A novel uncatalyzed insertion reaction of in situ formed trichlorosilyl cyanide with imines: A facile silicon mediated synthesis of α-aminonitriles

Article abstract of DOI:10.1081/SCC-120016363

The tetrachlorosilane-potassium cyanide reagent combination, (trichloro-silyl cyanide-in situ) is a new, safe and versatile hydrogen cyanide substitute. The reaction of this reagent with saturated aldimines 1a-j and benzophenone oxime 3, in absences of any catalyst, affords the corresponding α-aminonitriles 2a-j and 4, respectively, in excellent yield. Under the same condition the bisimine 5 yields the imidazo[1,2-a]quinoxaline derivative 6. Moreover, trichlorosilyl cyanide-in situ undergoes regiospecific addition onto α, β-unsaturated aldimines 7a-e to give the corresponding 1,2-addition products 8a-e, in high yield, under exceedingly mild conditions.

Full text of DOI:10.1081/SCC-120016363

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A Novel Uncatalyzed Insertion Reaction of In Situ
Formed Trichlorosilyl Cyanide with Imines: A Facile
Silicon Mediated Synthesis of α-Aminonitriles
Abdel-Aziz S. El-Ahl a
a Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt
Published online: 17 Aug 2006.
To cite this article: Abdel-Aziz S. El-Ahl (2003) A Novel Uncatalyzed Insertion Reaction of In Situ Formed Trichlorosilyl Cyanide
with Imines: A Facile Silicon Mediated Synthesis of α-Aminonitriles, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 33:6, 989-998, DOI: 10.1081/SCC-120016363
To link to this article: http://dx.doi.org/10.1081/SCC-120016363
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 6, pp. 989–998, 2003
A Novel Uncatalyzed Insertion Reaction
of In Situ Formed Trichlorosilyl Cyanide with
Imines: A Facile Silicon Mediated Synthesis
of a-Aminonitriles
*
Abdel-Aziz S. El-Ahl
Chemistry Department, Faculty of Science, Mansoura
University, Mansoura, Egypt
ABSTRACT
The tetrachlorosilane-potassium cyanide reagent combination,
(trichloro-silyl cyanide-in situ) is a new, safe and versatile hydrogen
cyanide substitute. The reaction of this reagent with saturated
aldimines 1a–j and benzophenone oxime 3, in absences of any
catalyst, affords the corresponding a-aminonitriles 2a–j and 4,
respectively, in excellent yield. Under the same condition the bis-
imine 5 yields the imidazo[1,2-a]quinoxaline derivative 6. More-
over, trichlorosilyl cyanide-in situ undergoes regiospecific addition
onto a, b-unsaturated aldimines 7a–e to give the corresponding
*Correspondence: Abdel-Aziz S. El-Ahl, Teachers College Mecca, Azzizia,
Mecca, Kingdom of Saudi Arabia; E-mail: elahl2002@yahoo.com.
989
DOI: 10.1081/SCC-120016363
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990
El-Ahl
1,2-addition products 8a–e, in high yield, under exceedingly mild
conditions.
Key Words: Tetrachlorosilane; Aldimines; Trichlorosilylcyanide;
Aminonitriles; Synthesis.
The synthetic exploitation of silicon reagents derived from tetra-
chlorosilane is considered to be an important undeveloped field in view
of the great variety of potential silicon reagents. The in situ generation of
safe and easily accessible silicon reagents, derived from tetrachlorosilane
and different soft nucleophiles, attracted our attention in the last few
years. Recently, we reported the in situ generation of triazidochlorosilane
from the reaction of tetrachlorosilane with sodium azide in dry acetoni-
trile. Triazidochlorosilane showed an enhanced reactivity in comparison
with other silyl azides.[1–4] This persuaded us to investigate in situ
generation of other silicon reagents via interaction of tetrachlorosilane
with different stoichiometeric amounts of various nucleophiles. Silyl
cyanides are good reagents for introducing cyano-functionality, protect-
ing and activating carbonyl compounds in organic synthesis.[5–8] The
reaction of various silyl cyanides with different functional groups
required the presence of either Lewis acids,[9–12] Lewis bases[13] or anionic
catalysts.[14–15] Besides, the preparation of various silyl cyanides takes
place under severe conditions; for example, dicyano-dimethylsilane was
prepared by reaction of dichlorodimethylsilane with silver cyanide at
120–140ꢀC for 5–7 days.[16] Due to their hydrolytic susceptibility, sensi-
tivity to air and decomposition on prolonged storage, most silicon
reagents should be freshly distilled or freshly prepared. Therefore, the
in situ preparation and evaluation of the synthetic potentialities of
simple, cheap and new silicon reagents is of great value. Thus, we
introduce here the trichlorosilyl cyanide (TCSC in situ) as a stoichio-
metric reagent combination, prepared in situ from equimolar amounts
of tetrachlorosilane and potassium cyanide in dry acetonitrile, and its
reaction with different aldimines.
In order to explore the synthetic potentiality of TCSC in situ we
decided to examine the cyanosilylation of imines as preliminary study,
since the products are important intermediates for synthesis of a-amio
acids. Although, it was reported that the in situ formed trimethylsilyl
cyanide failed to react with imines,[17] TCSC in situ reacts with N-
phenyl-N-(phenylmethylidene)amine 1a to give 2-anilino-2-phenylaceto-
nitrile 2a, in absence of any catalyst under exceedingly mild conditions, in
excellent yield (Sch. 1). To demonstrate functional group compatibility

