New chiral lithium aluminum hydrides based on biphenyl-2,2′- bisfenchol (BIFOL): Structural analyses and enantioselective reductions of aryl alkyl ketones

Article abstract of DOI:10.1016/j.tet.2006.01.060

A series of new chiral lithium aluminum hydrides based on BIFOL (biphenyl-2,2′-bisfenchol) and various alkyl alcohols (i.e., methanol, n-butanol, tert-butanol yielding BIFAl-H's) was synthesized and characterized by single crystal X-ray analyses. These investigations point to alkoxide redistribution for BIFAl-H-(O-tBu) (biphenyl-2,2′-bisfenchol aluminum hydride) species. The new BIFAl-H reagents are suitable to reduce aryl alkyl ketones with up to 62% ee. Computational transition structure analyses help to explain the experimentally observed enantioselectivities.

Full text of DOI:10.1016/j.tet.2006.01.060

Tetrahedron 62 (2006) 3704–3709
New chiral lithium aluminum hydrides based
0
on biphenyl-2,2 -bisfenchol (BIFOL): structural analyses
and enantioselective reductions of aryl alkyl ketones
D. A. Lange, J.-M. Neud o¨ rfl and B. Goldfuss
*
Institut f u¨ r Organische Chemie, Universit a¨ t zu K o¨ ln, Greinstrasse 4, D-50939 K o¨ ln, Germany
Received 23 November 2005; revised 17 January 2006; accepted 18 January 2006
Available online 28 February 2006
0
Abstract—A series of new chiral lithium aluminum hydrides based on BIFOL (biphenyl-2,2 -bisfenchol) and various alkyl alcohols (i.e.,
methanol, n-butanol, tert-butanol yielding BIFAl-H’s) was synthesized and characterized by single crystal X-ray analyses. These investigations
0
point to alkoxide redistribution for BIFAl-H-(O-tBu) (biphenyl-2,2 -bisfenchol aluminum hydride) species. The new BIFAl-H reagents are
suitable to reduce aryl alkyl ketones with up to 62% ee. Computational transition structure analyses help to explain the experimentally observed
enantioselectivities.
q 2006 Elsevier Ltd. All rights reserved.
1. Introduction
Ph
Ph
Lithium aluminum hydride (LAH) represents a powerful
reducing agent and is frequently employed in organic
synthesis, for example, for conversions of carbonyl
H
O
OH
OH
OH
OH
OH
OH
O
1
H
functions to alcohols. Appropriate chiral derivates of
LAH are widely used as reagents for enantioselective
Ph
Ph
2
reductions of prochiral ketones. The C -symmetric diols
2
1
2
3
3
BINOL (1) and TADDOL (2) (Scheme 1) are highly
4
(
M)-BINOL
(R,R)-TADDOL
(M)-BIFOL
effective for such enantioselective reductions in combi-
5
nation with an alkyl alcohol. The structural characteri-
Scheme 1. Chiral C -symmetric diols for chiral lithium aluminum hydrides.
2
zation of such modified LAH reagents is hence of major
interest. Some chiral LAH alkoxides tend to disproportio-
nation and aggravate the syntheses of a well defined
reducing species with high enantioselectivity. The use of
chelating diols decreases the appearance of multiple LAH
beneficial alignment of the fenchol moieties via hydrogen
bonding. As in BINOL (1), this flexible biaryl axis can
7
adopt to different metal sizes via rotation around the biaryl
axis. Modular aryl fenchols were successfully employed in
enantioselective palladium and copper catalyzed C–C-
5
species. IR structure analyses of BINOL modified LAH
alkoxides by Noyori et al. show that disproportionated LAH
8
9
couplings, in organo zinc catalysts and in chiral organo
1
0
5
species are detectable. Further examinations by Noeth et al.
lithium reagents. We here present structural analyses of
(M)-BIFOL modified lithium aluminum hydrides and their
application in enantioselective reductions of aryl alkyl
ketones.
2
7
by means of X-ray structure and Al NMR analyses show
the phenomenon of disproportionation and redistribution of
6
LAH alkoxides.
0
The chiral chelating diol (M)-BIFOL (biphenyl-2,2 -
bisfenchol, 3, Scheme 1), recently developed in our group,
exhibits a C -axis, which is induced and stabilized by
2
2. Results and discussion
A series of enantiopure lithium aluminum hydrides, that is,
BIFAl-H-(O-R) (4a,b,c), was prepared from LiAlH , (M)-
Keywords: Enantioselective reductions; Chiral lithium aluminum hydrides;
X-ray structure analyses; Transition structure analyses.
4
*
Corresponding author.
e-mail: goldfuss@uni-koeln.de
Fax:
C49
221
470
5057;
BIFOL and an alkyl alcohol, that is, methanol, n-butanol or
tert-butanol (Scheme 2).
0040–4020/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.01.060

