Bi(OTf)3- and Bi(TFA)3-catalyzed ring opening of epoxides with anilines under microwave irradiation

Article abstract of DOI:10.1246/cl.2004.304

A facile and environmentally friendly methodology for ring opening of epoxides with anilines has been developed in the presence of catalytic amounts of bismuth (III) triflate or bismuth (III) trifluoroacetate using microwave-assisted heating. The ease of this procedure allowed preparation of the β-aminoalcohols quickly and efficiently.

Full text of DOI:10.1246/cl.2004.304

304
Chemistry Letters Vol.33, No.3 (2004)
Bi(OTf)3- and Bi(TFA)3-Catalyzed Ring Opening of Epoxides
with Anilines under Microwave Irradiation
Ahmad R. Khosropour,ꢀ Mohammad M. Khodaei,ꢀ and Kazem Ghozati
Department of Chemistry, Faculty of Science, Razi University, Kermanshah 67149, Iran
(Received November 25, 2003; CL-031140)
A facile and environmentally friendly methodology for ring
Table 1. Bi(OTf)3 or Bi(TFA)3-catalyzed chemoselective
opening of epoxides with anilines under microwave irradiation16
opening of epoxides with anilines has been developed in the
presence of catalytic amounts of bismuth (III) triflate or bismuth
(III) trifluoroacetate using microwave-assisted heating. The ease
of this procedure allowed preparation of the ꢀ-aminoalcohols
quickly and efficiently.
Yield/%b / Time/s
Entry Productsa
Bi(OTf)3 Bi(TFA)3
NH
1
2
3
82/105
80/105
90/140
80/90e
78/90e
85/105
ꢀ-Aminoalcohols as versatile building blocks both in organ-
ic and pharmaceutical sciences,1–3 have attracted attention of
synthetic chemists. The most important synthetic routes to such
compounds are the ring opening of epoxides with amines by acid
catalyzed conditions.4–11 However, these methods often involve
the use of expensive reagents, extended reaction times, drastic
reaction conditions, unsatisfactory yields especially with ali-
phatic epoxides and entailed undesirable side reactions due to
polymerization or rearrengement of oxiranes. Acid catalyzed
ring opening of epoxides with anilines is limited and requires
the careful control of acidity to prevent side reactions. Therefore,
the introduction of a new and efficient method for this transfor-
mation is still in demand.
The applications of Bi(OTf)3 and Bi(TFA)3 as environmen-
tally friendly reagents for organic synthesis have been extensive-
ly investigated.12 This Lewis acid is easily available, inexpen-
sive, easy to handle and relatively insensitive to air and small
amounts of moisture.13 Also, we have reported that Bi(OTf)3
or Bi(TFA)3 are extraordinarily efficient catalysts for conversion
of epoxides to thiiranes and 1,3-dioxolanes.14
OH
NH
CH3
OH
NH
Br
OH
PhOCH2CH(OH)CH
2NH
4
5
6
92/80
86/90
90/85
82/150
78/135e
87/120f
PhOCH2CH(OH)CH
CH3
Br
2NH
PhOCH2CH(OH)CH
2NH
7
8
89/170
82/165f
PhOCH2CH(OH)CH2 NH
NO2
96/20c
93/25c
90/20d
92/20d
HOCH2CHNH
Ph
Microwave activation as a non-conventional energy source
has become an important method that can be used to carry out
a wide range of reactions within short reaction times and in high
yield and regioselectivity.15 There are no precedents of aminol-
ysis of epoxides with anilines under microwave irradiation, and
so we attempted this reaction in the presence of bismuth salts
such as Bi(OTf)3 and Bi(TFA)3. Combination of bismuth salts
and microwave irradiation served well in this reaction as well
(Scheme 1). When an acetonitrile solution of epoxide and aniline
was irradiated with microwave in the presence of a catalytic
amount of the bismuth salts, ring-opening proceeded smoothly
to afford the desired ꢀ-aminoalcohols in good to excellent yields
(Table 1).
HOCH2CHNH
Ph
CH3
9
HOCH2CHNH
Ph
Br
10
88/25c
90/40d
CH2=CHCH2OCH2CH(OH)CH2NH
11
12
80/150
79/140
79/150
75/165
NH
CH3(CH2)5CH(OH)CH2
CH3(CH2)2CH2OCH2CH(OH)CH2NH
ClCH2CH(OH)CH 2 NH
13
14
82/105
85/110
80/195
84/195
Bi(OTf)3 or Bi(TFA)3
MW/CH3CN
O
NHCH2CHR1
