Limonene oxide derived aziridinyl alcohols as highly efficient catalysts for asymmetric additions of organozinc species to aldehydes

Article abstract of DOI:10.1016/j.tetasy.2013.11.011

The facile synthesis of a series of novel catalysts bearing a tertiary hydroxyl group and an aziridine moiety as chelating centers constructed on the scaffold derived from limonene oxide is described. The newly prepared compounds have been tested for the enantioselective addition of diethylzinc and phenylethynylzinc to aryl and alkyl aldehydes, affording the corresponding chiral alcohols in very high chemical yields (up to 96%) and with excellent ee's of ca. 95%.

Full text of DOI:10.1016/j.tetasy.2013.11.011

Tetrahedron: Asymmetry xxx (2013) xxx–xxx
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Limonene oxide derived aziridinyl alcohols as highly efficient
catalysts for asymmetric additions of organozinc species to
aldehydes
⇑
Michał Rachwalski
´
´
Department of Organic and Applied Chemistry, University of Łódz, Tamka 12, 91-403 Łódz, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The facile synthesis of a series of novel catalysts bearing a tertiary hydroxyl group and an aziridine moi-
ety as chelating centers constructed on the scaffold derived from limonene oxide is described. The newly
prepared compounds have been tested for the enantioselective addition of diethylzinc and phenylethy-
nylzinc to aryl and alkyl aldehydes, affording the corresponding chiral alcohols in very high chemical
yields (up to 96%) and with excellent ee’s of ca. 95%.
Received 21 October 2013
Accepted 19 November 2013
Available online xxxx
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
The enantioselective carbon–carbon bond formation using
organometallic reagents is a commonly used methodology for the
synthesis of nonracemic compounds.1–5 Moreover, the asymmetric
additions of zinc organic species (especially diethylzinc) are recog-
nized as one of the most effective methods for generating enantio-
merically pure secondary alcohols.6–9 In the search for novel
enantiomerically pure ligands, the chiral pool of terpenes is of
enormous synthetic value.10–12 Recently we have described the
preparation of a series of enantiomerically pure aziridine alcohols
derived from salicylic and 2-hydroxymethylbenzoic acid, (S)-man-
delic and (S)-lactic acid, and their excellent catalytic effectiveness
in the asymmetric addition of diethylzinc and phenylethynylzinc
to aliphatic and aromatic aldehydes.5,13,14
According to literature protocols, some b-amino alcohols de-
rived from limonene oxide have been used for the asymmetric
addition of diethylzinc9 and phenylethynylzinc15 to aldehydes,
affording the corresponding chiral alcohols in high chemical yields
and with ee’s ranging from moderate15 (ca. 40%) to high9 (up to
80%).
2.1. Synthesis of the ligands
The straightforward one-step synthesis of our new ligands is
depicted in Scheme 1. As previously described,15,19 the cis-diaste-
reomer of (R)-(+)-limonene oxide can be isolated from the com-
mercially available (1:1) diastereomeric mixture of limonene
oxides. Selective epoxide ring opening with aziridines allows for
the formation of b-aziridine alcohols from the trans-epoxide. The
above 1:1 mixture of diastereomers of (+)-limonene oxide was re-
acted with a series of enantiomerically pure aziridines 3a–d in
refluxing water for 24 h in the presence of 1% triethylamine follow-
ing the protocols described by Steiner et al.9 and Spreitzer et al.20
The chemical yields and specific rotations of the newly synthe-
sized ligands 4a–d are collected in Table 1.
Using this reaction we were unable to synthesize a series of
(1S,2S,4R)-stereoisomers of b-amino alcohols 4a–d derived from
(+)-limonene oxide. This observation was in full agreement with
the literature report.9
With all of the aforementioned results in mind and taking into
account the fact that aziridines exhibit a special affinity to coordi-
nate with organozinc species,16–18 we decided to synthesize a
series of novel compounds using limonene oxide as a key building
block, and to test their catalytic activity toward the asymmetric
addition of diethylzinc and phenylethynylzinc to various aliphatic
and aromatic aldehydes.
2.2. Screening of the ligands
In order to check the catalytic activity of novel ligands 4a–d in
the asymmetric additions of organo zincs to aldehydes, we selected
the reactions of diethylzinc and phenylethynylzinc with benzalde-
hyde as the model transformations (Scheme 2). The reactions were
carried out under standard conditions (in toluene or THF, respec-
tively),21,22 and the results are shown in Table 2.
