Reductive detriflylation of N-triflylamides with Red-Al

Article abstract of DOI:10.1021/jo300947h

Reduction of cis-N-triflylaziridines with 10 equiv of Red-Al in toluene at-40 to 0 °C selectively afforded corresponding deprotected parent aziridines in good to high yields. N,N-Dialkyltriflylamides were also successfully cleaved under similar reaction conditions.

Full text of DOI:10.1021/jo300947h

Note
pubs.acs.org/joc
Reductive Detriflylation of N‑Triflylamides with Red-Al
Kazunori Miyamoto,* Md. Mahbubul Hoque, and Sayaka Ogasa
Graduate School of Pharmaceutical Sciences, University of Tokushima, 1-78 Shomachi, Tokushima 770-8505, Japan
S
* Supporting Information
ABSTRACT: Reduction of cis-N-triflylaziridines with 10 equiv of Red-Al in toluene at
−40 to 0 °C selectively afforded corresponding deprotected parent aziridines in good to
high yields. N,N-Dialkyltriflylamides were also successfully cleaved under similar reaction
conditions.
Table 1. Detriflylation of cis-2,3-Diphenyl-N-triflylaziridine
(4)
ziridines, useful for the construction of complex organic
molecules, are usually prepared by the direct aziridination
A
of alkenes.1 In the past decades, much attention has been paid
to the stereoselective aziridination reaction with a combination
of transition-metal catalysts and nitrenoid precursors.2 In many
cases, N-sulfonylimino-λ3-iodanes 1 [RSO2NIPh] (where R
= Me, p-tolyl, CCl3CH2O, etc.) served as nitrenoid progenitors
because of high reactivity, stability, and nontoxic nature of the
reagents.3 Recently, we synthesized a group 17 congener
aryl(trifluoromethanesulfonyl)imino-λ3-bromane 2 which can
serve as a versatile nitrenoid under metal-free conditions.4 For
example, reaction of various olefins with imino-λ3-bromane 2
smoothly undergoes at 0 °C or room temperature and produces
the corresponding aziridines in high yields with high stereo-
selectivity.4a Very recently, we have developed a one-pot
methodology where imino-λ3-bromane 2 is generated in situ
from diacetoxyaryl-λ3-bromane [p-CF3C6H4Br(OAc)2] and
trifluoromethanesulfonamide (TfNH2).5 From a synthetic
point of view, although these reactions potentially serve as a
useful choice for stereoselective synthesis of aziridines, there is
a difficult obstacle to overcome: triflylamides seem to be less
susceptible to reductive deprotection compared to common
arenesulfonamides. For example, the attempted reductive
deprotection of N-(alkyl)triflylamides using SmI2, which can
successfully deprotect N-alkylarenesulfonamides, is fruitless.6
There are only a limited number of methods available for
deprotection of N-alkyltriflylamides, and they usually requires
harsh reaction conditions (i.e., high temperature/excess
powerful reductant).7 Herein, we report a facile method for
reductive deprotection of N-triflylaziridines using sodium bis(2-
methoxyethoxy)aluminum dihydride (Red-Al) (3) under mild
conditions (at −40 or 0 °C).8 The reaction conditions were
also successfully applied to the deprotection of N,N-
dialkyltriflylamides.
a
yield (%)
entry Red-Al (equiv)
solvent
conditions
5
6
1
2
3
4
5
10
10
10
10
3
toluene
CH2Cl2
THF
−40 °C, 3 h, Ar
−40 °C, 3 h, Ar
−40 °C, 3 h, Ar
0 °C, 3 h, Ar
88
69
51
12
31
45
3
toluene
toluene
86 (82)
b
0 °C, 9.5 h, Ar
13
9
a
b
1H NMR yields. Isolated yield is shown in parentheses. SM (43%)
was recovered.
hand, use of 3 equiv of Red-Al was found to be fruitless,
yielding a low yield of 5 (entry 5).9 In these reactions, no cis−
trans isomerization of aziridine 5 was observed.
