A new class of C2 chiral photodimer ligands for catalytic enantioselective diethylzinc addition to arylaldehydes

Article abstract of DOI:10.1016/j.tet.2015.06.084

Abstract A new class of C₂ chiral materials was easily prepared by the photodimerization reaction of (R)-N-phenethyl-2-chromonecarboxamide followed by recrystallization. Reduction of the photodimer gave the corresponding alcohol stereoselectively. Both C₂ chiral materials worked effectively as ligands for enantioselective ethylation of arylaldehydes using diethylzinc.

Full text of DOI:10.1016/j.tet.2015.06.084

Tetrahedron 71 (2015) 6254e6258
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Tetrahedron
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A new class of C2 chiral photodimer ligands for catalytic
enantioselective diethylzinc addition to arylaldehydes
Yuki Ueda a, Fumitoshi Yagishita b, Hiroki Ishikawa a, Yuki Kaji a, Nozomi Baba a,
,
Yoshio Kasashima c, Takashi Mino a, Masami Sakamoto a
*
a Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, Yayoi, Inage, Chiba 263-8522, Japan
b Department of Chemical Science and Technology, The University of Tokushima, Minamijyousanjima, Tokushima 770-8506, Japan
c Education Center, Faculty of Engineering, Chiba Institute of Technology, Shibazono, Narashino, Chiba 275-0023, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 May 2015
Received in revised form 18 June 2015
Accepted 19 June 2015
Available online 27 June 2015
A new class of C2 chiral materials was easily prepared by the photodimerization reaction of (R)-N-
phenethyl-2-chromonecarboxamide followed by recrystallization. Reduction of the photodimer gave the
corresponding alcohol stereoselectively. Both C2 chiral materials worked effectively as ligands for
enantioselective ethylation of arylaldehydes using diethylzinc.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
C2 chiral ligand
Photodimer
Asymmetric ethylation
Diethylzinc
Diol ligand
1. Introduction
Many C2esymmetric chiral materials have been utilized for
several kinds of asymmetric syntheses as useful organic catalysts or
ligands of organometallics.1 However, most of these chiral mate-
rials are cumbersome to prepare and are expensive even if com-
mercially available. Considering the challenging new structural
motifs, we conceived of a convenient synthesis of C2 chiral mate-
rials that may spark the creative endeavors of many synthetic or-
ganic chemists.
Recently, we reported a convenient method to construct C2
chiral scaffolds by the diastereoselective photodimerization re-
action of chromone-2-carboxamides possessing a chiral substituent
on the nitrogen atom as shown in Scheme 1.2 The reaction provided
high product selectivity and stereochemistry, and the predominant
dimerization products had easily modifiable carbonyl and amide
groups.3 Here, we utilized the C2 chiral photodimers as ligands for
catalytic asymmetric synthesis of secondary alcohols using dieth-
ylzinc as a typical model reaction.4
Scheme 1. Diastereoselective photodimerization of chromone-2-carboxamides to
construct C2 chiral scaffolds.
2. Results and discussion
Chromone-2-carboxamides possessing chiral substituents were
easily obtained from the commercially available 2-
chromonecarboxylic acids and corresponding chiral amines.
* Corresponding author. E-mail address: sakamotom@faculty.chiba-u.jp (M.
Sakamoto).
http://dx.doi.org/10.1016/j.tet.2015.06.084
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

Y. Ueda et al. / Tetrahedron 71 (2015) 6254e6258
6255
When the amide 1 was irradiated in MeCN at room temperature
(20 ꢀC), the diastereoselective formation of the anti-head-to-head
type dimer occurred in 56% de. Decreasing the temperature of the
reaction to ꢁ40 ꢀC gave a better de value of 66%. Moreover, a higher
diastereoselectivity of 84% de was obtained by irradiation at ꢁ80 ꢀC
using propionitrile as a solvent.2
The absolute configuration of both dimers was established by X-
ray crystallographic analysis. The major isomer exhibited R con-
figuration of all carbon atoms in the cyclobutane ring, and the
minor isomer had an all S configuration in the four-membered ring.
