Enantioselective hydrophosphonylation of aldehydes using an aluminum binaphthyl Schiff base complex as a catalyst

Article abstract of DOI:10.1055/s-2007-984528

An aluminum binaphthyl Schiff base complex was found to be an efficient catalyst for enantioselective hydrophosphonylation of aldehydes. High enantioselectivities were obtained in reactions of both aromatic and aliphatic aldehydes (up to 84% and 86% ee, r

Full text of DOI:10.1055/s-2007-984528

1960
LETTER
Enantioselective Hydrophosphonylation of Aldehydes Using an Aluminum
Binaphthyl Schiff Base Complex as a Catalyst
Enantioselective
H
a
ydrophosp
t
honyla
s
tion of
A
ld
u
ehyde
s
ji Ito,*a Hironori Tsutsumi,a Masayuki Setoyama,a Bunnai Saito,b Tsutomu Katsukib
a
Department of Chemistry, Fukuoka University of Education, Akama, Munakata, Fukuoka 811-4192, Japan
Fax +81(940)351711; E-mail: itokat@fukuoka-edu.ac.jp
b
Department of Chemistry, Faculty of Science, Graduate School, Kyushu University 33, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
Received 23 April 2007
effective catalyst for enantioselective Baeyer–Villiger
oxidation, in which a Criegee intermediate chelates with a
cobalt ion (Scheme 1).6 Since the binaphthyl group is a
potent chiral auxiliary, we predicted that cobalt and
aluminum binaphthyl Schiff base complexes would also
exert asymmetric catalysis for hydrophosphonylation as
efficiently as the Al(N-methylsalalen) complex. More-
over, metal binaphthyl Schiff base complexes have the
advantage of high availability and modifiability.
Abstract: An aluminum binaphthyl Schiff base complex was found
to be an efficient catalyst for enantioselective hydrophosphonyla-
tion of aldehydes. High enantioselectivities were obtained in reac-
tions of both aromatic and aliphatic aldehydes (up to 84% and 86%
ee, respectively).
Key words: aluminum, asymmetric catalysis, binaphthyl Schiff
bases, hydrophosphonylation, a-hydroxy phosphonate
Optically active a-hydroxy phosphonic acids and phos-
phonates are useful compounds with wide pharmaceutical
application,1 and there have been multiple efforts to de-
velop a practical synthesis method. Of these methods, the
enantioselective addition of phosphite to aldehydes
(asymmetric Pudovik reaction) is the most straight-
forward one.2 However, successful enantioselective
addition has been limited thus far. Shibuya et al.2a,b re-
ported enantioselective hydrophosphonylation using the
La–Li–BINOL (LLB) complex as a catalyst and obtained
good enantioselectivity up to 82% ee in the reaction of
some aromatic aldehydes.2b On the other hand, Shibasaki
et al.2d,f achieved widely applicable, highly enantioselec-
tive hydrophosphonylation by complementarily using Al–
Li–BINOL (ALB) and LLB complexes as catalysts. These
unique catalysts are multifunctional and control the
hydrophosphonylation between aldehyde and phosphite.