With optimized reaction conditions in hand, detriflylation of
various triflylamides has been examined, and the results are
listed in Table 2. The reductive deprotection of cis-2,3-
disubstituted N-triflylaziridines 7a−c with Red-Al 3 smoothly
proceeded at 0 °C and afforded parent aziridines 8a−c in good
yields (entries 1−3). It should be noted that use of LiAlH4 (87
equiv)/Et2O/70 °C/14 h, instead of Red-Al 3, produced only a
ring-opended product for 7a (31%). In marked contrast, the
attempted deprotection of 2-octyl-N-triflylaziridine (18) with
Red-Al 3 gave only a ring-opened N-(1-methyl-1-nonyl)-
triflylamide (19), partly due to the smaller steric hindrance of
less substituted carbon atom of aziridine ring (Scheme 1).10
Furthermore, a variety of N,N-dialkyltriflylamides gave the
corresponding dialkylamines in high yields (entries 4−7).
Interestingly, the reductive cleavage of N,N-dialkyltriflylamides
is relatively slow and requires higher temperature and
prolonged reaction time (50 °C/9 h). In fact, the reduction
of N,N-dinonyltriflylamide (9) at room temperature gave only
53% of dinonylamine (10) after 5 h accompanied by the
recovery of 9 (40%). The lower reactivity of N,N-dialkyltri-
Table 1 summarizes the initial attempts for reductive
detriflylation of cis-N-triflylaziridine 4. Exposure of aziridine 4
to excess Red-Al 3 in toluene resulted in a smooth cleavage of
the N−S bond within 3 h at −40 °C and afforded parent cis-2,3-
diphenylaziridine (5) in 88% yield, along with the formation of
ring-cleaved triflylamide 6 (12%) (Table 1, entry 1). Changing
the solvent to CH2Cl2 and THF resulted in a decrease in the
yields of 5 (entries 2 and 3). Best selectivity was obtained when
the reaction was carried out at 0 °C (entry 4). On the other
Received: May 26, 2012
Published: August 9, 2012
© 2012 American Chemical Society
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Note
a
Chemical shifts (δ) are reported in parts per million (ppm) downfield
from internal Me4Si. Mass spectra (MS) were obtained on either a
ESIMS and GCMS instruments. Preparative thin-layer chromatog-
raphy (TLC) was carried out on precoated plates of silica gel with
fluorescent. Kieselgel 60 (230−400 mesh) was used for column
chromatography. Melting points were determined with melting points
apparatus and are uncorrected. Sodium bis(2-methoxyethoxy)alumi-
num dihydride 65 wt % solution in toluene was purchased from
commercial source.
Table 2. Detriflylation of N-Triflylaziridines with Red-Al.
Substrates. N-Triflylaziridines 4, 7a−c were prepared by
aziridination of the corresponding olefins with imino-λ3-bromane 2
according to the reported procedure.4a Triflylamides, 9, 11, 13, 15, and
17 were prepared from the corresponding amines by the reaction with
trifluoromethanesufonic anhydride according to the reported
procedure.15
General Procedure for Detriflylation of N-Triflylaziridines. A
Typical Example (Table 1, Entry 4). To a stirred solution of cis-2,3-
diphenyl-N-[(trifluoromethyl)sulfonyl]aziridine (5.3 mg, 0.016 mmol)
in toluene (1.0 mL) was added sodium bis(2-methoxyethoxy)-
aluminum dihydride 65 wt % in toluene (50 μL, 0.16 mmol) at 0
°C under argon, and the mixture was stirred for 3 h. After the addition
of 5% aqueous ammonium chloride (5 mL) at 0 °C, the mixture was
extracted with dichloromethane (each 7 mL, three times). The organic
layer was dried over anhydrous sodium sulfate and concentrated under
aspirator vacuum to give a white solid. 1H NMR analysis using 1,1,2,2-
tetrachloroethane as an internal standard showed the formation of cis-
2,3-diphenylaziridine (5) in 86% yield and ring-opened triflylamide 6
in 3% yield. Purification by preparative TLC (ethyl acetate−hexane
3:7, Rf = 0.3) gave aziridine 5 (2.3 mg, 82%) as colorless needles:5 mp
80−81 °C (lit.5 mp 80−81 °C, recrystallized from hexane); IR (KBr)
3276, 3060, 2974, 1602, 1497, 1446, 1416, 1195, 1072, 1056, 1027,
a
Conditions: 1:10 triflylamide/Red-Al 3, toluene, 0 °C, 3 h, Ar.
b
c
d
Isolated yields. Isolated as trifluoroacetate. N-(1-Phenylpropyl)-
e
triflylamide (17) (25%) was obtained. Yield of 10 at 50 °C for 9 h.
