Isomerization in the reaction of (aminoalkyl)trialkoxysilanes with carboxylic acid anhydrides

Article abstract of DOI:10.1134/S1070363207010070

The products of the reactions of monofunctional (3-aminopropyl) triethoxysilane and (3-aminopropyl)trimethoxysilane and bifunctional [3-N-(2-aminoethyl)aminopropyl]trimethoxysilane with acetic and maleic anhydrides in CCl₄ at varied reagent molar ratios are studied by ¹H and ¹³C NMR spectroscopy. It is shown that the reaction of [3-N-(2-aminoethyl)aminopropyl]trimethoxysilane with the above anhydrides leads to formation of diamides as mixtures of cisoid and transoid rotamers about both amide bonds. The reactions with monofunctional (aminopropyl) trialkoxysilanes give individual isomers.

Full text of DOI:10.1134/S1070363207010070

ISSN 1070-3632, Russian Journal of General Chemistry, 2007, Vol. 77, No. 1, pp. 47 54.
Pleiades Publishing, Ltd., 2007.
Original Russian Text V.A. Kovyazin, A.V. Nikitin, V.M. Kopylov, I.B. Sokol’skaya, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 1,
pp. 52 60.
Isomerization in the Reaction of (Aminoalkyl)trialkoxysilanes
with Carboxylic Acid Anhydrides
V. A. Kovyazin, A. V. Nikitin, V. M. Kopylov, and I. B. Sokol’skaya
State Research Institute of Chemisty and Technology of Organoelement Compounds,
sh. Entuziastov 38, Moscow, 111123 Russia
e-mail: vlkov@aport.ru
Received August 1, 2006
Abstract The products of the reactions of monofunctional (3-aminopropyl)triethoxysilane and (3-amino-
propyl)trimethoxysilane and bifunctional [3-N-(2-aminoethyl)aminopropyl]trimethoxysilane with acetic and
1
maleic anhydrides in CCl4 at varied reagent molar ratios are studied by H and 13C NMR spectroscopy. It is
shown that the reaction of [3-N-(2-aminoethyl)aminopropyl]trimethoxysilane with the above anhydrides leads
to formation of diamides as mixtures of cisoid and transoid rotamers about both amide bonds. The reactions
with monofunctional (aminopropyl)trialkoxysilanes give individual isomers.
DOI: 10.1134/S1070363207010070
The formation of isomers about the C(O) N bond
(rotamers) in the syntheses of organic amides has been
fairly well documented. An NMR study of the com-
position and structure of amides [1] showed that al-
kylamides of the general formula RC(O)NHR1 [R =
H, Me, Et, i-Bu; R1 = Me, Et, i-Pr, t-Bu, CH(OOH)
CH3, CH(OOH)CHR2CH3. PhCH2, CH(CH3)Ph]
are largely present as a transoid isomer (70 100%). In
the case of N,N-disubstituted alkylamides of the
general formula RC(O)NR1Me(R = H, Me, t-Bu, i-Bu;
R1 = Et, i-Pr, n-Bu, t-Bu and cyclo-C6H11), the
cisoid/transoid ratio is approximately 1:1, except for
HC(O)NMe(t-Bu) which contains 89% of the transoid
isomer. Obviously, such an isomer ratio is due to the
steric effect of the second substituent at the nitrogen
atom.
The amide form can exist as a mixture of the cisoid
and transoid rotamers, as shown by H NMR for N-
1
trimethylsilylated formamide and acetiamide [3], and
1
N-trimethylsilylanilides [4]. The H NMR spectrum
of N-trimethylsilylformanilide at 0 C displayed three
signals of the trimethylsilyl group ( 0.29, 0.18 and
0.06 ppm), two of which were assigned to rotamers
about the amide bond. It was also reported that the
rotation barrier about the amide bond in N-(trimethyl-
1
silyl)formamide is 3 kcal mol lower than in N-tert-
butylformamide [3].
