Indium(I) iodide promoted highly selective 1,2-addition of allyl and benzyl groups to α,β-unsaturated nitriles under sonication: A new synthesis of conjugated imines

Article abstract of DOI:10.1016/j.tetlet.2004.07.098

Allyl and benzyl bromides react with α,β-unsaturated nitriles in the presence of indium(I) iodide under sonication to produce the corresponding allylated and benzylated conjugated imines involving exclusive addition of the allyl/benzyl group to the C≡N moiety.

Full text of DOI:10.1016/j.tetlet.2004.07.098

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 6875–6877
Indium(I) iodide promoted highly selective 1,2-addition of allyl
and benzyl groups to a,b-unsaturated nitriles under sonication:
a new synthesis of conjugated imines
Brindaban C. Ranu* and Arijit Das
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India
Received 21 May 2004; revised 16 July 2004; accepted 22 July 2004
Available online 10 August 2004
Abstract—Allyl and benzyl bromides react with a,b-unsaturated nitriles in the presence of indium(I) iodide under sonication to
produce the corresponding allylated and benzylated conjugated imines involving exclusive addition of the allyl/benzyl group to
the C„N moiety.
ꢀ 2004 Elsevier Ltd. All rights reserved.
There has been increasing interest in the use of organo-
R
R3
InI
indium reagents in organic synthesis as they have made
significant contributions towards carbon–carbon bond
formation, novel rearrangements, and a variety of other
useful transformations over the past few years.1 The
addition of organoindium reagents to nitriles is very
interesting. Both allylindium sesquibromide and triallyl-
indium reagents react with a,b-unsaturated nitriles to
give 1,4-addition products, whereas similar reactions
with a,b-unsaturated carbonyl compounds lead to the
products of 1,2-addition.2a,b On the other hand, the
reaction of allylindium reagents with nitriles possessing
another electron withdrawing group at the a-position af-
fords the corresponding allylated enamines.2c As part of
our drive to explore new applications of organoindium
reagents,1e,3 we recently initiated an investigation on
the use of indium(I) iodide for useful chemical transfor-
mations.4 During this project we discovered another
novel application of InI, which demonstrated that allyl
and benzyl bromides react with a,b-unsaturated nitriles
in the presence of indium(I) iodide to produce the corre-
sponding allylated and benzylated conjugated imines
involving 1,2-addition exclusively (Scheme 1).
R1
R2
R3
CH2Cl2
RBr
NH
+
Sonication
R1 R2
CN
R= allyl, benzyl; R1, R2, R3 = H or alkyl
Scheme 1.
of substituted a,b-unsaturated nitriles toprovide the
corresponding imines through exclusive 1,2-addition.5
The results are presented in Table 1. Substituted allyl
bromides such as crotyl and cinnamyl bromides (entries
4–6) also participate in the reaction without any diffi-
culty producing only the products of a-addition. The
products were unequivocally identified as imines by their
characteristic IR absorptions (ꢀ3300 and 1650cmÀ1 due
to @NAH and C@N, respectively) and 1H NMR (d 6.0–
6.5 for NH) and 13C NMR (d 166–168 due tothe qua-
ternary C of C@N) spectral data. The mass spectromet-
ric data (LC–MS) of a couple of representative
compounds agree with the assigned structures. These
spectral data are also consistent with the values reported
for imines.6 Interestingly, the H and 13C NMR spectra
1
The experimental procedure is very simple.5 Several allyl
and benzyl bromides underwent reactions with a variety
of these products did not indicate the presence of any
tautomeric enamines.
The reactions are, in general, very clean. Without soni-
cation the reaction did not proceed at room temperature
under stirring. Although heating the reaction mixtures
under reflux leads to conversion, a longer period of time
is required than for reaction under sonication.
Keywords: Indium(I) iodide; Conjugated nitrile; 1,2-Addition; Imine;
Sonication.
*
Corresponding author. Tel.: +91 33 24734971; fax: +91 33
24732805; e-mail: ocbcr@iacs.res.in
0040-4039/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.07.098