Scheme 1
1
869 cm−1; H NMR (400 MHz, CDCl3) δ 7.19−7.04 (m, 10H), 3.60
(s, 2H); EIMS m/z (relative intensity) 195 (43, M+), 194 (42), 117
(13), 91 (100), 77 (5), 65 (22).
The authentic sample of triflylamide 6 was prepared from the
corresponding amine by reaction with trifluoromethanesulfonic
anhydride and triethylamine.15
Scheme 2
N-(1-Phenyl-2-phenethyl)trifluoromethanesulfonamide (6). Puri-
fication by preparative TLC (ethyl acetate−hexane 3:7, Rf = 0.7):
colorless prisms; mp 76−77 °C (recrystallized from dichloro-
methane−hexane at room temperature); IR (KBr) 3302, 3068, 3035,
1
1604, 1496, 1456, 1431, 1365, 1240−1160, 1144, 756, 696 cm−1; H
NMR (400 MHz, CDCl3) δ 7.39−7.29 (m, 4H), 7.29−7.22 (m, 2H),
7.16 (d, J = 7.5 Hz, 2H), 7.01−6.95 (m, 2H), 5.10 (d, J = 7.4 Hz, 1H),
4.90 (q, J = 7.4 Hz, 1H), 3.25−3.13 (m, 2H); 13C NMR (75 MHz,
CDCl3) δ 139.6 (q), 135.5(q), 129.8 (t), 128.8 (t), 128.6 (t), 128.2
(t), 127.2 (t), 126.2 (t), 119.3 (q, 1JCF = 318.4 Hz), 60.8 (t), 44.2 (d);
EIMS m/z (relative intensity) 238 [100%, (M-Bn)+], 181 (3), 91 (27),
77 (16); HRMS (ESI, negative) m/z calcd for C15H13F3NO2S [(M-
H)−] 328.0619, found 328.0622.
flylamides is responsible, at least in part, for the smaller leaving
group ability of N,N-dialkylamino group than that of aziridino
group.11 Our conditions worked well when the adjacent
protecting groups are base-labile benzyl group (entries 6 and
7).12
It should be noted that in the deprotection of N-
cyclopropyltriflylamide 15, no formation of β-fragmentation
products such as 20 was detected, which probably suggests that
the reactions do not involve a radical intermediate like 21 and/
or 22 (Scheme 2).13 A powerful thiol-like odor observed during
workup process suggests the formation of hydrogen sulfide,
which was confirmed by lead acetate paper test of acidified
aqueous layer.14 The formation of highly reduced sulfide ion
(S2−) is responsible, at least in part, for the requirement of
excess amount of reductant Red-Al 3.
Aziridine 8a was converted to the corresponding trifluoroacetate
salt by addition of excess trifluoroacetic acid to the combined organic
layer before evaporation.
cis-2,3-Dipropylaziridine (8a)·CF3CO2H: colorless oil (7.7 mg,
87%); IR (neat) 3300, 2962, 2937, 2875, 1672, 1460, 1433, 1373,
1
1228, 1192, 1149, 1026, 617 cm−1; H NMR (400 MHz, CDCl3) δ
2.86 (br t, J = 4.4 Hz, 2H), 1.80−1.70 (m, 4H), 1.62−1.47 (m, 4H),
1.01 (t, J = 7.1 Hz, 6H); 13C NMR (75 MHz, CDCl3) δ 40.2 (t), 26.1
(d), 20.4 (d), 13.4 (s); HRMS (ESI, positive) m/z calcd for C8H18N
[(M − CF3CO2)+] 128.1439, found 128.1442. After several attempts
to measure the 13C NMR spectrum, compound 8a·TFA salt was found
to be too labile to accumulate sufficient data to detect CF3CO2 group.
