Salicylhydrazone ligands for enantioselective addition of diethylzinc to aldehydes

Article abstract of DOI:10.1016/j.tetasy.2007.01.022

A family of chiral salicylhydrazones was designed and synthesized for application in asymmetric catalysis. The ability of newly prepared binaphthyl-containing salicylhydrazones as the chiral ligand was examined in the addition of diethylzinc to aldehyde, with the desired product obtained in up to 80% ee.

Full text of DOI:10.1016/j.tetasy.2007.01.022

Tetrahedron: Asymmetry 18 (2007) 165–169
Salicylhydrazone ligands for enantioselective addition
of diethylzinc to aldehydes
Takayoshi Arai,a,* Yoko Endob and Akira Yanagisawaa
aDepartment of Chemistry, Faculty of Science, Chiba University, Inage, Chiba 263-8522, Japan
bGraduate School of Science and Technology, Chiba University, Inage, Chiba 263-8522, Japan
Received 16 September 2006; revised 15 January 2007; accepted 18 January 2007
Abstract—A family of chiral salicylhydrazones was designed and synthesized for application in asymmetric catalysis. The ability of newly
prepared binaphthyl-containing salicylhydrazones as the chiral ligand was examined in the addition of diethylzinc to aldehyde, with the
desired product obtained in up to 80% ee.
ꢀ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
a stereogenic center at either end (nitrogen atom and/or
carbon atom) of the structure, a new family of chiral sali-
cylhydrazones was designed as depicted in Figure 1.
Since efficient catalytic asymmetric synthesis is an ideal
technology in modern chemistry, many chiral ligands have
been introduced aimed at high stereoselectivity.1 Among
the various types of chiral ligands reported, a series of
Schiff base derived from salicylaldehydes has been em-
ployed as the core function to construct the catalytic site
of metal complexes, as symbolized by the famous example
of salen ligands.2 Inspired by the successful use of Schiff
base, we herein report a diversity-oriented design and syn-
thesis of chiral salicylhydrazones, and demonstrate an
application in asymmetric catalysis.
2. Results and discussion
Toward the development of a chiral salicylaldehydes, we
decided to adopt a binaphthyl skeleton as the origin of chi-
rality, which has been well employed to create a promising
chiral reaction sphere.5
The synthesis of chiral binaphthyl-consisting salicyl-
hydrazones L1 was readily achieved by way of reaction
of salicylaldehyde with a chiral binaphthyl moiety in the
hydrazone-forming reaction (Scheme 1). Thus, after the
dehydrating condensation of salicylaldehydes and hydra-
zine, continuous alkylation by dibromomethyl-1,10-
binaphthyl6 provided the desired ligands L1a–L1d.
Hydrazones have been widely utilized for the derivatiza-
tion, characterization, and protection of carbonyl com-
pounds. Enders’ anion chemistry using RAMP and/or
SAMP represents the usefulness of chiral hydrazones in
asymmetric synthesis.3 Recently, Enders and Mino inde-
pendently reported pioneering work on P,N-hydrazone
ligands for the palladium-catalyzed asymmetric allylic
alkylation.4 Since it is possible for the hydrazones to have
Moreover, the idea of employing a binaphthyl moiety as
the origin of chirality led us to the synthesis of a different
type of ligand L2 as shown in Scheme 2. In this classifica-
tion, L1 consists of the binaphthyl skeleton at the end of
the nitrogen atom of hydrazone, while L2 has it on the car-
bon atom.
H
R2
*
N N
R1
*
R3
*
OH
Figure 1. Chiral salicylhydrazones (*: showing the chirality).
The synthesis of L2 was also achieved very readily by the
formation of hydrazone from 3,30-diformyl-2,20-binaph-
thol7 and N,N-dialkyl hydrazine; the target hydrazones
L2a and L2b were obtained in quantitative yields.
*
Corresponding author. Tel./fax: +81 43 290 2889; e-mail: tarai@faculty.
chiba-u.jp
0957-4166/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.01.022

166
T. Arai et al. / Tetrahedron: Asymmetry 18 (2007) 165–169
H
H
MgSO4
EtOH
O
O
R1
+
H2NNH2-H2O
OH
OH
OH
H2NNR2
+
CHCl3, r.t.