The publications describing reaction of carbofun-
ctional organisilicon amines with carboxylic acid an-
hydrides [5 14] include no information on their iso-
meric composition. Data on reaction of carboxylic
acids with [3-N-(2-aminoethyl)aminopropyl]trime-
thoxysilane are practically absent from the literature.
Burn et al. [2] found that the content of the tran-
soid rotamer of formanilide depends on the concentra-
tion of the compound in CDCl3 and grows from 45
to 73% as the concentration increases from 1.5 to
52.5 mol%. With other compounds [1], no definite
dependence was detected for the rotamer ratio on the
nature and polarity of solvents used in NMR spec-
troscopy and on the amide concentration in the solu-
tion.
In this connection we studied the composition and
structure of products formed by reactions of mono-
functional (3-aminopropyl)triethoxysilane (Ia) and (3-
aminopropyl)trimethoxysilane (Ib) and bifunctional
[3-N-(2-aminoethyl)aminopropyl]trimethoxysilane (Ic)
with acetic and maleic anhydrides at varied molar
reagent ratios. This work continues our studies re-
ported earlier [15].
At R1 = H or SiMe3, tautomeric isomerization is
possible with migration of the hydrogen atom to
oxygen:
The reactions of (aminoalkyl)alkoxysilanes with
anhydrides were carried out in carbon tetrachloride at
20 25 C. The reaction of Ia with equimolar
amount of acetic anhydride can lead to a mixture of
the cisoid and transoid rotamers of [3-N-(acetamido)-
propyl]triethoxyilane and elimination of acetic acid:
H(SiMe3)
OH(SiMe3)
R2
O
N
N
R
R
C
C
R2
47

48
KOVYAZIN et al.
(EtO)3Si(CH2)3NH2 + Ac2O
AcOH
Ia
H
C
H
O
+ (EtO)3CH2CH2CH2
N
+ (EtO)3CH2CH2CH2
N
O
C CH3
CH3
trans isomer
cis isomer
IIa
In an analysis of the spectra of the reaction pro-
proceeds without side processes and leads to forma-
tion of amido acid IIIa:
ducts of Ia with Ac2O, by analogy with [3, 4], the
formation of rotamers was determined from splitting
of the signal of the acetyl group, whose chemical shift
is very sensitive to conformational transformations.
As shows Fig. 1, near 2 ppm there are two signals,
but the downfield signal ( 1.97 ppm) is related to
acetic acid; the proton of the latter with the corres-
ponding integral intensity is registered far downfield
O
(EtO)3Si(CH2)3NH2 +
O
O
(EtO)3SiCH2CH2CH2NHCOCH=CHCOOH
(
10.27 ppm). The CH2N signal, too, provides no
IIIa
clear evidence for the presence of isomers, and as a
whole it corresponds to splitting due to coupling with
three protons of the neighboring CH2 and NH groups.
1
Like with acetic anhydride, the H NMR spectrum
reveals no signs of isomerism: The CH2N group is
registered as a quartet and the CH=CH group, as
The reaction of Ia with maleic anhydride also
3.0
2.6
Fig. 1. Fragment H NMR spectrum of compound IIa.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 1 2007
2.2
1.8
1.4
1.0
, ppm
1

ISOMERIZATION IN THE REACTION OF (AMINOALKYL)TRIALKOXYSILANES
49
3.2
2.8
2.4
2.0
1.6
1.2
0.98
0.4
, ppm
1
Fig. 2. Fragment H NMR spectrum of compound IVa.
two doublets. The carboxyl proton signal is broad and
appears very far downfield ( 15.48 ppm). The ab-
sence of isomerism and the presence of the carboxyl
proton signal give no evidence for the interaction of
the carboxy group with the amide nitrogen, we as-
sumed earlier [15], or this interaction is very
weak.
The reaction of compound IIIa with aminosilane
Ia leads to compound III which contains simultane-
ously an amide bond and ammonium group:
IIIa + (EtO)3SiCH2CH2CH2NH2
(EtO)3SiCaH2CbH2CcH2NHCOCH=CHCOO N+H3CdH2CeH2CfH2Si(OEt)3.