The results in Table 2 clearly show that all of new aziridines
4a–d derived from (+)-limonene oxide are prone to catalyze the
selected model reactions and give the desired products 5a and 6a
⇑
Tel.: +48 42 6355756.
E-mail address: mrach14@wp.pl
0957-4166/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetasy.2013.11.011
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2
M. Rachwalski / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
R1
R2
R2
R1
, Et3N
Me
Me
Me
O
O
OH
N
N
H
+
2
cis-
+
H2O, reflux
cis-2
trans-1
(1S,2S,4R)-4a-d
aziridines
Pri
Ph
Me
H
Pri
H
H
3a
(-)-(S)-2-phenylaziridine
3b (-)-(S)-2-methylaziridine
3c (-)-(S)-2-isopropylaziridine
3d
H
N
H
N
H
N
H
N
H
(+)-(R)-2-isopropylaziridine
3a
3d
3b
3c
Scheme 1. Synthesis of ligands 4a–d.
Table 1
OH
igand 4c, lEt2Zn
One-step synthesis of ligands 4a–d
RCHO
a
R
Entry
Ligand
Yield (%)
[a]
D
toluene
1
2
3
4
4a
4b
4c
4d
82
85
83
82
À14.8
+9.3
+11.2
À2.0
5a-e
OH
R
ligand 4c, Et2Zn
THF
RCHO
Ph
a
+
Ph
In chloroform at room temperature (c 1).
6a-e
OH
Scheme 3. Additions of diethyl- and phenylethynylzinc to aldehydes in the
presence of ligand 4c.
ligand, Et2Zn
PhCHO
Ph
toluene
5a
configuration of the chiral product5,13,14,21,22). A series of experi-
ments promoted by diastereomeric ligands 4c and 4d gave prod-
ucts that exhibited comparable specific rotation values and the
same absolute configuration (Table 2, entries 3 and 4) and this
supported this assumption. The absolute configurations at the
newly generated stereogenic centers of 5a and 6a were assigned
according to the literature reports.23
OH
Ph
ligand, Et2Zn
THF
Ph
Ph
PhCHO
+
6a
Scheme 2. Asymmetric addition of diethyl- and phenylethynylzinc to
benzaldehyde.
2.3. Asymmetric additions of diethyl- and phenylethynylzinc to
selected aldehydes in the presence of ligand 4c
in high chemical yields (>90%) and with excellent enantiomeric ex-
cesses of >90%. The results also suggest that the presence of a ste-
reogenic center on the aziridine moiety is not of crucial importance
with respect to the stereochemical outcome of the catalyzed
reactions (this is in contrast to our previous findings in which
the aziridine moiety exerts a decisive influence on the absolute
With the novel effective ligands in hand, we next determined
the scope of the catalytic activity of selected ligand 4c. Thus, it
was used for the title transformations performed with a series of
aldehydes as shown in Scheme 3. The results are summarized in
Table 3.
Table 2
Screening of ligands 4a–d
Entry
Ligand
Product 5a
ee (%)b
Product 6a
Abs. config.c
a
a
Yield (%)
[
a]
D
Abs. config.
Yield (%)
[a]
D
ee (%)b
1
2
3
4
4a
4b
4c
4d
96
92
97
91
+42.3
+40.5
+43.1
+40.0
94
90
96
89
(R)
(R)
(R)
(R)
92
89
96
88
À4.8
À4.5
À4.9
À4.4
91
88
95
86
(S)
(S)
(S)
(S)
a
b
c
In chloroform at room temperature (c 1).
Determined using chiral HPLC.
According to the literature data.23
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M. Rachwalski / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
3
Table 3
Additions of diethyl- and phenylethynylzinc to aldehydes in the presence of ligand 4c
Entry
R
Products 5a–e
Products 6a–e
a
a
Yield (%)
[
a]
D
ee (%)b
Abs. config.c
Yield (%)
[
a]
D
ee (%)b
Abs. config.c
1
2
3
4
5
Ph
97
95
92
94
92
+43.1
+48.1
À6.5
+8.3
96
92
93
95
93
(R)
(R)
(R)
(R)
(R)
96
91
92
93
90
À4.9
+7.5
À3.2
À3.9
95
90
92
94
91
(S)
(S)
(S)
(S)
(S)
2-MeOC6H4
n-Pr
4-BrC6H4
2-MeC6H4
+41.5
+11.3
a
b
c
In chloroform at room temperature (c 1).
Determined using chiral HPLC.