O
Br
Br
H
H
R2
N
H
N
R2
Et3N
THF
N N
OH
R1
L2a: R2 = CH3 (y. >99%)
OH
OH
L2b: R2, 2 = -(CH2)4- (y. >99%)
R
N
R2
L1a: R1 = H (y. 72%)
N
R2
L1b: R1 = 2-
L1c: R1 = 4-
t
Bu (y. 67%)
Bu (y. 45%)
H
t
L1d: R1 = 4-Br ( y. 17%)
Scheme 2. Synthesis of chiral hydrazone L2.
Scheme 1. Synthesis of chiral salicylhydrazone L1.
at ꢀ20 ꢁC caused a reduction of the catalyst efficiency
(entry 3), the reaction at 0 ꢁC gave the adduct in moderate
yield with 58% ee (entry 2). With regards to the effects of
the substituents, the bulky t-Bu at the 3-position drastically
diminished the enantioselectivity (entry 4). On the other
hand, the reaction using L2 proceeded smoothly even at
ꢀ20 ꢁC to give the adduct with improved enantioselectiv-
ity. For example, the reaction using L2a provided the prod-
uct in 61% yield with 71% ee under similar conditions with
those of entry 3.11 When the amount of L2a was increased
to 15 mol %, the adduct was obtained in 75% yield with up
to 80% ee (entry 9). The use of the 5-membered ring of pyr-
rolidine L2b showed similar results to those obtained by
L2a. Regarding the relationship of the absolute configura-
tion of the products with the chirality of ligands, an (R)-
enriched product was obtained by (R)-binaphthyl-contain-
ing L1, and an (S)-enriched product was obtained via the
use of (R)-binaphthyl-consisting L2.
With two types of chiral ligands L1 and L2 in hand, exam-
ination of their ability as the chiral ligand was started in the
well-studied nucleophilic addition of diethylzinc to benzal-
dehyde, in which chiral b-amino alcohols have been suc-
cessfully utilized.8,9 Although a number of Schiff base
ligands derived from salicylaldehydes have produced ele-
gant applications in asymmetric catalysis, the typical Schiff
base derived from the aldehyde, reacts with diethylzinc.10
Based on the aforementioned hydrazone-related anion
chemistry, we envisioned that the stability of the hydrazone
would allow the use of the N,O-framework of salicylhyd-
razone in the nucleophilic addition of diethylzinc.
When the newly prepared hydrazone L1a was examined in
the nucleophilic addition of diethylzinc to benzaldehyde at
room temperature, the target product was obtained in 48%
yield with 52% ee (Table 1, entry 1). Although the reaction
Table 1. Enantioselective diethylzinc addition to benzaldehydea
O
OH
(
R
)-L1 or (
R)-L2
H
+
Et2Zn
toluene
Entry
Ligand
Amount of ligand (mol %)
Temperature (ꢁC)
Time (h)
Yieldb (%)
eec (%)
Absolute configurationd
1
2
3
4
5
6
7
8
9
L1a
L1a
L1a
L1b
L1c
L1d
L2a
L2a
L2a
L2b
L2b
5
5
5
5
5
5
5
5
15
5
rt
0
5
22
22
22
22
22
6
6
6
6
6
48
40
16
28
31
36
57
61
75
51
80
52
58
49
14
56
48
45
71
80
72
76
(S)
(S)
(S)
(S)
(S)
(S)
(R)
(R)
(R)
(R)
(R)
ꢀ20
0
0
0
0
ꢀ20
ꢀ20
ꢀ20
ꢀ20
10
11
15
a The reaction was performed using 0.8 mmol of benzaldehyde and 1.6 mmol of diethylzinc in 0.9 mL of toluene.
b Isolated yield.
c ee was determined by HPLC analysis using a chiral stationary phase column (Daicel Chiralcel OD).
10b
d Absolute configuration was determined by specific rotation reported in Ref.
.

