distilled under N from sodium, potas-
2
sium, sodium/potassium alloy, or calcium hydride and degassed
twice prior to use or dispensed from an MBraun Solvent Purifier.
ingly, the reaction between aluminum hydride and various
methylsiloxanes did not go to completion but afforded the
novel reactive solid aluminumoxyhydride (HAlO). This
compound, which we have obtained as a nanosized amor-
phous solid, was found to be a mild reducing agent for
Solid LiAlH
4
, tetrahydrofuran (THF), or toluene solutions of
HSi) O, (n-Bu Sn) O, Me N(CH , and
3
cyclohexenone and Me
3
SiCl. Thin films of HAlO have also
LiAlH , Me SiCl, (Me
4
3
2
2
3
2
2
2 3
)11CH
2 2
been generated by thermal decomposition of (t-BuOAlH )
most organic substrates were obtained from commercial sources
on a metal surface in a chemical vapor deposition cold-wall
reactor. When a preliminary screening indicated that our
nanosized HAlO might be very selective and tolerant to
and used as received. (n-Bu Sn) O was purified by vacuum dis-
3
2
5
6
7
3
tillation. H Al‚NMe
3
and isophorone oxide were prepared ac-
cording to literature procedures. Hydride contents were determined
by hydrolysis of an aliquot with excess water and measurement of
2
the amount of H gas evolved. NMR spectra were recorded on a
*
To whom correspondence should be addressed. E-mail: rwehmsch@
fit.edu.
Varian Mercury 300 MHz, a Varian Unity Plus 400 MHz, a Bruker
AMX 360, or a Bruker Avance 400 MHz spectrometer, and H
†
Florida Institute of Technology.
1
‡
University of Oklahoma.
(
1) Vioux, P. A. Chem. Mater. 1997, 9, 2292-2299.
(2) Lafond, V.; Mutin, P. H.; Vioux, A. J. Mol. Catal. A 2002, 182-183,
(5) Veith, M.; Andres, K.; Faber, S.; Blin, J.; Zimmer, M.; Wolf, Y.;
Schn o¨ ckel, H.; K o¨ ppe, R.; de Masi, R.; H u¨ fner, S. Eur. J. Inorg. Chem.
2003, 4387-4393.
(6) Ruff, J. K.; Hawthorne, M. F. J. Am. Chem. Soc. 1960, 82, 2141-
2144.
81-88.
(3) Fooken, U.; Khan, M. A.; Wehmschulte, R. J. Inorg. Chem. 2001,
40, 1316-1322.
(4) Liu, S.; Fooken, U.; Burba, C. M.; Eastman, M. A.; Wehmschulte, R.
J. Chem. Mater. 2003, 15, 2803-2808.
(7) Wasson, R. L.; House, H. O. Org. Synth. 1963, 30-39, 552-553.
1
0.1021/ic061276w CCC: $33.50
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 21, 2006 8807
Published on Web 09/22/2006

Tewari et al.
Table 1. Reaction of HAlO with Carboxylic Acid Derivatives, Epoxides, and Nitrobenzene
HAlO,
mmol/mL
HAlO,
mmol
reaction
time, h
reaction
temp, °C
mass, g
(% yield)
entry
substrate
benzoic acid
benzonitrile
diethyloxalate
maleic anhydride
4-methyl acetanilide
N-methylpyrrolidinone
cyclooctene oxide
styrene oxide
g (mmol)
workup
product
1
2
3
4
5
6
7
8
9
0.34 (2.8)
0.41 (3.9)
0.35 (2.4)
0.14 (1.4)
0.56 (3.8)
0.49 (4.9)
0.87 (6.9)
0.30 (2.5)
0.59 (3.8)
0.81 (6.6)
0.8
1.0
0.3
0.6
0.6
1.0
0.7
0.5
0.6
1.0
8.0
7.9
5.0
5.8
5.6
9.8
6.9
4.9
7.5
19.6
30
130
22
60
96
115
182
30
20
95
24
60
60
60
60
60
60
60
22
60
acidic
acidic
acidic
acidic
basic
acidic
acidic
acidic
neutral
basic
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
no reaction
1- and 2-phenylethanol
0.29 (95)
isophorone oxide
nitrobenzene
no reaction at epoxide groupa
no reaction
10
a
For reduction of the keto group, see entry 18 in Table 2.