According to the literature data.4,21,22,24
The results shown in Tables 2 and 3 clearly indicate that the se-
(9.5 mmol) and triethylamine (0.15 mL) in deionized water (5 mL)
and the reaction mixture was refluxed for 24 h. Next, water was
evaporated in vacuo and the residue was subjected to column
chromatography (silica gel, hexane with ethyl acetate in gradient)
to afford the corresponding products 4a–d. Chemical yields and
optical rotation values are shown in Table 1.
lected ligand 4c should be considered as a highly effective catalyst
for the title reactions to give the desired chiral alcohols in excellent
yields and ee’s. Moreover, careful analysis of the absolute configu-
rations of the products shown in Table 3 suggests that the attack of
organozinc species promoted by 4c always takes place from the
same side, which is in full agreement with the literature transition
state models proposed for diethylzinc9 and alkynylzinc15 additions
catalyzed by alcohols derived from (+)-limonene oxide. The use of
limonene amino alcohols with a (1S,2S,4R)-absolute configuration
[from (+)-limonene oxide] led to the formation of the (R)-config-
ured adducts of diethylzinc and the (S)-configured adducts of
phenylethynylzinc, with the aldehydes being compatible with the
facial selectivity in both cases.9,15
Ligand 4a (colorless oil): 1H NMR (CDCl3): d = 1.40 (s, 3H), 1.55–
1.59 (m, 1H), 1.60 (s, 3H), 1.60–1.66 (m, 4H), 1.73–1.77 (m, 1H),
1.80–1.89 (m, 2H), 2.03 (d, J = 3.6 Hz, 1H), 2.18–2.25 (m, 1H),
2.43 (dd, J = 3.6, 5.5 Hz, 1H), 4.59–4.61 (m, 1H), 4.63–4.65 (m,
1H), 7.28–7.38 (m, 5H); 13C NMR (CDCl3): d = 20.9 (CH3), 26.7
(CH2az), 28.3 (CH2), 32.9 (CH3), 35.0 (CH2), 37.8 (CH), 38.1 (CH2),
41.1 (CHaz), 72.3 (CH), 73.6 (Cq), 108.3 (CH2), 126.3 (2Car), 126.7
(Car), 128.2 (2Car), 140.5 (Cq ar), 150.3 (Cq); MS (CI): m/z 272
(M+H); HRMS (CI): calcd for C18H25NO: 272.1246; found 272.1250.
Ligand 4b (colorless oil): 1H NMR (CDCl3): d = 1.21 (d, J = 6.0 Hz,
3H), 1.23 (d, J = 6.0 Hz, 1H), 1.33 (s, 3H), 1.37–1.39 (m, 1H), 1.44
(d, J = 3.6 Hz, 1H), 1.48–1.59 (m, 3H), 1.62–1.67 (m, 2H), 1.76 (s,
3H), 1.74–1.80 (m, 2H), 2.01–2.08 (m, 1H), 2.43–2.50 (m, 1H),
4.72–4.76 (m, 2H); 13C NMR (CDCl3): d = 18.2 (CH3az), 21.0 (CH3),
26.9 (CH2az), 28.2 (CH2), 30.8 (CH2), 32.1 (CH3), 35.1 (CH2), 37.9
(CH), 38.1 (CHaz), 72.0 (CH), 73.7 (Cq), 108.3 (CH2), 150.4 (Cq); MS
(CI): m/z 209 (M+H); HRMS (CI): calcd for C13H22NO: 209.1276;
found 209.1273.
3. Conclusion
Chiral ligands of type 4 derived from (+)-limonene oxide con-
taining five stereogenic centers were found to be highly efficient
catalysts for the enantioselective addition of diethyl- and phenyle-
thynylzinc to various aldehydes. The stereogenic centers located
on the terpene moiety exerted a decisive influence on the stereo-
chemistry of the reactions leading to the desired alcohols with
the same absolute configuration [(R) in the case of diethylzinc
and (S) in the case of phenylethynylzinc].
Ligand 4c (colorless oil): 1H NMR (CDCl3): d = 0.89 (d, J = 6.9 Hz,
3H), 1.02 (d, J = 6.7 Hz, 3H), 1.06 (d, J = 6.7 Hz, 1H), 1.33 (s, 3H),
1.38–1.47 (m, 4H), 1.49–1.53 (m, 1H), 1.59–1.64 (m, 3H),
1.65–1.72 (m, 1H), 1.75 (s, 3H), 1.78–1.83 (m, 1H), 1.93–2.00 (m,
1H), 2.44–2.50 (m, 1H), 4.73–4.74 (m, 2H); 13C NMR (CDCl3):
d = 18.1 (CH3az), 20.9 (CH3az), 21.3 (CH3), 26.6 (CH2az), 26.8 (CH2),
27.1 (CHaz), 29.8 (CH2), 32.1 (CH3), 35.1 (CH2), 38.1 (CHaz), 47.9
(CH), 72.6 (CH), 73.0 (Cq), 108.7 (CH2), 149.7 (Cq); MS (CI): m/z
238 (M+H); HRMS (CI): calcd for C15H27NO: 238.1650; found
238.1660.
