Effects of magnesium salts and amines on the stereoselectivity in the imine aldol reaction

Article abstract of DOI:10.1248/cpb.55.1773

In the imine aldol reactions of 1 with aromatic aldehydes using magnesium salts in the presence of amines, the threo/erythro ratios of products increased in the order Mg(ClO₄)₂>MgI₂>MgBr ₂>MgCl₂>Mg(OTf)₂ and N,N,N′, N′-tetramethylethylenediamine (TMEDA)>Et₃N. This increase in the threo/erythro ratios of products was estimated to be caused by a retro-imine aldol reaction under thermodynamic control.

Full text of DOI:10.1248/cpb.55.1773

December 2007
Notes
Chem. Pharm. Bull. 55(12) 1773—1775 (2007)
1773
Effects of Magnesium Salts and Amines on the Stereoselectivity in the
Imine Aldol Reaction
b
Kazuhiko HAYASHI, ,a Eiko KUJIME,a Hajime KATAYAMA,a Shigeki SANO,b and Yoshimitsu NAGAO
*
a College of Pharmacy, Kinjo Gakuin University; 2–1723 Omori, Moriyama-ku, Nagoya 463–8521, Japan: and b Graduate
School of Pharmaceutical Sciences, The University of Tokushima; Sho-machi, Tokushima 770–8505, Japan.
Received September 1, 2007; accepted October 1, 2007; published online October 2, 2007
In the imine aldol reactions of 1 with aromatic aldehydes using magnesium salts in the presence of amines,
the threo/erythro ratios of products increased in the order Mg(ClO4)2ꢂMgI2ꢂMgBr2ꢂMgCl2ꢂMg(OTf)2 and
N,N,Nꢀ,Nꢀ-tetramethylethylenediamine (TMEDA)ꢂEt3N. This increase in the threo/erythro ratios of products
was estimated to be caused by a retro-imine aldol reaction under thermodynamic control.
Key words magnesium; reverse stereoselectivity; imine aldol reaction; thermodynamic control; retro-imine aldol reaction
In previous studies, we have reported aldol-type reactions in yields (Table 2, threo/erythroꢀ43/57→75/25). The change
with magnesium salts in the presence of amines and shown in the threo/erythro ratios of products with time suggested
the unique stereo- and chemoselectivity.1—6) In the course of that Mg(ClO4)2 gave the threo product 3a under thermody-
study, we were interested in the effects of magnesium salts by namic control, not via the ‘boat-metal transition state’. Be-
differences in the counter anions and the effects of the cause, if the reaction using Mg(ClO4)2 proceeded via the
amines. It is known that the counter anions of metal salts and ‘boat-metal transition state’, the threo/erythro ratios of prod-
amines affect the reactivity and/or selectivity in some aldol- ucts wouldn’t change from the beginning of the reaction. In
type reactions.7—11) This paper describes the effects of mag- the case of Mg(OTf)2, the lack of change in the threo/erythro
nesium salts and amines on the stereoselectivity of the imine ratios of products with time suggested that the reaction pro-
aldol reaction3) of N-cyclohexylimine 1 with aromatic alde- ceeded under kinetic control and afforded the erythro-excess
hydes.
Table 1. Stereoselectivity in the Imine Aldol Reaction of 1 with 2a
Imine aldol reactions with PhCHO (2a) using various
magnesium salts in the presence of Et3N were examined, and
the results are summarized in Table 1.
