Synthesis, characterization, and structures of arylaluminum reagents and asymmetric arylation of aldehydes catalyzed by a titanium complex of an N-sulfonylated amino alcohol

Article abstract of DOI:10.1016/j.tetasy.2009.05.018

A series of phenylaluminum reagents AlPh_xEt_3-x(L) (x = 1-3) containing adduct ligand L [Et₂O, THF, OPPh₃, or 4-dimethylaminopyridine (DMAP)] were synthesized and characterized. NMR studies showed that AlPh_x

Full text of DOI:10.1016/j.tetasy.2009.05.018

Tetrahedron: Asymmetry 20 (2009) 1407–1412
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Tetrahedron: Asymmetry
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Synthesis, characterization, and structures of arylaluminum reagents
and asymmetric arylation of aldehydes catalyzed by a titanium
complex of an N-sulfonylated amino alcohol
Shuangliu Zhou a,b, Da-Wei Chuang a, Shih-Ju Chang a, Han-Mou Gau a,
*
a Department of Chemistry, National Chung Hsing University, Taichung 402, Taiwan
b Anhui Key Laboratory of Molecule-based Materials, College of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of phenylaluminum reagents AlPhxEt3Àx(L) (x = 1–3) containing adduct ligand L [Et2O, THF,
OPPh3, or 4-dimethylaminopyridine (DMAP)] were synthesized and characterized. NMR studies showed
that AlPhxEt3Àx(L) (x = 1 or 2) exists as an equilibrium mixture of 3–4 species in solution. Solid-state
structures of the phenylaluminum reagents reveal a distorted tetrahedral geometry. Asymmetric addi-
tions of phenylaluminum to 2-chlorobenzaldehyde were examined employing a titanium(IV) complex
[TiL*(OPri)2]2 10 (H2L* = (1R,2S)-2-(p-tolylsulfonylamino)-1,3-diphenyl-1-propanol) as a catalyst precur-
sor. It was found that the adduct ligand L had a strong influence on the reactivity and the enantioselec-
tivity in asymmetric phenyl additions to aldehydes. The phenylaluminum reagents with OPPh3 or DMAP
were unreactive toward aldehydes, and AlPh3(THF) was found to be superior to AlPh3(OEt2) or
AlPhEt2(THF). Asymmetric aryl additions of AlAr3(THF) to aldehydes employing a loading of 5 mol % tita-
nium(IV) complex 10 with a strategy of a slow addition of the aldehydes over 20 min were conducted,
and the reactions produced optically active secondary alcohols in high yields with excellent enantioselec-
tivities of up to 94% ee.
Received 16 April 2009
Accepted 15 May 2009
Available online 24 June 2009
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
The solid-state triarylaluminum compounds were, in general,
prepared as dimeric species.7 However bulky substituted aryl
The enantioselective addition of organometallic reagents to car-
bonyl compounds is an important synthetic method that gives
optically active alcohols via carbon–carbon bond formation.1
Among the methods available for the synthesis of chiral diaryl-
methanols, the asymmetric addition of phenylzinc reagents has
achieved much success owing to wide substituent tolerances and
mild reaction conditions.2 Although high enantioselectivities have
been achieved, the non-substituted phenyl group as a transferring
nucleophile limited the application. Subsequently, a method where
arylzinc reagents were prepared by transmetallation between aryl-
boronic acids or arylboranes and diethylzinc as aryl transfer re-
agents was reported by Bolm et al.3 and later by others.4 This
modified methodology broadened the scope of the aryl transfer
reaction using functionalized arylzinc reagents as nucleophiles.
The arylzinc reagents could also be prepared from reactions of zinc
halides with metallic aryl reagents.5 In contrast to zinc reagents,
organoaluminum compounds have proven to be more reactive
nucleophiles,6 owing to the greater Lewis acidity of the aluminum
center.
ligands have been used for the preparation of highly reactive
monomeric compounds. The first three-coordinate monomeric tri-
mesitylaluminum was reported by Oliver et al.8 The bulky mesityl
ligands provided significant steric hindrances to the metal center
and prevented the formation of the dimeric aluminum complex.