OH
NHCHCH2OH
R1
R2
or
R2
R2
NH2
aAll products were characterised by 1H NMR, 13C NMR, and IR.
bIsolated and unoptimized yields. c1 mol %. d3 mol %. e4 mol %.
f6 mol %.
+
R1
Scheme 1.
A variety of epoxides were converted to the ꢀ-aminoalco-
hols efficiently and chemoselectivity irrespective of functional
groups like PhO-, RO-, ClCH2-, and allyl ether. The presence
of electron-donating and -withdrawing groups on aniline ring
had no effect on this reaction (Entries 1–3, 4–7, and 8–10). When
cyclohexene oxide was treated with aniline in the presence of
Bi(OTf)3 and Bi(TFA)3 under microwave irradiation, trans-2-
(anilino)cyclohexan-1-ol was regioselectively obtained in 82%
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.3 (2004)
305
and 89% yields, respectively (Entries 1 and 2). This reaction pro-
ceeded indeed rapidly to finish within 105 second of irradiation.
The reactions of styrene oxide with aniline gave rise to the for-
mation of ꢀ-aminoalcohol derived from ꢁ-attack of aniline
(Entries 8-10). These high reactivity and selectivity are similar
to those observed in the case of strong Lewis acids.10a,b,11
Very recently, Cepanec has reported that the epoxide-open-
ing reaction with nitroaniline gave unsatisfactory result in spite
of the prolonged reaction time.5 In sharp contrast to this, our pro-
tocol promoted by microwave irradiation furnished the desired
ꢀ-aminoalcohol in a good yield even in the short reaction time
(<175 s) (Entry 7).
In conclusion, the present microwave-assisted procedure
provides an efficient and very simple methodology for the prep-
aration of 2-anilinoalkanols using Bi(OTf)3 or Bi(TFA)3 as very
cheap, low toxic, and oxygen and moisture tolerant catalysts in
very short reaction times. In addition, high selectivity described
in this report is another merit of this method.
Synthesis, 2003, 1387. b) I. Mohammadpoor-Baltork and A.
R. Khosropour, Monatsh. Chem., 133, 189 (2002). c) S.
Repichet, A. Zwick, L. Vendier, C. Le Roux, and J. Dubac,
Tetrahedron Lett., 43, 993 (2002). d) M. N. Leonard, L. C.
Wieland, and R. S. Mohan, Tetrahedron, 58, 8373 (2002).
13 I. Mohammadpoor-Baltork, A. R. Khosropour, H. Aliyan, J.
Chem. Res., Synop., 7, 780 (2001). and cited therein.
14 a) I. Mohammadpoor-Baltork and A. R. Khosropour,
Molecules, 6, 996 (2001). b) I. Mohammadpoor-Baltork
and A. R. Khosropour, Synth. Commun., 22, 3411 (2001).
15 R. S. Pottorf, N. K. Chadha, M. Katkevics, V. Ozola, E.
Suna, H. Ghane,T. Regberg, and M. R. Player, Tetrahedron
Lett., 44, 175 (2003).
16 Experimental Procedure: To a solution of epoxides (1 mmol)
and anilines (1 mmol) in CH3CN (2 mmol) was added Bi(O-
Tf)3 (0.02 mmol) or Bi(TFA)3 (0.05 mmol). The mixture was
irradiated with microwave oven for the appropriate time ac-
cording to Table. After completion of the reaction, as indi-
cated by TLC or GLC, the solvent was evaporated and wash-
ed with 0.5N HCl (25 mL). The mixture was extracted with
CH2Cl2 (3 ꢁ 10 mL) and the organic layer was dried with
Na2SO4. The solvent was evaporated and the crude product
purified by plate or column chromatography on a silica gel to
afford the pure ꢀ-aminoalcohols in 75–96% yields. Spectro-
scopic data for compounds: 2: trans-2-(4-Methylanilino)cy-
clohexan-1-ol: Viscous liquid, IR (KBr, cmꢂ1) 3618–3104
All of the products refer to pure 2-anilinoalkanols. The ap-
paratus used for these reactions was a Samsung domestic micro-
wave oven (900 W) without any modification.
We are thankful to the Razi University Research Council for
partial support of this work.
References and Notes
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32.2, 35.5, 51.0, 70.7, 114.0, 118.7, 129.7, 151.6, Analysis
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´
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Published on the web (Advance View) February 14, 2004; DOI 10.1246/cl.2004.304