As shown in Table 1, variable stereoselectivity was ob-
served. In the presence of Et3N, the use of MgCl2, MgBr2,
and Mg(OTf)2 gave erythro-excess products (threo/erythroꢀ
7/93, 10/90 and 5/95), while the employment of MgI2 and
Mg(ClO4)2 resulted in the formation of threo-excess prod-
ucts (threo/erythroꢀ57/43 and 75/25). Interestingly, the
threo/erythro ratios of products increased in the reaction with
N,N,Nꢁ,Nꢁ-tetramethylethylenediamine (TMEDA) instead of
Et3N, and the reversal of stereoselectivity was observed in
the cases of MgCl2 and MgBr2 (threo/erythroꢀ7/93→91/9
and 10/90→81/19). Previously, we reported that the threo
product was obtained under thermodynamic control, and the
erythro product was formed under kinetic control in the
imine aldol reaction of 1 with aromatic aldehydes using
MgBr2 in the presence of Et3N.3) In contrast, H. X. Wei and
co-workers have proposed the ‘boat-metal transition state’12)
as the intermediate in the aldol reaction of ketones with alde-
hydes using MgI2 in the presence of amine to explain the
generation of threo products.11) Thus, in order to identify the
cause of the variable stereoselectivity depending on the kinds
of magnesium salts and amines, the reactions employing
Mg(OTf)2 and Mg(ClO4)2 in the presence of Et3N, in which
the reversal stereoselectivity was observed (Table 1,
threo/erythroꢀ5/95 and 75/25), were carried out and the
change in the threo/erythro ratios of products with time was
confirmed. As shown in Table 2, the use of Mg(OTf)2 af-
forded the erythro-excess products regardless of the reac-
tion time (threo/erythroꢀ5/95). However, in the case of
Mg(ClO4)2, the erythro-excess products were converted to
the threo-excess products as time advanced without a change
Et3Nb)
TMEDAb)
threo/erythroc)
(yield %)d)
MgXna)
threo/erythroc)
(yield %)d)
MgF2
MgCl2
MgBr2
MgI2
Mg(OTf)2
Mg(ClO4)2
MgSO4
— ( 0)
7/93 (90)
10/90 (92)
57/43 (91)
5/95 (72)
75/25 (84)
— ( 0)
—
91/ 9 (93)
81/19 (82)
78/22 (84)
16/84 (22)
90/10 (90)
—
a) 1.2 mol eq of the magnesium salt was employed. b) 2.4 mol eq of the amine was
employed. The “mol eq” was based on the nitrogen atom of amine. c) Determined by
200 MHz 1H-NMR analysis. d) Isolation yield of a mixture of 3a and 4a.
Table 2. Change in the Stereoselectivity with Time in the Reaction of 1
with 2a Employing Mg(OTf)2 and Mg(ClO4)2
a)
a)
Mg(OTf)2
Mg(ClO4)2
Reaction time
threo/erythrob)
threo/erythrob)
(yield %)c)
(yield %)c)
10 min
30 min
16 h
5/95 (9)
—
5/95 (72)
43/57 (83)
58/42 (89)
75/25 (84)
a) Reaction condition: magnesium salts (1.2 mol eq)–Et3N (2.4 mol eq) at ꢃ45 °C.
b) Determined by 200 MHz 1H-NMR analysis. c) Isolation yield of a mixture of 3a
and 4a.
∗ To whom correspondence should be addressed. e-mail: hayashi@kinjo-u.ac.jp
© 2007 Pharmaceutical Society of Japan

1774
Vol. 55, No. 12
products. Further, the reactions using Et3N and TMEDA in of 1 with 2a—f, though TMEDA had only a limited effect on
the presence of MgCl2 were also examined, as shown in the imine aldol reaction.
Table 3. The use of Et3N gave rise to the erythro-excess prod-
Experimental
ucts, regardless of the reaction time, and TMEDA changed
the threo/erythro ratios of products with time like Mg(ClO4)2
All melting points were determined on a Yanaco micro melting point ap-
paratus and are uncorrected. IR spectra were obtained on a JASCO FT/IR-
420 IR Fourier transform spectrometer. 1H-NMR (200, 400 MHz) spectra
were recorded on a JEOL JNM-FX200 or JEOL JNM-GSX400 spectrome-
ter. Chemical shifts are given in d values (ppm) using tetramethylsilane
(TMS) as an internal standard. Electron impact (EI)-MS were recorded on a
JEOL JMS SX-102A spectrometer. Elementary combustion analyses were
performed on a Yanaco CHN CORDER MT-5. All reactions were monitored
by TLC employing 0.25-mm silica gel plates (Merck 5715; 60 F254). Column
chromatography was carried out on silica gel [Kanto Chemical 60N; 63—
210 mm]. All reagents were used as purchased.