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Trichlorosilyl Cyanide
991
Scheme 1.
for this process and to define the scope of the method, the reaction of
several functionalized aldimines has been investigated. In most cases a-
aminonitriles 2a–j (Table 1) are isolated in pure form after aqueous work
up. Also, in the number of systems studied the addition process has been
devoid of side reactions such as imine cleavage (Sch. 4). In addition, by
this method Lewis acids, Lewis bases and various anionic catalysts, which
could have a deleterious effect on several functional groups, may be
avoided.
The structures of a-aminonitriles were based on spectral and elemen-
tal analyses. The IR spectrum of 2a showed a characteristic weak absorp-
tion band at 2235 cmꢁ1 for cyano-group and a strong band at 3337 cmꢁ1
for NH group. The 1H NMR of 2a showed two doublets, one proton each
at ꢀ 4.1 and ꢀ 5.42 for the NH (D2O exchangeable) and CH–protons
respectively. The other a-aminonitriles have spectral data compatible
with their structures (Table 2). The cyanosilylation of the sterically hin-
derded bezophenone oxime 3 was, also, investigated. The reaction
afforded the corresponding a-aminonitrile 4, in high yield, in the presence
of one mole excess of tetrachlorosilane. Therefore, it seems that the reac-
tion proceeds via the O-silylated intermediate A, followed by addition of
trichlorosilyl cyanide to the carbon-nitrogen double bond (Sch. 1).
Compound 4 does not exhibit a nitrile absorption in the IR. However,
this is expected, since nitriles that have an electron-withdrawing group a
to the nitrile group often have very weak or undetectable absorption in
the 2000–2400 cmꢁ1 region.[18]
The reaction of the bis-imine 5 with two equivalents of TCSC pro-
duced the imidazo[1,2-a]quinoxaline 6 in good yield. The structure of 6
was based on analytical and spectral data. The IR spectrum of 6 lacks
any absorption bands for carbonitrile groups, instead it showed two
absorption bands due to symmetric and asymmetric stretching frequen-
cies of primary amine at3368 and 3290 cm ꢁ1. The 1H NMR spectrum of
6 displayed a broad, two-proton singlet (D2O exchangeable) at ꢀ 5.5,

