PFAM catalyzed enantioselective diethylzinc addition to imines

Article abstract of DOI:10.3906/kim-1410-29

Chiral amines are important starting materials for the synthesis of biologically important compounds. Enantioselective addition of dialkylzinc reagents to imines is a reliable method for the synthesis of these compounds. Different chiral catalysts were de

Full text of DOI:10.3906/kim-1410-29

Turk J Chem
Turkish Journal of Chemistry
(2015) 39: 290 – 296
¨
http://journals.tubitak.gov.tr/chem/
˙
c
⃝ TUBITAK
doi:10.3906/kim-1410-29
Research Article
PFAM catalyzed enantioselective diethylzinc addition to imines
∗
¨
˘
˘
Ozdemir DOGAN , Eda C¸ AGLI
Department of Chemistry, Middle East Technical University, Ankara, Turkey
Received: 12.10.2014
•
Accepted/Published Online: 26.11.2014
•
Printed: 30.04.2015
Abstract: Chiral amines are important starting materials for the synthesis of biologically important compounds.
Enantioselective addition of dialkylzinc reagents to imines is a reliable method for the synthesis of these compounds.
Different chiral catalysts were developed and used for this method. Phosphorous based PFAM catalysts were tried
for the first time in the enantioselective synthesis of amines by reacting diethylzinc with N-sulfonyl imines and N-
diphenylphosphinoyl imines. Chiral amines were isolated with moderate to acceptable yields and enantioselectivities.
Key words: Chiral amines, diethylzinc addition, sulfonyl imines, phosphinoyl imines
1. Introduction
Chiral amines have a capability of providing high density structural information and H-bonding. Therefore,
they are important starting materials for the synthesis of biologically active molecules.1,2 The catalytic enan-
tioselective addition of dialkylzinc reagents to the C=N double bond of imines is a reliable and important
process for the synthesis of optically active amines containing a chirality center at the α-position. How-
ever, synthesis of those amines is not easy most of the time.3,4 Due to the lower reactivity of imine carbon
enantioselective addition to C=N double bonds needs to be carried out in the presence of either chiral aux-
iliaries or chiral ligands. Soai reported the first enantioselective addition of dialkylzinc reagent to the C=N
double bond of N-diphenylphosphinoyl imines via the use of a stoichiometric amount of chiral amino alcohol
as a promoter and obtained high enantioselectivity.5 Tomioka was the first to report the same reaction us-
ing N-sulfonyl imines with copper-chiral amidophosphine catalyst.6 Since then different groups have studied
this reaction with different chiral catalysts; for example, Gong et al. used copper-bidentate and tridentate
bisoxazolines,7 Wang et al. used copper-chiral ferrocenyl amidophosphines,8 Shi et al. used copper-chiral
binaphthylthiophosphoramides,9 and Suziki et al. used copper-N -heterocyclic carbenes.10 As examples of
diethylzinc addition to N-diphenylphosphinoyl arylaldimine, Charatte et al. used copper-chiral phosphine
ligands,11 Ha et al. used copper-diphosphine and thiophosphoramide ligands,12 Wang et al. used copper-
ferrocenyl amino ketones,13 Liao et al. used copper-chiral tert-butanesulfinylphosphines,14 and Yus et al. used
polymer-supported L-prolinol catalyst.15 Besides N -sulfonyl and N -phosphinoyl imines, this reaction was also
studied with BOC and formyl protected imines by Aleksakis et al.16 and Feringa et al.17 respectively.
However, due to the unavailability of some chiral ligands, impractical reaction conditions, use of ex-
cess amounts of dialkylzinc (3–5 equivalents), and the catalytic turnover problems, researchers are still trying
to develop a better catalyst system and practical reaction conditions for this reaction. In our previous re-
∗Correspondence: dogano@metu.edu.tr
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ports, highly enantioselective copper-catalyzed conjugate addition of diethylzinc to substituted chalcones18 and
silver-catalyzed enantioselective 1,3-dipolar cycloaddition reactions of azomethine ylides19 were accomplished
using phosphino ferrocenyl aziridinyl methanol (PFAM) and phosphineoxy ferrocenyl aziridinyl methanol
(POFAM) ligands, respectively. Therefore, we were also interested in investigating the performance of these
phosphorous-based ligands for copper-catalyzed enantioselective diethylzinc addition to N -sulfonyl and N -
diphenylphosphinoyl imines.