T. Arai et al. / Tetrahedron: Asymmetry 18 (2007) 165–169
167
For accessing the catalyst structure and the reaction mech-
anism, the non-linear effects were examined in Figure 2.12
ligand was examined in the nucleophilic addition of dieth-
ylzinc to aldehydes (up to 80% ee). Since various N,N-dial-
kyl hydrazines and substituted salicylaldehydes are readily
accessible, the synthesis of a large number of binaphthyl-
consisting salicylhydrazones is possible. Further applica-
tion of the chiral salicylhydrazones to various asymmetric
catalyses is now in progress.
4. Experimental
4.1. General
Analytical TLC was carried out on pre-coated (0.25 mm)
silica gel plates. Column chromatography was conducted
with 100–210 lm silica gel 60 N (Spherical, neutral). Infra-
red (IR) spectra were recorded on a FTIR spectrometer. 1H
NMR spectra were recorded on 400 MHz spectrometers.
1
Chemical shifts of H NMR spectra were reported relative
to tetramethylsilane (d 0) or chloroform (d 7.26). Splitting
patterns are indicated as s, singlet; d, doublet; t, triplet; q,
quartet; m, multiplet; br, broad. 13C NMR spectra were
recorded on 100 MHz spectrometers. Chemical shifts of
13C NMR spectra were reported relative to CDCl3 (d 77.0).
Figure 2. Non-linear effect in the addition of diethylzinc to benzaldehyde.
The catalysis using L1a showed moderately positive non-
linear effects of asymmetric amplification as normally
observed in the nucleophilic addition of diethylzinc to
aldehyde.13 In contrast, the reaction using L2a showed
negative non-linear effect of asymmetric depletion. This
result would shows us that the catalysis using the relatively
bulky L2a still proceeds with keeping the aggregated
structure.
4.2. 2-((E)-((R)-6,7-Dihydro-5H-dinaphtho[2,1-c;10,20-e]-
azepin-6-ylimino)methyl)phenol L1a
To a mixture of salicylaldehyde (1 mmol, 106 lL) and
hydrazine monohydrate (1.0 equiv, 51 lL) in ethanol
(2 mL) was added MgSO4 (2.0 equiv, 250 mg), and the
resulting suspension stirred at 50 ꢁC for 30 min. Then,
Et3N (2.2 equiv, 330 lL) and 2,20-bromomethyl binaphtha-
lene (1.1 equiv, 484 mg) in THF (2 mL) were added to the
mixture. After being stirred at 50 ꢁC for 19 h, the insoluble
residue was removed by filtration, and concentrated in
vacuo. To the residue was added saturated NaHCO3
(30 mL), and the solution was extracted with CH2Cl2
(15 mL · 4). The combined extract was dried over Na2SO4.
After removal of the solvent, the residue was purified by
column chromatography on neutral silica gel (AcOEt/
hexane, 1:15) to give L1a (29.9 mg, 72%) as a solid. Rf
Other catalytic enantioselective additions of diethylzinc to
aldehydes were also examined in Table 2. The reaction of
aliphatic aldehyde was also catalyzed to provide the adduct
in moderate enantiomeric excess, although the chemical
yield was decreased.
Table 2. (R)-L2a-catalyzed diethyl zinc addition to aldehydesa
(R)-L2a
(15 mol %)
OH
1
+
RCHO
Et2Zn
0.6 (AcOEt/hexane = 1:4); H NMR (CDCl3) d: 11.68 (s,
R
toluene
1H), 7.94–7.91 (m, 4H), 7.58 (d, J = 8.5 Hz, 2H), 7.50–
7.43 (m, 5H), 7.28–7.22 (m, 2H), 7.16–7.11 (m, 1H),
7.00–6.94 (m, 2H), 6.80–6.75 (m, 1H), 4.47 (d,
J = 12.3 Hz, 2H), 3.76 (d, J = 12.3 Hz, 2H); 13C NMR
(CDCl3) d 56.6, 77.2, 116.4, 119.0, 119.5, 125.9, 126.1,
127.2, 127.4, 128.4, 129.0, 129.1, 131.4, 132.5, 133.3,
134.8, 139.9, 157.3; IR (ATR): 3050, 3010, 2927, 2854,
Entry RCHO
Temperature Time Yield
eeb
(%)
(ꢁC)
(h)
(%)
1c
2c
3c
4d
5c
4-MeOC6H4CHO
4-ClC6H4CHO
PhCHCHCHO
2-Naphthaldehyde
PhCH2CH2CHO
0
ꢀ10
ꢀ20
0
96
24
24
96
42
Quant. 31
74
89
81
38
36
42
37
58
ꢀ10
1267, 746 cmꢀ1
; FAB-HRMS calcd for C29H22ON2
414.1732, found 414.1723.
a The reaction was performed using 0.2 mmol of aldehyde and 0.4 mmol of
diethylzinc in 250 lL of toluene.
b ee was determined by HPLC analysis using a chiral stationary phase
column (Daicel Chiralcel OD).
