Asymmetric synthesis of both enantiomers of α-trifluoromethyl substituted homoallylamine

Article abstract of DOI:10.1055/s-2002-35631

An efficient asymmetric synthesis of both enantiomers of α-trifluoromethylated homoallylamine via nucleophilic allylation of trifluoroacetaldehyde SAMP- or RAMP-hydrazone, followed by benzoylation and SmI₂-promoted nitrogen-nitrogen single bond cleavage is described.

Full text of DOI:10.1055/s-2002-35631

SPECIAL TOPIC
2585
Asymmetric Synthesis of Both Enantiomers of -Trifluoromethyl Substituted
Homoallylamine
Enantiomeric
-Tr
a
ifluorometh
z
yl Substitu
u
ted Homoally
m
lamine
s
asa Funabiki,*a Masashi Nagamori,a Masaki Matsui,a Dieter Endersb
a
Department of Materials Science and Technology, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
Fax +81(58)2301893; E-mail: kfunabik@apchem.gifu-u.ac.jp
b
Institut für Organische Chemie, Rheinisch-Westfälische Technische Hochschule, Professor-Pirlet-Straße 1, 52074 Aachen, Germany
Received 2 September 2002
was first examined (Scheme 2). The results are summa-
rized in Table 1.
Abstract: An efficient asymmetric synthesis of both enantiomers
of -trifluoromethylated homoallylamine via nucleophilic allyla-
tion of trifluoroacetaldehyde SAMP- or RAMP-hydrazone, fol-
lowed by benzoylation and SmI2-promoted nitrogen-nitrogen single
bond cleavage is described.
O
O
Sn
4
1.
N
N
N
HN
Key words: asymmetric synthesis, fluorine, hydrazone, allylation,
amines
2. PhLi, Et2O
0 - 62%
F3C
F3C
1b
2b
Scheme 2
The asymmetric nucleophilic allylation of imino deriva-
tives has received increasing attention in organic synthe-
sis,1,2 because the resulting enantio-enriched homo-
allylamines or hydrazines are promising precursors of -
amino substituted aldehydes, ketones, epoxides, and cal-
boxylic acid derivatives. Although some reports recently
described the asymmetric synthesis of -alkyl, aryl or vi-
nyl substituted -trifluoromethylamines,3 to the best of
our knowledge, there is no report on the stereoselective al-
lylation of -trifluoromethylated imines or hydrazones
leading to enantiopure -trifluoromethylated homoallyl-
amines despite their synthetic utility.4,5 As a part of our
studies directed towards the asymmetric synthesis of or-
ganofluorine compounds by means of the SAMP- or
RAMP-hydrazone method,6 we describe herein the first
asymmetric synthesis of both enantiomers of -trifluoro-
methylated homoallylamine (R)- or (S)-4a using trifluoro-
acetaldehyde SAMP- or RAMP-hydrazone (S)- or (R)-1a
as a starting substrate (Scheme 1).
Table 1 Screening of the Reaction Conditions for the Allylation of
Trifluoroacetaldehyde Morpholinohydrazone (1b)
Entrya
Tetraallyltin (equiv)
PhLi (equiv)
Yield of 2b (%)b
7 (38)
1
3
3
4
2
1
21 (35)
47
3
2
8
4
3
12
3
62
5
3c
0.3
20 (15)
0 (quant)
6d
0
a The reaction was performed with hydrazone 1b (1 mmol) in Et2O (11
mL).
b Yields of isolated products. Values in parentheses stand for the re-
covered 1b.
c Allyltriphenyltin was used in place of tetraallyltin.
d The reaction was carried out in the presence of 5 mol% of Yb(OTf)3
in MeCN at r.t.
nucleophilic
allylation
NHBz
N
N
F3C
As shown in Scheme 2, treatment of tetraallyltin (3 equiv)
with phenyllithium (12 equiv) at room temperature, fol-
lowed by addition of trifluoroacetaldehyde morpholino-
hydrazone (1b) at –78 °C, gave the corresponding
trifluoroacetaldehyde morpholinohydrazine 2b in 62%
yield (entry 4). The reaction of 1b with three equivalents
of tetraallyltin and PhLi, respectively, was extremely
sluggish, providing only a trace amount of 2b (entry 1).