NMR chemical shift values were determined relative to the residual
protons in C or CDCl as an internal reference (δ 7.15 or 7.26
ppm). 13C NMR spectra were referenced to the solvent signal (δ
and the mixture began to turn cloudy after 30 min. The reaction
mixture was stirred overnight to give a colorless suspension. The
solid was collected on a glass frit, washed with hexanes (3 × 50
mL), and dried in vacuo. Yield: 0.35 g. FTIR (mineral oil, KBr
D
6 6
3
1
28.0 or 77.0 ppm). IR spectra were recorded in the range 4000-
-1
-1
400 cm using a Nicolet Nexus 470 or a Nicolet Magna 550 FTIR
plates): ν(Si-H) 2110 (m), ν(Al-H) 1900 (st) cm . The active
spectrometer.
hydride content of this solid was determined as 21.6 mmol/g (cf.
Reaction of H
3
Al‚NMe
. A solution of H
in benzene (30 mL) was treated with Me
2.0 mmol) and stirred for 30 min at room temperature. After a
slight concentration of the solution under reduced pressure,
Me HSi) O (1.59 g, 11.9 mmol) was added, and the reaction
3
with (Me
Al‚NMe
N(CH
2
HSi)
2
O in the Presence of
(0.89 g, 9.9 mmol)
(2.56 g,
22.7 mmol/g for pure HAlO).
2
Me N(CH
2
)11CH
3
3
3
Method B. A solution of LiAlH
diluted to 24 mL, and Me SiCl (1.53 mL, 1.30 g, 12.0 mmol) was
2
added via syringe at room temperature. After 30 min, (Me HSi) O
4
(1.0 M, 12 mL) in THF was
2
2
)
11CH
3
3
1
2
(2.50 mL, 1.88 g, 14.0 mmol) was added via syringe, and the
mixture began to turn cloudy after 30 min. The reaction mix-
ture was refluxed for 20 h. The active H content of the suspension
was determined as 0.42 mmol/mL. The solid was collected on a
glass frit, washed with hexanes (2 × 50 mL), and dried in vacuo.
Yield: 0.98 g. FTIR (mineral oil, KBr plates): ν(Si-H) 2286 (m),
(
2
2
mixture was stirred at room temperature for 24 h. Because no visible
change was observed, the clear colorless solution was heated at 60
C for 16 h, during which time a fine colorless precipitate formed.
The solid was collected on a glass frit, washed with hexanes (3 ×
0 mL), and dried in vacuo. Yield: 1.15 g. FTIR (mineral oil, CsI
plates): ν(Si-H) 2101 (w), ν(Al-H) 1885 (st), δ(Al-H) and
ν(Al-O) 800 (vst) cm . FTIR (KBr pellet): 2959 (st), 2927 (st),
856 (m), ν(Si-H) 2114 (w), ν(Al-H) 1894 (st), 1480 (m), 1468
m), 1261 (m), 1018 (st), 800 (vst) cm-1. The volatile material was
distilled off the filtrate, and the remaining clear colorless oil was
°
-
1
5
ν(Al-H) 1877 (st) cm . The active hydride content of this solid
was determined as 7.8 mmol/g.
-
1
DAlO from LiAlD . DAlO was prepared analogously to HAlO
4
2
(
starting from a THF solution (15 mL) of LiAlD (0.45 M, 6.7
mmol). Yield: 0.50 g. The active hydride content of the suspension
was 0.45 mmol/mL and that of the isolated solid 9.2 mmol/g. FTIR
4
1
identified as pure Me
Reaction of H Al‚NMe
Al‚NMe (1.34 g, 15.0 mmol) in benzene (60 mL) was treated
dropwise with (n-Bu Sn) O (8.94 g, 15.0 mmol) at room temper-
2
N(CH
2
)
11CH
3
by H NMR spectroscopy.
(mineral oil, KBr plates): ν(Si-H) 2105 (m), ν(Al-H) 1882 (m),
-
1
with (n-Bu Sn) O. A solution of
3
2
ν(Al-D) 1375 (st) cm
Reaction with Excess Disiloxane. A solution of LiAlH
M, 6 mL) in THF was diluted to 12 mL, and Me SiCl (0.77 mL,
0.65 g, 6.0 mmol) was added via syringe at room temperature. After
30 min, (Me HSi) O (4.4 mL, 3.35 g, 25.0 mmol) was added via
.