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992
El-Ahl
Table 1. Characterization and IR (cmÀ1) data of compounds 2a–j, 4, 6, 8a–e and
10a,b.
ꢀ
M.p. C
Compd. no. R
Ar
Ph
[lit. M.p.] Yield (%) IR cmꢁ1
2a
Ph
85–87
[85–86][24]
99–100a
116
87
2235, 3337
2b
2c
4-BrC6H4
4-ClC6H4
Ph
Ph
92
89
2225, 3348
2246, 3301
[112–13][24]
130b
2d
2e
2f
4-BrC6H4
4-MeOC6H4 Ph
4-MeC6H4 Ph
4-BrC6H4
89
96
84
87
2241, 3359
2225, 3329
2240, 3308
2243, 3320
108c
94–95d
123–25
[121–22][25]
2g
Ph
4-ClC6H4
2h
2i
Ph
4-MeOC6H4 73–75
85
89
2233, 3337
2245, 3348
[74–76][24]
84–85
4-ClC6H4
4-MeC6H4
[81–82][25]
124–26e
162–64f
155–56g
128–29h
137–38i
142–44j
108–10k
116–17l
54–56
2j
4
2-HOC6H4 Ph
76
93
72
94
96
93
92
94
95
2240, 3312, 3364
3343, 3280, 3050
3368, 3290, 3191
2255, 3345
2230, 3350
2250, 3375
6
8a
8b
8c
8d
8e
10a
H
Cl
Br
Me
MeO
2228, 3360
2232, 3346
4-BrC6H4
4-ClC6H4
[55–58][26]
45–46
10b
92
[47–50][26]
aAnal. calcd. for C14H11BrN2: C, 58.56; H, 3.86. Found: C, 58.92; H, 3.47.
bAnal. calcd. for C14H10Br2N2: C, 45.94; H, 2.75. Found: C, 45.56; H, 3.01.
cAnal. calcd. for C15H14N2O: C, 75.61; H, 5.92. Found: C, 75.14; H, 6.39.
dAnal. calcd. for C15H14N2: C, 81.05; H, 6.35. Found: C, 80.79; H, 6.83.
eAnal. calcd. for C14H12N2O: C, 74.98; H, 5.39. Found: C, 75.32; H, 5.68.
fAnal. calcd. for C14H12N2O: C, 74.98; H, 5.39. Found: C, 74.45; H, 5.53.
gAnal. calcd. for C22H16Cl2N4: C, 64.88; H, 3.96. Found: C, 65.28; H, 4.25.
hAnal. calcd. for C16H14N2: C, 82.02; H, 6.02. Found: C, 81.59; H, 6.43.
iAnal. calcd. for C16H13ClN2: C, 71.51; H, 4.88. Found: C, 71.27; H, 5.25.
jAnal. calcd. for C16H13BrN2: C, 61.36; H, 4.18. Found: C, 61.69; H, 3.93.
kAnal. calcd. for C17H16N2: C, 82.23; H, 6.49. Found: C, 82.68; H, 6.05.
lAnal. calcd. for C17H16N2O: C, 77.25; H, 6.10. Found: C, 76.87; H, 6.52.

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Trichlorosilyl Cyanide
993

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994
El-Ahl
attributable to primary amino-group. The 13C NMR spectrum of 6
showed two characteristic peaks at ꢀ 138.9 and 151.9 assignable for C-
3a and C-2 respectively.[19] The plausible mechanism for formation of 6 is
shown in (Sch. 2). It seems that the reaction proceeds via the formation of
the double addition intermediate A, followed by cyclization and
tautomerisation to give intermediate C. Further cyclization of C affords
intermediate D, which on tautomerisation and hydrolysis yields
compound 6 (Sch. 2).
The regioselectivity of the addition of TCSC in situ onto a, b-unsat-
urated imines was also investigated. When (E )-N-phenyl-N-(3-phenyl-2-
propenylidene)amine 7a reacts with TCSC in situ, (E )-1-anilino-3-
phenyl-2-propenyl cyanide 8a was obtained as a sole product in nearly
quantitative yield (Sch. 3). Similarly, the reaction of different substituted
a, b-unsaturated imines 7b–e with TCSC afforded exclusively the 1,2-
addition products 8b–e. The structures of compounds 8a–e were based
1
on spectral and analytical data. The H NMR of 8c displayed a one
proton doublet at ꢀ 4.06(exchangeable with D2O) attributable to NH
group; one proton doublet of doublet at ꢀ 5.01 assignable to Hc;
Scheme 2.

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Trichlorosilyl Cyanide
995
Scheme 3.
Scheme 4.
a doubletof doubletat ꢀ 6.25 assignable to Hb and a doublet integrated
for one proton at ꢀ 7.04 attributed to Ha.
The conjugate addition products 9a–e were not detected in the
1
reaction mixture either by TLC or by H NMR analyses of the crude
products. The regiospecifity of this reaction is of great value for prepara-
tion of b, g-unsaturated-a-aminonitriles, which are important intermedi-
ates for synthesis of b, g-unsaturated-a-amino acids and amino alcohols.
The 1H NMR of 8b,c showed that the formed b, g-unsaturated-a-amino-
nitriles are completely in the E-form and this demonstrates that there
is no E-Z-isomerization under the reaction condition.
It is noteworthy that the treatment of different Schiff bases with
tetrachlorosilane in acetonitrile at room temperature afforded the
corresponding aldehyde and the amine hydrochloride after aqueous
work up (Sch. 4).
However, the free aldehydes or its cyanohydrin were not detected
in the worked up reaction mixture of different aldimines with TCSC in
situ. It is postulated that the activation energy of the addition of TCSC