IVa
The absence of isomers in compounds IIIa and
IVa allows the trimethylene CH2 proton signals to be
assigned to the amide and ammonium parts of com-
pound IVa (Fig. 2). The broad signal ( 3.21 ppm) is
related to CH2NHC(O) methylene protons, while the
triplet at 2.83 ppm, to CH2N+H3 methylene protons.
The quintet at 1.74 ppm probably belongs to the
CHb2 group in the amide part, while the quintet at
1.61 ppm, to the CHc2 group in the ammonium part of
compound IVa. From the integral intensity ratio, the
broadened singlet at 7.61 ppm can be assigned to
+NH3 protons and the singlet at 9.50 ppm, to an
NHCO proton.
Thus, we conclude that the position of methylene
proton signals depends on the chemical nature of the
nitrogen atom. The strongest downfield shift is chara-
cteristic of the amide nitrogen in amido acid II (
=
0.85 ppm for CH2N protons) as compared with
the amine nitrogen in compound Ia.
Similar results were obtained with (3-aminopropyl)-
1
trimethoxysilane (Ib). The H NMR spectra (Table 1)
showed that the products of the reactions of Ib with
acetic and maleic anhydrides (IIb and IIIb, respec-
tively), undergo no isomerism at the given tempera-
ture, and that the structure of their organic parts is
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50
KOVYAZIN et al.
(MeO)3Si(CH2)3NH2 + Ac2O
AcOH + (MeO)3Si(CH2)3NHAc,
Ib
IIb
O
(MeO)3Si(CH2)3NHCOCH=CHCOOH.
Ib +
O
IIIb
O
completely the same as the respective structures of
compounds IIa and IIIa.
sible number of isomeric reaction products depends
on the molar ratio of the initial reagents. At a 1:1
molar ratio, a mixture of amides, acetates, and amido-
acetates of various structure and composition can form:
In the reaction of diamine Ic with Ac2O, the pos-
(MeO)3Si(CH2)3NH(CH2)2NH2 + Ac2O
(MeO)3Si(CH2)3N(CH2)2N+H3 OAc
Ac
Ic
IIc
+ (MeO)3Si(CH2)3N+H2(CH2)2NHAc + (MeO)3Si(CH2)3N(CH2)2NHAc
OAc
Ac
IIIc
IVc
+ (MeO)3Si(CH2)3N+H2(CH2)2N+H3OAc .
OAc
Vc
The acetyl group can occupy positions at the first
or second nitrogen atom (structures IIc and IIIc, res-
pectively), that is, two geometric isomers are possible.
Each isomer can exist as two rotamers about the
amide bond. Furthermore, double amide IVc and
double salt Vc can form. Compound IVc includes two
centers of isomerism and can exist as four isomers.
Compound Vc should not have isomers due to the free
proton exchange between the ammonium groups.
Thus, at equimolar reagent ratio, nine isomers can
be present in the reaction mixture.
At a 1:2 diamine:Ac2O molar ratio, this reaction
can afford four isomers of [3-[N-(2-acetamidoethyl)-
acetamido]propyl]trimethoxysilane (IVc) which has
two centers of isomerism, and free acetic acid.
Table 1. 1H chemical shifts in the reaction systems involving compounds Ia or Ib
Chemical shift, , ppm
Reaction systems
and compounds
SiCH2
CH2CH2CH2
CH2N
NHn
CH3CH2O
CH2CH2O COOH RCOOH
(EtO)3Si(CH2)3NH2
(EtO)3Si(CH2)3NH2
+ AcOH
(EtO)3Si(CH2)3NH2
+ Ac2O
(EtO)3Si(CH2)3NH2
+ MA
(MeO)3Si(CH2)3NH2 0.48 t
+ Ac2O
0.45 t
0.56 t
1.37 quintet
1.66 quintet
2.51 t
2.72 t
1.05 s
8.21 s
1.05 t
1.13 t
3.64 q
3.72 q
3.73 q
3.77 q
1.81 s
0.54 t
0.62 t
1.54 quintet
1.68 quintet
1.44 quintet
1.62 quintet
3.14 q
3.32 q
3.03 q
3.26 q
6.36 s
8.56 s
1.14 t
1.13 t
10.27
15.48
10.74
16.92
1.90 s
1.97 s
6.24 d
6.48 d
1.82 s
1.88 s
6.21 d
6.54 d
6.77 s 3.39 s CH3O
8.63 s 3.46 s CH3O
(MeO)3Si(CH2)3NH2 0.57 t
+ MA
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 1 2007

ISOMERIZATION IN THE REACTION OF (AMINOALKYL)TRIALKOXYSILANES
51
(MeO)3Si(CH2)3NH(CH2)2NH2 + 2Ac2O
Ic
(MeO)3Si CaH2 CbH2 CcH2 N1 CdH2 CeH2 N2H Ac + 2AcOH.