3
3
H
3
3
4
(1.0
3
2
3
ature. The mixture warmed slightly upon stannoxane addition, and
a fine colorless precipitate formed immediately. The mixture was
stirred at room temperature for another 16 h. The solid was collected
on a glass frit, washed with hexanes (3 × 30 mL), and dried in
2
2
syringe, and the mixture began to turn cloudy after 30 min. The
reaction mixture was refluxed for 22 h. The active H content of
the suspension was determined as 0.18 mmol/mL. The solid
was collected on a glass frit, washed with hexanes (2 × 50 mL),
and dried in vacuo. Yield: 0.64 g. FTIR (mineral oil, KBr plates):
1
vacuo. Yield: 1.26 g. Because the H NMR spectrum of a suspen-
sion of this solid still showed a significant amount of impurities,
3
mainly n-Bu SnH, the solid was stirred in hexanes (50 mL) for
-
1
2
0 h, followed by filtration and drying in vacuo. This purified
ν(Si-H) 2283 (m), ν(Al-H) 1875 (st) cm . The active hydride
content of this solid was determined as 1.3 mmol/g.
1
solid contained only minor amounts of (n-Bu
3
Sn)
and 13C{ H} NMR spectroscopy. FTIR (mineral oil, CsI plates):
ν(Al-H) 1885 (st), δ(Al-H) and ν(Al-O) 800 (vst) cm-1
2
according to H
1
Reaction of HAlO with Organic Substrates. A solution of a
substrate in THF was treated with a measured amount of a freshly
prepared HAlO suspension (method B, above), and the resulting
mixture was stirred at the desired temperature for 12 h or more.
The progress of the reactions was monitored by determination of
the hydride content of aliquots of the reaction mixture. The reaction
mixtures were quenched with water, the precipitated aluminum salts
were dissolved in acid or base, and the organic phase was separated.
.
Removal of the solvents from the combined filtrates and washing
afforded a colorless liquid (7.60 g), which was identified as a
1
13
1
119
mixture of n-Bu
3
SnH and (n-Bu
3
Sn)
2
using H, C{ H}, and
-
1
8
Sn{ H} NMR spectroscopy.
HAlO from LiAlH . Method A. Me
0.0 mmol) was added via syringe to a solution of LiAlH
1.0 M, 10 mL, 10 mmol) in toluene at room temperature to
4
3
SiCl (1.18 mL, 1.08 g,
‚2THF
1
(
4
The aqueous phase was extracted with Et
2
O, the combined organic
give an immediate formation of a fine colorless solid (LiCl). The
solid was allowed to settle, the supernatant liquid was decanted,
phases were dried with MgSO , and the solvent was removed under
4
reduced pressure to give the crude products. If needed, the products
were purified using standard procedures. The identities of the
2 2
(Me HSi) O (1.94 mL, 1.48 g, 11.0 mmol) was added via syringe,
1
products were established by H NMR spectroscopy and occasion-
(8) Wrackmeyer, B. Ann. Rep. NMR Spectrosc. 1985, 16, 73-186.
ally mass spectrometry. Details are given in Tables 1 and 2 and
8808 Inorganic Chemistry, Vol. 45, No. 21, 2006

Aluminumoxyhydride
Table 2. Reaction of HAlO with Aldehydes and Ketones
HAlO,
mmol/mL
HAlO,
mmol
reaction
time, h
reaction
temp, °C
mass, g
(% yield)
entry
substrate
g (mmol)
workup
product
9-anthrol
cinnamyl alcohol
1-hexanol
benzhydrol
2-heptanol
11
12
13
14
15
16
17
18
19
9-anthraldehyde
cinnamaldehyde
hexanal
benzophenone
2-heptanone
0.77 (3.8)
0.40 (3.0)
0.48 (4.8)
0.85 (4.7)
0.45 (3.9)
0.58 (3.0)
0.15 (1.50)
0.59 (3.8)
0.31 (1.0)
0.5
0.4
0.6
0.9
0.9
0.4
0.4
0.6
0.6
3.8
6.0
4.8
9.4
7.8
5.0
4.5
7.5
1.2
12
12
5
24
24
20
20
20
20
22
25
26
22
22
60
60
22
22
acidic
acidic
acidic
acidic
acidic
basic
basic
neutral
acidic
0.55 (71)
0.30 (77)
0.43 (86)
0.78 (90)
0.33(71)
0.40 (70)
0.11 (71)
0.39 (66)
a
â-ionone
â - ionol
2,4- pentanedione
isophorone oxide
progesterone
2,4-pentanediol
see Scheme 1
see Scheme 1
a
Crude yield: 03.1 g. See the experimental procedure.