Employing tetraallyltin (1–2 equiv) and PhLi (4–8 equiv)
produced 2b in 21–47% yields along with recovered 1b
(entries 2 and 3). The use of allyltriphenyltin (3 equiv) in
place of tetraallyltin with PhLi (3 equiv) was also ineffec-
tive, giving 2b in 20% yield together with 15% of 1b (en-
try 5). Yb(OTf)3-catalyzed (5 mol%) allylation7 with 0.3
equivalent of tetraallyltin in acetonitrile at room tempera-
OMe
F3C
(R)-4a or
(S)-4a
(S)-1a or
(R)-1a
ee = >99%
Scheme 1
For the screening of the allylation reaction of trifluoro-
acetaldehyde hydrazones, the reaction between trifluoro-
acetaldehyde morpholinohydrazone (1b) and tetraallyltin
Synthesis 2002, No. 17, Print: 02 12 2002.
Art Id.1437-210X,E;2002,0,17,2585,2588,ftx,en;c04902SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

2586
K. Funabiki et al.
SPECIAL TOPIC
ture did not occur at all, and 1b was recovered in quanti- lated homoallylamine based on the nuleophilic allylation
tative yield (entry 6). With other allylic nucleophiles e.g., of trifluoroacetaldehyde SAMP- or RAMP-hydrazone
allylmagnesium bromide (3 equiv, –78 °C to r.t., over- (ee >99%).
night), only trace amount of 2b was formed with 67% re-
covery of 1b.
Optical rotaions were measured in Uvasol grade CHCl3 (Merck) on
a HORIBA SEPA-300 instrument. HPLC was carried out using
Next, in order to synthesize diastereo- and enantiomeri-
Daicel CHIRALCEL OD (Daicel, 0.46 cm
25 cm) column on a
cally pure hydazines, the stereoselective allylation of trif-
luoroacetaldehyde SAMP- or RAMP-hydrazone (S)- or
(R)-1a was carried out (Scheme 3).8
Shimadzu LC-4A liquid chromatograph. Melting points were ob-
tained on a Yanagimoto MP-S2 micro melting point apparatus and
are uncorrected. IR spectra were recorded on a Shimadzu FT-IR
8100A spectrometer. 1H NMR spectra were measured with a JEOL
-400 (400 MHz) FT-NMR spectrometer in CDCl3 with tetrameth-
ylsilane as the internal standard. 13C NMR spectra were obtained on
a JEOL -400 (100 MHz) FT-NMR spectrometer in CDCl3 with te-
tramethylsilane as the internal standard. 19F NMR spectra were re-
corded on a JEOL -400 (376 MHz) FT-NMR spectrometer in
CDCl3 solution using trifluoroacetic acid as the external standard.
Mass spectra were taken on a Hitachi QP 1000 spectrometer (70
eV). HRMS were measured on a JEOL JMS-700 mass spectro-
meter. SmI2 (0.1 M THF solution) and tetraallyltin were purchased
from Aldrich Chemical Co.
NHBz
N
N
F3C
(R)-4a
OMe
F3C
(S)-1a
ee = >99%
a
c
80% (67%)
75% (98%)
OMe
OMe
Allylation of Trifluoroacetaldehyde SAMP-Hydrazone (S)-1a;
(2R,2 S)-2-[2 -(Methoxymethyl)pyrrolidin-1 -yl]amino-1,1,1-
trifluoropent-4-ene [(2R,2 S)-2a]; Typical Procedure
N
N
b
NH
NBz
68% (61%)
F3C
F3C
A solution of PhLi (5.7 mL of a 1.06 M cyclohexane–Et2O solution,
6 mmol) was slowly added to an anhyd Et2O solution (10 mL) of tet-
raallyltin (0.424 g, 1.5 mmol) at r.t. under argon. After the reaction
mixture was stirred at r.t. for 1 h and cooled to –78 °C, an anhyd
Et2O solution (1 mL) of trifluoroacetaldehyde SAMP-hydrazone
(S)-1a (0.105 g, 0.5 mmol) was slowly added at –78 °C. After stir-
ring at –78 °C for 4 h, the resulting mixture was quenched with a
cold sat. aq NaHCO3 solution (50 mL), and the precipitate formed
was removed by suction filtration with hexane (30 mL). The mix-
ture was extracted with Et2O (2 30 mL), dried (Na2SO4), and the
solvent was removed under reduced pressure. After the isomer ratio
of the products was determined by 19F NMR, the residue was puri-
fied by flash chromatography on silica gel (hexane–Et2O, 10:1) to
give hydrazine 2a (80%, 0.102 g); yield: 80%; Rf 0.28 (hexane–
Et2O, 10:1); [ ]D21 –34.8 (c = 0.97, CHCl3).