4.3. 2-tert-Butyl-6-((E)-((R)-6,7-dihydro-5H-dinaphtho-
[2,1-c;10,20-e]azepin-6-ylimino)methyl)phenol L1b
c The absolute configuration was based on Ref. 10b.
d The absolute configuration was based on Ref. 14.
To
a mixture of 3-tert-butyl-2-hydroxybenzaldehyde
(0.3 mmol, 51.4 lL) and hydrazine monohydrate
(1.05 equiv, 15.3 lL) in ethanol (0.6 mL) was added
MgSO4 (2.0 equiv, 74 mg), and the resulting suspension
was stirred at 30 ꢁC for 2 h. Then, Et3N (2.0 equiv,
90 lL) and 2,20-bromomethyl binaphthalene (1.0 equiv,
3. Conclusion
In conclusion, a series of binaphthyl-containing salicylhyd-
razones was synthesized, and their potential as chiral

168
T. Arai et al. / Tetrahedron: Asymmetry 18 (2007) 165–169
132 mg) in THF (1.5 mL) was added to the mixture. After
being stirred at 60 ꢁC for 8 h, insoluble residue was
removed by filtration, and concentrated in vacuo. The
residue was purified by column chromatography on neutral
silica gel (AcOEt/hexane, 1:15) to give L1b (94.6 mg,
67%) as a solid. Rf 0.28 (AcOEt/hexane = 1:15); 1H
NMR (CDCl3) d: 12.15 (s, 1H), 7.97–7.93 (m, 4H), 7.62
(d, J = 8.2 Hz, 2H), 7.56 (s, 1H), 7.50–7.45 (m, 4H), 7.3–
7.26 (m, 2H), 7.18 (dd, J = 1.5, 7.8 Hz, 1H), 6.89 (dd,
J = 1.6, 7.7 Hz, 1H), 6.74 (t, J = 15.3 Hz, 1H), 4.52 (d,
J = 12.3 Hz, 2H), 3.78 (d, J = 12.3 Hz, 2H), 1.48 (s, 9H);
13C NMR (CDCl3) d: 29.5, 34.9, 56.4, 118.2, 119.4, 125.8,
126.1, 126.6, 127.3, 127.4, 127.7, 128.4, 129.0, 131.4,
132.6, 133.4, 134.8, 136.7, 141.1, 156.7; IR (ATR): 3052,
3008, 2958, 2864, 1495, 752 cmꢀ1; FAB-HRMS calcd for
C33H30N2O 470.2358, found 470.2338.
7.20–7.10 (m, 3H), 7.00 (d, J = 2.5 Hz, 1H), 6.75 (d,
J = 8.7 Hz, 1H), 4.4 (d, J = 12.4 Hz, 2H), 3.7 (d,
J = 12.3 Hz, 2H); 13C NMR (CDCl3) d: 56.5, 110.7,
118.2, 121.3, 126.0, 126.1, 126.7, 127.1, 127.4, 128.4,
129.1, 131.1, 131.4, 131.4, 132.2, 133.3, 134.7, 156.2; IR
(ATR): 3055, 3010, 2924, 2852, 1477, 1267, 754 cmꢀ1
;
FAB-HRMS calcd for C29H21BrN2O (M+) 492.0837,
found 492.0777.
