NMR, IR, and ESR spectroscopic investigation of reaction of αα-silylamines with carbon tetrachloride

Article abstract of DOI:10.1002/poc.1442

This paper reports about high reactivity of α-silylamines in the reaction with CCl₄. Unlike Et₃N, α-silylamines rapidly react with CCl₄ upon irradiation with daylight to form α-silylamine hydrochloride salts in 92-98% yiel

Full text of DOI:10.1002/poc.1442

Research Article
Received: 30 April 2008,
Revised: 28 July 2008,
Accepted: 5 August 2008,
Published online in Wiley InterScience: 24 September 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1442
NMR, IR, and ESR spectroscopic investigation
of reaction of a-silylamines with carbon
tetrachloride
a
a
a
*
Nataliya F. Lazareva , Tamara I. Vakul’skaya and Igor M. Lazarev
This paper reports about high reactivity of a-silylamines in the reaction with CCl4. Unlike Et3N, a-silylamines rapidly
react with CCl4 upon irradiation with daylight to form a-silylamine hydrochloride salts in 92–98% yields. The influence
of structure of a-silylamines and solvent on the degree of conversion was displayed. The interaction of a-silylamines
with CCl4 was studied by NMR, ESR, and IR spectroscopy. C-centered radicals of a-silylamines were detected by ESR
spectroscopy with spin traps (MNP, ND, and PBN) in reaction mixtures in CH3CN and C6H6 and it show the radical
character of this reaction. Both CH3CN and C6H6 serve as solvents as well as reagents for this reaction. A mechanism of
an interaction between a-silylamines and CCl4 is discussed. Copyright ß 2008 John Wiley & Sons, Ltd.
Keywords: a-silylamines; carbon tetrachloride; C-centered radical of a-silylamine; ESR; NMR; IR-spectroscopy
The radical cations of arylamines were identified by charac-
INTRODUCTION
teristic absorption spectra and the paths of their further
transformations were studied.[18] Application of the CIDNP
method proved the radical nature of the reaction between
CCl4 and triethylamine.[27–29] Enhanced absorption in the CIDNP
spectra was observed for the signals of chloroform and terminal
olefinic protons of diethylvinylamine during photolysis of the
charge–transfer complex Et3N—CCl4 in acetonitrile or methanol.
This suggests the formation of the radical pair Et2NC0HCH3—’CCl3
in the first stage of the reaction and confirms the earlier proposed
Carbon tetrachloride (CCl4) is widely used in various fields of
organic chemistry as a solvent[1,2] and, in particular, for
investigation of properties of amines.[3–7] The first data about
instability of solutions of amines in CCl4 appeared in the middle of
the previous century. Their solutions in glass vessels turned
yellow upon storage, the amine hydrochlorides precipitated, and
chloroform was formed.[8,9] When studying this phenomenon
using the methods of UV, IR, and NMR spectroscopy and X-ray
analysis, the formation of weak charge–transfer complexes of
amines with polyhalogenomethanes was found.[10–23] It was
assumed that complexation of amine with C(Hal)4 (Hal ¼
halogen) was due to the interaction of the lone pair on the
[11]
mechanism for the reaction of triethylamine with CCl4
(Scheme 1). Diethylvinylamine is an extremely unstable com-
pound which polymerizes to afford a product with the system of
conjugated bonds. The formation of this polymer may account
for the coloring of the reaction mixture.[28,29]
*
nitrogen atom of the amine with the s -orbital of the C—Hal
bond.[20] According to the X-ray date, in crystalline amine–
polyhalogenomethane complexes, the nitrogen atoms of the
amine molecule coordinate with the halogen atoms, forming
discrete donor–acceptor pairs.[16,17,21] Of special interest is
chemical transformation of these complexes under the action
of light which is the reason of instability of the amine solutions in
CCl4. Investigation of the mechanism of this reaction started
practically simultaneously with studying the process of complex
formation of amines with polyhalogenomethanes.[8,11,14,18] Such
factors as UV irradiation, addition of salts of transition metals, and
effect of the glass surface facilitate the reaction.[14,15,22,23] Kosover
was the first who offered the mechanism of the reaction
between Et3N and CCl4[11] (Scheme 1). By now, it is unequivocally
established that in weak complexes of amines with tetrahalo-
genomethanes upon irradiation in the near UV field the electron
transfers from the amine to tetrahalogenomethane with the
formation of the pair of the radical cation of amine and the radical
anion of tetrahalogenomethane (reference [18] and references
therein). The latter usually undergoes irreversible degradation to
Recently, we have found that the induction period of the
reaction of 1-dialkylaminomethylsilatranes R2NCH2Si(OCHRCH2)3
N (R ¼ Me, (CH2)5) with CCl4 upon irradiation with the daylight is
0.2 h, whereas for the reaction of Et3N with CCl4 under similar
conditions this period reaches 96–144 h.[30] The yields of the
corresponding salts [R2NþHCH2Si(OCHRCH2)3N]Clꢀ and [Et3NHþ]Clꢀ
are 42–48% (20 h) and 0.5% (164 h) accordingly. We have
assumed that such a drastic difference in the reactivity of
1-dialkylaminomethylsilatranes and triethylamine is caused by
the abnormally high basicity of the exocyclic nitrogen atom[31]
due to
a strong electron donating inductive effect of
*
Correspondence to: N. F. Lazareva, A.E. Favorsky Irkutsk Institute of Chemistry,
Siberian Branch of the Russian Academy of Sciences, 1 Favorsky Street, Irkutsk
664033, Russia.