(2R,2'S)-2a
(2R,2'S)-3a
Scheme 3 Reagents and conditions: (a) tetraallyltin, PhLi, r.t., then
–78 °C; (b) catalytic DMAP, Et3N, PhCOCl, r.t.; (c) SmI2, THF/
DMPU, r.t.
When SAMP-hydrazone (S)-1a was treated with a mix-
ture, obtained by the reaction between 3 equivalents of tet-
raallyltin and 12 equivalents of PhLi at room temperature,
in Et2O at –78 °C for 4 hours, the allylated hydrazine
(2R,2 S)-2a was produced with high diastereoselectivty
(93:7), and diastereomerically pure 2a was easily obtained
by flash column chromatography in 80% yield. Hydra-
zone (R)-1a also nicely underwent the allylation reaction
to give (2S,2 R)-2a (>98% de) in 67% yield after column
chromatography.
IR (KBr): 3293, 2977, 1645, 1458, 1379, 1273, 1173, 1125, 920,
762 cm–1.
1H NMR (400 MHz, CDCl3):
= 1.50–1.59 (m, 1 H,
CH2CHAHBCH), 1.64–1.77 (m, 2 H, CHA HB CHAHBCH), 1.86–
1.95 (m, 1 H, CHA HB CH2CH), 2.25–2.33 (m, 1 H, CHAHB), 2.42–
2.49 (m, 1 H, CHAHB) 2.43 (q, J = 8.78 Hz, 1 H, NCHAHB), 2.63–
2.70 (m, 1 H, NCHAHB), 2.70 (m, 1 H, CF3CH), 3.30 (br s, 1 H,
NH), 3.35 (s, 3 H, OCH3), 3.38 (AB quartet, J = 9.27, 5.37 Hz, 1 H,
CHAHBO), 3.44–3.49 (m, 1 H, NCH), 3.47 (AB quartet, J = 9.27,
The obtained diastereomerically pure hydrazines 2a were
easily converted to the enantiopure homoallylamines. Af-
ter benzoylation of SAMP- or RAMP-hydrazine 2a with
an excess amount of triethylamine and benzoyl chloride in
the presence of a catalytic amount of DMAP at room
temperature, respectively, SmI2-induced cleavage of the 5.37 Hz, 1 H, CHAHBO), 5.12 (ddt, J = 9.75, 1.71, 1,71 Hz, 1 H,
CH=CHAHB), 5.16 (ddt, J = 17.08, 1.71, 1.71 Hz,
CH=CHAHB), 5.85 (ddt, J = 17.08, 9.75, 7.31 Hz, 1 H, CH=CH2).
13C NMR (100 MHz, CDCl3): = 20.92 (s), 26.19 (s), 32.61 (s),
57.45 (s), 58.98 (s), 61.09 (q, JCF = 25.91 Hz), 66.58 (s), 75.14 (s),
126.21 (q, JCF = 282.30 Hz), 133.60 (s).
1 H,
nitrogen–nitrogen single bond of the hydrazides 3a gave
the corresponding enantiopure -trifluoromethylated ho-
moallylamines (R)- and (S)-4a in 75–98% yield with
>99% ee without any detectable epimerization or racem-
ization.9 The benzoyl group is essential for the cleavage of
nitrogen–nitrogen single bond of 3a. Thus, treatment of
SAMP-hydrazine 2a with SmI2 under the same conditions
19F NMR (CDCl3): = 3.41 (d, JFH = 7.63 Hz, 3 F).
MS (EI): m/z (%) = 252.0 (M+, 7.9), 207.0 (M+ – CH2OCH3, 100.0),
gave only a trace amount of homoallylamine 4a, and hy- 129.0 (M+ – CF3CHCH2CHCH2, 8.1).
drazine 2a was recovered quantitatively.
HRMS (CI): m/z calcd for C11H19ON2F3 (M + H), 253.1529; found,
253.1520.
