A novel decyanogenative coupling of α-cyanoimines mediated by samarium. A facile route to α-diketimines

Article abstract of DOI:10.1246/cl.2004.102

Conversion of α-cyanoimines 1 into α-diketimines 2 has been achieved successfully by using samarium diiodide in dry tetrahydrofuran in high yields without formation of anilinoanil 3, 1,2-diamines 4 or anilides 5.

Full text of DOI:10.1246/cl.2004.102

102
Chemistry Letters Vol.33, No.2 (2004)
A Novel Decyanogenative Coupling of ꢀ-Cyanoimines Mediated by Samarium.
A Facile Route to ꢀ-Diketimines
Ashim J. Thakur, Dipak Prajapatiꢀ, and Jagir S. Sandhuy
Department of Medicinal Chemistry, Regional Research Laboratory, JORHAT- 785 006, Assam, India
yDepartment of Chemistry, Panjabi University, Patiala-147 002, Punjab
(Received October 20, 2003; CL-030991)
Conversion of ꢀ-cyanoimines 1 into ꢀ-diketimines 2 has
4 or anilides 5.
In a typical procedure, to a solution of ꢀ-cyanoimine 1a
been achieved successfully by using samarium diiodide in dry
tetrahydrofuran in high yields without formation of anilinoanil
3, 1,2-diamines 4 or anilides 5.
(0.412 g, 2 mmol) in dry THF (15 mL), was added two equiva-
lent of freshly prepared15 SmI2 (2 mmol) and the mixture was re-
fluxed for 18 h. After completion (monitored by tlc), THF was
distilled off and diethyl ether (20 mL) was added to it. The or-
ganic layer was then washed with sodium thiosulphate solution
and dried over anhydrous Na2SO4. Evaporation of the solvent
on a rotary evaporator gives the ꢀ-diketimine 2a in 80% yield,
without formation of any side products as reported by earlier
workers.11,12 The structure of the diketimine thus obtained was
unambiguously identified on the basis of its physicochemical da-
ta (IR, NMR, and MS) and finally by preparing 1,2-diketones by
the acid hydrolysis of the corresponding ꢀ-diketimines.16 2a mp
143–144 ꢁC, 1H NMR ꢂ 7.10–7.35 (m, 20H, aromatics). Mþ
360; analysis C26H20N2 calcd. C, 86.67; H, 5.55; N, 7.78, found
C, 86.72; H, 5.60; N, 7.66. Similarly ꢀ-cyanoimines 1b–k were
reacted and the corresponding ꢀ-diketimines 2b–k were ob-
tained in good yields. The results and scope of this reaction
are shown in the Table 1. Enhancing further the reaction time
gave no improvement in yield but rather decomposition of start-
ing material occured. An inspection of Table 1 shows that this
method is fairly general and the substituents in Ar1 and Ar2
can vary considerably. The ꢀ-cyanoimines derived from benzal-
dehyde and other aromatic amines (besides aniline) such as 4-
chloroaniline, 2 and 4-toluidine, 3 and 4-anisidines, smoothly
underwent this reaction to afford the corresponding dianils in
good yields. The reaction, however, fails with benzylidine cyclo-
hexylamine, p-hydroxybenzylidineaniline and p-nitrobenzylid-
ineanilines. It is notable that the cyanide ion catalyzed coupling
Recently, samarium metal, its salts and organosamarium
compounds have been widely employed as useful reagents or
catalysts in organic synthesis.1 Since the pioneering studies by
Kagan2 and co-workers demonstrated the particular effective-
ness of SmI2, which is a powerful single-electron transfer reduc-
tant in the lanthanide series, the utilisation of SmI2 in synthetic
organic chemistry has been dramatically documented.3 The ver-
satility of SmI2 is emphasized by the fact that it can be used to
generate both C-centered radicals and carbanions. It possesses
excellent chemoselectivity in the reduction of carbonyl, alkyl
halide, and ꢀ-heterosubstituted carbonyl substrates, promotes
Barbier type coupling reactions, hetero-alkene coupling reac-
tions, carbon–carbon fragmentation/ring expansion chemistry
of some aliphatic 1,4-diketones,4 radical cyclizations,5 deoxyge-
native coupling of benzamides,6 and mediates cascade radical
cyclizations7 in the synthesis of peoxylactone ꢁ etc. But samari-
um has not been employed for the dimerization of ꢀ-cyano-
imines so far though it is used for the coupling of aldimines8
to 1,2-diamines. In continuation to our studies on metal mediated
organic transformations,9 we report herein a novel one-pot pro-
cedure for the decyanogenative coupling of ꢀ-cyanoimines to
form ꢀ-diketimines using samarium diiodide as a mediator.
However, to our knowledge there are no literature reports on
the coupling of ꢀ-cyanoimines into ꢀ-diketimines and this is
the first ever report of this kind. Here the reaction proceeds effi-
ciently in high yields at the reflux temperature of tetrahydrofur-
an. These ꢀ-diketimines are useful key intermediates for the
preparation of 1,2-diketones by the acid hydrolysis.