2. Results and discussion
Various metal salts and chiral ligands were tested for diethylzinc addition to N -sulfonyl and N -phosphinoyl
arylaldimines by different groups. Based on the literature results, copper salts have advantages over the other
transition metal salts. Therefore, we tested the performance of chiral phosphino ferrocenyl aziridinyl ketones
(PFAM1-2) and phosphino ferrocenyl aziridinyl methanols (PFAM3-6) with copper salts for diethylzinc
addition to aldimines (Figure).
Figure. Structures of PFAM1-6 ligands.
In order to screen the ligands we chose N -sulfonyl imine as the model substrate and adopted the literature
procedures7,13 as the starting point. These studies are summarized in Table 1.
Ligand screening studies (Table 1, entries 1–6) showed that the ligand PFAM2 formed the product in
highest ee. Therefore, further optimizations were done by using this ligand. Changing the reaction solvent
from toluene to THF, DCM, or 1,2-dichloroethane (entries 7–9) did not improve enantioselectivity and so we
decided to stay with toluene. After ligand and solvent screening, we also looked at the effect of reaction
concentration. It was seen that at a concentration lower than 0.05 M (entry 10), the product was obtained
in lower yield with no enantioselectivity. At higher concentration (entries 11 and 12), yields were acceptable
but the enantioselectivities were low. From the concentration studies, 0.05 M was found to be optimum for
this catalyst system. Additives are also commonly used for these reactions. Therefore, in order to see the
effect of additives on our catalyst system i Pr2 NEt, Et3 N, HMPA, DABCO, MeOH, and TMEDA were tried
(entries 15–20). After observing that TMEDA increased the enantioselectivity, it was decided to do further
optimization by changing the amount and also the type of copper salt. These studies are summarized in Table
2. Addition of molecular sieves did not change the enantioselectivity but increased the yield more than 10%
(compare Table 1 entry 20 with Table 2 entry 1). By increasing the amount of copper salt from 15 mol % to 30
mol %, yield increased further but enantioselectivity decreased (entry 2). When the amount of copper salt was
decreased both the yield and enantioselectivity were increased (entry 3). After seeing the effect of molecular
sieves, it was decided to dry copper salts by a heat gun under a vacuum line. This resulted in a big jump in
enantioselectivity (entry 4). Under dry conditions and a further decrease in the amount of copper salt to 3.5
mol %, enantioselectivity remained almost the same but the yield was low. Therefore, 7 mol % copper salt was
determined to be the optimum amount for this catalyst system. We also wanted to find out whether the amount
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Table 1. Optimization studies.
Chiral
ligand
Conc.
(M)
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
Temp. Yielda eeb
Entry
Solvent
Additive
(◦C)
–20
–20
–20
–20
–20
–20
–20
–20
–20
–20
–20
–20
0
(%)
60
78
48
< 5
30
28
20
60
99
39
52
66
86
29
31
59
58
74
70
59
(%)
9
17
4
n.d.
6
4
10
11
5
rac
6
1
2
3
4
5
6
7
8
PFAM1 Toluene
PFAM2 Toluene
PFAM3 Toluene
PFAM4 Toluene
PFAM5 Toluene
PFAM6 Toluene
PFAM2 THF
-
-
-
-
-
-
-
-
-
-
-
-
-
-
PFAM2 DCM
PFAM2 1,2-DCE 0.05
9
10
11
12
13
14
15
16
17
18
19
20
PFAM2 Toluene
PFAM2 Toluene
PFAM2 Toluene
PFAM2 Toluene
PFAM2 Toluene
PFAM2 Toluene
PFAM2 Toluene
PFAM2 Toluene
PFAM2 Toluene
PFAM2 Toluene
PFAM2 Toluene
0.025
0.1
0.2
6
0.05
0.05
0.05
0.05
0.05
0.05
0.05
0.05
rac
rac
15
rac
rac
rac
3
–50
iPr2NEt –20
Et3N
HMPA
–20
–20
DABCO –20
MeOH –20
TMEDA –20
20
aIsolated yield.
bDetermined by chiral HPLC.
of diethylzinc could be lowered. Running the experiment with 1.5 equivalents of diethylzinc, yield remained
the same but the ee improved (entry 7). Reducing the diethylzinc to 1.0 equivalent, however, the product was
obtained in lower yield and enantioselectivity (entry 8). We also tried longer and shorter reaction times but the
results did not improve (entries 11 and 12). Finally, changing the diethyzinc addition time was also tried but
with no success.