Scheme 1. Selected HAlO Reductions
C6H6, 60 °C, 20 h
H Al‚NMe + (Me HSi) O
8
3
3
2
2
HAlO + 2Me SiH + NMe (1)
2
2
3
obtained. Only on one occasion did a toluene suspension
become opalescent, suggesting particle sizes close to the
wavelength of visible light. Given that soluble small particles
generally display a higher reactivity than insoluble ag-
gregates, we attempted to solubilize nanometer-sized particles
by coating them with the long-chain aliphatic amine
Me
2
N(CH
2
)
11CH
3
. A solution of H in benzene was
3
Al‚NMe
3
treated with 1.2 equiv of Me
2
N(CH )11CH , stirred for 30
2 3
min, concentrated slightly to remove some NMe
3
, and fin-
ally treated with (Me HSi) O. However, a fine colorless
2
2
solid was obtained after 16 h at 60 °C, analogous to the
reactions without the long-chain amine. The mass of the solid
powder was more than twice the amount expected for
HAlO, and according to IR spectroscopy, this increase was
due to Me
2 2 3
N(CH )11CH . The strong and sharp Al-H stretch
-
1
was observed at its normal position at 1894 cm ac-
-
1
companied by a weak Si-H absorption at 2114 cm ,
resulting from residual silane (Me SiH ) or siloxy groups
-OSiHMe ). NMR spectra collected from samples sus-
pended in C showed only hexane signals (used for
2
2
(
2
6 6
D
washing) even after heating to reflux for a couple of min-
utes, suggesting that the amine is either bound tightly to the
surface or encapsulated inside the HAlO particles. The clear
Scheme 1. The reduction of progesterone (entry 19) resulted in the
formation of a mixture of seven compounds. A fraction of the
mixture was separated by column chromatography, and the two
major isomers were identified as 3-hydroxy-4-pregnen-20-one (1)
2 2 3
and colorless filtrate contained only Me N(CH )11CH after
removal of the volatile materials under reduced pressure.
There was no indication of soluble amine-coated HAlO
particles.
and 4-pregnene-3,20-diol (2) in an approximate 6:1 ratio. 1: eluted
1
with 1:1 EtOAc/hexane, R
f
) 0.50. H NMR (CDCl
3
, 400 MHz):
δ 0.63 (s, CH , 3H), 1.05 (s, CH
OHCHCHdC, 1H), 5.29 (s, CHdC, 1H). MS (DART): M + H ,
m/z 317.2726 (calcd 317.2475 for C21
3
3
, 3H), 2.11 (s, CH
3
, 3H), 4.14 (m,
+
Because the reaction time for the preparation of HAlO
with 10-12 h is relatively long, the potential of employing
stannoxanes instead of siloxanes was investigated. The Sn-O
bond is longer and weaker than the Si-O bond [1.940 Å in
+
H
33
2
O ); (M + H ) - H
2
O,
m/z 299.2393 (calcd 299.2375 for C21
H
33
O
2
). 2: eluted with 1:1
, 400 MHz): 0.77 (s,
1
EtOAc/hexane, R
f
) 0.45. H NMR (CDCl
3
CH , 3H), 1.05 (s, CH
OHCHCHdC, 1H), 5.25 (s, OHCHCHdC, 1H). MS: (M + H )
3
3
, 3H), 1.12 (d, CH
3
, 3H), 4.15 (m,
1
0
11
+
(Me
for Me
compound bis(tributyltin)oxide (Bu
available and air-stable. Furthermore, the presence of co-
valently bound -OSnBu groups on the particle surface could
3
Sn)
SnOEt vs 438 kJ/mol for (Me
Sn) O is commercially
2 3 2
O vs 1.626(5) Å in (Me Si) O and 276 kJ/mol
1
2
12
9
3
3 2
Si) O ], and the
-
2
2H O, m/z 283.2434 (calcd 283.2426 for C21H31).