D. A. Lange et al. / Tetrahedron 62 (2006) 3704–3709
3705
1. LiAlH4
OH
OH
2. R-OH
O
O
H
Li
Al
THF/RT
OR
3
(M)-BIFOL)
(M)-BIFAl-H-(O-R)
4a: R=Me
4b: R=nBu
4c: R=tBu
Scheme 2. Synthesis of chiral, BIFOL-based aluminum hydrides, (M)-
BIFAl-H-(O-R).
X-ray structures of 4a and 4b (Figs. 1 and 2) reveal the
formation of the expected reactive hydride species. As
5
6
observed by the groups of Noyori and Noeth, aging of the
reaction solution may give rise to redistribution of hydride
components (LiAlH4Kn(RO) , Scheme 3). Indeed, the
formation of (M)-BIFAl-(O-tBu) 5 (Fig. 3) from 4c points
2
to such a redistribution.
Figure 2. X-ray crystal structure of 4b. Hydrogen atoms are omitted except
H1. Thermal ellipsoids represent a 20% probability. Selected atom
˚
distances (in A): Al–H1 1.36, Al–O1 1.78, Al–O2 1.70, Al–O3 1.78, Li–
n
0
O1 1.97, Li–O3 1.95, Li–O4 1.99, Li–O5 1.96. Biaryl angle: C
0
2
–C
1
–C
1
–
C
2
K106.38.
˚
1.95 A). In 5 the Li-ion is coordinated by two THF and two
(
oxygen from the tert-butanol moieties. The structure of 5
˚
exhibits a shorter Li–O distance to the tert-butoxy (1.99 A
˚
˚
O3), 1.93 A (O4)) than to the two THF molecules (2.04 A
(
˚
(O5), 2.05 A (O6)). All examined BIFAl-H-(O-R) structures
crystallize in (M) conformations of the chiral biaryl axis.
The computed relative energy of the optimized (M)-6-calc
structure is 20.3 kcal/mol lower than its diastereomer (P)-6-
calc (Table 1, Figs. 4 and 5). The dihedral angels of the
chiral C -axis are very similar (4a: K106.38; 4b: K106.38,
2
5
: K107.48), which suggest that the influence of the alkyl
alcohols are rather small and the steric interactions between
the bulky fenchone moieties are high. Hence, the minus
biaryl conformations of fenchone moieties in (M)-BIFAl-H-
(O-R) are more favorable than in the (P)-conformations.
This parallels the situation in (M)-BIFOL, where additional
hydrogen bonding contributes to the stabilization of the (M)-
conformation. Computations and X-ray analyses of BIFAl-
H-(O-R) tends to a 100% de of the (M) biaryl axis. Similar
7
results were obtained for (M)-BIFOL. Enantioselective
reduction of aryl alkyl ketones like butyrophenon with (M)-
BIFAl-H-(O-nBu) yields (S)-1-phenylbutane-1-ol in an
enantiomeric excess of 62% (Table 2, Scheme 4).
Reductions of acetophenon, methylacetophenon and pro-
piophenon result in a range of 3–50% ee.
Figure 1. X-ray crystal structure of 4a. Hydrogen atoms are omitted except
H1. Thermal ellipsoids represent a 20% probability. Selected atom
˚
distances (in A): Al–H1 1.59, Al–O1 1.73, Al–O2 1.73, Al–O3 1.78, Li–
To predict the sense of enantioselectivity for aryl alkyl
ketone reductions, Noyori et al. proposed a six-membered
transition state model for the reduction of aryl alkyl ketones
0
O3 1.90, Li–O4 1.97, Li–O5 1.95, Li–O6 1.97. Biaryl angle: C
0
2
–C
1
–C
1
–
C
2
K106.38.
5
with (M)-BINAl-H-(O-R). Repulsive p–p interactions
between the oxygen lone pairs and the delocalized p-system
of the phenyl rest favores (S)-configuration of the secondary
Molecular structures of 4a, 4b and 5 (Figs. 1–3) show all
tetra coordinated lithium and aluminum ions. In the
molecular structure of 4a (Fig. 1) the Li-ion coordinates
to three oxygen atoms of three THF ligands with Li–O
5
alcohol (Scheme 5).
˚
distances of 1.97 (O4), 1.95 (O5) and 1.97 A (O6). The Li–
This qualitative transition state model was adopted for
quantitative transition structure computations of the
reduction of different prochiral aryl alkyl ketones with
(M)-BIFAl-H-(O-R). Each reducing agent and the appro-
priate substrate yielding the highest enantioselectivity were
chosen for transition state optimization. In all three cases the
˚
O3 distance to the BIFOL ligand is shorter (1.90 A) than the
distances to the solvent THF. The molecular structure of 4b
shows a Li-ion, which is coordinated by two THF molecules
˚
with Li–O distances of 1.96 (O3) and 1.99 A (O5), one
˚
oxygen of the BIFOL ligand (2.06 A) and the n-butoxy unit