In summary, we have achieved the first efficient asym-
metric synthesis of both enantiomers of -trifluoromethy-
Synthesis 2002, No. 17, 2585–2588 ISSN 0039-7881 © Thieme Stuttgart · New York

SPECIAL TOPIC
Enantiomeric -Trifluoromethyl Substituted Homoallylamines
2587
(2S,2 R)-2-[(2 -Methoxymethyl)pyrrolidin-1 -yl]amino-1,1,1-
trifluoropent-4-ene [(2S,2 R)-2a]
This compound was prepared staring from (R)-1a according to the
above typical procedure; yield: 67%; Rf 0.25 (hexane–Et2O, 10:1);
[ ]D22 +43.62 (c = 0.96, CHCl3).
SmI2-Induced Cleavage of the N–N Single Bond of SAMP-Hy-
drazide 3a; (R)-N-(1,1,1-Trifluoropent-4-en-2-yl)benzamide
[(R)-4a]; Typical Procedure
A THF solution of SmI2 (13.6 mL, 0.1 M, 1.4 mmol) was slowly
added to a THF solution (3.1 mL) of SAMP-hydrazide (2R,2 S)-3a
(0.161 g, 0.45 mmol) in the presence of DMPU (0.78 mL) at r.t. un-
der argon. After 30 min the reaction mixture was quenched with a
sat. aq NaHCO3 solution (50 mL), extracted with CH2Cl2 (3 30
mL), and dried (Na2SO4). After the solvent was removed under re-
duced pressure, the residue was purified by flash chromatography
on silica gel (hexane–Et2O, 5:1) to give the amide 4a (75%, 0.082
g); yield: 75%; Rf 0.13 (hexane–Et2O, 5:1); mp 153.0–154.5 °C;
[ ]D22 +6.65 (c = 1.01, CHCl3).
2-(Morpholin-1 -yl)amino-1,1,1-trifluoropent-4-ene (2b)
This compound was prepared starting from 1b following the above
typical procedure; yield: 62%; Rf 0.23 (hexane–Et2O, 10:1).
IR (KBr): 3278, 2859, 1646, 1453, 1358, 1275, 1113, 878 cm–1.
1H NMR (400 MHz, CDCl3): = 2.09 (ddd, J = 14.64, 9.50, 9.50
Hz, 1 H, CHAHB), 2.38 (br, 1 H, NH), 2.40–2.45 (m, 1 H, CHAHB),
2.54–2.59 (m, 4 H, CH2NCH2), 3.17–3.24 (m, 1 H, CF3CH), 3.58–
3.65 (m, 4 H, CH2OCH2), 5.12–5.15 (m, 2 H, CH=CH2), 5.65–5.73
(m, 1 H, CH=CH2).
IR (KBr): 3281, 1647, 1537, 1375, 1264, 1179, 1103, 924, 708
cm–1.
1H NMR (400 MHz, CDCl3): = 2.35–2.43 (m, 1 H, CHAHB),
2.56–2.63 (m, 1H, CHAHB), 4.83–4.93 (m, 1 H, CF3CH), 5.13 (ddt,
J = 8.05, 1.47, 1.47 Hz, 1 H, CH=CHAHB), 5.15 (ddt, J = 14.40,
1.47, 1.47 Hz, 1 H, CH=CHAHB), 5.73 (ddt, J = 14.40, 9.76, 8.05
Hz, 1 H, CH=CH2), 6.04 (d, J = 9.27 Hz, 1 H, NH), 7.38–7.41 (m,
2 H, C6H5-Hm), 7.46–7.50 (m, 1 H, C6H5-Hp), 7.69–7.72 (m, 2 H,
C6H5-Ho).
13C NMR (100 MHz, CDCl3): = 31.96 (s), 56.32 (s), 58.28 (q,
JCF = 28.12 Hz), 66.12 (s), 120.55 (s), 125.62 (q, JCF = 284.50 Hz),
132.42 (s).
19F NMR (CDCl3): = 1.85 (d, JFH = 6.11 Hz, 3 F).
MS (EI): m/z (%) = 224.0 (M+, 8.7), 101.0 (M+
–
CF3CHCH2CHCH2, 100.0).
13C NMR (100 MHz, CDCl3): = 32.87 (s), 49.88 (q, JCF = 30.32
Hz), 119.68 (s), 125.08 (q, JCF = 243.15 Hz), 127.05 (s), 128.65 (s),
131.43 (s), 131.07 (s), 133.41 (s), 167.33 (s).