6876
B. C. Ranu, A. Das / Tetrahedron Letters 45 (2004) 6875–6877
potential of indium(I) iodide8 and presents great prom-
ise for more useful applications.
Table 1. Synthesis of conjugated imines
Entry
R
R1 R2 R3
Time (h) Yield (%)a
1
2
H
H
H
CH3 8.20
73
74
Ph
H
H
11.30
10
Acknowledgements
3
4
73
We are pleased toacknowledge the financial support
from CSIR, New Delhi [Grant No. 01(1739)/02] for this
investigation. A.D. is also thankful to CSIR for his
fellowship.
H
H
11.10
12
71b
5
6
H
H
H
H
70b
71b
Ph
Ph
CH3 12.20
7
8
9
PhCH2
PhCH2
PhCH2
H
H
H
H
H
8
CH3 8.30
84
82
73
H
Ph
References and notes
H
11.10
1. (a) Cintas, P. Synlett 1995, 1087; (b) Li, C.-J. Tetrahedron
1996, 52, 5643; (c) Li, C.-J.; Chan, T. H. Tetrahedron 1999,
55, 11149; (d) Chauhan, K. K.; Frost, C. G. J. Chem. Soc.,
Perkin Trans. 1 2000, 3015; (e) Ranu, B. C. Eur. J. Org.
Chem. 2000, 2347; (f) Podlech, J.; Maier, T. C. Synthesis
2003, 633.
2. (a) Wang, L.; Sun, X.; Zhang, Y. Synth. Commun. 1998, 28,
3263; (b) Araki, S.; Horie, T.; Kato, M.; Hirashita, T.;
Yamamura, H.; Kawai, M. Tetrahedron Lett. 1999, 40,
2331; (c) Fujiwara, N.; Yamamoto, Y. Tetrahedron Lett.
1998, 39, 4729; (d) Fleming, F. F.; Wang, Q. Chem. Rev.
2003, 103, 2035.
3. (a) Ranu, B. C.; Hajra, A.; Jana, U. J. Org. Chem. 2000, 65,
6270; (b) Ranu, B. C.; Hajra, A.; Jana, U. Tetrahedron Lett.
2000, 41, 531; (c) Ranu, B. C.; Samanta, S.; Hajra, A.
Synlett 2002, 987; (d) Ranu, B. C.; Das, A.; Samanta, S.
Synlett 2002, 727; (e) Ranu, B. C.; Dey, S. S.; Hajra, A.
Tetrahedron 2002, 58, 2529; (f) Ranu, B. C.; Hajra, A.; Dey,
S. S.; Jana, U. Tetrahedron 2003, 59, 813; (g) Ranu, B. C.;
Samanta, S. J. Org. Chem. 2003, 68, 7130; (h) Ranu, B. C.;
Das, A.; Hajra, A. Synthesis 2003, 1012; (i) Ranu, B. C.;
Samanta, S. Tetrahedron 2003, 59, 7901.
10
11
PhCH2
H
10
80
80
PhCHCH3
H
11
a Yields refer to those of pure isolated products characterized by
spectroscopic data (IR, 1H and 13C NMR).
b Only the a-addition product was obtained.
Usually, the reaction proceeds efficiently with 20mol%
of InI, however, with a smaller amount (5–10mol%)
the reaction takes a relatively long period of time to
go to completion. Although other solvents such as
THF, acetonitrile may be used, the best results in terms
of yield were obtained using methylene chloride.
The exact mechanism for this 1,2-addition is not clearly
understood at this moment, however, it may be specu-
lated that the reaction goes via the following path
(Scheme 2).
4. (a) Ranu, B. C.; Mandal, T.; Samanta, S. Org. Lett. 2003, 5,
1439; (b) Ranu, B. C.; Mandal, T. Synlett 2004, 1239.
5. Representative experimental procedure (entry 7): A mixture
of acrylonitrile (53mg, 1mmol) and benzyl bromide
(171mg, 1mmol) in dry methylene chloride (3mL) was
sonicated in an ultrasonic bath (Eyela, Japan) in the
presence of indium(I) iodide (50mg, 20mol%) for 8h
(TLC). The reaction mixture was quenched with a few
drops of H2O and extracted with diethyl ether (3 · 10mL).
The organic extract was washed with brine, dried (Na2SO4)
and evaporated to leave the crude product, which was
purified by column chromatography over silica gel (hexane/
ether 3:1) togive the pure product (122mg, 84%) as a white
solid, mp 51ꢁC; IR (KBr) 3286, 3066, 1652, 1539,
In conclusion, the present procedure using indium(I)
iodide provides an easy access to allylated and benzyl-
ated conjugated imines, which are of synthetic utility
in organic reactions.7 Moreover, the exclusive 1,2-addi-
tion of allyl and benzyl groups to a,b-unsaturated
nitriles promoted by InI is of significance in the context
of the typical 1,4-addition promoted by triallylindium,2b
allylindium sesquibromide2a and other organometallic
reagents.2d Nevertheless, this reaction demonstrates the
1
1242cmÀ1; H NMR d 4.42 (AB q, J1 = 21.8, J2 = 5.8Hz,
Br
2H), 5.58 (dd, J1 = 2.0, J2 = 9.8Hz, 1H), 6.09–6.28 (m, 2H),
6.64 (br s, NH), 7.22–7.31 (m, 5H); 13C NMR d 43.8, 127.0,
127.8, 128.1 (2C), 129.1 (2C), 131.1, 138.5, 166.1; LC–MS:
molecular ion 145.020. Anal. Calcd for C10H11N: C, 82.71;
H, 7.63. Found: C, 82.55; H, 7.71. This procedure is
followed for all the reactions listed in Table 1. All the
compounds were satisfactorily characterized from their
spectroscopic data (IR, 1H NMR and 13C NMR) and
elemental analysis.
InI
+
RBr
R
In
I
..
R1
R2
N
C
R3
R
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Tetrahedron Lett. 1971, 12, 4897.
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326; (b) Serckx-Poncin, B.; Hezbain-Frisque, A. M.;
Ghosez, L. Tetrahedron Lett. 1982, 23, 3261; (c) Boger,
In
N
I
R
R3
R1
R1
R2
InI
C
+
N
R2
R3
Scheme 2.

B. C. Ranu, A. Das / Tetrahedron Letters 45 (2004) 6875–6877
6877
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