3706
D. A. Lange et al. / Tetrahedron 62 (2006) 3704–3709
O
O
H
O
O
H
H
O
O
O-tBu
O-tBu
2
Al
Li
Al
Li
+
Al
Li
O-tBu
4c
5
Scheme 3. Redistribution of (M)-BIFAl-H-(O-tBu) according to the attempted synthesis of 4c and the subsequent detection of 5 via X-ray crystal structure
analyses.
Figure 4. MNDO optimized structure of the (M)-BIFAL-H-(O-Me) anion.
0
0
K100.78.
Biaryl angle: C
2
–C
1
–C
1
–C
2
Figure 3. X-ray crystal structure of 5. Hydrogen atoms are omitted.
Thermal ellipsoids represent a 20% probability. Selected atom distances (in
˚
A): Al–O1 1.74, Al–O2 1.74, Al–O3 1.79, Al–O4 1.78, Li–O3 1.99, Li–O4
0
0
1.93, Li–O5 2.04, Li–O6 2.05. Biaryl angle: C
2
–C
1
–C
1
–C
2
K107.48.
Table 1. Computed relative energies (Erel, kcal/mol) of (P)- and (M)-
conformations of BIFAl-H-(O-R)
a
R
(M)-BIFAl-H-(O-R)-calc
(P)-BIFAl-H-(O-R)-calc
Me (6)
nBu
tBu
0
0
0
25.0
23.4
23.2
a
MNDO optimized, see Refs. 11–13. Relative energies without ZPE
correction.
Figure 5. MNDO optimized structure of (P)-BIFAL-H (O-Me) anion.
0
0
90.78.
Biaryl angle: C
2
–C
1
–C
1
–C
2
(S)-configuration of the corresponding alcohol is more
favorable than (R)-configuration (Table 3; Figs. 6
and 7).
1
1,12,13
ketones with moderate to fairly good enantioselectivities.
X-ray structure analyses and computations reveal only (M)-
conformations of these BIFOL modified lithium aluminum
3. Conclusions
hydrides. The appearance of BIFAl(O-tBu) (5) provides
2
The new (M)-BIFAl-H-(O-R) reagents are efficiently
accessible and suitable for the reduction of aryl alkyl
evidence for hydride–alkoxide redistributions, according to
earlier studies on alkoxide modified lithium aluminum