4.6. (R)-3,30-Diformyl-2,20-binaphthol bis-N-(dimethyl-
amino)imine L2a
To a solution of 3,30-diformyl binaphthol (0.3 mmol,
103 mg) in CHCl3 (6 mL) was added 1,1-dimethyl hydra-
zine (2.2 equiv, 51 lL). After being stirred at rt for 16 h,
the reaction mixture was concentrated in vacuo to give
L2a (133.1 mg, 99%) as a yellow solid. Rf 0.25 (AcOEt/hex-
1
4.4. 4-tert-Butyl-2-((E)-((R)-6,7-dihydro-5H-dinaphtho-
[2,1-c;10,20-e]azepin-6-ylimino)methyl)phenol L1c
ane = 1:2); H NMR (CDCl3) d: 11.75 (s, 2H), 7.82–7.76
(m, 4H), 7.66 (s, 2H), 7.28–7.15 (m, 6H), 2.95 (s, 12H);
13C NMR (CDCl3) d: 42.6, 116.6, 121.9, 123.1, 124.7,
126.7, 128.0, 128.2, 129.0, 133.4, 136.8, 152.3; IR (ATR):
3055, 3001, 2956, 2871, 2792, 1435, 750 cmꢀ1; FAB-HRMS
calcd for C26H26O2N4 426.2056, found 426.2021.
To
a mixture of 5-tert-butyl-2-hydroxybenzaldehyde
(0.2 mmol, 34.3 lL) and hydrazine monohydrate (1.0 equiv,
9.7 lL) in ethanol (0.4 mL) was added MgSO4 (2.0 equiv,
48 mg), and the resulting suspension stirred at rt for 2 h.
Then, Et3N (2.0 equiv, 60.2 lL) and 2,20-bromomethyl
binaphthalene (1.0 equiv, 88 mg) in THF (0.4 mL) was
added to the mixture. After being stirred at 60 ꢁC for
26 h, the insoluble residue was removed by filtration, and
concentrated in vacuo. The residue was purified by column
chromatography on neutral silica gel (AcOEt/hexane, 1:15)
to give L1c (42.3 mg, 45%) as a solid. Rf 0.55 (AcOEt/hex-
4.7. (R)-3,30-Diformyl-2,20-binaphthol bis-N-(pyrrolidinyl)-
imine L2b
To a solution of 3,30-diformyl binaphthol (0.23 mmol,
80 mg) in CHCl3 (4 mL) was added 60% aq N-aminopyroli-
zine (4.0 equiv, 150 lL). After being stirred at rt for 6 h, the
reaction was quenched by adding saturated NaHCO3
(30 mL), and the aqueous layer was extracted with CHCl3
(15 mL · 4). The combined extract was dried over Na2SO4.
After removal of the solvent, the residue was purified by
column chromatography on neutral silica gel (AcOEt/hex-
ane, 1:2) to give L2b (110 mg, 99%) as a white solid in yield.
1
ane = 1:4); H NMR (CDCl3) d: 11.48 (s, 1H), 7.95 (dd,
J = 3.5, 8.3 Hz, 4H), 7.60 (d, J = 8.5 Hz, 2H), 7.56 (s,
1H), 7.49–7.45 (m, 4H), 7.30–7.25 (m, 2H), 7.19 (dd,
J = 2.3, 8.6 Hz, 1H), 7.02 (d, J = 2.2 Hz, 1H), 6.90 (d,
J = 8.7 Hz, 1H), 4.50 (d, J = 12.1 Hz, 2H), 3.78 (d,
J = 12.3 Hz, 2H), 1.25 (s, 9H); 13C NMR (CDCl3) d: 31.4,
33.9, 56.6, 116.0, 118.7, 125.8, 125.9, 126.1, 126.3, 127.2,
127.4, 128.4, 129.0, 131.4, 132.6, 133.4, 134.8, 140.6,
1
Rf 0.4 (AcOEt/hexane = 1:2); H NMR (CDCl3) d: 11.69
(s, 2H), 7.79 (d, J = 8.2 Hz, 2H), 7.73 (s, 2H), 7.55 (s,
2H), 7.26–7.15 (m, 6H), 3.33–3.29 (m, 8H), 1.97–1.90 (m,
8H); 13C NMR (CDCl3) d: 23.4, 50.8, 116.5, 122.4, 123.0,
124.7, 126.4, 127.8, 128.2, 128.2, 133.2, 135.7, 152.1; IR
141.7, 155.1; IR (ATR): 3052, 2954, 2870, 1431, 750 cmꢀ1
;
FAB-HRMS calcd for C33H30N2O 470.2358, found
470.2326.