Major isomer 2 was easily optically resolved by crystallization on
the basis of the big difference in crystallinity. When a 56% de
mixture of photodimers was recrystallized from a mixed solvent of
CHCl3 and hexane, a pseudo-racemic crystal composed of a 1:1
mixture of 2 and 3 with higher crystallinity crystallized from the
first crystallization. The minor isomer 3 was nearly removed by this
first crystallization, and enantiopure 2 was easily obtained by
a second crystallization from the mother liquor. In other words,
enantiopure 2 could be isolated by crystallization twice from the
photoreaction mixture. Further purification was not required to use
2 in subsequent asymmetric syntheses. On the other hand, optical
resolution by preparative HPLC was required to obtain optically
pure 3.
For enantioselective alkylations using dialkylzinc, diols have
commonly been used as effective chiral ligands.5 The chromone
dimer has a convertible functional carbonyl group, and reduction
was examined to provide chiral alcohols. When optically pure 2 was
reacted with NaBH4 in THF and the reaction mixture was separated
by column chromatography on silica gel, enantiopure 4 was ob-
tained in 63% yield without any cumbersome experimental pro-
cedures. The stereochemistry of the newly formed chiral center was
determined as (R) by X-ray structural analysis. The hydride attacked
from the vacant side so as to avoid the bulk of the chromone
structure (Scheme 2, Fig. 1).
Fig. 1. Perspective view of reduced chromone dimer 4.
Scheme 3. Asymmetric ethylation of benzaldehyde using chromone ligands.
Table 1
Asymmetric ethylation of benzaldehyde using chromone ligands
Entry
Ligand
Yield (%)a
ee (%)b
Configc
1
2
3
4
1
2
3
4
20
75
81
77
0
70
d
R
S
ꢁ65
71
R
a
Isolated yield.
b
c
Determined by HPLC analysis using a CHIRALCEL OD column.
Determined by comparison with an authentic sample.
It is known that many diols serve as good ligands for catalytic
Scheme 2. Synthesis of diol 4 by stereoselective reduction of 2.
asymmetric alkylation using diethylzinc, and it was expected that
the dimeric diol 4 would be more effective for the asymmetric re-
action.5 The chiral diol 4 was slightly more stereoselective for the
asymmetric reaction (entry 4), and gave the (R)-alcohol in 71% ee.
We then examined the effect of the reaction temperature by
using the dimer ligands, 2 and 4 (Scheme 4). Decreasing the
temperature to 0 ꢀC from 20 ꢀC resulted in decreased reactivity
(Table 2, entries 3 and 4), but the ee value was almost the same
(entries 2 and 4). On the other hand, increasing the temperature
decreased the stereoselectivity, and the ee values were reduced
(entries 5 and 6). These results showed that a temperature of 20 ꢀC
was the best condition for ethylation using photodimer ligands.
Furthermore, there was little difference between the reactions
We herein report the behavior of these chiral chromone de-
rivatives 2e4 as a new class of ligands in the enantioselective al-
kylation of aryl aldehydes as a representative model asymmetric
reaction because many chiral ligands effectively catalyze the
asymmetric alkylation of aldehydes with high ees.6
Before the asymmetric reaction using the chiral photodimers,
we tried the reaction using the monomer 1; however, 1 did not
work as an effective ligand for the asymmetric ethylation (Scheme
3, Table 1, entry 1). Next, when chiral dimer 2 was used, 70% ee of
(R)-1-phenyl-1-propanol was obtained in 75% yield (entry 2). Di-
astereomeric dimer 3, with a different stereochemistry of the
cyclobutane ring, also worked, and 65% ee of enantiomeric (S)-al-
cohol was obtained in a better chemical yield of 81%. The chiral
phenethyl group did not affect the enantioselectivity, suggesting
that the dimer structure controlled the enantioselectivity
considerably.
using
2 and the diol 4 in the chemical yields, ees, and
stereochemistry.
Next, enantioselective addition using a variety of arylaldehydes
was examined with the photodimer ligands at 20 ꢀC (Scheme 5,
Table 3). In the case of the reaction of 2-chlorobenzaldehyde, both
ligands worked and gave moderate chemical yields and ee values

6256
Y. Ueda et al. / Tetrahedron 71 (2015) 6254e6258
We provided a new class of C2 chiral ligands that were easily
prepared by photodimerization, recrystallization, and reduction
without cumbersome procedures. The difference between photo-
dimers 2 and 3 was the stereochemistry of the cyclobutane ring.