Typical Procedure for the Reaction of 1 with 2a Using Mg(OTf)2 in
the Presence of Et3N (Table 1) To a suspension of 1.2 mol eq of
Mg(OTf)2 (726 mg, 2.3 mmol) in MeCN (9 ml) was added a solution of 1
(346 mg, 1.9 mmol) and 2.4 mol eq of Et3N (0.645 ml, 4.6 mmol) in MeCN
(4 ml) at ꢃ45 °C under argon. After being stirred at ꢃ45 °C for 1 h, a solu-
tion of 1 mol eq of 2a (205 mg, 1.9 mmol) in MeCN (3 ml) was added and
stirred at ꢃ45 °C for 16 h. The reaction mixture was quenched with 1 mol/l
AcOH at room temperature for 1.5 h and extracted with CH2Cl2. The CH2Cl2
extract was washed with saturated NaHCO3 and brine, dried over MgSO4,
did. These results were rationalized by the same reasons de-
scribed above. Thus, the magnesium salts and amines were
speculated to cause a retro-imine aldol reaction under ther-
modynamic control and to increase the threo/erythro ratios of
products in the order Mg(ClO4)2ꢂMgI2ꢂMgBr2ꢂMgCl2ꢂ
Mg(OTf)2 and TMEDAꢂEt3N, as summarized in Fig. 1.
Table 4 shows representative results of the reaction of 1
with aromatic aldehydes 2a—f under four kinds of condi-
tions (A—D). The reactions under condition A using
Mg(OTf)2 produced lower threo/erythro ratios of products
than those under condition B employing Mg(ClO4)2, and the
reversal of stereoselectivity was observed except in the reac-
tion with 2d (entry 4). The only reaction with 2a and 2b
under condition C showed lower threo/erythro ratios of prod-
ucts than those under condition D using TMEDA (entries 1,
2). The effect of Mg(ClO4)2 was observed in the all reactions and evaporated in vacuo. The residue of threo 3a and erythro 4a in the ratio
of 5 : 95 was purified by column chromatography (eluent AcOEt–n-hexane,
Table 3. Change in the Stereoselectivity with Time in the Reaction of 1
with 2a Employing Et3N and TMEDA
3/7) to give the mixture of threo 3a and erythro 4a (282 mg, 72%). Ratio de-
termination was carried out by 1H-NMR analysis (200 MHz, CDCl3).
Compound 3a13): Pale yellow needles (CH2Cl2–n-hexane), mp 73—74 °C.
1H-NMR (400 MHz, CDCl3) d: 1.2—1.3 (1H, m), 1.5—1.7 (3H, m), 1.7—
1.8 (1H, m), 2.0—2.1 (1H, m), 2.3—2.4 (1H, m), 2.4—2.5 (1H, m), 2.6—
2.7 (1H, m), 3.98 (1H, d, Jꢀ2.9 Hz), 4.78 (1H, dd, Jꢀ2.9, 8.8 Hz), 7.3—7.4
(5H, m). IR (KBr) cmꢃ1: 3555, 1703, 1448, 1133, 1061, 986, 702, 677. EI-
MS m/z: 204.1142 (Calcd for C13H16O2: 204.1150).
Et3Na)
TMEDAa)
threo/erythrob)
(yield %)c)
Reaction time
threo/erythrob)
(yield %)c)
10 min
2 h
16 h
7/93 (50)
—
7/93 (90)
7/93 (60)
32/68 (79)
91/ 9 (93)
Compound 4a13): Colorless needles (CH2Cl2–n-hexane), mp 105—107 °C.