E-mail: nataly_lazareva@irioch.irk.ru
a
N. F. Lazareva, T. I. Vakul’skaya, I. M. Lazarev
A.E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch of the Russian
Academy of Sciences, 1 Favorsky Street, Irkutsk 664033, Russia
.
afford the halogen anion and the radical CHal3.[18,24–26]
J. Phys. Org. Chem. 2009, 22 144–154
Copyright ß 2008 John Wiley & Sons, Ltd.

NMR, IR, AND ESR SPECTROSCOPIC INVESTIGATION
ClCCl3
ClCCl3
..
ν
h
Et2NCH2CH3
ClCCl3
+
..
Et2NCH2CH3
Et2NCH2CH3
- H
..
.
..
e-transfer
ClCCl3
CCl3 Et NCHCH
Et2NCH2CH3
Et2NCHCH3
+
2
2
.
Cl
H
+ HCl
.
+ CCl3
Et3N
Et3N
.
Et2NCH=CH2
+
Et3N HCl
Scheme 1. The mechanism of the reaction between Et3N and CCl4 proposed by Kosover[11]
silatranylmethyl group (s ¼ ꢀ2.24[32,33]). Introduction of the
second silatranylmethyl group to the nitrogen atom results in a
substantial increase of its basicity in N-methyl-N, N-bis-
(silatranylmethyl)amine MeN[CH2Si(OCH2CH2)3N][31] and, as a
consequence, in its reactivity.[34,35] So far, the reaction of this
compound with CCl4 was not studied. In this work, we discuss the
results of investigation of a-silylamines in their reaction with CCl4
obtained by IR, ESR, and NMR spectroscopy. As the objects for this
goal, a-silylamines RR0NCH2SiX3 1–7 and, for comparison,
triethylamine 15 were chosen.
methylene chloride extracts from these reactions contained
7–12% of 1-chlorosilatrane, it was determined by integration of
the intensities of the CH2N and CH2O proton signals of the
silatrane ring. In acetonitrile the reaction with CCl4 was
completed in 36–72 h and the yield of salts 8–14 was almost
quantitative. The compounds 1–7 react with CCl4 in benzene
more slow than in acetonitrile. After the same period, the yields of
compounds 8–14 in acetonitrile were much higher than in
benzene. For example, the yield of compound 8 in CH3CN
and C6H6 after 2 h comprises 41 and 18% accordingly. The yield of
by-products in these solvents is slight. The structure of the
a-silylamine has a pronounced effect on the yield provided that
all other things are equal. On the one hand, the structure of the
silyl group is of importance, since the yield of the silatranyl-
containing products 8–11 is higher than 12–14. On the other
hand, an important factor is the number of silylmethyl groups at
the nitrogen atom. Thus, introduction of a second silylmethyl
group, irrespective of its structure, increases the yield provided
that all other things are equal. For example, the yield of
compounds 8 and 9 containing two silatranylmethyl groups is
higher than that of compounds 10, 11, containing only one such
group. Similarly, the yield of the compound 13, containing two
trimethylsilylmethyl groups is larger than that of compound 12
containing only one such group.
*
RESULTS AND DISCUSSION
Compounds 1–7 reacts with CCl4 when exposed to daylight at
room temperature in benzene, acetonitrile, or in an excess of CCl4
resulting in the formation of the amine hydrochloride products
8–14 as shown in Scheme 2.
In all cases the induction period does not exceed 0.5–15 min
depending on the solvent and the structure of a-silylamine. The
yield of products 8–14 in various solvents as a function of
the reaction time are given in Table 1. The yield of 8–14 in CCl4 as
the solvent is less than 50% after 24 h and a large amount of
poorly soluble, colored products is formed, that agrees well with
our previous results.[30] We failed to identify the colored products
from the reaction mixtures with compounds 5–7. However, from
the reaction mixtures with silatranes 1–4, extraction of the solid
residue obtained after removal of excess CCl4, with boiling
methylene chloride afforded the mixtures of salts 8–11 with
1-chlorosilatrane. As observed from the 1H NMR spectra, the
In the presence of hydroquinone or in the dark the reaction of
compound 1 with CCl4 proceeds extremely slowly. Unlike the
above a-silylamines, the mixture of triethylamine 15 with CCl4 is
stable when exposed to light during several days. However, upon
the UV irradiation of this mixture triethylamine hydrochloride 16
is formed as soon as in 2 h.