Each reaction using 2 or 3 gave opposite selectivity (Table 1, entries
2 and 3), which indicated that the chiral phenethyl group did not
affect the enantioselectivity and the dimer structure controlled the
stereoselectivity. Furthermore, there was no significant difference
in the reactivity and the stereoselectivity between the reactions
using ligands 2 and 4, as the results show in Tables 1e3. In these
reactions, we cannot provide the exact structure of the active in-
termediate complex without precise evidence by X-ray or spectral
techniques.
Scheme 4. Asymmetric ethylation of benzaldehyde using chromone ligands.
Table 2
Asymmetric ethylation of benzaldehyde using chromone ligands
Entry
Ligand
Temp (ꢀC)
Yield (%)a
ee (%)b
Configc
1
2
3
4
5
6
2
4
2
4
2
4
20
20
0
0
40
40
75
77
Trace
38
74
70
71
d
R
R
d
3. Conclusion
71
63
46
R
R
R
77
We provided a convenient method to construct a C2 chiral
scaffold by the diastereoselective photodimerization reaction of
chiral chromone-2-carboxamides followed by recrystallization and
reduction with sodium borohydride. Enantioselective ethylation of
arylaldehydes was examined using the new class of C2 chiral li-
gands as a typical model of an asymmetric reaction. It was con-
firmed that the C2 chiral dimer ligands could be utilized as ligands
for catalytic asymmetric ethylation of arylaldehydes using
diethylzinc.
a
Isolated yield.
b
c
Determined by HPLC analysis using a CHIRALCEL OD column.
Determined by comparison with an authentic sample.
(entries 3 and 4). However, introduction of a substituent at the
ortho-position prevented the reaction and decreased the chemical
yields. In the case of 4-chlorobenzaldehyde, better chemical yields
were obtained with almost the same ee values (entries 5 and 6). An
electron-rich methyl group at the ortho-position decreased the
4. Experimental section
4.1. General
reactivity (entries
7 and 8), whereas the reaction with 4-
methylbenzaldehyde gave moderate chemical yields and ees (en-
tries 9 and 10). In the cases of the reactions with an electron-rich
methoxy group at the 2- or 4-position, lower reactivity and ster-
eoselectivity resulted (entries 11e14). On the other hand, the re-
action with 1-naphthaldehyde gave the best results of 95% ee by the
use of 2 and 4 with good chemical yields (entries 15 and 16). 2-
Naphthaldehyde gave moderate stereoselectivity. In all cases, the
stereochemistry of the resulting alcohol was the R-configuration.
NMR spectra were recorded in CDCl3 solution on a Bruker 300
instrument operating at 300 MHz for 1H and 13C NMR spectroscopy.
Chemical shifts are reported in parts per million (ppm) relative to
TMS as an internal standard. IR spectra were recorded on a JASCO
FT/IR-230 spectrometer. Specific rotation was measured using a DIP
370 polarimeter (JASCO). X-ray single crystallographic analysis was
conducted using a SMART APEX II (Bruker AXS).
4.2. Preparation and photochemical procedure of 2-
chromonecarboxamides (1)2
Scheme 5. Asymmetric alkylation of arylaldehydes using chromone dimer ligands.
Chromone-2-carboxamide 1 was obtained in 85% yield accord-
ing to the reported procedure.2
A 0.02 M solution of 2-
chromonecarboxamide 1 was deoxygenated by bubbling argon
for 20 min and was irradiated for 2 h at 20 ꢀC. After irradiation the
solvent was removed in vacuo and dimeric products were obtained
in 98% yield at 85% conversion. The diastereoselectivity of 56% was
determined on the basis of NMR spectra. Crystallization of a mix-
ture of 56% de of diastereomeric dimers from a mixed solvent of
CHCl3 and hexane gave pseudo-racemic crystals composed of a 1:1
mixture of 2 and 3 in 35% yield. Crystallization from the mother
liquor afforded optically pure 2 in 42% yield.