1H-NMR (400 MHz, CDCl3) d: 1.5—1.6 (1H, m), 1.6—1.8 (3H, m), 1.8—
1.9 (1H, m), 2.0—2.1 (1H, m), 2.3—2.5 (2H, m), 2.5—2.6 (1H, m), 3.08
(1H, d, Jꢀ2.4 Hz), 5.39 (1H, br s), 7.2—7.4 (5H, m). IR (KBr) cmꢃ1: 3503,
1687, 1131, 777, 746, 704, 553. EI-MS m/z: 204.1131 (Calcd for C13H16O2:
204.1150).
a) Reaction condition: MgCl2 (1.2 mol eq)–amines (2.4 mol eq) at ꢃ45 °C. The
“mol eq” of the amine was based on the nitrogen atom. b) Determined by 200 MHz
1H-NMR analysis. c) Isolation yield of a mixture of 3a and 4a.
1
Compound 3b14): Colorless prisms (Et2O–n-hexane), mp 79—80 °C. H-
NMR (400 MHz, CDCl3) d: 1.2—1.3 (1H, m), 1.5—1.8 (4H, m), 2.0—2.1
(1H, m), 2.3—2.4 (1H, m), 2.4—2.5 (1H, m), 2.5—2.6 (1H, m), 3.80 (3H,
s), 3.94 (1H, d, Jꢀ2.9 Hz), 4.74 (1H, dd, Jꢀ2.9, 8.8 Hz), 6.88 (2H, d,
Jꢀ8.8 Hz), 7.24 (2H, d, Jꢀ8.8 Hz). IR (KBr) cmꢃ1: 3462, 1708, 1515, 1252,
1174, 1029, 829, 559. EI-MS m/z: 234.1260 (Calcd for C14H18O3:
234.1256).
Compound 4b14): Colorless needles (Et2O–n-hexane), mp 126—127 °C.
1H-NMR (400 MHz, CDCl3) d: 1.5—1.6 (1H, m), 1.6—1.9 (4H, m), 2.0—
2.1 (1H, m), 2.3—2.5 (2H, m), 2.5—2.6 (1H, m), 3.04 (1H, d, Jꢀ2.9 Hz),
3.80 (3H, s), 5.32 (1H, t, Jꢀ2.9 Hz), 6.87 (2H, d, Jꢀ8.3 Hz), 7.22 (2H, d,
Fig. 1. Summary of the Stereoselectivity in the Imine Aldol Reaction of 1
with 2a
Table 4. Stereoselectivity in the Imine Aldol Reaction with Various Aldehydes
Condition Aa)
Condition Ba)
threo/erythrob)
(yield %)c)
Condition Ca)
threo/erythrob)
(yield %)c)
Condition Da)
threo/erythrob)
(yield %)c)
Entry
R
Products
threo/erythrob)
(yield %)c)
1
2
3
4
5
6
H
MeO
Me
Ph
Cl
3a/4a
3b/4b
3c/4c
3d/4d
3e/4e
3f/4f
5/95 (72)
8/92 (30)
5/95 (64)
9/91 (71)
8/92 (86)
20/80 (84)
75/25 (84)
75/25 (73)
76/24 (81)
33/67 (88)
67/33 (84)
65/35 (89)
7/93 (90)
12/88 (70)
9/91 (82)
7/93 (91)
17/83 (86)
24/76 (90)
91/ 9 (87)
68/32 (55)
9/91 (85)
8/92 (87)
7/93 (91)
15/85 (93)
MeO2C
a) Condition A: Mg(OTf)2 (1.2 mol eq)–Et3N (2.4 mol eq) at ꢃ45 °C for 1 h, condition B: Mg(ClO4)2 (1.2 mol eq)–Et3N (2.4 mol eq) at ꢃ45 °C for 1 h, condition C: MgCl2
(1.2 mol eq)–Et3N (2.4 mol eq) at ꢃ45 °C for 1 h, condition D: MgCl2 (1.2 mol eq)–TMEDA (2.4 mol eq) at ꢃ45 °C for 1 h. The “mol eq” of amine was based on the nitrogen atom.
b) Determined by 200 MHz 1H-NMR analysis. c) Isolation yield of a mixture of 3a—f and 4a—f.