Recently, Kee et al. reported the hydrophosphonylation of
aromatic aldehydes using Al(salen)3a,c and Al(salan)
complexes3b bearing a cyclohexanediamine unit, which
showed moderate enantioselectivity up to 61% ee.3b The
X-ray analysis of the Al(salan) complex revealed that it
possesses a di-m-hydroxo structure and the salan ligand
adopts a cis-b-conformation.3d These results also suggest-
ed that a chiral complex possessing two coordination sites
in a cis-relation could be a catalyst for asymmetric hydro-
phosphonylation. Moreover, we reported that the chiral
Al(N-methylsalalen) complex that takes a trigonal bipyra-
midal configuration and possesses a chiral nitrogen atom
close to the metal center was an excellent catalyst for
enantioselective hydrophosphonylation of various types
of aldehydes and imines.4 On the other hand, we have
reported that the chiral cobalt binaphthyl–Schiff base
complex 1 that has a cis-b-configuration5 serves as an
O
O
O
1a (5 mol%)
UHP
Ph
Ph
79% ee, 96%
1a: M = Co, R1 = R2 = F, X = I
1b: M = Co, R1 = t-Bu, R2 = NO2, X = I
(R)
2a: M = Al, R1 = R2 = F, X = Cl
2b: M = Al, R1 = t-Bu, R2 = NO2, X = Cl
2c: M = Al, R1 = R2 = t-Bu, X = Cl
2d: M = Al, R1 = t-Bu, R2 = H, X = Cl
2e: M = Al, R1 = t-Bu, R2 = Br, X = Cl
2f: M = Al, R1 = t-Bu, R2 = I, X = Cl
2g: M = Al, R1 = t-Bu, R2 = OMe, X = C
N
O
N
O
M
X
5
R2
5′R2
3
3′
R1
R1
Scheme 1
We first examined hydrophosphonylation of p-chloro-
benzaldehyde with dimethyl phosphite in THF at room
temperature in the presence of 1a or 1b, however, the de-
sired reaction did not occur. Thus, we next examined the
more oxygenophilic Al(chloro) complex 2a, bearing the
same ligand as 1a, as a catalyst7 and 2a was found to
catalyze the desired reaction, albeit slowly and with low
enantioselectivity (Table 1, entry 1). Encouraged by
this result, we examined the catalysis of several other
Al(chloro) complexes and found that the use of complexes
(2b–g)8 that possess a tert-butyl group at C3 and C3¢ re-
markably improved chemical yield and enantioselectivity
(entries 2–7). The substituents at C5 and C5¢ also affected
yield and enantioselectivity to a lesser extent. Complex 2e
bearing a bromo substituent was found to be the catalyst
of choice in terms of enantioselectivity (84% ee, entry 5).9
Complex 2f bearing an electron-donating methoxy group
also showed high enantioselectivity, but the chemical
yield was diminished (entry 7). Other solvents were
examined with complex 2e but did not improve the
SYNLETT 2007, No. 12, pp 1960–1962
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Advanced online publication: 25.06.2007
DOI: 10.1055/s-2007-984528; Art ID: U03907ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Enantioselective Hydrophosphonylation of Aldehydes
1961
Table 1 Enantioselective Hydrophosphonylation of p-Chloro-
benzaldehyde Using Aluminum Binaphthyl Schiff Base Complexes
as Catalystsa
Table 2 Enantioselective Hydrophosphonylation of Various Alde-
hydes Using the Aluminum Binaphthyl Schiff Base Complex 2e as
Catalysta
O
OH
OH
O
P
2a–g
(10 mol%)
O
O
P
2e (10 mol%)
OMe
OMe
OMe
OMe
P
H
R
P
+
+
R
H
*
OMe
H
OMe
H
THF, r.t.
THF, r.t.
OMe
OMe
O
O
Cl
Cl
Entry R in RCHO
Time (d) Yield (%) ee (%) Config.b
Entry
Cat.
2a
2b
2c
Time (d) Yield (%) ee (%)b
Config.c
1
2
Ph
5
5
2
2
2
2
1
1
2
2
62
69
55
82
69
79
71
86
79
82
79c
82c
79c
80c
80c
75c
83c
86d
86d
64c
R
1
2
3
4
5
6
7
4
4
3
5
3
3
3
29
86
11
76
65
68
84
78
83
R
R
R
R
R
R
R
p-FC6H4
p-MeOC6H4
p-MeC6H4
o-FC6H4
o-MeC6H4
PhCH2CH2
c-Hex
3
R
R
100
83
4
2d
2e
5
78
6
2f
69
7
2g
68
a All reactions were carried out at r.t. with a 1:1.1:0.1 molar ratio of p-
chlorobenzaldehyde:dimethyl phosphite:2.