4. Experimental
4.1. General
Unless otherwise specified, all reagents were purchased from
commercial suppliers and used without further purification. Tetra-
hydrofuran and toluene were distilled from sodium benzophenone
ketyl radical. 1H and 13C NMR spectra were recorded on a Bruker
instrument at 600 and 151 MHz, respectively, with CDCl3 as the sol-
vent and relative to TMS as the internal standard. Data are reported
as s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet,
b = broad. Optical rotations were measured on a Perkin–Elmer
241 MC polarimeter with a sodium lamp at room temperature
(c 1). Melting points were determined on a MELTEMP apparatus
and are uncorrected. Column chromatography was carried out
using Merck 60 silica gel. TLC was performed on Merck 60 F254 silica
gel plates. Visualization was accomplished with UV light (254 nm)
or using iodine vapor. The enantiomeric excess (ee) values were
determined by chiral HPLC (Knauer, Chiralcel OD). Aziridines
3a–d were prepared according to the literature procedures.25,26
Ligand 4d (colorless oil): 1H NMR (CDCl3): d = 0.83 (d, J = 6.8 Hz,
3H), 1.04 (d, J = 6.7 Hz, 3H), 1.08 (d, J = 6.7 Hz, 1H), 1.35 (s, 3H),
1.41–1.48 (m, 4H), 1.49–1.52 (m, 1H), 1.61–1.66 (m, 3H), 1.67–
1.73 (m, 1H), 1.76 (s, 3H), 1.80–1.85 (m, 1H), 1.95–2.01 (m, 1H),
2.46–2.51 (m, 1H), 4.75–4.76 (m, 2H); 13C NMR (CDCl3): d = 18.1
(CH3az), 20.9 (CH3az), 21.3 (CH3), 26.6 (CH2az), 26.9 (CH2), 27.1
(CHaz), 29.8 (CH2), 32.1 (CH3), 35.1 (CH2), 38.1 (CHaz), 47.9 (CH),
72.6 (CH), 73.0 (Cq), 108.7 (CH2), 149.7 (Cq); MS (CI): m/z 238
(M+H); HRMS (CI): calcd for C15H27NO: 238.1650; found 238.1653.
4.3. Asymmetric addition of diethylzinc to aldehydes; general
procedure21
Chiral catalysts of type 4 (0.1 mmol) in dry toluene (5 mL) were
placed in a round-bottomed flask. The mixture was cooled to 0 °C
and a solution of diethylzinc (1.0 M sln. in hexane, 3.0 mmol)
was added under argon. After stirring for 30 min, an aldehyde
(1.0 mmol) was added at 0 °C, and the mixture was stirred at room
4.2. Synthesis of the ligands 4a–d—general procedure9,20
The cis/trans-(+)-limonene oxide mixture (0.23 g, 1.5 mmol)
was mixed with the corresponding enantiomerically pure aziridine
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M. Rachwalski / Tetrahedron: Asymmetry xxx (2013) xxx–xxx
temperature overnight. Next, a 5% HCl aqueous solution was
added, the layers were separated, and the aqueous phase was ex-
tracted with diethyl ether (4 Â 10 mL). The combined organic lay-
ers were washed with brine (10 mL) and dried over anhydrous
MgSO4. The solvents were evaporated off to afford the crude alco-
hols 5a–e, which were purified via column chromatography on sil-
ica gel (hexane with ethyl acetate in gradient). The yields, specific
rotations, enantiomeric excess values, and the absolute configura-
tions of products 5a–e are collected in Table 3. The spectroscopic
data were in full agreement with those reported in the litera-
ture.4,21–23
and the absolute configurations of the products 6a–e are collected
in Table 3. The spectroscopic data were in full agreement with
those reported in the literature.4,21–24
4.4.1. (S)-(À)-1,3-Diphenyl-prop-2-yn-1-ol 6a
Enantiomeric excess was determined by HPLC with a Chiralcel
OD–H column (98:2 hexane/isopropanol, 1 mL/min); major enan-
tiomer tR = 11.8 min, minor enantiomer tR = 21.2 min; ee = 95%.