C6H6 or CH3CN;
36 - 72 h
RR'NCH2SiX3
+
CCl4
RR'NHCH2SiX3 . Cl
+
HCCl3
1 - 7
8 - 14
R = Me, R' = CH2SiX3, SiX3 = Si(OCH2CH2)3N (1, 8);
R = PhCH2, R' = CH2SiX3, SiX3 = Si(OCH2CH2)3N (2, 9);
R = R' = Me, SiX3 = Si(OCH2CH2)3N (3, 10);
RR' = (CH2)5, SiX3 = Si(OCH2CH2)3N (4, 11);
R = R' = Me; SiX3 = SiMe3 (5, 12);
R = Me, R' = CH2SiX3,SiX3 = SiMe3 (6, 13);
R = Me, R' = CH2SiX3, SiX3 = SiMe2OMe (7, 14);
Scheme 2. The reaction of a-silylamines 1–7 with CCl4
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2009, 22 144–154

N. F. LAZAREVA, T. I. VAKUL’SKAYA AND I. M. LAZAREV
resonance of the NCH2Si group in compound 8, 9 represents an
AB quartet (JAB ¼ 14.0 Hz) and the proton resonance of
the NCH2C (a skeleton of silatranes) is an AB quartet, where
each component is split into a triplet with a vicinal coupling
Table 1. Yields of [RR0NHþCH2SiX3]Clꢀ is function of the
reaction time
Yields of [RR0NHþCH2SiX3]Clꢀ
3
constant (JAB ¼ 12.5 Hz, J ¼ 6.2 Hz). Protonation of the nitrogen
atom in N-methyl-N,N-bis(silatranylmethyl)amine
1 and 2
produces a prochirality[37] of NCH2Si groups and these methylene
protons becomes diastereotopic.[38] Diastereotopic atoms of
hydrogen have a various chemical environment (Scheme 3) and
differ by chemical and physical properties. In particular,
methylene protons are anisochronous, that is, these atoms have
Solvent t, h
8
9
10 11 12 13 14
16c
CCl4
CCl4
2
8
2
11
27
18
34
57
91
41
76
89
12
8
9
7
2
traces
26 17 19
17 11 12
36 29 28 12 23 17
58 39 41 26 35 29
61
39 29 31
79 67 69 17 51 44
90 79 80 29 78 71
5
2
16 14
11 8
C6H6
C6H6
C6H6
C6H6
CH3CN
CH3CN
CH3CN 24
CH3CN 24
CH3CN 24
CH3CN 36
CH3CN 48
CH3CN 72
1
a various chemical shift in the H NMR spectra.[39,40] The typical
8
24
48
2
example of prochiral molecule with diastereotopic hydrogen
atoms is 2-butanol MeCHAHBCH(OH)Me.[38]
The 1H NMR monitoring of the reaction mixture of compounds
1, 3, and 7 with CCl4 in acetonitrile shows that the singlet signal of
chloroform (d ¼ 7.28 ppm) appears in the spectra within 1 h. Its
intensity depends on the structure of the a-silylamine (1 > 3 > 7)
and increases in time (Fig. 1).
8
25 19
3
8
7a
3b
97
94 98 92
94
IR spectroscopy was employed to investigate the reaction of
compounds 1 and 3 with CCl4 in acetonitrile. These two
compounds were chosen for the IR study because of the presence
of the NMe group in the molecules. The symmetric stretching
vibration band n(C—H) of the NCH3 group at 2750–2760 cmꢀ1
was chosen as an analytical marker.[41] This band is absent in the
ammonium salts MeRR0R00NþClꢀ, so that the decrease of its
intensity allows to determine the degree of the quaternization
reaction. The intensity of the NMe band in the IR spectrum of the
reaction mixture of compound 3 with CCl4 decreases non-
uniformly (Fig. 2). The spectrum remains practically unchanged
within 10 min after mixing of the reagents, that is in line with the
existence of the induction period in this reaction.[30] Then, during
8 h, it gradually decreases to 30% of the original value. No
96
98
a The reaction mixture was kept in the dark.
b Hydroquinone was added to CH3CN before mixing.
c UV irradiation.
The structures of products 8–14 were assigned by multinuclear
NMR spectroscopy (Tables 2–4). A downfield shift of the signals of
the NCH3 and NCH2Si groups in the 1H NMR spectra of 8–14 with
respect to the starting compounds 1–7 (Table 2) is indicative of
the quaternization of the nitrogen center.[35,36] The proton
Table 2. 1H NMR data for the a-silylamines R2NCH2SiX3 and the corresponding ammonium salts [R2NHþCH2SiX3]Yꢀ in CD3CN
R2NCH2SiX3
[R2NHþCH2SiX3]Yꢀ
d, ppm
d, ppm
No.
R–N
N–CH2–Si
1.89 (s)
SiX3
No.
R–N
N–CH2–Si
SiX3
1
2.17 (s)
2.83 (t)
8
2.83 (s)
2.17 (JAB ¼ 14.0 Hz)
3.03 (JAB ¼ 12.5 Hz,
3J ¼ 6.2 Hz);
3.82 (t)
3.79 (t)
2.77 (t)
3.79 (t)
2.78 (t)
3.74 (t)
3.77 (t)
2.80 (t)
5.79 (NHþ)
4.18 (d) (3J ¼ 5.38 Hz)
7.44 (m) 6.02 (NHþ)
2.86 (s)
2.39
2.16 (JAB ¼ 14.43 Hz) 2.36
2
3.52 (s)
7.15 (m)
2.20 (s)
1.94 (s)
1.63 (s)
1.80 (s)
9
2.95 (t)
3.74 (t)
2.96 (t)
3.82 (t)
3.83 (t)
2.91 (t)
3
10
2.13 (s)
2.57 (NHþ)
1.57 (m)
3.05 (m)
4a
1.59 (m)
2.41 (m)
11a
2.17 (s)
10.15 (NHþ)
2.60 (s)
5
6
7
2.22 (s)
2.10 (s)
2.17 (s)
1.85 (s)
1.82 (s)
1.86 (s)
0.06 (s)
0.18 (s)
0.02 (s)
12
13
14
2.31 (s)
2.60 (s)
2.61 (s)
0.19 (s)
0.24 (s)
9.83 (NHþ)
2.74 (s)
11.55 (NHþ)
2.70 (s)
0.24 (s)
3.36
10.78 (NHþ)
a Solvent CDCl3.
J. Phys. Org. Chem. 2009, 22 144–154
Copyright ß 2008 John Wiley & Sons, Ltd.
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NMR, IR, AND ESR SPECTROSCOPIC INVESTIGATION
Table 3. 13C{1H} data for the a-silylamines R2NCH2SiX3 and the corresponding ammonium salts [R2NHþCH2SiX3]Yꢀ in CD3CN
R2NCH2SiX3
[R2NHþCH2SiX3]Yꢀ
d, ppm
d, ppm
No.