By providing an additional neutral ligand, a series of four-coordi-
nate triarylaluminum complexes, such as Al(o-tolyl)3(OEt2),7a
AlMes3(THF) (Mes = mesityl),9 AlPh3(THF),10 AlMes3(4-picoline)Á
11
(C7H8)0.5
,
and AlPh3(E(SiMe3)3) (E = P or As)12 were also synthe-
sized and characterized.
Although many arylaluminum compounds have been synthe-
sized and characterized, there is scarce research on the asymmetric
arylation of aldehydes or ketones using the arylaluminum com-
pounds as aryl sources, this led us to investigate asymmetric aryl
additions of arylaluminum compounds to aldehydes or ketones.
We discovered that arylaluminum compounds are effective re-
agents in asymmetric aryl additions in short reaction times to alde-
hydes catalyzed by the titanium catalyst of 10 mol % (R)-H8-BINOL
or 20 mol % 1,3-bis[N-sulfonyl-(1R,2S)-1,3-diphenyl-2-aminopro-
panol]benzene.13 Furthermore, the AlAr3(THF) compounds have
been proven to be highly enantioselective aryl transfer reagents
for additions to aromatic ketones affording tertiary alcohols in
excellent enantioselectivities14 in addition to being coupling
* Corresponding author. Tel.: +886 4 22878615; fax: +886 4 22862547.
E-mail address: hmgau@dragon.nchu.edu.tw (H.-M. Gau).
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.05.018

1408
S. Zhou et al. / Tetrahedron: Asymmetry 20 (2009) 1407–1412
reagents with aryl halides.15 Subsequently, von Zezschwitz et al.16
and Hoveyda et al.17 reported the asymmetric 1,2- or 1,4-additions
of arylaluminum reagents to cyclic enones by using AlPhMe2 pre-
pared in situ, respectively. Alexakis et al. also reported a copper-cat-
alyzed asymmetric conjugate addition of AlArEt2 reagents to
enones.18
In order to further explore arylaluminum reagents for asym-
metric catalysis, we herein report the synthesis and structures of
a series of arylaluminum reagents containing an adduct of Et2O,
THF, OPPh3, or DMAP and their asymmetric aryl additions to alde-
hydes employing a loading of 5 mol % titanium(IV) complex 10 as a
catalyst precursor.
Bn
Ph
OPri
OPri
Ts
Ts
iPrO
N
O
Ti
Ti
O
N
iPrO
Ph
10
Bn
2. Results and discussion
Figure 1. Molecular structure of AlPh3(OEt2) 1. Hydrogen atoms are omitted for
clarity.
2.1. Syntheses and spectroscopic studies of the aluminum
reagents
The aluminum complex AlPh3(OEt2) 1 was obtained in high
yield from a reaction of AlCl3 with 3 equiv PhMgBr in Et2O. Further
reaction of 1 or AlPh3(THF) 2 in toluene with OPPh3 or 4-dimethyl-
aminopyridine (DMAP) produced AlPh3(OPPh3) 3 and AlPh3(D-
MAP)ÁC7H8 4, respectively. Reactions of AlPhEt2(THF) 513c with
OPPh3 or DMAP afforded AlPhEt2(OPPh3) 6 or AlPhEt2(DMAP) 7
(Scheme 1). The 1H NMR spectra of 6 and 7 revealed three sets of
ethyl resonances and three sets of phenyl signals, indicating that
6 and 7 in CDCl3 solution contained a mixture of four species rep-
resented as AlPhxEt3Àx(OPPh3) and AlPhxEt3Àx(DMAP) (x = 0, 1, 2, or
3), respectively. Recrystallization of 6 or 7 from toluene gave crys-
talline solids of AlPh2Et(OPPh3) 8 and AlPh2Et(DMAP) 9, respec-
tively. It is noteworthy that the 1H NMR spectra of 8 and 9
showed two sets of ethyl resonances and three sets of phenyl sig-
nals, indicating that both 8 and 9 in CDCl3 solution partly con-
verted to AlPhEt2(L) and AlPh3(L). Structures of complexes 1, 3, 4,
and 9 were further determined by single-crystal X-ray analyses.