December 2007
1775
1H-NMR (400 MHz, CDCl3) d: 1.3—1.4 (1H, m), 1.5—1.7 (3H, m), 1.7—
1.8 (1H, m), 2.0—2.1 (1H, m), 2.3—2.4 (1H, m), 2.4—2.5 (1H, m), 2.5—
2.6 (1H, m), 3.91 (3H, s), 4.04 (1H, d, Jꢀ2.9 Hz), 4.85 (1H, dd, Jꢀ2.9,
Jꢀ8.3 Hz). IR (KBr) cmꢃ1: 3441, 1696, 1511, 1250, 1170, 834. EI-MS m/z:
234.1256 (Calcd for C14H18O3: 234.1256).
1
Compound 3c14): Colorless prisms (Et2O–n-hexane), mp 91—92 °C. H-
8.8 Hz), 7.40 (2H, d, Jꢀ8.3 Hz), 8.02 (2H, d, Jꢀ8.3 Hz). IR (KBr) cmꢃ1
:
NMR (400 MHz, CDCl3) d: 1.2—1.3 (1H, m), 1.5—1.7 (3H, m), 1.7—1.8
(1H, m), 2.0—2.1 (1H, m), 2.3—2.4 (1H, m), 2.34 (3H, s), 2.4—2.5 (1H,
m), 2.6—2.7 (1H, m), 3.92 (1H, d, Jꢀ2.9 Hz), 4.75 (1H, dd, Jꢀ2.9, 8.9 Hz),
7.15 (2H, d, Jꢀ8.3 Hz), 7.20 (2H, d, Jꢀ8.3 Hz). IR (KBr) cmꢃ1: 3517, 1689,
1297, 1130, 1044, 823, 554. EI-MS m/z: 218.1292 (Calcd for C14H18O2:
218.1307).
3526, 1717, 1694, 1446, 1313, 1283, 1117, 750. EI-MS m/z: 262.1202
(Calcd for C15H18O4: 262.1205).
Compound 4f15): Colorless needles (Et2O–n-hexane), mp 132—134 °C.
1H-NMR (400 MHz, CDCl3) d: 1.4—1.5 (1H, m), 1.6—1.8 (3H, m), 1.8—
1.9 (1H, m), 2.0—2.1 (1H, m), 2.3—2.5 (2H, m), 2.6—2.7 (1H, m), 3.14
(1H, d, Jꢀ2.9 Hz), 3.91 (3H, s), 5.45 (1H, t, Jꢀ2.9 Hz), 7.38 (2H, d,
Jꢀ8.3 Hz), 8.01 (2H, d, Jꢀ8.3 Hz). IR (KBr) cmꢃ1: 3535, 1699, 1313, 1282,
1120, 1106, 1091, 712. EI-MS m/z: 262.1216 (Calcd for C15H18O4:
262.1205).
Compound 4c14): Colorless prisms (Et2O–n-hexane), mp 111—113 °C.
1H-NMR (400 MHz, CDCl3) d: 1.5—1.6 (1H, m), 1.6—1.8 (3H, m), 1.8—
1.9 (1H, m), 2.0—2.1 (1H, m), 2.3—2.4 (1H, m), 2.34 (3H, s), 2.4—2.5
(1H, m), 2.5—2.6 (1H, m), 2.99 (1H, d, Jꢀ2.9 Hz), 5.35 (1H, t, Jꢀ2.9 Hz),
7.14 (2H, d, Jꢀ8.3 Hz), 7.19 (2H, d, Jꢀ8.3 Hz). IR (KBr) cmꢃ1: 3471, 1697,
1133, 1120, 1091, 985, 825, 533. EI-MS m/z: 218.1304 (Calcd for C14H18O2:
218.1307).
References
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Compound 3d: Colorless needles (CH2Cl2–n-hexane), mp 142—143 °C.