D. A. Lange et al. / Tetrahedron 62 (2006) 3704–3709
3707
Table 2. Enantioselective reductions of aryl alkyl ketones with BIFAl-H-
(O-R) (4a,b,c)
a
Ketone
Alk
(M)-4a
(M)-4b
ee (%) conf
(M)-4c
ee (%)
Ar
conf
ee (%) conf
Me
Me
Et
Pr
tBu
Ph
pTol
Ph
Ph
Ph
R
S
S
R
S
37
22
23
24
50
R
S
S
S
S
38
39
32
62
37
R
S
R
R
S
15
38
4
17
3
a
Enantioselectivities were determined by chiral GC (Chiraldex-GTA
column). Absolute configurations were determined by POLAR LmP-WR
polarimeter. Isolated yields (not optimized) are up to 20%.
O
OH
(M)-BIFAl-H-(O-R)
Ar Alk
1. –78 °C; 18 h
Ar * Alk
2. hydrolytic work-up
THF
Figure 6. B3LYP/6-31G*//ONIOM (B3LYP/6-31G* (Al, Li, O, C, H):
MNDO) optimized transition state leading to (R)-1-phenylbutanol
R=Me, n-Bu, t-Bu
Alk=Me, Et, Prop, t-Bu
Ar=Ph, pTol
(
disfavored TS).
Scheme 4. (M)-BIFAl-H-(O-R) for enantioselective reductions of prochiral
ketones.
O
R
C
O
Al
Al
C
H
O
H
O
R
Li
Li
O
O
O
O
R
R
favored
disfavored
O
R
C
O
Al
Al
O
O
R
C
H
O
H
R
Li
Li
O
O
O
R
Figure 7. B3LYP/6-31G*//ONIOM (B3LYP/6-31G* (Al, Li, O, C, H):
MNDO) optimized transition state leading to (S)-1-phenylbutanol (favored
TS).
favored
disfavored
Scheme 5. Transition structures of the (M)-BINAl-H-(O-R) and (M)-
BIFAl-H-(O-R) reduction of prochiral aryl alkyl ketones.
4. Experimental
All reactions and crystallization approaches were carried
out under argon atmosphere using long necked Schlenk-
tubes dried by heatgun. THF was always freshly distilled
over sodium under argon atmosphere before use. Methanol
was distilled over magnesium under argon. n-Butanol and
tert-butanol (HPLC-Grade) were used without drying.
Lithium aluminum hydride (LAH) was used as a 1 M
solution in THF commercially available from Aldrich.
Enantiomeric excess of the chiral secondary alcohols were
measured on a Hewlett Packard HP 6890 GC (chiral
capillary column: Chiraldex-GTA, 30 m, 0.25 mm,
0.25 mm); Absolute configuration of the chiral secondary
alcohols were determined by POLAR LmP-WR polarimeter
(wavelenght: 589 nm); X-ray analysis were recorded
with Nonius Kappa CCD diffractometer (Mo Ka; wave-
Table 3. Computed relative energies (Erel, kcal/mol) and imaginary
frequencies (i, cm ) of six-membered transition structures for alkyl aryl
ketone redutions (for 4b cf. Figs. 6 and 7)
K1
a
BIFAL-HCalkyl aryl
ketones
(S) Erel(kcal/mol)
(R) Erel (kcal/mol)
4aCtBu, Ph
4bCPr, Ph
4cCMe, pTol
0 (i 846)
0 (i 716)
0 (i 691)
1.4 (i 807)
0.9 (i 439)
7.9 (i 569)
a
B3LYP/6-31G*//ONIOM (B3LYP/6-31G* (Al, Li, O, C, H): MNDO),
see Refs. 11–13. Relative energies without ZPE correction.
hydride disproportionations. Transition state modeling can
reflect some origins of enantioselectivity. Besides their
application as hydride transfer reagents, the BIFAl-H
species are promising chiral Lewis acids. Such properties
are currently under investigation.
˚
length lZ0.71073A).