2.2. Molecular structures of complexes 1, 3, 4, and 9
Suitable crystals of complexes 1 (Fig. 1), 4 (Fig. 3), and 9 (Fig. 4)
for structure determinations were obtained by slowly cooling their
respective toluene solutions to 0 °C. Crystals of complex 3 (Fig. 2)
were obtained by liquid diffusion of hexane/CH2Cl2 solution. The
selected bond lengths and angles are listed in Table 1.
A typical feature of all the structures described herein is that
three carbon atoms and an oxygen atom (or a nitrogen atom) sur-
round the metal atom in a distorted tetrahedral geometry. The
C–Al–O and C–Al–N angles formed around the metal center range
Figure 2. Molecular structure of AlPh3(OPPh3) 3. Hydrogen atoms are omitted for
clarity.
from 102° to 108° (Table 1) which are obviously smaller than the
C–Al–C angles in the range of 111–117°. Similar results were also
observed in complexes such as Al(o-Tol)3(OEt2),7a AlMes3(THF),9
11
AlPh3(THF),10 and AlMes3(4-picoline)Á(C7H8)0.5
.
From Table 1, it is clear that averaged Al–C bond distances in
complexes 1 (1.986(2) Å), 3 (1.986(2) Å), 4 (1.989(2) Å), and 9
(1.990(2) Å) are nearly identical. These averaged Al–C distances
are comparable to those in AlPh3(THF) (1.983(2) Å) and Al(o-To-
l)3(OEt2) (1.990(2) Å), but shorter than that of 2.017(7) Å and
2.009(12) Å found in AlMes3(THF) and AlMes3(4-picoline)Á
(C7H8)0.5, respectively. The Al–O distance of 1.924(1) Å in complex
1 is longer than those of 1.819(1) Å in 3 and 1.897(1) Å in AlPh3(THF),
but shorter than that of 1.969(5) Å in AlMes3(THF). The Al–N dis-
tance (1.962(1) Å) in complex 4 is comparable to that of 1.978(2) Å
in 9, but shorter than that of 2.045(8) Å in AlMes3(4-pico-
line)Á(C7H8)0.5. The observed variations of bond distances result from
both steric interactions and electronic effect.
Et2O
0 °C
L
AlCl3 + PhMgBr
AlPh3(L)
AlPh3(Et2O)
Toluene
80 °C
1
3: L = OPPh3
4: L = DMAP
recrystallization
L
AlPhEt2(THF)
AlPhEt2(L)
AlPh2Et(L)
Toluene
80 °C
6: L = OPPh3
7: L = DMAP
5
8: L = OPPh3
9: L = DMAP
Scheme 1. Syntheses of the aluminum reagents.

S. Zhou et al. / Tetrahedron: Asymmetry 20 (2009) 1407–1412
1409
2-chlorobenaldehyde were examined using 10 mol % complex 10
as a catalyst precursor. The results are summarized in Table 2,
and it was found that the adduct ligands of the aluminum reagents
have a strong influence on both reactivities and enantioselectivities
in asymmetric phenylation reactions. Both AlPh3(OEt2) 1 and
AlPh3(THF) 2 showed excellent reactivities giving diarylmethanol
12a in excellent yields of 100% and 91% (entries 1 and 2),
respectively. However, the addition of AlPh3(OEt2) gave 12a in a
low enantioselectivity of 9% in comparison to 65% for the
AlPh3(THF) addition reaction. The addition of AlPhEt2(THF) 5 to
2-chlorobenzaldehyde gave diarylmethanol 12a in 64% yield with
a low enantioselectivity of 32%, and an ethyl addition product
was also obtained as a by-product in an 11% yield (entry 5). In con-
trast, the aluminum reagents 3, 4, 8, and 9 were unreactive toward
2-chlorobenaldehyde when they were coordinated by a strong Le-
wis base such as OPPh3 or DMAP (entries 3, 4, 6, and 7). When
the reaction was carried out under the reaction conditions of
10 mol % 10, 1.25 equiv Ti(OPri)4, 1.2 equiv AlPh3(THF) in THF, the