(ATR): 3055, 2958, 2924, 2854, 1435, 1340, 750 cmꢀ1
;
FAB-HRMS calcd for C30H30O2N4 478.2369, found
478.2366.
4.5. 4-Bromo-2-((E)-((R)-6,7-dihydro-5H-dinaphtho-
[2,1-c;10,20-e]azepin-6-ylimino)methyl)phenol L1d
4.8. General procedure for the asymmetric addition of
diethylzinc to benzaldehyde
To a mixture of 5-bromosalicylaldehyde (2 mmol, 402 mg)
and hydrazine monohydrate (1.0 equiv, 97 lL) in THF
(4 mL) was added MgSO4 (5.0 equiv, 600 mg). After being
stirred at rt for 21 h, the insoluble residue was removed by
filtration, and the residue was purified by column chroma-
tography on neutral silica gel (AcOEt/hexane, 1:4) to give
the corresponding hydrazone (97.4 mg, 22% yield). Then,
the hydrazone (0.45 mmol, 97 mg) and Et3N (2 equiv,
135 lL) in THF (1.8 mL) were added to 2,20-bromomethyl
binaphthalene (1.0 equiv, 198 mg). After being stirred at
40 ꢁC for 24 h, the insoluble residue was removed by filtra-
tion, and the residue was purified by column chromatogra-
phy on neutral silica gel (AcOEt/hexane, 1:15) to give L1d
(44.4 mg, 19% yield). Rf 0.3 (AcOEt/hexane = 1:10); 1H
NMR (CDCl3) d: 11.55 (s, 1H), 7.90–7.80 (m, 4H), 7.48
(d, J = 8.5 Hz, 2H), 7.41–7.36 (m, 4H), 7.28 (s, 1H),
To a solution of L2a (17 mg, 0.04 mmol) in anhydrous tol-
uene was added diethylzinc (1.6 mL 1.0 M solution in hex-
ane) under Ar and stirred for 10 min at room temperature.
The solution was cooled at ꢀ20 ꢁC, and benzaldehyde
(81.3 lL, 0.8 mmol) was added slowly. After being stirred
for 6 h at ꢀ20 ꢁC, the reaction was quenched with 1 M
aqueous HCl (20 mL). The mixture was extracted with
Et2O, and the combined organic layer dried over Na2SO4.
After removal of the solvent under reduced pressure, the
residue was purified by column chromatography on silica
gel to afford 1-phenyl-1-propanol (81.7 mg, 75%) as a col-
orless liquid. The enantiomeric excess was determined by
HPLC analysis using a chiral stationary phase column
(Daicel Chiralcel OD, Hex/i-PrOH = 9:1).

T. Arai et al. / Tetrahedron: Asymmetry 18 (2007) 165–169
169
Int. Ed. 2001, 40, 3284–3308; (d) Pu, L. Tetrahedron 2003, 59,
9873–9886.
Acknowledgments
9. Selected examples of catalytic enantioselective dialkylzinc
addition to aldehydes without use of co-activator (e.g.,
Ti(OiPr)4): (a) Noyori, R.; Suga, S.; Kawai, K.; Okada, S.;
Kitamura, M.; Oguni, N.; Hayashi, M.; Kaneko, T.;
Matsuda, Y. J. Organomet. Chem. 1990, 382, 19–37; (b)
Soai, K.; Yokoyama, S.; Hayasaka, T. J. Org. Chem. 1991,
56, 4264–4268; (c) Kitajima, H.; Ito, K.; Katsuki, T. Chem.
Lett. 1996, 343–344; (d) Huang, W.-S.; Hu, Q.-S.; Pu, L. J.
Org. Chem. 1998, 63, 1364–1365; (e) Ding, K.; Ishii, A.;
Mikami, K. Angew. Chem., Int. Ed. 1999, 38, 497–501; (f)
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Science, Sports,
Culture and Technology of the Japanese Government. We
are highly grateful to Japan Hydrazine Company Inc. for
the generous gift of N-aminopyrrolidine.
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