3708
D. A. Lange et al. / Tetrahedron 62 (2006) 3704–3709
4
(
.1. Synthesis and crystallization of (M)-BIFAl-H-
O-Me) 4a$3THF
of lithium aluminum hydride. After stirring for 30 min,
1 equiv of a primary alkyl alcohol, respectively,
methanol, butanol or tert-butanol was added slowly
under a rapid stream of hydrogen gas. The solution
was stirred again for 30 min and cooled down to
K78 8C. Then 0.5 equiv of the aryl alkyl ketone was
dissolved in 2 ml abs THF was added drop wise. The
solution was stirred for 18 h and quenched with 0.5 ml
methanol followed by acidic work-up with 2 N HCl and
(
M)-BIFOL (1 mmol) was dissolved in 5 ml abs THF in a
schlenk-tube and 1.2 equiv of a 1 M solution of lithium
aluminum hydride in THF was added under few elusion of
hydrogen. The mixture was stirred for 30 min. Then 1 mmol
abs methanol was added slowly under a rapid stream of
hydrogen gas and stirred again for 30 min. For crystal-
lization the solution was cooled down to 4 8C for 3 days and
then cooled down to K28 8C for 5 days. White crystals
separated. Yield: 0.26 g (35%). X-ray crystal data of 4a:
C H AlLiO ; MZ738.91; space group orthorhombic, aZ
neutral work-up with saturated NaSO -solution. The
4
secondary alcohol was extracted with diethyl ether. The
organic phase was washed with saturated NaCl-solution
and dried over NaSO . The resulting secondary alcohol
4
5
68
6
4
˚
˚
˚
˚
9
9
0
.9449(8) A, bZ19.389(1) A, cZ22.483(1) A, aZ90, bZ
was distilled and analyzed by chiral GC (Chiraldex-GTA
column) and polarimetry.
3
0, gZ90; VZ4335.3(6) A ; ZZ4; TZ293(2) K; mZ
.091 mm ; reflections total: 24407, unique: 9202,
K1
observed: 3695 (IO2s(I)); parameters refined: 577; R1Z
0
.0549, wR2Z0.1097; GOFZ0.895.
5. Computational details
4
(
.2. Synthesis and crystallization of (M)-BIFAl-H-
O-nBu) 4b$1THF
All structures were fully optimized and characterized by
frequency computations using Gaussian 03 with standard
11
1
basis sets 6-31G* and the B3LYP hybrid-DFT method.
2
13
(M)-BIFOL (1 mmol) was dissolved in 5 ml abs THF in a
schlenk-tube and 1.2 equiv of a 1 M solution of lithium
aluminum hydride in THF was added under few elusion of
hydrogen. The mixture was stirred for 30 min. Then 1 mmol
n-butanol was added slowly under a rapid stream of hydrogen
gas and stirred again for 30 min. For crystallization the
solution was cooled down to K28 8C for 3 days. White
crystals separated. Yield: 0.10 g (14%). X-ray crystal data of
Oniom method is used for transition state computations
(
Scheme 6).
4
b: C H AlLiO ; MZ708.89; space group monoclinic, aZ
44 66 5
˚
˚
˚
O
Al
R
C
Li
O
9
.4880(10) A, bZ19.6400(10) A, cZ11.6700(10) A, aZ90,
3
˚
bZ113.157(5), gZ90; VZ1999.4(3) A ; ZZ2; TZ
O
H
K1
1
2
00(2) K; mZ0.094 mm ; reflections total: 5582, unique:
216, observed: 1485 (IO2s(I)); parameters refined: 466;
O
R
R1Z0.0757, wR2Z0.1783; GOFZ1.049. All THF were
disordered. Only one orientation is shown.
Scheme 6. B3LYP/6-31G*//ONIOM (B3LYP/6-31G* (Al, Li, O, C, H):
MNDO) optimized transition state. B3LYP/6-31G* optimized area is
framed.
4
5
.3. Synthesis and crystallization of (M)-BIFAl-(O-tBu)2
$2THF
6. Supplementary material
(
M)-BIFOL (1 mmol) was dissolved in 3 ml abs THF in a
Crystallographic data for the structural analysis has been
deposited with the Cambridge Crystallographic Data
Centre, CCDC No. 271631 for compound 4a; No. 271632
for compound 4b and No. 271633 for compound 5. Copies
of this information may be obtained free of charge from the
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ,
UK (fax: C44 1223 336 33) or deposit@ccdc.cam.ac.uk or
http://www.ccdc.cam.ac.uk
schlenk-tube and 1.2 equiv of a 1 M solution of lithium
aluminum hydride in THF was added under few elusion of
hydrogen. The mixture was stirred for 30 min. Then 1 mmol
tert-butanol was added slowly under a rapid stream of
hydrogen gas and stirred again for 30 min. For crystal-
lization the solution was cooled down to 4 8C for 3 days and
then cooled down to K28 8C for 3 days. White crystals
separated. Yield: 0.13 g (17%). X-ray crystal data of 4c:
C H AlLiO ; MZ789.99; space group monocline, aZ
4
8
74
6
˚
˚
˚
1
0.4959(2) A, bZ19.2992(4) A, cZ11.9120(2) A, aZ90,
3
˚
bZ111.476(1), gZ90; VZ2245.39(7) A ; ZZ2; TZ
2
K1
93(2) K; mZ0.091 mm ; reflections total: 18192,
Acknowledgements
unique: 9791, observed: 7781 (IO2s(I)); parameters
refined: 566; R1Z0.0390, wR2Z0.1218; GOFZ0.656.
We are grateful to the Fonds der Chemischen Industrie for
financial support as well as for a Dozenten-Stipendium to
B.G. We especially thank the Deutsche Forschungsge-
meinschaft (DFG) for funding (GO-930/9, GO-930/7 and
GO-930/5) as well as the Bayer AG, the BASF AG, the
Wacker AG, the Degussa AG, the Raschig GmbH, the
Symrise GmbH, the Solvay GmbH and the OMG AG for
generous support.
4
.4. Enantioselective reduction of the acetophenon,
methylacetophenon, propiophenon, butyrophenon,
pivalophenon
The respective (M)-BIFAl-H-(O-R) reagent was prepared
from 2.5 mmol (M)-BIFOL and 1 equiv of a 1 M solution

D. A. Lange et al. / Tetrahedron 62 (2006) 3704–3709
3709
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