phenyl addition to 2-chlorobenzaldehyde afforded the correspond-
ing diarylmethanol 12a in 98% yield and 86% ee (entry 8). To exam-
ine the requirement of Ti(OPri)4 in the catalytic system, a reaction
without an addition of Ti(OPri)4 was carried out, and 12a was ob-
tained in a low conversion of 11% (entry 9), suggesting that 10 is
not the actual catalyst. Instead, the dititanium complex 10 is a cata-
lyst precursor. In the catalytic solution, Ti(OPri)4 exchanges a phenyl
group with AlPh3(THF),13a and the resulting species reacts further
with 10 to give an active species which has a structure similar to
the active species proposed for the asymmetric ZnEt2 addition to
aldehydes employing the same 10/Ti(OPri)4 catalytic system.19d
Tuning the amount of Ti(OPri)4 to 1.0 and 1.5 equiv led to prod-
uct 12a in yields of 78% and 100% with enantioselectivities of 72
and 88% (entries 10 and 11), respectively. When Ti(OPri)4 was kept
at 1.5 equiv and AlPh3(THF) was altered to 1.0 and 1.4 equiv (en-
tries 12 and 13), the reactions afforded the product with enantiose-
lectivities of 85% and 82%, respectively. In order to suppress the
racemic background reactions that would lower the enantioselec-
tivities of the addition product, a strategy of a slow addition of
the aldehyde to a solution containing the catalytic system and
the arylaluminum reagent significantly improved the enantioselec-
tivity to 98% (entry 14). To examine the efficiency of the catalytic
system, the loading of 10 was decreased to 5 mol%, producing
the product in 100% yield and an excellent 92% ee (entry 15).
Reducing the loadings of 10 further to 2.5 and 1 mol % resulted
in decreasing enantioselectivities to 86% and 62% (entries 16 and
17), respectively. While employing the 10/Ti(OPri)4 catalytic sys-
tem, it was found that, among the AlPhxEt3Àx(L) reagents,
AlPh3(THF) was the best phenyl source for additions to aldehydes.
The asymmetric aryl transfer of AlAr3(THF) to a variety of alde-
hydes was subsequently performed in the presence of 5 mol % 10
with the slow addition of the aldehyde over 20 min; the results
are listed in Table 3. Regardless of the electronic nature or the
Figure 3. Molecular structure of AlPh3(DMAP)ÁC7H8 4. Hydrogen atoms and solvent
molecule are omitted for clarity.
Figure 4. Molecular structure of AlPh2Et(DMAP) 9. Hydrogen atoms are omitted for
clarity.
2.2.1. Asymmetric addition of aluminum reagents to aldehydes
catalyzed by a titanium(IV) complex of an N-sulfonylated amino
alcohol
It has been established by us that AlAr3(THF) and AlPhEt2(THF)
compounds containing
a THF adduct are excellent arylation
reagents of aldehydes.13 We also demonstrated that derivatives of
amino alcohols were excellent ligands for multiple types of asym-
metric carbon–carbon bond formation reactions.19 In order to study
reactivities and enantioselectivities of AlPhxEt3Àx(L) (L = Et2O, THF,
OPPh3, DMAP), asymmetric phenyl additions of AlPhxEt3Àx(L) to
Table 1
Selected bond lengths (Å) and bond angles (°) for 1, 3, 4, and 9
1
3
4
9
Al–O
1.924(1)
1.981(2)
1.987(2)
1.989(2)
1.986(2)
1.819(1)
1.981(2)
1.986(2)
1.990(2)
1.986(2)
Al–N(1)
Al–C(1)
Al–C(7)
Al–C(13)
Al–C(av)
1.962(1)
1.988(2)
1.985(2)