cis-2-Methyl-3-phenylaziridine (8b):16 purification by preparative
TLC (ethyl acetate−hexane 5:5, Rf = 0.5); colorless prisms (5.4 mg,
60%); mp 41 °C (lit.16 mp 41−43 °C, recrystallized from
dichloromethane); IR (neat) 3315, 2920, 2850, 1496, 1452, 1261,
In conclusion, Red-Al was shown to be an excellent
deprotecting agent for N-triflylaziridines and N,N-dialkyltri-
flylamides.
EXPERIMENTAL SECTION
General Information. IR spectra were recorded on a FT-IR
instrument. Nuclear magnetic resonance (1H NMR and 13C NMR)
spectra were obtained on either a 300 or 400 MHz spectrometers.
■
1
737, 700 cm−1; H NMR (400 MHz, CDCl3) δ 7.37−7.20 (m, 5H),
3.27 (d, J = 6.6 Hz, 1H), 2.43 (dq, J = 6.6, 5.9 Hz, 1H), 0.92 (d, J = 5.9
Hz, 3H); ESIMS (positive) m/z 134 [(M + H)+].
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Note
cis-9-Azabicyclo[6.1.0]nonane (8c):17 purification by preparative
TLC (methanol−dichloromethane 5:5, Rf = 0.4); colorless oil (1.2 mg,
58%); IR (neat) 3259, 2925, 2854, 1469, 1371, 1186, 924, 849, 781,
739 cm−1; 1H NMR (400 MHz, CDCl3) δ 2.23−1.96 (m, 3H), 1.82−
1.53 (m, 4H), 1.53−1.34 (m, 4H), 1.24−1.09 (m, 2H); ESIMS
(positive) m/z 126 [(M + H)+]. For 8c, reactions were repeated three
times to obtain reasonable amount of product.
ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for Young
Scientists (B) (JSPS).
REFERENCES
■
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̈
1
3273, 3116, 2914, 2848, 1469, 1126, 901, 868, 717 cm−1; H NMR
(400 MHz, CDCl3) δ 2.58 (t, J = 7.4 Hz, 4H), 1.47 (quint, J = 7.4 Hz,
4H), 1.35−1.18 (m, 24H), 0.88 (t, J = 7.2 Hz, 6H); EIMS m/z
(relative intensity) 269 (3, M+), 156 (100), 142 (4), 126 (2), 112 (5),
98 (7), 84 (7), 70 (9).
Diheptylamine (12):7f colorless oil (12.3 mg, 99%); IR (neat) 3286,
2925, 2854, 1466, 1377, 1130, 725 cm−1; 1H NMR (400 MHz,
CDCl3) δ 2.60 (t, J = 7.1 Hz, 4H), 1.48 (quint, J = 7.1 Hz, 4H), 1.37−
1.17 (m, 16H), 0.88 (t, J = 6.7 Hz, 6H); EIMS m/z (relative intensity)
213 (5, M+), 142 (16), 128 (100), 98 (6), 84 (6), 70 (9), 57 (24).
N-Benzylcyclohexylamine (14):18 colorless oil (9.2 mg, 78%); IR
(neat) 3307, 2925, 2852, 1495, 1362, 1348, 1122, 1028, 733, 698
́
(3) (a) Dauban, P.; Saniere, L.; Tarrade, A.; Dodd, R. H. J. Am. Chem.
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1
cm−1; H NMR (400 MHz, CDCl3) δ 7.38−7.20 (m, 5H), 3.82 (s,
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H.; Shiro, M.; Hayashi, S.; Nakanishi, W. J. Am. Chem. Soc. 2007, 129,
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2H), 2.55−2.45 (m, 1H), 1.97−1.88 (m, 2H), 1.78−1.55 (m, 2H),
1.34−1.07 (m, 6H); EIMS m/z (relative intensity) 189 (21, M+), 146
(78), 91 (100), 65 (14).