3
2
Results and Discussion
3
HAlO Synthesis from Isolated H
3
3
Al‚NMe . We have
reported previously that the reaction of aluminum hydride
(
9) D’Incan, E.; Loupy, A.; Restelli, A.; Seyden-Penne, J.; Viout, P.
H
3
Al‚NMe with 1,1,3,3-tetramethyldisiloxane [(Me HSi) O]
3
2
2
Tetrahedron 1982, 38, 1755-1759.
in a benzene solution at 60 °C for 20 h gave the most
(
(
10) Vilkov, L. V.; Tarasenko, N. A. Zh. Strukt. Khim. 1969, 10, 1102.
11) Barrow, M. J.; Ebsworth, E. A. V.; Harding, M. M. Acta Crystallogr.,
Sect. B 1979, B35, 2093-2099.
3
consistent yields of HAlO according to eq 1.
Using toluene or THF as the solvent did not significantly
alter the appearance or the IR spectrum of the HAlO
(
12) Baldwin, J. C.; Lappert, M. F.; Pedley, J. B.; Poland, J. S. J. Chem.
Soc., Dalton Trans. 1972, 1943-1947.
Inorganic Chemistry, Vol. 45, No. 21, 2006 8809

Tewari et al.
enhance the particle solubility. The reaction of H
3
Al‚NMe
3
is approximately twice that from the reactions that used
with (Bu Sn) O is fast and exothermic. A fine precipitate of
3
2
H
3
3
Al‚NMe as the starting material or in which LiCl was
HAlO was formed immediately after the stannoxane addition.
Further stirring at room temperature did not lead to any ob-
vious changes. Although the stannoxane reaction is signifi-
cantly faster than the siloxane reaction, a major drawback is
separated prior to HAlO formation. In addition, the amount
of Si-H groups remaining in this solid is somewhat higher
than that in the HAlO obtained from LiCl free samples as
shown in the IR spectra. Use of LiAlD
an Al-D stretching frequency with a value of 1375 cm
close to the value expected for a simple spring model (1330
4
afforded DAlO with
-
1
the difficulty of removing the byproducts Bu
3
SnH and
Bu SnSnBu and occasionally unreacted stannoxane. Because
3
3
-
1
of their high boiling points their removal requires repeated
extraction with hexanes or ether. Even after three washings,
cm ). The active hydride content of the suspensions varies
depending on the reaction time, with reproducible results ob-
these compounds can still be detected easily in C
6
D
6
4
tained after 20 h at 60 °C. Typically, 0.5 M LiAlH solu-
1
suspensions of the powder by H NMR spectroscopy. The
distannane Bu SnSnBu is formed through base-catalyzed
NMe ) decomposition of Bu
SnH.13 It should also be
mentioned that an excess of stannoxane did not lead to the
formation of Al , an indication of the decreased reactivity
of HAlO compared to other aluminum hydrides.
HAlO Synthesis from H Al‚L Formed in Situ. The
synthesis of crystalline H Al‚NMe requires the use of
tions result in HAlO suspensions with active hydride con-
centrations of around 0.42-0.44 M, corresponding to a
hydride yield of 84-88%. To ensure that there is no soluble
hydride content, the solid HAlO/LiCl mixture was allowed
to settle and the supernatant was hydrolyzed, yielding no
3
3
(
3
3
2 3
O
H
2
gas. Prolonged refluxing (2-3 days) does not alter the
3
hydride content. Use of a large excess of (Me HSi) O
2
2
3
3
eventually leads to the formation of a solid that has a
significantly reduced hydride content, but its identity was
not investigated in much detail. The IR spectrum of the
isolated solid displays a reduced Al-H and an increased
Si-H absorption.