8
9
n-Hex
b Determined by HPLC analysis using chiral stationary phase column
(Daicel Chiralpak AS-H; hexane–i-PrOH = 4:1).
10
(E)-PhCH=CH
R
c Absolute configuration was determined by chiroptical comparison
with the published value (ref. 2d).
a All reactions were carried out at r.t. with a 1:1.1:0.1 molar ratio of
aldehyde:dimethyl phosphite:2e.
b Absolute configuration was determined by chiroptical comparison
with the published value (ref. 2d).
c Determined by HPLC analysis using chiral stationary phase column
(Daicel Chiralpak AS-H; hexane–i-PrOH = 4:1).
d Determined by HPLC analysis using chiral stationary phase column
(Daicel Chiralpak AS-H; hexane–i-PrOH = 9:1) after benzoylation of
the product.
enantioselectivity (toluene, 81% ee; CH2Cl2, 74% ee;
N,N-dimethylformamide, 73% ee). The reaction with 2e
was examined at several reaction temperatures and room
temperature was found to be optimal (0 °C: 44% yield,
83% ee; 30 °C: 79% yield, 80% ee; 40 °C: 90% yield, 78%
ee).
With 2e as a catalyst, we next examined the reactions of
various aromatic aldehydes under the optimized condi-
tions (Table 2, entries 1–6). Irrespective of the electronic
nature of the aryl substituent and their location, the
reactions of aromatic aldehydes showed high enantio-
selectivities (ca. 80% ee), except for the reaction of o-
tolualdehyde that showed a slightly reduced 75% ee
(entries 1–6). To our delight, the reaction of linear and
a-substituted aliphatic aldehydes proceeded smoothly
with good enantioselectivities greater than 80% ee (en-
tries 7–9). The reaction of cinnamaldehyde, however,
showed moderate enantioselectivity (64% ee, entry 10).
In conclusion, we have demonstrated that a suitably sub-
stituted aluminum binaphthyl Schiff base complex is a
promising catalyst for enantioselective hydrophosphonyl-
ation of both aromatic and aliphatic aldehydes, though
these are preliminary results. Further studies on the opti-
mization of the ligand and the reaction mechanism are in
progress in our laboratory.
Acknowledgment
This work was partially supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports, and
Culture of Japan (No. 16750083).
Although the reaction mechanism of this reaction is un-
clear, the electronic nature of the C5(5¢) substituents does
not greatly affect the enantioselectivity and the presence
of a tert-butyl group at C3(3¢) is essential for achieving
good enantioselectivity and acceptable yield. These re-
sults suggest that the tert-butyl group may play an impor-
tant role in the regulation of the transition-state structure
of this reaction. On the other hand, aliphatic aldehydes are
better substrates than aromatic aldehydes in terms of
enantioselectivity, but further study is required to fully
understand the mechanism of asymmetric induction.
References and Notes
(1) Reviews: (a) Wiemer, D. F. Tetrahedron 1997, 53, 16609.
(b) Groger, H.; Hammer, B. Chem. Eur. J. 2000, 6, 943; and
references cited therein.
(2) (a) Yokomatsu, T.; Yamagichi, T.; Shibuya, S. Tetrahedron:
Asymmetry 1993, 4, 1779. (b) Yokomatsu, T.; Yamagishi,
T.; Shibuya, S. Tetrahedron: Asymmetry 1993, 4, 1783.
(c) Rath, N. P.; Spilling, C. D. Tetrahedron Lett. 1994, 35,
227. (d) Arai, T.; Bougauchi, M.; Sasai, H.; Shibasaki, M. J.