HRMS (CI): m/z calcd for C9H16ON2F3 (M + H), 225.1216; found,
225.1210.
19F NMR (CDCl3): = 2.60 (d, JFH = 7.63 Hz, 3 F).
MS (EI): m/z (%) = 243.0 (M+, 13.6), 202.0 (M+ – CH2CHCH2, 0.8),
Benzoylation of SAMP-Hydrazine 2a; (2R,2 S)-N-[2 -(Meth-
oxymethyl)pyrrolidin-1 -yl]]-N-(1,1,1-trifluoropent-4-en-2-
yl)benzamide [(2R,2 S)-3a]; Typical Procedure
105.0 (PhCO, 100.0).
A mixture of SAMP-hydrazine (2R,2 S)-2a (0.179 g, 0.709 mmol),
benzoyl chloride (0.996 g, 7.09 mmol), Et3N (0.716 g, 7.09 mmol),
and a catalytic amount of DMAP (1 crystal) in CH2Cl2 (2 mL) was
stirred at r.t. for 3 d under argon. The resultant mixture was
quenched with a sat. aq NaHCO3 solution (50 mL), extracted with
Et2O (3 30 mL), washed with a sat. aq NaHCO3 solution (2 30
mL), and dried (Na2SO4). After removal of the solvent under re-
duced pressure, the residue was purified by flash chromatography
on silica gel (hexane–Et2O, 20:1, followed by hexane–Et2O, 5:1) to
give the hydrazide 3a (68%, 0.173 g); yield: 68%; Rf 0.13 (hexane–
Et2O, 5:1); [ ]D22 –39.06 (c = 1.00, CHCl3).
(S)-N-(1,1,1-Trifluoro-4-penten-2-yl)benzamide [(S)-4a]
This compound was prepared staring from (2S,2 R)-3a according to
the above typical procedure; yield: 98%; Rf 0.13 (hexane–Et2O,
5:1); mp 148.8–151.0 °C; [ ]D22 –3.36 (c = 1.02, CHCl3).
Acknowledgments
K.F. is grateful to the Alexander von Humboldt Foundation for a
postdoctoral fellowship (2000–2001). This work was supported by
the Saijiro Endo Foundation. We thank Professors Hiroki Yamana-
ka and Takashi Ishihara as well as Dr. Tsutomu Konno of the Kyoto
Institute of Technology for the HRMS measurements of 2a,b.
IR (KBr): 2979, 1667, 1601, 1447, 1348, 1321, 1173, 1120, 924,
702 cm–1.
1H NMR (400 MHz, CDCl3): = 1.52–1.90 (m, 4 H, CH2CH2),
2.77–2.88 (m, 1 H, NCHAHB), 3.07 (s, 1 H, NCHAHB), 3.14–3.20
(m, 1 H, CHAHB), 3.22–3.27 (m, 1 H, CHAHB), 3.36 (s, 3 H, OCH3),
3.44–3.53 (m, 1 H, NCH), 3.48 (AB quartet, J = 9.52, 4.15 Hz, 1
H, CHAHBO), 3.79–3.83 (m, 1 H, CHAHBO), 4.19 (m, 1 H, CF3CH),
5.13 (dd, J = 16.34, 1.22 Hz, 1 H, CH=CHAHB), 5.17 (dd, J = 9.27,
1.22 Hz, 1 H, CH=CHAHB), 5.59 (ddt, J = 16.34, 9.27, 6.83 Hz, 1
H, CH=CH2), 7.32–7.51 (m, 5 H, C6H5).
13C NMR (100 MHz, CDCl3): = 24.31 (s), 27.95 (s), 30.48 (s),
54.87 (s), 58.60 (s), 62.43 (q, JCF = 28.67 Hz), 63.46 (s), 74.65 (s),
124.31 (q, JCF = 265.20 Hz), 126.13 (s), 128.60 (s), 129.65 (s),
131.65 (s), 136.33 (s), 171.09 (s).
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Synthesis 2002, No. 17, 2585–2588 ISSN 0039-7881 © Thieme Stuttgart · New York

2588
K. Funabiki et al.
SPECIAL TOPIC
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Synthesis 2002, No. 17, 2585–2588 ISSN 0039-7881 © Thieme Stuttgart · New York