vacuum lines and a drybox. To develop a synthetic procedure
for HAlO that can be performed with only minor precautions
in practically any synthetic laboratory, we have investigated
several methods starting from commercially available LiAlH
solutions. It has been reported previously that clean and
reactive THF solutions of H Al can be obtained by the careful
addition of 100% sulfuric acid to a LiAlH solution in
4
Overall, this system has proven to be robust and simple
enough for employment in a standard synthetic laboratory,
and its reactivity toward 18 different organic substrates was
investigated. Suspensions of HAlO were reacted with a
variety of nonprotic organic substrates, and the results are
listed in Tables 1 and 2 as well as in Scheme 1. HAlO is
reactive only toward aldehydes and ketones, which were
reduced cleanly to the corresponding alcohols. No reactions
were observed with carboxylic acids (after deprotonation),
esters, amides, anhydrides, nitriles, nitrobenzene, and ep-
oxides even after prolonged refluxing. Among this group,
only styrene oxide was eventually reduced after 30 h at 60
°C. Enolizable ketones were reacted with an extra 1 equiv
of HAlO. Unfortunately, the regioselectivity of the HAlO
reductions was rather poor. For example, in the reductions
of isophorone oxide and styrene oxide, two isomers in close
to 1:1 ratios were obtained, and 2,4-pentanediol was isolated
as both the meso and racemic isomers in an approximately
1:1 ratio. On the other hand, progesterone was reduced
primarily at the conjugated carbonyl function. The crude
product consisted of several compounds, which were sepa-
rated by column chromatography. The two main products,
singly reduced 3-hydroxy-4-pregnen-20-one and doubly
reduced 4-pregnene-3,20-diol, were found in an approximate
6:1 ratio. Single reduction at the acetyl group was not
observed, and the double reduction product is most likely
due to the use of a 20% excess of HAlO. Considering that
HAlO consists of insoluble particles in the tens of nanometer
range, the observed preference may be due to steric reasons.
An inspection of progesterone models indicates the conju-
gated keto group to be more accessible.
3
4
1
4,15
THF.
LiAlH
Because the thought of adding a strong acid to
may be disconcerting to many chemists, we inves-
4
tigated the potential of Me
successfully in the synthesis of primary alanes RAlH
3
SiCl, which has been applied
from
3
the respective alanates RAlH Li. Initial experiments were
2
16,17
aimed at the isolation of HAlO and were thus conducted in
benzene or ether to separate the LiCl side product. Generally,
a suspension of purified LiAlH
of amine was treated with Me
4
in the presence of 2 equiv
SiCl at room temperature
3
followed by filtration or decanting to remove the lithium
chloride. The resulting H Al‚NR solutions were then reacted
with the siloxane (Me HSi) O as described in the previous
3
3
2
2
section to generate HAlO. However, this method suffered
from the difficulty to generate suspensions of finely divided
LiAlH
had only relatively low contents of active hydride. Better
results were obtained when a 1 M solution of LiAlH ‚2THF
4
, and the resulting HAlO generally was impure and
4
in toluene was employed. Under these conditions, HAlO was
isolated in 95% yield.
For reductions of organic substrates, the presence of LiCl
should not be a factor, and HAlO was generated in situ from
a THF solution using a typical commercially available 1.0
M THF solution of LiAlH . Here, the LiCl byproduct is
4
soluble and is eventually precipitated together with HAlO.
Hence, the mass of the product isolated from this reaction
(
(
(
(
(
13) Davies, A. G. Organotin Chemistry; Wiley-VCH: Weinheim, Ger-
many, 2004.
14) Brown, H. C.; Yoon, N. M. J. Am. Chem. Soc. 1966, 88, 1464-
1472.
15) Gorrell, I. B.; Hitchcock, P. B.; Smith, J. D. Chem. Commun. 1993,
Summary
189-190.
16) Wehmschulte, R. J.; Grigsby, W. J.; Schiemenz, B.; Bartlett, R. A.;
Power, P. P. Inorg. Chem. 1996, 35, 6694-6702.
HAlO can be obtained from crystalline H
3
Al‚NMe
3
or,
17) Wehmschulte, R. J.; Power, P. P. Inorg. Chem. 1994, 33, 5611-5612.
more conveniently, through in situ generated H
3
Al‚THF by
8810 Inorganic Chemistry, Vol. 45, No. 21, 2006

Aluminumoxyhydride
reaction with a slight excess of 1,1,3,3-tetramethyldisiloxane.
Through the latter procedure, a suspension of HAlO can be
obtained in a straightforward manner in any synthetic lab-
oratory, and its reactivity and selectivity match those of
Acknowledgment. Financial support for this work from
the American Chemical Society Petroleum Research Fund
is gratefully acknowledged.
IC061276W
18
3
Li(t-BuO) AlH. These HAlO suspensions selectively reduce
aldehydes and ketones and are not affected by other nonprotic
functional groups.
(
18) March, J. AdVanced Organic Chemistry: Reactions, Mechanisms, and
Structure, 4th ed.; John Wiley & Sons: New York, 1992.
Inorganic Chemistry, Vol. 45, No. 21, 2006 8811