1.993(2)
1.989(2)
1.978(2)
2.001(2)
1.983(2)
1.978(3)
1.990(2)
Al–C(1)
Al–C(7)
Al–C(13)
Al–C(av)
O–Al–C(1)
O–Al–C(7)
O–Al–C(13)
C(1)–Al–C(7)
C(7)–Al–C(13)
C(1)–Al–C(13)
105.00(6)
103.36(6)
103.28(6)
115.07(7)
114.76(7)
113.48(7)
108.80(7)
104.97(7)
103.94(7)
113.29(8)
112.60(7)
112.44(7)
N(1)–Al–C(1)
N(1)–Al–C(7)
N(1)–Al–C(13)
C(1)–Al–C(7)
C(7)–Al–C(13)
C(1)–Al–C(13)
102.47(8)
106.21(7)
103.91(7)
111.73(7)
114.04(8)
116.82(8)
102.82(8)
103.47(8)
106.08(10)
110.81(9)
116.95(12)
114.84(12)

1410
S. Zhou et al. / Tetrahedron: Asymmetry 20 (2009) 1407–1412
Table 2
Optimizations of asymmetric AlPhxEt3Àx(L) to 2-chlorobenzaldehyde catalyzed by 10/Ti(OPri)4 catalysta
Cl
O
Cl
OH
Ph
10/Ti(OPri)4
H
AlPhxEt3-x(L)
+
0°C, 30 min
11a
12a
Entry
10 (mol %)
Solvent
AlPhxEt3Àx(L)
AlPhxEt3Àx(L) (equiv)
Ti(OPri)4 (equiv)
Conv.c (%)
eed (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14b
15b
16b
17b
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
THF
THF
THF
THF
THF
AlPh3(OEt2) 1
AlPh3(THF) 2
AlPh3(OPPh3) 3
AlPh3(DMAP) 4
AlPhEt2(THF) 5
AlPh2Et(OPPh3) 8
AlPh2Et(DMAP) 9
AlPh3(THF) 2
AlPh3(THF) 2
AlPh3(THF) 2
AlPh3(THF) 2
AlPh3(THF) 2
AlPh3(THF) 2
AlPh3(THF) 2
AlPh3(THF) 2
AlPh3(THF) 2
AlPh3(THF) 2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.0
1.4
1.2
1.2
1.2
1.2
1.25
1.25
1.25
1.25
1.25
1.25
1.25
1.25
—
1.0
1.5
1.5
1.5
100
91
0
9
65
—
—
32
—
—
86
—
72
88
85
82
98
92
86
62
0
64 (11)e
0
0
98
11
78
100
85
100
100
100
100
100
THF
THF
THF
THF
1.5
1.5
1.5
1.5
2.5
1
THF
a
0.50 mmol 2-ClC6H4CHO, THF (3 mL), 0 °C, equiv of Ti(OPri)4 and AlPh3(THF) are relative to 2-chlorobenzaldehyde.
b
c
2-ClC6H4CHO (0.50 mmol) in 0.8 mL THF was added dropwise over 20 min; reaction time of 30 min including the addition time of the substrate to the catalytic solution.
Conversions based on 1H NMR spectra.
d
e
The ee values were determined by HPLC.
The value in parenthesis was conversion of the ethyl addition product.
steric effect of the substituent on the aryl groups, asymmetric phe-
nyl additions to aromatic aldehydes afforded diarylmethanols in
high yields with excellent enantioselectivities of 90–94% (entries
1–11). In addition to aromatic aldehydes, the phenyl addition to
cyclic 2-furylaldehyde gave the addition product 12m in an
excellent 93% yield and a 90% ee (entry 13). For aliphatic alde-
hydes, the phenyl addition to ButCHO or PriCHO produced the cor-
responding secondary alcohols in excellent yields but in moderate
enantioselectivities of 74% and 77% (entries 14 and 15), respec-
tively. The additions of substituted aryl to benzaldehyde afforded
the desired products in good or excellent stereoselectivities of
86% and 91% (entries 16 and 17).
the
a,b-unsaturated (E)-cinnamaldehyde was studied to furnish
the secondary alcohol 12l in 92% yield with a good enantioselectiv-
ity of 87% (entry 12). In contrast, the phenyl addition to the hetero-
Table 3
3. Conclusion
Asymmetric AlAr3(THF) addition to aldehydes catalyzed by 10/Ti(OPri)4 catalysta
5 mol % 10
A series of phenylaluminum reagents AlPhxEt3Àx(L) (L = Et2O,
THF, OPPh3, or DMAP) were synthesized. 1H NMR studies show
the existence of 3–4 species for AlPhxEt3Àx(L) (x = 1 or 2) in solu-
tion, while solid-state structures reveal a tetrahedral geometry.