Ac
IVc
1
The H NMR spectrum of the reaction product of
Ic with Ac2O at a 1:2 molar ratio shows five acetyl
signals ( 1.80, 1.83, 1.89, 1.94, and 1.96 ppm), one
of which belongs to acetic acid. The 13C NMR spec-
trum of compound IV also confirms the presence in
the reaction mixture of four isomers: It contains four
upfield from that of the transoid carbon atom. This
fact suggests that Ce ( 44.64 ppm) is cisoid and Cd
(
51.17 ppm) is traCnsoid to the carbonyl group.
C
Consequently, the signals at
47.82 and 47.19 ppm
C
relate to transoid-Ce and cisoid-Cd (Fig. 3a). Hence,
the preferable molecular conformation of Vc is [trans-
3-[cis-N-(2-acetamidoethyl)acetamido]propyl]trime-
thoxysilane.
signals of the carbonyl group ( 171.4 172.2 ppm)
C
and four signals of the CH3C(O) group (
20.6
22.2 ppm). Note that each carbon atom in Cthe tri-
methylene bridge gives two signals. This fact suggests
that that the chemical shift of the trimethylene carbon
signals is affected by the conformation of the amide
bond of the nearest nitrogen atom. The NCH2CH2N
methylene carbons are registered as four signals,
rather than eight expected, two signals for each isomer.
Thus, the reaction of diamine Ic with two mole-
cules of acetic anhydride leads to formation of the
maximum number of possible isomers.
1
The H NMR spectrum of the reaction mixture of
Ic and Ac2O in a 1:1 molar ratio is more complicated
and does not allow judging about the number of iso-
mers, because the acetyl spectral region which is in
the most informative in this case contains overlapping
signals. However, the 13C NMR spectrum attests the
presence of practically all isomers: The carbonyl
region contains nine peaks, eight of which belong to
the acetamido groups ( 170.3 171.5 ppm) and one
to the acetoxy anion ( C 177.4 ppm). The acetoxy
carbonyl carbon signal oCf the reaction product of the
The 13C NMR spectrum allows estimation of the
conformer ratio for different amide groups. The in-
tegral intensity ratio of the 13C(a), 13C(c), 13C(d), and
13C(e) signals of the isomers (Fig. 3) gives evidence
showing that the conformer ratio varies from 1:3 to
1:4 and is equal for both nitrogen atoms (Fig. 3a).
It has been shown on an example of N,N-dibutyl-
formamide [16] that the signal of the -carbon atom
in the cisoid position to the carbonyl oxygen is shifted
diamine with acetic acid falls in the same region (
C
177.5 ppm) (Tables 2 and 3).