R–N
N–CH2–Si
47.46
SiX3
No.
R–N
N–CH2–Si
46.69
SiX3
1
54.51
50.96
57.26
50.91
58.26
8
53.52
50.07
57.02
50.25
57.91
2
62.16
129.39
129.49
128.41
139.74
53.02
57.34
9
60.49
129.87
131.23
133.46
49.95
3
50.37
51.42
51.69
58.12
51.92
58.43
10
51.11
48.58
49.86
50.29
57.19
50.12
57.27
4a
24.76
26.77
59.01
49.74
56.76
55.33
11a
22.18
25.27
58.87
48.69
52.94
52.73
5
6
7
43.40
50.94
50.66
ꢀ1.49
ꢀ1.21
ꢀ1.16
12
13
14
40.98
46.36
48.15
0.63
ꢀ0.57
ꢀ0.78
50.59
a Solvent CDCl3.
I, %
100
Table 4. 29Si data for the a-silylamines R2NCH2SiX3 and the
corresponding ammonium salts [R2NHþCH2SiX3]Yꢀ in CD3CN
80
60
40
20
1
No.
R2NCH2SiX3
No.
[R2NHþCH2SiX3]Yꢀ
.
3
1
2
3
4a
5
6
7
ꢀ81.53
ꢀ81.11
ꢀ75.31
ꢀ73.23
ꢀ1.55
8
9
ꢀ84.99
ꢀ85.04
ꢀ81.52
ꢀ82.39
ꢀ1.78
.
10
11a
12
13
14
7
.
.
.
ꢀ1.86
ꢀ0.94
ꢀ1.99
ꢀ0.95
t, h
a Solvent CDCl3.
2
4
6
8
10
Figure 1. The dependence of the change of the integrated intensity of
chloroform signal with time in reaction mixtures 1 (D) [or 3 (*) or 7
(o)]–CCl4–CH3CN
induction period was observed in the reaction of compound 1
with CCl4. The band at 2753 cmꢀ1 started to gradually drop
immediately after mixing of the solutions of compound 1 and
CCl4 in acetonitrile to reach 20% of its starting value after 8 h
(Fig. 2a). The excess of CCl4 has no effect on the degree of
conversion of the starting a-silylamine. The absorption bands of
the silatranyl group are retained in the products 8, 10 (Table 5),
whereas the n(C—H) bands of the NCH3 group at 2700–
2800 cmꢀ1 are absent. New bands appearing at 2400–2700 cmꢀ1
belong to the stretching vibrations of the Nþ—H group. The IR
spectra of compounds 8, 10 are identical to those of
HA
R
HB
X
Y
R
X
Y
X
R
Y
HB
HA
HB
HA
Z
Z
Z
Scheme 3. Nonequivalence of diastereotopic hydrogen atoms in methylene group of compounds RCHAHBEXYZ (E ¼ C, N)
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J. Phys. Org. Chem. 2009, 22 144–154

N. F. LAZAREVA, T. I. VAKUL’SKAYA AND I. M. LAZAREV
Figure 2. The change of IR spectra of the reaction mixture with time (0–8 h) (a) 1–CCl4; (b) 3–CCl4 (solvent CH3CN)
hydrochlorides of N-methyl-N,N-bis(silatranylmethyl)amine and
N,N-dimethyl(silatranylmethyl)amine.[30,35]
CCl4-radical cation of amine) pair which is further transformed
into the a-radical of amine and ꢁCCl3. Based on the pathway
shown in Scheme 1, we anticipate the observation of some of
these intermediates by ESR spectroscopy. Earlier, the use of this
method allowed us to identify the formation of the radical cation
of amine 1 in its reaction with AgNO3.[35] However, no signals in
the ESR spectra were detected in the reaction mixtures of
compounds 1, 2, and 4 with CCl4. This may be due to small
As noted above, the reaction of a-silylamines with CCl4
proceeds in the light and is retarded by hydroquinone, that is
indicative of its radical character and is in compliance with the
known data on the reactions of organic amines with poly-
halogenomethanes.[18] According to Scheme 1, the reaction of
amine with CCl4 begins with the formation of the (radical anion of
Table 5. Characteristica IR frequencies for the amines, MenN[CH2Si(OCH2CH2)3N]3ꢀn [n ¼ 1 (1), n ¼ 2 (3)] and the corresponding
hydrochloride salts 8 and 10
Compounds
1
8
3
10
Assignment
585 (m)
633 (m)
655 (m)
790 (s)
815 (s)
914 (m)
940 (m)
589 (m)
638 (m)
659 (m)
791 (s)
814 (s)
916 (m)
943 (m)
585 (m)
621 (m)
580 (m)
628 (m)
ns(SiO3)
779 (m)
815 (s)
783 (s)
791 (s)
907 (m)
930 (m)
ns(SiOC)
911 (m)
938 (m)
945 (m)
1090 (s)
1101 (s)
1124 (s)
1276 (m)
n(CC), n(CO), n(CN)
1090 (vs)
1115 (s)
1085 (vs)
1113 (s)
1077 (s)
1110 (s)
nas(COSi)
1277 (m)
1274 (m)
2498 (w)
2531 (w)
2605 (w)
2636 (w)
2674 (w)
1273 (m)
2480 (m)
2520 (m)
2590 (w)
2700 (w)
d(NCC)
n(NþH)
2755 (m)
2755 (m)
2808 (m)
2873 (m)
2914 (m)
2934 (m)
2969 (m)
2994 (w)
n((CH3)N), n((CH3)2N)
n(CH2,CH3)
2880 (m)
2937 (m)
2950 (m)
2882 (m)
2939 (m)
2977 (m)
2878 (m)
2930 (m)
2950 (m)
a s, strong; m, medium; w, weak; vs, very strong.