N-Benzylcyclopropylamine (16):13b colorless oil (12.5 mg, 93%);
IR (neat) 3319, 3086, 3026, 2006, 2927, 2819, 1495, 1454, 1373, 1346,
1213, 1014, 733, 698 cm−1; 1H NMR (400 MHz, CDCl3) δ 7.35−7.20
(m, 5H), 3.83 (s, 2H), 2.19−2.11 (m, 1H), 0.48−0.41 (m, 2H), 0.41−
0.35 (m, 2H); MS m/z (relative intensity) 147 (M+, 7), 146 (22), 132
(16), 118 (7), 105 (6), 91 (100), 65 (17).
The authentic sample of triflylamide 17 was prepared from
corresponding amine by the reaction with trifluoromethanesulfonic
anhydride and triethylamine.15
(5) Hoque, M. M.; Miyamoto, K.; Tada, N.; Shiro, M.; Ochiai, M.
Org. Lett. 2011, 13, 5428.
(6) Ankner, T.; Hilmersson, G. Org. Lett. 2009, 11, 503.
N-(1-Phenylpropyl)triflylamide (17): colorless plates (2.4 mg, 25%,
entry 2); mp 62−63 °C (recrystallized from dichloromethane); IR
(KBr) 3286, 2972, 2879, 1441, 1371, 1356, 1230, 1186, 1151, 1028,
(7) (a) Hendrickson, J. B.; Bergeron, R. Tetrahedron Lett. 1973, 39,
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1
937, 704 cm−1; H NMR (400 MHz, CDCl3) δ 7.43−7.29 (m, 3H),
7.29−7.22 (m, 2H), 5.08 (br d, J = 7.7 Hz, 1H), 2.01−1.84 (m, 2H),
0.93 (t, J = 7.2 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 140.0 (q),
1
129.0 (t), 128.3 (t), 126.2 (t), 119.4 (q, JCF = 319.0 Hz), 61.5 (t),
́
́ ́
int, A.-M.; Bodor, A.; Gomory, A.; Vekey, A.;
̈ ̈
1997, 119, 22. (f) Bal
30.8 (d), 10.5 (s); HRMS (ESI, negative) m/z calcd for
Szabo, D.; Rabai, J. J. Fluorine Chem. 2005, 126, 1524.
́
́
C10H11F3NO2S [(M − H)−] 266.0463, found 266.0459.
(8) A preliminary study on the deprotection of cis-2,3-diphenyl-N-
triflylaziridine using Red-Al 3 has been described in ref 5.
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oil (8.1 mg, 95%); IR (neat) 3303, 2929, 2858, 1460, 1431, 1230,
1194, 1151, 1024, 617 cm−1; 1H NMR (400 MHz, CDCl3) δ 4.56 (br
d, J = 8.5 Hz, 1H), 3.72−3.58 (m, 1H), 1.53 (q, J = 7.4 Hz, 2H), 1.44−
1.17 (m, 15H), 0.89 (t, J = 7.4 Hz, 3H); 13C NMR (75 MHz, CDCl3)
δ 115.4 (q, 1JCF = 318.3 Hz), 52.8 (t), 37.8 (d), 31.9 (d), 29.4 (d), 29.2
(d, 2C), 25.6 (d), 22.7 (d), 22.2 (s), 14.1 (s); HRMS (ESI, negative)
m/z calcd for C11H21F3NO2S [(M − H)−] 288.1245, found 288.1236.
(11) Aziridines exhibit weaker basicity than alkylamines. The pKa
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ASSOCIATED CONTENT
■
S
* Supporting Information
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726. The partial formation of elemental sulfur was observed when the
hexane extract of the acidified aqueous layer was left at room
temperature under air for hours, which also supported the generation
of S2− anion during the course of the reaction.
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1H NMR spectra of 5, 6, 8a·TFA, 8b, 8c, 10, 12, 14, 16, 17, and
19 and 13C NMR spectra of 6, 8a, 17, and 19. This material is
available free of charge via Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kmiya@ph.tokushima-u.ac.jp. Tel: (+81)88-633-9532.
Fax: (+81)88-633-9504.
Notes
The authors declare no competing financial interest.
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Note
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