After deciding on the optimized conditions (toluene as the solvent, 0.5 M reaction concentration, 1.5 equiv-
alents of diethylzinc, –20 ◦ C as the reaction temperature, TMEDA as the additive, and 7 mol % Cu(OTf)2),
substrate screening studies were carried out. The results of these studies are summarized in Table 3. As can be
seen, p-methoxy, or p-bromo phenyl groups on the substrate 1, formed the product in lower yields and enan-
tioselectivities (entries 2 and 3). Changing the aryl group to 1-naphthyl or 2-naphthyl on the substrate resulted
in lower ee (entries 3 and 4). When phosphinoyl imine was used as the substrate, only 2-naphthylphosphinoyl
imine formed the product in acceptable yield and ee. For the others (2g–i) yields were low and so the enantios-
electivities were not determined (entries 7–9).
In conclusion, we tested the performance of a new catalyst system, copper-PFAM, for enantioselective
diethylzinc addition to both sulfonyl and phosphinoyl imines. This catalyst system served better for sulfonyl
imines than phosphinoyl imines. It was found that the reaction had to be carried out under dry conditions.
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Table 2. Additive and copper salt screening.
Ligand
(mol %) (equiv)
Et2Zn
Cu salt
(mol %) (%)
Yielda eeb
Entry
Cu salt
(%)
22
6
1c
10
10
10
10
10
10
10
10
5
20
10
10
10
10
3.0
3.0
3.0
3.0
3.0
3.0
1.5
1.0
1.5
1.5
1.5
1.5
1.5
1.5
Cu(OTf)2 15
Cu(OTf)2 30
73
80
90
56
40
66
55
47
99
50
48
30
67
53
2c
3c
Cu(OTf)2
Cu(OTf)2
7
7
30
73
72
65
77
53
25
53
70
65
56
72
4c,d
5c,d
6c,d
7
Cu(OTf)2 3.5
Cu(OAc)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
Cu(OTf)2
7
7
7
7
7
7
7
7
7
8
9
10
11e
12e
13f
14f
aIsolated yield.
bDetermined by chiral HPLC.
c
˚
Reaction was carried out with 4 A molecular sieves.
dCu(OTf)2 was dried via heat gun.
eReaction time for entries 11 and 12 was 42 h and 15 h, respectively; for the rest it was 21 h.
f Et2Zn addition time for entries 13 and 14 was 1 h and 4.5 h, respectively; for the rest it was 3 h.
Table 3. Arylaldimine screening studies.
Entry
R
Ph
R’
Ts
Product Yielda (%) eeb (%)
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
55
43
50
60
30
50
77
43
56
26
12
52
4-MeOC6H4 Ts
4-BrC6H4
1-Naphtyl
2-Naphtyl
2-Naphtyl
Ph
Ts
Ts
Ts
Ph2PO 2f
Ph2PO 2g
< 5
< 5
< 5
n.d
n.d
n.d
4-MeOC6H4 Ph2PO 2h
4-BrC6H4 Ph2PO 2i
aIsolated yield.
bDetermined by chiral HPLC. Comparing the HPLC retention times with the literature,6 absolute configurations of 2a6
and 2f11 were assigned as S.
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3. Experimental
3.1. General
All reactions were carried out in flame-dried glassware under reduced pressure and charged with argon or
nitrogen unless otherwise stated. Air- and moisture-sensitive imines and chiral phosphorous containing ligands
were stored under inert atmosphere. They were transferred via syringe to the reactor after being dissolved in
reaction solvent under inert medium. During the work-up procedure for the synthesis of derivatives of imines,
TiO2 was filtered through a Celite pad (Merck Celite 545) and washed with dichloromethane. They were
further purified by recrystallization with an appropriate dry mixture of solvents if needed. Commercial copper
salts, Cu(OAc)2 and Cu(OTf)2 , were benzene-azeotroped or dried under vacuum with a heating gun before
use. Toluene was dried with sodium and charged with nitrogen. Tetramethylethylenediamine (TMEDA) was
distilled over sodium under vacuum and stored with potassium hydroxide pellets under inert atmosphere. Just
˚
before use, 4 A molecular sieves were activated at high temperature.