J. Phys. Org. Chem. 2009, 22 144–154
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NMR, IR, AND ESR SPECTROSCOPIC INVESTIGATION
Figure 3. ESR spectrum of the reaction mixture 4–CCl4–C6H6 with MNP
after 2 h
Figure 4. ESR spectrum of the reaction 1–CCl4–C6H6 in the presence of
ND after 1 h
concentration of the complex between the amine and
polyhalogenomethane[18] that would give rise to low concen-
tration of the intermediates formed upon its decomposition. A
short lifetime of the intermediates could also affect the detection
of the ESR signal. With this in mind, we made an attempt to study
the reaction between a-silylamine and CCl4 using the ESR
spin-trapping method.[42] 2-Methyl-2-nitrosopropane (MNP),
nitrosodurol (ND), and a-phenyl-N-tert-butylnitron (PBN) are
well-known spin traps[43] widely used for trapping of unstable
carbon-centered radicals[44] including ꢁCCl3. We used all of these
spin traps assuming that their different selectivity[45] will allow us
to register all radicals formed in the reactions of a-silylamines
with CCl4. Indeed, in the ESR spectra of the reaction mixtures of
compounds 1, 2, 4, and 15 with CCl4 in benzene in the presence
of MNP the signals of different intensity were registered. The ESR
signals obtained from the reaction of 4 with CCl4 in C6H6 in the
presence of MNP were relatively broad triples of doublets (Fig. 3)
caused by the interaction of the unpaired electron with one
nucleus with spin 1 (nitrogen) and one nucleus with spin ½
(proton). Their characteristics are given in Table 5. With regard to
the literature data,[44] the observed signals belong to the spin
adducts of MNP to the C-centered radicals of the type R0R00NꢁCHR.
Computer simulation of the spectra has shown, that their
asymmetry is due to superposition of the triplet signal from
t-Bu2NOꢁ radical. MNP eliminates the tert-butyl radical which is
trapped with another molecule of MNP to give t-Bu2NOꢁ
radical.[42,44] The intensity of the latter considerably increases
in time, and it is easily identified by the value of the hyperfine
splitting constant aN ¼ 1.556 mT. No other spin adducts with MNP
were found.
Figure 5. High-resolution ESR spectrum of the reaction mixture
2–CCl4–C6H6 in the presence of ND after 12 h
In the ESR spectra of the reaction mixtures of 1 and 4 with CCl4
and ND the overlapping signals of the spin adducts from the two
radicals are observed (Figs. 4–6). The value of the hyperfine
splitting constant of these triplets of doublets (Table 6) is typical
for spin adducts of ND with carbon-centered radicals containing
proton in the a-position to the radical center,[44] which are
formed by deprotonation of radical cations of amines (Scheme 1).
The second signal appears on the background of the main triplet
of doublets (Table 6, Fig. 4) and its characteristics are determined
from the spectrum with high resolution after 12 h (Figs. 5 and 6)
and coincides with those of the spin adduct, ND/ꢁPh.[44] This
assignment was corroborated by the fact that when benzene was
replaced by deuterobenzene the signal of the spin adduct
ND/ꢁC6D5 was split to a triplet from the nitrogen center with a
constant of 1.0 mT. Further splitting to a doublet on the a-proton
Figure 6. ESR spectrum of spin adduct ND/ꢁPh observed in the reaction
mixtures of compounds 1, 2, 4, and 15 with CCl4 in benzene in the
presence of ND: (a) experimental after 24 h and (b) simulated ESR
spectrum
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2009, 22 144–154

N. F. LAZAREVA, T. I. VAKUL’SKAYA AND I. M. LAZAREV
Table 6. ESR spectral parameters for the spin adducts formed in the reaction of amines 1, 2, 4, and 15 with CCl4 in the presence of
spin traps
Spin trap
MNP
NDa
PBN
Amine Solvent
aN, mT aH(1), mT g-factor aN, mT aH(1), mT g-factor aN, mT
aH(1),b mT
g-factor
1
CCl4/C6H6
1.378
0.456
2.0067
1.234
0.648
2.0054
1.420
1.388
0.220 (0.110)
0.156 (0.088)
2.0062
2.0063
CCl4, C6D6
CCl4
CCl4, C6H6
1.236
1.233
1.222
0.650
0.644
0.600
2.0054
2.0054
2.0055
1.388
1.420
1.388
1.388
1.422
1.388
1.388
1.388
1.388
1.388
1.388
1.388
0.156 (0.100)
0.218 (0.110)
0.156 (0.088)
0.156 (0.100)
0.222 (0.088)
0.156 (0.088)
0.156 (0.088)
0.156 (0.067)
0.156 (0.088)
0.156 (0.088)
0.156 (0.100)
0.156 (0.100)
2.0063
2.0062
2.0063
2.0063
2.0062
2.0063
2.0063
2.0063
2.0063
2.0063
2.0063
2.0063
2
4
1.365
1.398
0.478
0.447
2.0066
2.0064
CCl4
CCl4, C6H6
1.333
1.244
0.622
0.623
2.0055
2.0055
CCl4, C6H6, hnc
CCl4, C6D6
CCl4
CCl4, hnc
CCl4, C6H6
CCl4
1.377
1.377
0.445
0.445
2.0064
2.0064
1.249
1.248
1.288
1.400
1.400
0.622
0.624
0.622
0.666
0.666
2.0055
2.0055
2.0055
2.0054
2.0054
d
d
15
1.360
1.355
0.410
0.440
a Multicomponent signal of the spin adduct of ND with phenyl radical identified by the value of hyperfine splitting constant aN 10.11;
a
H(3) 2.78; aH(2) 0.95;[47] and g ¼ 2.00614[46] is overlapped with the main triplet of doublets in all cases in the system CCl4–C6H6 in the
presence of ND.
b The width of a line is given in brackets.
c After UV irradiation for 5–7 min in a quartz cell.
d One more signal was observed (not identified).