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996
El-Ahl
in situ onto imines is lower than that of addition of tetrachlorosilane due
to the fact is that the silicon–carbon bond is much more weaker than the
silicon–chlorine one.[20] Therefore the imine cleavage was not observed
under the reaction condition.
In conclusion, the value of this work is not only for introducing a
simple cheap method for preparation of a-aminonitriles, but
for introducing the reagent combination: tetrachlorosilane–potassium
cyanide as a simple, cheap and safe reagenteasily obaitnable
from commercially available materials.
EXPERIMENTAL
Melting points (ꢀC) (uncorrected) were determined using Griffin
melting point apparatus. IR spectra were recorded on MATTSON
5000 FTIR Spectrometer. 1H NMR and 13C NMR spectra were recorded
on a GE Omega GN-300 instrument at 300 and 75 MHz, respectively,
using CDCl3 as solvent. Chemical shifts are expressed in ꢀ relative to
TMS as an internal standard. Elemental analyses were carried out at
the microanalytical unit, Faculty of Science, Mansoura University.
General Procedure for the Reaction of Imines 1a–i and 7a–e
with the Reagent Combination of Tetrachlorosilane and
Potassium Cyanide: Synthesis of 2-Arylamino-2-
Arylacetonitrile 2a–i and (E)-1-Arylamino-3-Aryl-2-
Propenyl Cyanide 8a–e
A mixture of tetrachlorosilane (5 mmol, 0.6 mL) and potassium
cyanide (5 mmol, 0.325 g) in 20 mL acetonitrile was stirred at room
temperature for one half hour with exclusion of moisture. Then,
a (5 mmol) solution of each of the imines 1a–i[21] and 7a–e[22] in
15–25 mL of dry acetonitrile was added over a period of 15 min. The
reaction mixture was stirred at room temperature for 1–3 h, then
poured onto crushed-ice (40 g), extracted with chloroform, dried over
sodium sulfate and distilled off to give the a-aminonitriles 2a–i and
8a–e, which were recrystallized from aqueous-ethanol (Table 1).
The benzophenone oxime 3 and N-phenyl-N-(4-hydroxy-phenylmethyli-
dene)amine 1j were reacted in the same manner except that 5 mmoles
more of tetrachlorosilane were added for protection of the hydroxyl
group.

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Trichlorosilyl Cyanide
Reaction of Tetrachlorosilane–Potassium Cyanide Reagent
997
Combination With the Bis-Imine 5: Synthesis of
1,4-Di(4-Chlorophenyl)-3,5-Dihydroimidazo [1,2-a]Quinoxalin-
2-yl Amine 6
A mixture of tetrachlorosilane (10 mmol, 1.2 mL) and potassium
cyanide (10 mmol, 0.65 g) in 35 mL acetonitrile was stirred at room tem-
perature for one half hour, with exclusion of moisture. Then, a solution
of N1, N2-di-[1-(4-chlorophenyl)methylidene]-1,2-benzenediamine 5[23]
(5 mmol, 1.77 g) in 20 mL acetonitrile was added over a period of
15 min. The reaction mixture was stirred for 4 h and processed in the
same manner to give the imidazo[1,2-a]quinoxaline 6 which was recrys-
tallized from ethanol (Table 1).
Reaction of Imines 1b,c with Tetrachlorosilane
A mixture of tetrachlorosilane (5 mmol, 0.6 mL) and/or imine 1b,c
(5 mmol) in 20 mL dry acetonitrile was stirred at room temperature for
6 h, then poured onto crushed-ice (20 g), extracted with chloroform
(25 mL). The chloroform extract was washed with water, dried over
sodium sulfate and distilled off to give pure 4-chloro- and 4-bromobenz-
aldehydes 10a,b respectively. The aqueous layer was basified with sodium
hydroxide and extracted with ether. The ether extract was dried and
distilled off. The oily residue was identified as aniline (TLC and it gave
the usual reactions of aniline).
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Received in the UK August 16, 2001