Asymmetric additions of phenylaluminum reagents to 2-chloro-
benzaldehyde catalyzed by the titanium(IV) complex 10 of an
N-sulfonylated amino alcohol suggest that the adduct ligands play
a key role in both the reactivity and the enantioselectivity in asym-
metric phenylations of aldehydes. The phenylaluminum reagents
containing a strong Lewis base of OPPh3 or DMAP were unreactive
toward aldehydes. For the 10/Ti(OPri)4 catalytic system, AlPh3
(THF) was superior to AlPh3(OEt2) or AlPhEt2(THF). In this study,
5 mol % 10 was used for the additions of the highly reactive
AlAr3(THF) to aldehydes by the strategy of slow addition of alde-
hydes over 20 min, furnishing optically active secondary alcohols
12 in high yields with excellent enantioselectivities of up to 94%.
OH
O
1.5 equiv Ti(OPri)4
+ AlAr3(THF)
1.2 equiv
R
R
H
Ar
THF, 0°C, 30 min
11
12
Entry
R
Ar
Product
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
2-ClC6H4
4-ClC6H4
2-BrC6H4
4-BrC6H4
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
p-Tolyl
12a
12b
12c
12d
12e
12f
12g
12h
12i
12j
12k
12l
12m
12n
12o
12h’
92
91
94
96
94
98
95
93
80
96
94
92
93
93
92
91
94
93 (R)
90 (R)
93 (R)
90 (R)
94 (R)
90 (R)
91 (R)
92 (R)
90 (R)
91 (R)
92 (R)
87 (S)
90 (R)
74 (S)
77 (S)
91 (S)
86 (S)
2-MeOC6H4
4-MeOC6H4
2-MeC6H4
4-MeC6H4
4-CF3C6H4
1-Naphthyl
2-Naphthyl
(E)-Cinnamyl
2-Furyl
But
Pri
Ph
Ph
4. Experimental
2-Napthyl
12k’
4.1. General remarks
Substrate/10/AlAr3(THF)/Ti(OPri)4 = 0.50/0.025/0.60/0.75 mmol, substrate in
0.8 mL THF was added dropwise over 20 min; reaction time including the addition
time of the substrate to the catalytic solution.
Isolated yields after column chromatography.
The ee values were determined by HPLC, Absolute configurations were obtained
by comparison with the HPLC data of known compounds.
a
All syntheses and manipulations of air- and moisture-sensitive
materials were performed under a dry nitrogen atmosphere using
standard Schlenk techniques or in a glovebox. Solvents were re-
fluxed and distilled over sodium benzophenone ketyl (THF, Et2O,
b
c

S. Zhou et al. / Tetrahedron: Asymmetry 20 (2009) 1407–1412
1411
toluene, or hexane) or P2O5 (dichloromethane) under nitrogen
atmosphere prior to use. Aldehydes were dried over MgSO4 or
molecular sieves and distilled before use. Ketones, OPPh3, and
DMAP were used as received. 1H NMR and 13C NMR spectra in
CDCl3 were recorded on a Varian Mercury-400 spectrometer with
chemical shifts given in ppm from the internal TMS. AlAr3(TH-
F)9a,13a and AlPhEt2(THF)13c were synthesized according to the lit-
erature procedure.
solid of AlPh2Et(OPPh3) 8. The 1H NMR spectrum indicates that
complex 8 in CDCl3 solution contains a mixture of three major
species of AlPhEt2(OPPh3), AlEtPh2(OPPh3), and AlEtPh2(OPPh3).
Anal. Calcd for C32H30OPAl 8: C, 78.67; H, 6.19. Found: C, 78.19;
H, 6.67.