*
occupied lone pair orbital with the s (C—Si) orbital. The
was not manifested in the spectrum because the hyperfine
constant for deuterium is seven times less than that for proton.
In the ESR spectra of the reaction mixture of 4/CCl4 in benzene
in the presence of PBN (Fig. 7) similar signals to those of
increased basicity and the low ionization potential are the key
properties of a-silylamines, which are responsible for the drastic
difference of their reactivity from that of organic amines. If it is
the mixture of the spin adducts PBN/ꢁCCl3 and PBN/ꢁC6H5
[46]
were observed. Because of strong overlapping the signals cannot
be separated in the X-range. Indeed, computer simulation has
shown the observed ESR spectra to be superposition of the two
triplets of doublets which belong to spin adducts of PBN with
trichloromethyl and phenyl radicals (Table 6). In the course of the
reaction the intensity of the signal of the spin adduct PBN/ꢁCCl3 is
increased so that the contribution of PBN/ꢁC6H5 becomes
negligible. In the absence of benzene, only the spin adduct
PBN/ꢁCCl3 is registered in the ESR spectra (Table 6, Fig. 7b).
a-Silylamines belong to
a large group of compounds
XCH2M with heteroatoms X ¼O, S, or N in the geminal position
to the element of the 14th group (M ¼ Si, Ge, and Sn), which
possess unique reactivity and unusual spectroscopic proper-
ties.[48–52] Various models are invoked to explain the nature of this
a-effect.[53–58] Amines having the silyl group in the a-posi-
tion, R2NCH2SiX3, show an increased basicity as compared with
their carbon analogs and their ionization potential is lower than
that of trialkylamines.[59–62] According to the literature,[53,63] the
increase of the energy of the lone pair of heteroatom (and, hence,
low ionization potential) is due to destabilizing four-electron
geometrically dependent interaction of the lone pair with the
s(C—Si) orbital,[64] whereas the radical cation center on the
heteroatom is stabilized by hyperconjugation of the half-
Figure 7. ESR spectra of the reaction mixture 4–CCl4–C6H6 in the pre-
sence PBN: (a) 2 h and (b) 36 h
J. Phys. Org. Chem. 2009, 22 144–154
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NMR, IR, AND ESR SPECTROSCOPIC INVESTIGATION
granted that the formation of complex of amine and CCl4 occurs
due to interaction of the lone pair of nitrogen atom with the
use of ESR spectroscopy. These facts corroborate the participa-
tion of solvents (benzene and acetonitrile) as donors of hydrogen
atom in these reactions. Apparently, the C-centered radicals of
a-silylamines eliminate hydrogen atom from the molecule of the
solvent to recover the starting a-silylamine with the transfer of
the radical center to the rest of the solvent molecule. This allows
to explain quantitative transformation of a-silylamines into the
corresponding hydrochlorides in aprotic solvents. The above
analysis allows to assume the following tentative mechanism of
the reaction of a-silylamines with CCl4 in aprotic solvents
(Scheme 4. The products and intermediates identified by us in
this reaction are given in red).
When excess of CCl4 is used as a solvent the yields of
silylamines do not exceed 50%. The main reason of the decrease
of the yield is that in the absence of the H-donor solvent the
C-centered radicals of a-silylamines are stabilized by recombina-
tion or decomposition. The products of the recombination of a
radical RR0NCꢁHSiX3 were not found in the reaction mixtures of
compound 1–7 with CCl4. We have found that 1-chlorosilatrane is
present among the products of the reaction of compounds 1–4
with excess of CCl4. The insoluble, yellowy–brown solid residues
of polymeric products were obtained after extraction of
compounds 8–14 from the reaction mixtures. The absorption
bands at 1660–1685 cmꢀ1 in IR spectra of polymeric products
[20]
*
s (C—Cl)-orbital
it must depend on the basicity of the
nitrogen atom. The low ionization potential of the a-silylamines
provides an explanation for their propensity to single electronic
transfer reactions under the conditions of photochemical initia-
tion with the formation of the corresponding radical cations.[65,66]
A similar process takes place in the complex RR0R00N—CCl4 under
the action of daylight. The experiments showed that the
reactivity of the studied a-silylamines 1–7 depends on the
number and structure of organosilicon substituents being
changed in the order 1 >> 2 > 3 >> 4 > 6 > 7 > 5. Both the
basicity and the potential of oxidation of amines are governed by
their structure, that is, by inductive and steric effects of the
substituents at the nitrogen atom.[67–72] The donor inductive
effect of the CH2Si(OCH2CH2)3N group is much larger than that of
the CH2SiMe3 group (sI are ꢀ0.36 and ꢀ0.05, respectively),[32,33,73]
that makes the exocyclic nitrogen atom of a-silatranylamines
more basic than in a-trialkylsilylamines. The degree of pyrami-
dalization of the nitrogen atom is another major factor
determining the electron donating ability of its lone pair.[72,74]
Apparently, a steric effect of silatranyl group caused the change
of degree of pyramidalization of nitrogen atom. It is not excluded,
that the sum of these two factors provides higher reactivity of
a-silatranylamines 1–4 in comparison with a-silylamines 5–7.