Table 1. SmI2 catalyzed dimerisation of ꢀ-cyanoimines 1 into
ꢀ-diketimines 2
Product
2
Ar1
Ar2
Time M.p.
/h
/ꢁC
18.0 143–145 144–4520
19.0 149–151 150–15120 72
15.0 156–157 155–15610 74
Lit.M.p. Yielda
Walia10 and Becker11 have reported a alkalicyanide mediat-
ed oxidative dimerization of aromatic aldimines to ꢀ-diketi-
mines, but it involves an aerial oxidation of dianilinostilbene
or anilinoanil formed during the reaction course. In another re-
port12 benzil dianil has been obtained in 16% yield by heating
aniline and benzil in the presence of P2O5 at 200 ꢁC. Conse-
quently, there is a need for the development of more selective
method of general applicability. In a recent report Fujiwara et
al.13 have reported the dehydrogenative coupling of aldimines
to vicinal diimines mediated by ytterbium. Though, several re-
ports on the coupling of imines to 1,2-diamines are available14
but the dehydrogenative coupling of imines to ꢀ-diimines are
very rare. Our novel finding couples with ease a wide variety
of ꢀ-cyanoimines directly to form the corresponding ꢀ-diketi-
mines without formation of any anilinoanil 3 or dianilinostilbene
/ꢁC
/%
80
a
b
c
d
e
f
g
h
i
Ph
Ph
Ph
Ph
p-Anisyl
p-Chloro
Ph
m-Chloro
p-Tolyl
Ph
p-Anisyl 17.5 160–162 162–16310 80
Ph
Ph
15.5 215–216 216–21710 72
18.5 147–149 147–14820 70
p-Tolyl 19.0 157–158 157–15810 70
m-Anisyl 16.0 134–135 134–13510 75
p-Chloro 19.0 170–172 172–17320 70
Ph
Ph
j
k
o-Tolyl
Ph
Ph
o-Tolyl 21.0 145–146 145–14721 55
21.0 180–181 181–18221 50
aYields refer to pure isolated products, fully characterised by 1H
NMR and IR spectroscopy.
Copyright Ó 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.2 (2004)
103
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Ar1 C=N Ar2
Ar1 C=N Ar2
Ar1 C=N Ar2
SmI2
4
5
Ar1 C=N Ar2
CN
Ar1 C
H
NHAr2
THF
1
2
3
Ar1 C NHAr2
Ar1CONHAr2
Ar1 C NHAr2
5
6
7
8
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4
Scheme 1.
of benzylidine-p-toluidine in liquid ammonia gives a colorless
crystalline dimer anilinoanil.17 In contrast, with SmI2 in THF
we obtained directly the corresponding dianil 2g instead of ani-
linoanil. Furthermore, the reaction conditions are tolerant to the
ether group (Entries D and H) and aromatic chloro group, which
showed selectivity to give the ꢀ-diketimines without any de-
chlorination (Entry I). Although the detailed mechanism is not
clear at this stage, it is likely that the reaction starts by an elec-
tron transfer from samarium diiodide18 to ꢀ-cyanoimine 1. As
cyanide is a good leaving group, first the reagent SmI2 generates
the radical by one electron reduction which then forms the orga-
nosamarium species followed by coupling with second equiva-
lent of ꢀ-cyanoimines produced the ꢀ-diketimine 2 via a sama-
rium diketimine complex intermediate.
In conclusion, we have provided a novel and efficient meth-
od for the coupling of ꢀ-cyanoimines into ꢀ-diketimines em-
ploying samarium diiodide in dry THF which involves a simple
workup and will make a useful and important addition in the ex-
isting methodologies. The ꢀ-diketimines thus obtained can be
easily hydrolysed in acidic media16 to the corresponding 1,2-
diketones in excellent yields. Of particular interest is that this
procedure also provides a novel and convenient method for the
synthesis of 1,2-dicarbonyl functionality, which has recently at-
tracted19 a growing attention.
9
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We thank the Department of Science and Technology
(DST), New Delhi for financial support. One of us (AJT) thanks
CSIR, New Delhi for the award of a Senior Research Fellowship.
We also thank Director, Regional Research Laboratory, Jorhat
for his keen interest and constant encouragement.
15 As the commercially available samarium diiodide is expen-
sive and is also hygroscopic and air-sensitive, we freshly pre-
pared this reagent in an inert atmosphere under very stringent
conditions and used directly. For a typical case, iodine (5.1 g,
20 mmol) was added with stirring to a mixture of Sm powder
(3.1 g, 22 mmol) in dry THF. The initial mild exothermic re-
action subsided within several minutes to form a yellow sus-
pension of SmI3. The mixture was then refluxed with stir-
ring. The colour of the suspension gradually turns to blue.
Refluxing overnight gives 0.1 M/L of SmI2.
References and Notes
1
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filtered and the melting point was compared with authentic
samples for all the compounds.
2
3
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Published on the web (Advance View) December 27, 2003; DOI 10.1246/cl.2004.102