4.6. Synthesis of 7 and 9
To a toluene (30 mL) solution of DMAP (0.488 g, 4.00 mmol)
was added 5 (0.937 g, 4.00 mmol) at room temperature. The mix-
ture was stirred for 12 h at 80 °C. The solvent was removed under
reduced pressures to give an oily solid of 7. 1H NMR (CDCl3,
400 MHz): AlEt3(DMAP), d 8.07 (d, J = 6.8 Hz), 6.52 (d, J = 6.8 Hz,
2H), 3.05 (s, 6H), 0.95 (t, J = 7.6 Hz, 9H), À0.16 (q, J = 8.0 Hz,
6H) ppm; AlPhEt2(DMAP), d 8.07 (d, J = 6.8 Hz, 2H), 7.62–7.59 (m,
2H), 7.28–7.16 (m, 3H), 6.52 (d, J = 6.8 Hz, 2H), 3.05 (s, 6H), 1.06
(t, J = 8.2 Hz, 6H), 0.08 (q, J = 8.2 Hz, 4H) ppm; AlPh2Et(DMAP), d
8.07 (d, J = 6.8 Hz, 2H), 7.66–7.32 (m, 4H), 7.28–7.16 (m, 6H),
6.52 (d, J = 6.8 Hz, 2H), 3.05 (s, 6H), 1.13 (t, J = 8.2 Hz, 3H), 0.35
(q, J = 8.0 Hz, 2H); AlPh3(DMAP), d 8.07 (d, J = 6.8 Hz, 2H), 7.72–
7.70 (m, 6H), 7.28–7.16 (m, 9H), 6.52 (d, J = 6.8 Hz, 2H), 3.05 (s,
6H) ppm. Recrystallization of the crude product 7 from toluene
gave colorless crystals of AlPh2Et(DMAP) 9. The 1H NMR spectrum
indicates that complex 9 in CDCl3 solution contains a mixture of
three major species of AlPhEt2(DMAP), AlPh2Et(DMAP), and
AlPh3(DMAP). Anal. Calcd for C21H25N2Al 9: C, 75.88; H, 7.58; N
8.43. Found: C, 75.52; H, 7.39; N 8.69.
4.2. Synthesis of AlPh3(OEt2) 1
A solution of phenylmagnesium bromide (90.0 mmol) in Et2O
was slowly added to a solution of AlCl3 (4.00 g, 30.0 mmol) in
Et2O at 0 °C. The mixture was stirred at room temperature for
12 h and the solvent was removed under reduced pressure to af-
ford a residue, which was extracted with toluene (2 Â 50 mL).
The extracts were combined and concentrated to about 50 mL. Col-
orless crystals of 1 (8.58 g, 86.0% yield) were obtained by cooling
the concentrated solution at 0 °C. 1H NMR (CDCl3, 400 MHz): d
7.80–7.78 (m, 6H), 7.32–7.30 (m, 9H), 4.13 (q, J = 7.2 Hz, 4H),
1.18 (t, J = 7.2 Hz, 6H) ppm. 13C{1H} NMR (CDCl3, 100 MHz): d
146.99, 138.23, 127.62, 127.21, 67.64, 13.35 ppm. Anal. Calcd for
C22H25OAl: C, 79.49; H, 7.58. Found: C, 78.90; H, 7.32.
4.3. Synthesis of AlPh3(OPPh3) 3
A toluene (10 mL) solution of OPPh3 (0.278 g, 1.00 mmol) was
added to a toluene (20 mL) solution of 1 (0.332 g, 1.00 mmol) at
room temperature. The mixture was stirred for 12 h at 80 °C. The
solvent was removed under reduced pressure to afford a powder.
Colorless crystals of 3 (0.477 g, 89.0% yield) were obtained by li-
quid diffusion of hexane/CH2Cl2 solution. 1H NMR (CDCl3,
400 MHz): d 7.61–7.55 (m, 9H), 7.49–7.44 (m, 6H), 7.40–7.35 (m,
6H), 7.16–7.10 (m, 9H) ppm. 13C{1H} NMR (CDCl3, 100 MHz): d
151.72, 138.16, 133.69, 132.70 (d, J(C–P) = 10.9 Hz), 129.11 (d, J(C–CP)
= 13.7 Hz), 127.21, 126.53, 126.34 ppm. Anal. Calcd for C36H30O-
PAl: C, 80.58; H, 5.64. Found: C, 80.33; H 6.01.
4.7. X-ray crystallography
Suitable crystals of complexes 1, 3, 4, and 9 were mounted un-
der nitrogen atmospheres in sealed capillaries. Diffraction was
performed on a Bruker AXS SMART 1000 or an Oxyford Gemini
S diffractometer using graphite-monochromated Mo K
(k = 0.71073 Å); temperature 293(2) K for complexes 1, 3, and 9,
and 100(2) K for complex 4; u and scan technique; SADABS effects
a radiation
x
and empirical absorption were applied in the data corrections. All
structures were solved by direct methods (SHELXTL-97),20 com-
pleted by subsequent difference Fourier syntheses, and refined
4.4. Synthesis of AlPh3(DMAP)ÁC7H8 (4)
by full-matrix least squares calculations based on F2
(SHELXTL-97).