The absolute configuration was ascribed by comparison of the
specific rotation value with the reported data.29
4.4.2. (S)-(+)-1-(2-Methoxyphenyl)-3-phenyl-prop-2-yn-1-ol 6b
Enantiomeric excess was determined by HPLC with a Chiralcel
OD column (98:2 hexane/isopropanol, 1 mL/min); major enantio-
mer tR = 22.6 min, minor enantiomer tR = 16.8 min; ee = 90%. The
absolute configuration was ascribed by comparison of the specific
rotation value with the reported data.29,30
4.3.1. (R)-(+)-1-Phenylpropanol 5a
Enantiomeric excess was determined by HPLC with a Chiralcel
OD–H column (99:1 hexane/isopropanol, 1 mL/min); major enan-
tiomer tR = 15.5 min, minor enantiomer tR = 18.1 min; ee = 96%.
The absolute configuration was ascribed by comparison of the spe-
cific rotation value with the reported data.27
4.4.3. (S)-(À)-1-Phenyl-hex-1-yn-3-ol 6c
4.3.2. (R)-(+)-1-(2-Methoxyphenyl)-1-propanol 5b
Enantiomeric excess was determined by HPLC with a Chiralcel
OD column (98:2 hexane/isopropanol, 1 mL/min); major enantio-
mer tR = 16.2 min, minor enantiomer tR = 6.5 min; ee = 92%. The
absolute configuration was ascribed by comparison of the specific
rotation value with the reported data.30
Enantiomeric excess was determined by HPLC with a Chiralcel
OD–H column (98:2 hexane/isopropanol, 1 mL/min); major enan-
tiomer tR = 30.3 min, minor enantiomer tR = 27.4 min; ee = 92%.
The absolute configuration was ascribed by comparison of the spe-
cific rotation value with the reported data.28
4.4.4. (S)-(À)-1-(4-Bromophenyl)-3-phenyl-prop-2-yn-1-ol 6d
Enantiomeric excess was determined by HPLC with a Chiralcel
OD column (98:2 hexane/isopropanol, 1 mL/min); major enantio-
mer tR = 40.1 min, minor enantiomer tR = 10.8 min; ee = 94%. The
absolute configuration was ascribed by comparison of the specific
rotation value with the reported data.30
4.3.3. (R)-(-)-3-Hexanol 5c
Enantiomeric excess was determined by HPLC with a Chiralcel
OD–H column (99:1 hexane/isopropanol, 1 mL/min); major enan-
tiomer tR = 24.7 min, minor enantiomer tR = 30.6 min; ee = 93%.
The absolute configuration was ascribed by comparison of the spe-
cific rotation value with the reported data.21
4.4.5. (S)-(+)-1-(2-Methylphenyl)-3-phenyl-prop-2-yn-1-ol 6e
Enantiomeric excess was determined by HPLC with a Chiralcel
OD column (98:2 hexane/isopropanol, 1 mL/min); major enantio-
mer tR = 22.1 min, minor enantiomer tR = 9.6 min; ee = 91%. The
absolute configuration was ascribed by comparison of the specific
rotation value with the reported data.29
4.3.4. (R)-(+)-1-(4-Bromophenyl)-1-propanol 5d
Enantiomeric excess was determined by HPLC with a Chiralcel
OD–H column (98:2 hexane/isopropanol, 1 mL/min); major enan-
tiomer tR = 25.5 min, minor enantiomer tR = 22.1 min; ee = 95%.
The absolute configuration was ascribed by comparison of the spe-
cific rotation value with the reported data.28
Acknowledgements
4.3.5. (R)-(+)-1-(2-Methylphenyl)-1-propanol 5e
Enantiomeric excess was determined by HPLC with a Chiralcel
OD–H column (98:2 hexane/isopropanol, 1 mL/min); major enan-
tiomer tR = 20.2 min, minor enantiomer tR = 23.1 min; ee = 93%.
The absolute configuration was ascribed by comparison of the spe-
cific rotation value with the reported data.21
Financial support by the National Science Centre (NCN), Grant
No. 2012/05/D/ST5/00505 for M.R. is gratefully acknowledged.
´
The scientific award of the Foundation of University of Łódz for
M.R. is also acknowledged.
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Please cite this article in press as: Rachwalski, M. Tetrahedron: Asymmetry (2013), http://dx.doi.org/10.1016/j.tetasy.2013.11.011