The electron transfer in weak complexes of amines with
polyhalogenomethanes gives rise to the formation of the pair of
the radical cation of amine and radical anion of CCl4.[18]
Unfortunately, we failed to detect these intermediates when
studying the reaction mixtures RR0CH2NSiX3—CCl4 by ESR. Yet,
the use of the spin traps allowed us to register the C-centered
radicals of a-silatranylamines. Under the same conditions, we also
detected the trichloromethyl radical formed by degradation of
the radical anion of CCl4. The ꢁCCl3 radical forms chloroform
molecule that proved by NMR spectroscopy. These results agree
well with the data known for organic amines.[18] One of the most
probable paths of degradation of the radical cation of tertiary
amine is the process of a-deprotonation with the formation of the
C-centered radical.[75–80] Despite of a great deal of publications
devoted to the problem of degradation of radical cations of
amines, so far there is no distinct understanding of the process of
deprotonation. Apparently, it depends on the structure and
basicity of amine and includes several elementary steps. The
direction of degradation of radical cations of a-silylamines
RR0NCH2SiMe3 is determined by the unique structure of the
SiCH2N fragment and depends on the structure of the
substituents and the nature of the solvent used.[81–83] In aprotic
solvents (acetonitrile) a-deprotonation predominates whereas
desilylation proceeds mainly in polar protic solvents (methanol).
The reaction of compounds 1–7 with CCl4 in benzene or
acetonitrile results in the formation of the corresponding amines
hydrochlorides 8–14 in close to quantitative yields. This may be
only if the solvent takes part in the reaction as an H-donor to
radical ꢁCH(NRR0)SiX3 to return the starting amine to the reaction
cycle. Acetonitrile is known to be hydrogen atom donor.[84,85] The
—
belong to NC C groups (polyconjugated bonds). This result
—
coincides with data which have been received at the studying of
the by-products of oxidation reaction of amines.[28] The element
analysis showed the high contents of chlorine in the polymers
(30–65%). Apparently, one of the ways of stabilization of the
radical of a-silylamine may be the process of desilylation with the
—
formation of ClSiX and polymeric products containing the NC
—
C group (Scheme 5). Probably, the reaction proceeding at the
3
mixing of the a-silylamine with an excess of CCl4 is more complex
RR'NCH2SiX3
RR'NCH2SiX3
+
CCl4
CCl4
h
ν
e-
RR'NCH2SiX3
CCl4
RR'NCH2SiX3 . . . CCl4
- Cl
- H
RR'NCHSiX3
CCl3
- H
C6H6
or
amine
C6H6
C6H5
ꢁ
radical CH2CN formed after expulsion of hydrogen atom from
acetonitrile can dimerize to give succinonitrile. Indeed, we
observed the appearance of new signals in the 1H (2.76 ppm) and
13C (14.8 and 119.6 ppm) NMR spectra of the reaction mixtures of
compounds 1 and 3 with CCl4 in acetonitrile solution after 1 day.
The formation of phenyl radical was observed when studying the
reactions of a-silylamines with CCl4 in benzene as a solvent by the
HCCl3
[RR'NHCH2SiX3] Cl
Scheme 4. A possible mechanism for the reaction between a-silylamines
and CCl4 (C6H6 solvent)
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N. F. LAZAREVA, T. I. VAKUL’SKAYA AND I. M. LAZAREV
[RR'NC=CCl2] HCl
[RR'NC=CCl2] HCl
EXPERIMENTAL
polymer
1H (400.13 MHz), 13C{1H} (100.62 MHz), and 29Si{1H} DEPT
(79.5 MHz) NMR spectra were recorded on a Bruker DPX-400
spectrometer. Chemical shifts are given in parts per million, using
internal references (TMS or cyclohexane). IR spectra were run on a
Specord IR-75 spectrometer (KBr pellets). ESR spectra of the
reaction mixtures were recorded in vacuum cells at ambient
temperature on a SE/X-2547 spectrometer (Radiopan, Poland)
equipped with a magnetometer and a high frequency instru-
ment. Simulated spectra were obtained using the WINEPR
SimFonia 1.25 program (Bruker, Inc. 1996). The concentration of
spin traps was used about 10ꢀ2 ꢀ 10ꢀ3 mol Lꢀ1. The solvents
were purified and were dried according to standard pro-
cedures.[86] CCl4 was storing in a closed vessel with the molecular
sieve 4A. N-methyl- and N-benzyl-N,N-bis(silatranylmethyl)amines
(1 and 2) have been synthesized by the method given in article.[87]
1-Dimethylamino- and 1-piperidinomethylsilatranes (3 and 4) have
been synthesized by the literature method.[88] The a-silylamines
5–7 have been synthesized by the literature method.[89] All
experiments were performed in a flame-dried apparatus under a
static atmosphere of dry argon.
[RR'NC=CCl2] HCl
ClSiX3
+
RR'NCHCCl3
CCl4
RR'NCHSiX3
-
+
RR'NCH2SiX3 . . . CCl4
CCl3
- Cl
- H
General procedure of the interaction of
a-silylamines 1–7 with CCl4
[RR'NHCH2SiX3] Cl
a-Silylamine (0.1 mmol) was dissolved in 3 mL of a solvent in the
Schlenk tube. The solution was degased and was added the 0.15
mmol (an excess) of the previously degassed CCl4 by the syringe.