A toluene (10 mL) solution of DMAP (0.122 g, 1.00 mmol) was
added to a toluene (20 mL) solution of 1 (0.332 g, 1.00 mmol) at
room temperature. The mixture was stirred for 12 h at 80 °C. The
solvent was removed under reduced pressures to afford a powder
which was extracted with toluene (15 mL). Colorless crystals of 4
(0.424 g, 90.0% yield) were obtained by cooling the toluene solu-
tion at 0 °C. 1H NMR (CDCl3, 400 MHz): d 8.12 (m, 2H), 7.72 (m,
6H), 7.28–7.25 (m, 12H), 7.16 (m, 2H), 6.50 (m, 2H), 3.04 (s, 6H),
2.35 (s, 3H) ppm. 13C{1H} NMR (CDCl3, 100 MHz): d 155.78,
149.55, 147.57, 138.48, 137.86, 129.04, 128.23, 126.99, 126.95,
125.30, 106.70, 39.28, 21.42 ppm. Anal. Calcd for C32H33N2Al: C,
81.33; H, 7.04; N, 5.96. Found: C, 80.93; H, 7.14; N 5.67.
Crystallographic data (excluding structure factors) for the struc-
tures in this paper have been deposited with the Cambridge Crys-
tallographic Data Centre as supplementary publication numbers
CCDC-710552 (1), CCDC-710554 (3), CCDC-710553 (4), and
CCDC-710551 (9). Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [fax: +44(0)-1223-336033 or e-mail: deposit@ccdc.cam.
ac.uk].
4.8. General procedure for the asymmetric aryl addition of
aldehydes
Under a dry nitrogen atmosphere, 10 (0.027 g, 0.025 mmol) and
Ti(OPri)4 (0.22 mL, 0.75 mmol) were mixed in dry THF (3 mL) at
room temperature. After stirring for 30 min, AlPh3(THF) (0.198 g,
0.600 mmol) in dry THF (2 mL) was added at 0 °C. The mixture
was stirred for another 10 min, and an aldehyde (0.50 mmol) in
THF (0.8 mL) was added dropwise to the resulting solution over
20 min at 0 °C. The mixture was allowed to react for 10 min at this
temperature, and then quenched with 2 M NaOH. The aqueous
phase was extracted with diethyl ether (3 Â 10 mL), dried over
MgSO4, filtered, and concentrated. The residue was purified by col-
umn chromatography to give the secondary alcohol. Enantiomeric
excesses of products were determined by HPLC using suitable chi-
ral columns from Daicel.
4.5. Synthesis of 6 and 8
To a toluene (30 mL) solution of OPPh3 (0.834 g, 3.00 mmol)
was added 5 (0.703 g, 3.00 mmol) at room temperature. The mix-
ture was stirred for 12 h at 80 °C. The solvent was removed under
reduced pressures to give an oily solid of 6. 1H NMR (CDCl3,
400 MHz): AlEt3(OPPh3), d 0.92 (t, J = 8.2 Hz, 9H), À0.33 (q,
J = 8.2 Hz, 6H) ppm; AlPhEt2(OPPh3), d 7.63–7.48 (m, br, 17H),
7.15–7.11 (m, 3H), 0.99 (t, J = 8.2 Hz, 3H), À0.09 (q, J = 8.0 Hz,
2H) ppm; AlPh2Et(OPPh3), d 7.63–7.48 (m, br, 19H), 7.15–7.11
(m, 6H), 1.03 (t, J = 8.2 Hz, 3H), À0.09 (m, 2H) ppm; AlPh3(OPPh3),
d 7.63–7.48 (m, br, 24H), 7.15–7.11 (m, 6H) ppm. Recrystalliza-
tion of the crude product 6 from toluene gave colorless crystalline

1412
S. Zhou et al. / Tetrahedron: Asymmetry 20 (2009) 1407–1412
1017–1021; (c) Kim, J. G.; Walsh, P. J. Angew. Chem., Int. Ed. 2006, 45, 4175–
Acknowledgment
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2712.
Financial support under the grant number of NSC 96-2113-M-
005-007-MY3 from the National Science Council of Taiwan is
appreciated.
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