Table 3
Asymmetric ethylation of arylaldehydes using chromone dimer ligands
Entry
R
Ligand
Yield (%)a
ee (%)b
Configc
1
2
3
4
5
6
7
8
C6H5
C6H5
2-ClC6H4
2-ClC6H4
4-ClC6H4
4-ClC6H4
2-MeC6H4
2-MeC6H4
4-MeC6H4
4-MeC6H4
2-MeOC6H4
2-MeOC6H4
4-MeOC6H4
4-MeOC6H4
1-Naphthyl
1-Naphthyl
2-Naphthyl
2-Naphthyl
2
4
2
4
2
4
2
4
2
4
2
4
2
4
2
4
2
4
75
77
63
60
72
83
36
42
69
76
51
51
41
56
82
88
63
90
70
71
68
67
68
67
72
73
63
70
58
67
30
50
95
95
62
74
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
4.3. Reduction of photodimer 2 by NaBH4 leading to diol (4)
9
10
11
12
13
14
15
16
17
18
After NaBH4 (4.0 equiv) was added to a THF solution of the
photodimer 2 portion by portion, the solution was stirred for 2 h at
room temperature. Aqueous HCl was added and treated with usual
work up process. The diol 4 was separated by column chromatog-
raphy on silica gel with the mixed solvent of hexane and ethyl ac-
etate as an eluent. Solid diol 4 was recrystallized from a mixture of
hexane and diethyl ether. The chemical yield was 63%.
a
Isolated yield.
4.3.1. ((5aR,5bR,11R,11aS,11bS,12R)-11,12-dihydroxy-11,11a,11b,12-
tetrahydrocyclobuta[1,2-b:4,3-b0]dichromene-5a,5b-diyl)bis((((R)-1-
phenylethyl)-l2-azanyl)methanone) (4). Colorless crystal; mp
b
c
Determined by HPLC analysis using a chiral column.
Determined by comparison with an authentic sample. The major enantiomer in
all cases had the (R)-configuration.

Y. Ueda et al. / Tetrahedron 71 (2015) 6254e6258
6257
20
106e107 ꢀC; [
a
]
ꢁ20.23 (c 0.48, CHCl3); IR: (cmꢁ1, KBr) 3420,
propanol¼98:2, 1 ml/min, 254 nm UV detector), tR¼12.94 min for
(R) and tR¼14.18 min for (S).
D
3292, 1671; 1H NMR: (300 MHz, CDCl3)
d
1.62 (d, J¼6.8 Hz, 6H), 3.74
(s, 2H), 4.60 (m, 4H), 5.14 (m, 2H), 6.95 (d, J¼8.1 Hz, 2H), 6.87 (t,
J¼7.9 Hz, 4H), 7.06 (t, J¼7.4 Hz, 2H), 7.19e7.29 (m, 12H), 7.45 (d,
20
4.5.6. 1-(20-Methoxyphenyl)-1-propanol (Table 3, entry 12). [
a]
D
J¼7.5 Hz, 2H); 13C NMR: (75 MHz, CDCl3)
d
21.26, 49.42, 112.24,
þ13.16 (c 0.50, CHCl3), ([
a]
26 þ23.7 (c 1.40, CHCl3) for 95% ee (R));11
D
118.05, 124.26, 125.94, 126.04, 126.34, 127.89, 134.47, 141.85, 154.58,
155.14, 158.34, 178.08; HRMS(ESI-MS) m/z calcd for C36H34N2O6þH
591.2490, found 591.2471.
67% ee by HPLC analysis (CHIRALCEL OD-H column, hexane:2-
propanol¼99:1, 0.5 ml/min, 254 nm UV detector), tR¼66.91 min
for (R) and tR¼59.80 min for (S).
20
4.5.7. 1-(40-Methoxyphenyl)-1-propanol (Table 3, entry 14). [
a]
D
4.4. X-ray diffraction analysis data of diol (4)
þ22.70 (c 0.59, CHCl3), ([
a]
26 þ38.9 (c 1.23, CHCl3) for 96% ee (R));11
D
50% ee by HPLC analysis (CHIRALCEL OD column, hexane:2-
propanol¼97:3, 1 ml/min, 254 nm UV detector), tR¼19.71 min for
(R) and tR¼24.23 min for (S).