Org. Chem. 1996, 61, 2926. (e) Yokomatsu, T.; Yamagichi,
T.; Matsumoto, K.; Shibuya, S. Tetrahedron 1996, 52,
Synlett 2007, No. 12, 1960–1962 © Thieme Stuttgart · New York

1962
K. Ito et al.
LETTER
(6) (a) Uchida, T.; Katsuki, T. Tetrahedron Lett. 2001, 42,
6911. (b) Uchida, T.; Katsuki, T.; Ito, K.; Akashi, S.; Ishii,
A.; Kuroda, T. Helv. Chim. Acta 2002, 85, 3078.
(7) Complexes 2 were prepared according to the reported
procedure (ref. 4).
(8) The cationic complex of 2c has been reported to be an
efficient catalyst for enantioselective aldol reactions of
aldehydes and 5-alkoxyoxazoles. See: Evans, D. A.; Janey,
J. M.; Magomedov, N.; Tedrow, J. S. Angew. Chem. Int. Ed.
2001, 40, 1884.
11725. (f) Sasai, H.; Bougauchi, M.; Arai, T.; Shibasaki, M.
Tetrahedron Lett. 1997, 38, 2717. (g) Yokomatsu, T.;
Yamagishi, T.; Shibuya, S. J. Chem. Soc., Perkin Trans. 1
1997, 1527. (h) Groaning, M. D.; Rowe, B. J.; Spilling, C.
D. Tetrahedron Lett. 1998, 39, 5485. (i) Cermak, D. M.;
Du, Y.; Wiemer, D. F. J. Org. Chem. 1999, 64, 388.
(j) Yamagishi, T.; Yokomatsu, T.; Suemune, K.; Shibuya, S.
Tetrahedron 1999, 52, 12125. (k) Skropeta, D.; Schmidt, R.
R. Tetrahedron: Asymmetry 2003, 14, 265.
(3) (a) Duxbury, J. P.; Cawley, A.; Thornton-Pett, M.; Wantz,
L.; Warne, J. N. D.; Greatrex, R.; Brown, D.; Kee, T. P.
Tetrahedron Lett. 1999, 40, 4403. (b) Ward, C. V.; Jiang,
M.; Kee, T. P. Tetrahedron Lett. 2000, 41, 6181.
(c) Duxbury, J. P.; Warne, J. N. D.; Mushtaq, R.; Ward, C.;
Thornton-Pett, M.; Jiang, M.; Greatrex, R.; Kee, T. P.
Organometallics 2000, 19, 4445. (d) Nixon, T. D.;
Dalgarno, S.; Ward, C. V.; Jiang, M.; Halcrow, M. A.;
Kilner, C.; Thornton-Pett, M.; Kee, T. P. C. R. Chim. 2004,
7, 809.
(4) (a) Saito, B.; Katsuki, T. Angew. Chem. Int. Ed. 2005, 44,
4600. (b) Saito, B.; Egami, H.; Katsuki, T. J. Am. Chem. Soc.
2007, 129, 1978.
(5) Fe and Mn binaphthyl–Schiff base complexes have been
reported to possess cis-b configuration: Cheng, M.-C.;
Chang, M. C.-W.; Peng, S.-M.; Cheung, K.-K.; Che, C.-M.
J. Chem. Soc., Dalton Trans. 1997, 3479.
(9) Typical Procedure: Hydrophosphonylation of p-Chloro-
benzaldehyde
Complex 2e (8.2 mg, 10 mmol) was placed in a flask under
nitrogen and THF (0.5 mL) and dimethyl phosphite (10.1
mL, 0.11 mmol) were added. After stirring for 10 min at r.t.,
p-chlorobenzaldehyde (14.1 mg, 0.1 mmol) was added.
After stirring for another 3 d, the reaction was quenched with
1 N HCl and extracted with EtOAc. The organic extract was
dried over anhyd MgSO4 and concentrated. Silica gel
chromatography of the residue (hexane–EtOAc, 7:3 to 1:1)
gave the desired product (19.5 mg, 78%) as an oil. The ee of
the product was determined to be 84% by HPLC using chiral
stationary phase column as described in the footnote to
Table 1.
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