1H-NMR (400 MHz, CDCl3) d: 1.3—1.4 (1H, m), 1.5—1.7 (3H, m), 1.8—
1.9 (1H, m), 2.0—2.1 (1H, m), 2.3—2.4 (1H, m), 2.4—2.5 (1H, m), 2.6—
2.7 (1H, m), 4.01 (1H, d, Jꢀ2.4 Hz), 4.84 (1H, dd, Jꢀ2.4, 8.8 Hz), 7.35 (1H,
t, Jꢀ7.8 Hz), 7.40 (2H, d, Jꢀ7.8 Hz), 7.44 (2H, t, Jꢀ7.8 Hz), 7.58 (2H, d,
Jꢀ7.8 Hz), 7.59 (2H, d, Jꢀ7.8 Hz). IR (KBr) cmꢃ1: 3493, 1693, 1488, 1128,
846, 776, 762, 699. EI-MS m/z: 280.1455 (Calcd for C19H20O2: 280.1463).
Anal. Calcd for C19H20O2: C, 81.39; H, 7.19. Found: C, 81.21; H, 7.24.
Compound 4d: Colorless needles (Et2O–n-hexane), mp 138—139 °C. 1H-
NMR (400 MHz, CDCl3) d: 1.5—1.6 (1H, m), 1.6—1.9 (4H, m), 2.0—2.1
(1H, m), 2.3—2.4 (1H, m), 2.4—2.5 (1H, m), 2.6—2.7 (1H, m), 3.08 (1H, d,
Jꢀ2.4 Hz), 5.44 (1H, t, Jꢀ2.4 Hz), 7.34 (1H, t, Jꢀ7.8 Hz), 7.38 (2H, d,
Jꢀ7.8 Hz), 7.44 (2H, t, Jꢀ7.8 Hz), 7.58 (2H, d, Jꢀ8.3 Hz), 7.60 (2H, d,
Jꢀ8.3 Hz). IR (KBr) cmꢃ1: 3449, 1699, 1488, 1110, 1093, 750, 692. EI-MS
m/z: 280.1483 (Calcd for C19H20O2: 280.1463). Anal. Calcd for C19H20O2: C,
81.39; H, 7.19. Found: C, 81.28; H, 7.23.
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Compound 3e13): Colorless prisms (CH2Cl2–n-hexane), mp 97—98 °C.
1H-NMR (400 MHz, CDCl3) d: 1.2—1.3 (1H, m), 1.5—1.7 (3H, m), 1.7—
1.8 (1H, m), 2.0—2.1 (1H, m), 2.3—2.4 (1H, m), 2.4—2.5 (1H, m), 2.5—
2.6 (1H, m), 4.00 (1H, d, Jꢀ2.9 Hz), 4.76 (1H, dd, Jꢀ2.9, 8.8 Hz), 7.26 (2H,
d, Jꢀ8.3 Hz), 7.32 (2H, d, Jꢀ8.3 Hz). IR (KBr) cmꢃ1: 3420, 1698, 1486,
1128, 1090, 1013, 831, 556. EI-MS m/z: 238.0756 (Calcd for C13H15O2Cl:
238.0761).
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Compound 4e13): Colorless prisms (CH2Cl2–n-hexane), mp 119—120 °C.
1H-NMR (400 MHz, CDCl3) d: 1.5—1.6 (1H, m), 1.6—1.7 (3H, m), 1.8—
1.9 (1H, m), 2.0—2.1 (1H, m), 2.3—2.4 (1H, m), 2.4—2.5 (1H, m), 2.5—
2.6 (1H, m), 3.09 (1H, d, Jꢀ2.9 Hz), 5.35 (1H, t, Jꢀ2.9 Hz), 7.24 (2H, d,
Jꢀ8.3 Hz), 7.30 (2H, d, Jꢀ8.3 Hz). IR (KBr) cmꢃ1: 3530, 1703, 1491, 1314,
1129, 1087, 833, 542. EI-MS m/z: 238.0753 (Calcd for C13H15O2Cl:
238.0761).
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Compound 3f15): Colorless needles (Et2O–n-hexane), mp 109—110 °C.