This mixture was degassed once again and was stirred on a
magnetic stirrer at room temperature and upon irradiation with
the daylight with time which indicated in Table 1. After a solvent
was removed on the rotor evaporator, a solid residue was drying
in the vacuum. The crude product was then purified by the
recrystallization.
Scheme 5. A possible mechanism for the reaction between a-silylamines
and excess of CCl4
than is represented by Scheme 5. However, the scheme provides
an explanation for the formation of the 1-chrlorosilatrane in the
reaction mixture.
It is necessary to note, that the reaction CCl4 with the
compounds 5–7 is extremely sensitive in the presence of a trace
of H2O at a reaction mixture. If this reaction is to be carried out in
the wet solvents preferentially the formation of the products of
the cleavage Si—C bond is observed.
CONCLUSION
The results presented in this account demonstrate the higher
reactivity of the a-silylamines 1–7 with CCl4 compared
to Et3N. Unlike Et3N, the interaction of a-silylamines with CCl4
begins within few minutes after their mixing upon the irradiation
of the daylight. This reaction has a radical character and it is in
compliance with the known data on the reactions of organic
amines with polyhalogenomethanes. C-centered radicals of
a-silylamines were detected by ESR spectroscopy in reaction
mixtures in CH3CN and C6H6. The a-silylamines hydrochlorides
are formed with high yield, if the reaction was running in these
solvents. If the reaction was running in an excess of CCl4, then the
yield of a-silylamines hydrochlorides decreases significantly and
simultaneously with the increasing of yield of by-products. The
investigation of the reaction mixtures by spectroscopy confirmed
that the solvents (CH3CN and C6H6) were also reagents for the
reaction. Probably, the radicals of a-silylamines break loose an H
atom from solvent. This possibility is absent, if the reaction run in
excess of CCl4 and a decay of radicals of a-silylamines takes place
with the formation of a great quantity of by-products.
N-Methyl-N,N-bis(silatranylmethyl)amine hydrochloride (8)
The reaction required 36 h for full completion of the reaction. The
yield of the crude product was 97% and the yield after
recrystallization (chloroform–hexane, 1:1) was 86%. Anal. calcd.
for C15H32ClN3O6Si2: C 40.76; H 7.30; N 9.51; found: C 40.64; H 7.11;
N 9.60.
N-benzyl-N,N-bis(silatranylmethyl)amine hydrochloride (9)
The reaction required 36 h for full completion of the reaction. The
yield of the crude product was 94% and the yield after
recrystallization (chloroform–hexane, 1:1) was 81%. Anal. calcd.
for C21H36ClN3O6Si2: C 48.68; H 7.00; N 8.11; found: C 48.89, H 7.31,
N 8.06.
Finally, CCl4 is relatively inert organic liquid, which is used
extensively as a solvent in spectroscopic investigations, in
synthetic organic chemistry and in extraction processes.
However, it is necessary to remember at use of CCl4 that this
solvent sometimes can react with a dissolved compound.
N-(silatranylmethyl)diamine hydrochloride (10)
The reaction required 36 h for full completion of the reaction. The
yield of the crude product was 99% and the yield after
recrystallization (chloroform–hexane, 1:1) was 88%. Anal. calcd.
J. Phys. Org. Chem. 2009, 22 144–154
Copyright ß 2008 John Wiley & Sons, Ltd.
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NMR, IR, AND ESR SPECTROSCOPIC INVESTIGATION
for C9H21ClN2O3Si: C 40.21; H 7.87; N 10.42; found: C 40.37; H 8.05;
N 10.63.
of the sample, the glassware was wrapped in black paper. To a
1 mL solution of the a-silylamine (1.5 mmol Lꢀ1) was added a spin
trap (MNP, ND, or PBN).[39] This solution was seated in the
ampoule by a syringe and was degassed by several freeze-thaw
cycles. Then the equimolar amount of the previously degassed
CCl4 was added to the mixture of a-silylamine with the spin trap
by the syringe. This mixture was degassed once again by several
freeze-thaw cycles and the ampoule was sealed. The ampoule
was thawed and the solution was mixed carefully. The ampoule
was irradiated by the daylight for 5 min just before their
submission in the resonator of an ESR spectrometer.
N-(silatranylmethyl)piperidine hydrochloride (11)
The reaction required 36 h for full completion of the reaction. The
yield of the crude product was 92% and the yield after
recrystallization (chloroform–hexane, 1:1) was 83%. Anal. calcd.
for C12H25ClN2O3Si: C 46.66; H 8.16; N 9.07; found: C 46.82; H 7.96;
N 9.15.
N-(trimethylsilylmetyl)dimethylamine hydrochloride (12)
The reaction required 72 h for full completion of the reaction. The
yield of the crude product was 98% and the yield after
recrystallization (chloroform–hexane, 1:1) was 90%. Anal. calcd.
for C6H18ClNSi2: C 42.96; H 10.82; N 8.35; found: C 43.11; H 10.98; N
8.47.
Acknowledgements
We are grateful to Dr. Esfir Brodskaya for her skilful help in IR
spectroscopic investigation of this work and to Professor Bagrat
Shainyan for useful discussion and his help at the preparation of
manuscript. We are grateful to the Russian Foundation for Basic
Research (no. 01–03–32723) for financial support.
N-methyl-N,N-bis(trimethylsilylmetyl)amine hydrochloride (13)
The reaction required 48 h for full completion of the reaction. The
yield of the crude product was 96% and the yield after
recrystallization (chloroform–hexane, 1:1) was 89%. Anal. calcd.
for C9H26ClNSi2: C 45.05; H 10.92; N 5.84; found: C 45.14; H 11.11; N
5.69.
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