Ethyl addition products were purified via flash column chromatography on Silica Gel 60 (E. Merck,
particle size: 0.040–0.063 mm, 230–400 mesh ASTM). TLC analyses were performed on 250-µm Silica Gel
60 F254 plates. Enantiomeric excess (ee) was determined by chiral HPLC. 1 H and 13 C NMR spectra were
recorded on a Bruker Spectrospin Avance DPX-400 Ultra shield instrument at 400 and 100 MHz, respec-
tively, relative to TMS. Optical rotations were measured by Rudolph Research Analytical Autopol III Po-
larimeter. N -arylmethylsulfonamides and N -aryldiphenylphosphinoylamides were synthesized by the literature
procedures.20,21 Chiral ligands PFAM1-6 were synthesized as in the literature.18,19
3.2. General procedures for asymmetric diethylzinc addition to arylaldimines
˚
In a 10-mL flame-dried Schlenk tube purged with nitrogen, Cu(OTf)2 (7.0 mg, 0.020 mmol) and 4 A molecular
sieves were added and dried under reduced pressure via heating. Toluene (3.5 mL) was added at room
temperature. Then, chiral ligand (13.0 mg, 0.026 mmol) was dissolved in toluene (0.5 mL) and the reaction
was stirred for 1 h at room temperature. After 1 h, imine (68.0 mg, 0.230 mmol) and TMEDA (8.0 µL, 0.04
mmol) were added consecutively. After complete dissolution of imine, the reaction mixture was cooled to –20
◦ C. Et2 Zn in 1.1 M toluene (0.47 mL, 0.34 mmol) was added dropwise at regular time intervals over 3 h and
the reaction mixture was stirred overnight. At this point, before work-up, the reaction mixture was analyzed
by taking the 1 H NMR, which showed no imine proton at 9 ppm corresponding to the starting material. The
reaction was quenched with saturated NH4 Cl aqueous solution (4.0 mL). The organic phase was extracted twice
with DCM (2 × 10.0 mL) and the combined organic phase was dried over Na2 SO4 . The crude product was
purified by flash column chromatography on silica gel (pentane:acetone 10:1). The pure product was obtained
in moderate to good yields. The enantioselectivity was determined by HPLC using a Chiralcel.
3.2.1. N -[1-phenylpropan-1-yl]-4-methylbenzenesulfonamide (2a)
Rf = 0.40 pentane/acetone 5:1; [α]2D1 = –84.2 (c 0.25, CH2 Cl2) for 77% ee and –96.2 (c 0.25, CH2 Cl2) for
81% ee. Lit.9 [α]D25 = –52.2 (c 2.97, CHCl3) for 86% ee. 1 H NMR δ 6.99–7.52 (m, 9H), 4.81 (d, J = 7.5 Hz,
1H), 4.18 (q, J = 7.4 Hz, 1H), 2.31 (s, 3H), 1.75 (dq,J =15.8 Hz, J = 7.9 Hz, 2H), 0.78 (t, J = 7.5 Hz, 3H);
13 C NMR δ 142.9, 141.5, 139.9, 137.6, 129.3, 128.8, 128.7, 128.3, 128.1, 128.0, 127.3, 126.0, 55.0, 26.7, 21.3,
10.4. HPLC: Chiralcel OD column, UV detection at 254 nm, eluent: hexane/iPrOH 10:1, flow 0.7 mL min−1
,
tR = 15.1 min (minor) and 21.3 min (major).
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3.2.2. N -[1-(4-methoxyphenyl)propan-1-yl]-4-methylbenzenesulfonamide (2b)
Rf = 0.43 pentane/acetone 5:1; [α]D21 = –7.6 (c 0.25, CH2 Cl2) for 43% ee. Lit.9 [α]2D5 = – 80.1 (c 1.50,
CHCl3) for 90% ee. 1 H NMR δ 7.54 (d, J = 7.9 Hz, 2H), 7.13 (d, J = 7.4 Hz, 2H), 6.92 (d, J = 7.9 Hz,
2H), 6.68 (d, J = 8.2 Hz, 2H), 4.90 (d, J = 7.0 Hz, 1H), 4.13 (q, J = 7.2 Hz, 1H), 3.74 (s, 3H), 2.37 (s,
3H), 1.74 (dq, J = 45.4 Hz,J =8.3 Hz, 2H), 0.76 (t, J = 7.2 Hz, 3H); 13 C NMR δ 142.9, 132.8, 129.2, 127.7,
127.2, 114.8, 113.7, 59.6, 55.3, 30.5, 21.4, 10.4. HPLC: Chiralcel OD-H column, UV detection at 254 nm, eluent:
hexane/iPrOH 10:1, flow 0.6 mL min−1 , tR = 29.8 min (minor) and 34.4 min (major).