Colorless prismatic crystals from etherehexane, including
each equimolar amount of ether and water, monoclinic space
ꢀ
ꢀ
ꢀ
group C2, a¼24.9503(3) A, b¼10.01740(10) A, c¼17.3961(2) A,
3
ꢀ
b
m
¼124.4275(4)ꢀ, V¼3586.38(7) A , Z¼4,
r
¼1.265 g/cm3,
4.5.8. 1-(10-Naphthyl)-1-propanol (Table 3, entry 15). [
a
]
20 þ53.66 (c
D
¼0.714 mm-1, Flack parameter¼0.03(3) The structure was
0.50, CHCl3), ([
a]
25 þ51.10 (c 4.10, CHCl3) for 98% ee (R));11 95% ee by
D
solved by the direct method of full matrix least-squares, where
the final R and wR were 0.0315 and 0.0875 for 5753 reflections.
CCDC1400554.
HPLC analysis (CHIRALCEL OD column, hexane:2-propanol¼90:10,
1 ml/min, 254 nm UV detector), tR¼15.40 min for (R) and
tR¼8.80 min for (S).
4.5. Asymmetric ethylation of arylaldehyde with diethylzinc
4.5.9. 1-(20-Naphthyl)-1-propanol (Table 3, entry 18). [
a]
20 þ30.53 (c
D
0.57, CHCl3), ([
a]
20 þ35.1 (c 2.4, CHCl3) for 92% ee (R));12,13 74% ee by
D
Diethylzinc (1.1 ml, 1 M solution in hexane) was added to
a stirred solution of ligand (0.033 mmol) in toluene (1.0 ml) under
Ar at room temperature. After stirring the resulting reaction mix-
ture at this temperature for 15 min, benzaldehyde (0.05 ml,
0.55 mmol) was added. Stirring was continued for 24 h at room
temperature. At the end of this period, the reaction mixture was
diluted with ethyl acetate (10 ml), quenched with a saturated
aqueous solution of NH4Cl (10 ml), and the two layers were sepa-
rated. The aqueous layer was extracted with ethyl acetate
(2ꢂ10 ml). The combined organic layers were dried over Na2SO4,
concentrated and purified by flash column chromatography on
silica gel using hexaneeethyl acetate. 1-Phenyl-1-propanol was
obtained in 81% yield (61 mg, 0.446 mmol). Chiral HPLC was used to
determine the ee. Other asymmetric reaction using various alde-
hydes and ligands were examined with the same procedure.
HPLC analysis (CHIRALCEL OD column, hexane:2-propanol¼95:5,
1 ml/min, 254 nm UV detector), tR¼18.34 min for (R) and
tR¼16.13 min for (S).
Acknowledgements
This work was supported by Grants-in-Aid for Scientific Re-
search (No. 25288017) from the Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of the Japanese
Government.
Supplementary data
Supplementary data (Crystal structure for compound 4 and
copies of 1H and 13C NMR spectra of new compounds.) related to
this article can be found at http://dx.doi.org/10.1016/
j.tet.2015.06.084.
20
4.5.1. 1-Phenyl-1-propanol (Table 2, entry 2). [
a]
þ33.68 (c 0.47,
D
CHCl3), ([
a]
25 þ42.4 (c 2.50, CHCl3) for 99% ee (R));7 71% ee by HPLC
D
analysis (CHIRALCEL OD column, hexane:2-propanol¼98:2, 1 ml/
min, 254 nm UV detector), tR¼14.72 min for (R) and tR¼20.04 min
for (S).
References and notes
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D
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a
]
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D
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a
]
20 þ40.28 (c 0.51,
CHCl3), ([
a
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D þ60.1 (c 5.0, CHCl3) for 95% ee (R));10D73% ee by HPLC
analysis (CHIRALPAK AD-H column, hexane:2-propanol¼99:1,1 ml/
min, 254 nm UV detector), tR¼15.30 min for (R) and tR¼18.58 min
for (S).
20
4.5.5. 1-(40-Tolyl)-1-propanol (Table 3, entry 10). [
a
]
þ30.25 (c
D
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a
]
25 þ37.2 (c 1.00, CHCl3) for 85% ee (R));11 70% ee by
D
analysis
(CHIRALPAK
AD-H
column,
hexane:2-
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