3.2.3. N -[1-(4-bromophenyl)propan-1-yl]-4-methylbenzenesulfonamide (2c)
Rf = 0.41 pentane/acetone 5:1; [α]D21 = –20.9 (c 0.25, CH2 Cl2) for 56% ee. Lit.9 [α]2D5 = –25.5 (c 1.25,
CHCl3) for 92% ee. 1 H NMR δ 7.49 (d, J = 7.9 Hz, 2H), 7.26 (d, J = 7.3 Hz, 2H), 7.13 (d, J = 7.9 Hz,
2H), 6.88 (d, J = 7.3 Hz, 2H), 4.80 (d, J = 7.0 Hz, 1H), 4.17 (q, J = 7.2 Hz, 1H), 2.38 (s, 3H), 1.70 (dq, J =
45.0 Hz, J = 7.6 Hz, 2H), 0.78 (t, J = 7.5 Hz, 3H); 13 C NMR δ 141.2, 131.4, 129.3, 128.4, 126.9, 121.2, 59.2,
30.3, 21.4, 10.4. HPLC: Chiralpak AS-H column, UV detection at 254 nm, eluent: hexane/iPrOH 75:25, flow
0.7 mL min−1 , tR = 30.7 min (minor) and 54.8 min (major).
3.2.4. N -[1-(1-naphthylmethylene)propan-1-yl]-4-methylbenzenesulfonamide (2d)9
Rf = 0.52 pentane/acetone 5:1. 1 H NMR δ 7.82–7.23 (m, 9H), 6.88 (d, J = 8.0 Hz, 2H), 5.23 (d, J = 7.7
Hz, 1H), 5.04 (q, J = 7.2 Hz, 1H), 2.45 (s, 3H), 1.94 (dq, J =14.6 Hz,J =4.3 Hz, 2H), 0.85 (t, J = 7.5 Hz,
3H); 13 C NMR δ 142.8, 142.6, 133.7, 131.3, 131.2, 130.6, 129.8, 129.1, 128.9, 128.7, 127.8, 127.3, 126.8, 126.7,
126.1, 126.0, 125.5, 125.2, 125.1, 123.9, 123.2, 122.6, 55.8, 45.4, 30.4, 21.2, 10.7. HPLC: Chiralpak AD column,
UV detection at 254 nm, eluent: hexane/iPrOH 85:15, flow 0.7 mL min−1 , tR = 19.1 min (minor) and 24.2
min (major).
3.2.5. N -[1-(2-naphthylmethylene)propan-1-yl]-4-methylbenzenesulfonamide (2e)7
Rf = 0.47 pentane/acetone 5:1, [α]2D1 = –8.8 (c 0.25, CH2 Cl2) for 12% ee. 1 H NMR δ 7.69–7.27 (m, 8H),
7.06 (dd, J = 8.6 Hz,J =1.84 Hz, 1H), 6.84 (d, J = 8.1 Hz, 2H), 4.94 (d, J = 7.6 Hz, 1H), 4.28 (q, J = 7.3
Hz, 1H), 2.09 (s, 3H), 1.78 (dq, J = 40.4 Hz, J = 7.53 Hz, 2H), 0.76 (t, J = 7.4 Hz, 3H); 13 C NMR δ 142.8,
142.6, 135.8, 128.5, 128.4, 127.7, 127.5, 127.0, 126.5, 126.4, 126.0, 125.9, 124.9, 124.8, 124.7, 124.0, 122.6, 59.9,
46.4, 30.4, 21.5, 10.8. HPLC: Chiralpak AD column, UV detection at 254 nm, eluent: hexane/iPrOH 85:15,
flow 0.7 mL min−1 , tR = 21.5 min (minor) and 29.0 min (major).
3.2.6. N -[1-(2-naphthyl)propyl]-P,P-diphenylphosphinic amide (2f)11
Rf = 0.30 pentane/acetone 5:1. 1 H NMR δ 7.91–7.75 (m, 3H), 7.65 (d, J = 7.7 Hz, 2H), 7.47–7.22 (m, 10H),
6.91 (d, J = 8.0 Hz, 2H), 5.04 (q, J = 7.0 Hz, 1H), 4.97 (d, J = 7.3 Hz, 1H), 1.96 (dq, J = 14.4 Hz, J =3.5
Hz, 2H), 0.86 (t, J = 7.5 Hz, 3H); 13 C NMR δ 133.2, 133.1, 133.1, 133.0, 131.7, 128.9, 128.8, 128.7, 58.1, 31.4
(JC−P = 4.8 Hz), 18.2 (JC−P = 5.3 Hz), 10.8. HPLC: Chiralpak AD column, UV detection at 254 nm, eluent:
hexane/iPrOH 80:20, flow 1.0 mL min−1 , tR = 10.3 min (minor) and 12.7 min (major).
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Acknowledgments
¨
˙
We thank the Scientific and Technological Research Council of Turkey (TUBITAK, Grant No. TBAG-110T073)
and the Middle East Technical University Research Foundation for the financial support.
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