Table 2. 1H chemical shifts in the reaction systems involving compound Ic
Chemical shift, , ppm
Reaction mixtures
and compounds
SiCH2
CH2CH2CH2 CH2N
NCH2
2.34 t
CH2N
MHn RCOOH CH3O
3.25 s
(MeO)3Si(CH2)3NH(CH2)2NH2
(MeO)3Si(CH2)3NH(CH2)2NH2
+ Ac2O
0.35 t
0.29 t
0.36 t
1.29 quintet 2.46 t
2.30 t 0.81 s
1.39 m
2.59 m 3.05 m 3.14 m 7.28 s 1.81 s 3.28 s
1.49 quintet 2.75 t
7.57 s
8.33 s
9.57 s
(MeO)3Si(CH2)3NH(CH2)2NH2
+ 2Ac2O
0.45 t
1.54 m
3.14
3.26 m 3.34 m 7.01 s 10.9 s 2.08 s
7.17 s
7.26 s
(MeO)3Si(CH2)3NH(CH2)2NH2 0.49 quintet 1.56 quintet 2.78 t
3.11
3.63
3.50
8.25 s
8.93 s 6.20 m
9.62 s
10.75 s
8.49 s
5.80
3.50 s
3.53 s
+ MA
0.61 quintet
1.72 m
2.93 t
(MeO)3Si(CH2)3NH(CH2)2NH2
+ 2MA
0.55 t
1.66 quintet 3.34 t
6.19
6.75 m
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52
KOVYAZIN et al.
(a)
50
(b)
46
42
38 , ppm
, ppm
8.0
6.0
4.0
13
d
e
c
a
Fig. 3. Fragment C NMR spectrum of compound IVc for the (a) NC H C H N and NC H and (b) SiC H groups.
2
2
2
2
Besides, the carbon signals of each CH2 group in
a total of six signals should be observed.
the trimethylene fragment is registered as six peaks,
providing evidence for the assumption that the 13Ca c
chemical shifts are affected exclusively by the con-
formation of the amide bond of the nearest nitrogen
atom. For example, the Ca atom of the SiCH2 group
in compounds IIc and IVc should give 2 signals,
while that in compounds IIIc and Vc, one signal, i.e.
In the reaction of diamine Ic with maleic anhydride,
the number of possible reaction products should also
be defined by the molar ratio of the reagents. At an
equimolar ratio, the reaction mixture can contain
compounds VIc and VIIc, diamido diacid VIIIc, and
parent diamine Ic:
O
(MeO)3Si(CH2)3NH(CH2)2NH2 +
(MeO)3SiCH2CH2CH2NHCH2CH2NHC(O)CH=CHCOOH
O
Ic
VIc
O
+ (MeO)3SiCH2CH2CH2N1CH2CH2N2HC(O)CH=CHCOOH + Ic
+ (MeO)3Si(CH2)3N(CH2)2NH2
C(O)CH=CHCOOH
VIIIc
C(O)CH=CHCOOH
VIIc
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ISOMERIZATION IN THE REACTION OF (AMINOALKYL)TRIALKOXYSILANES
53
Table 3. 13C chemical shifts in the reaction systems involving compound Ic
Chemical shift, , ppm
Reaction mixtures
and compounds
SiCH2
CH2CH2CH2
CH2N
NCH2CH2N RCOOH CH3C(O)N C(O)N
CH3O
49.77
(MeO)3Si(CH2)3NH
(CH2)2NH2
6.16
22.67
41.37
51.89, 51.93
(MeO)3Si(CH2)3NH
(CH2)2NH2 + Ac2O
5.77, 6.03, 19.50, 19.78, 35.68, 35.76, 44.67, 46.03, CH3CO
6.09, 7.20, 19.92, 20.23, 36.08, 36.94, 46.83, 47.03, 177.38
7.24, 7.41 20.33, 20.71 37.67, 38.44 47.20, 47.25,
47.68, 50.23, CH3CO
20.87,
20.94,
22.30,
22.56
170.33, 49.43,
170.49, 49.84,
170.56, 50.06,
170.84,
171.03,
171.10,
171.52
50.12
50.29, 51.27, 23.19
49.67
(MeO)3Si(CH2)3NH
(CH2)2NH2 + 2Ac2O
5.70, 5.96 20.20, 20.34 37.55, 38.14 44.64, 47.19, CH3CO
20.58,
20.66,
22.09,
22.21
171.38, 50.07,
47.82, 51.17 174.18
171.49,
171.57,
172.19
50.13
CH3CO
21.57
(MeO)3Si(CH2)3NH
(CH2)2NH2 + 2MA
5.70, 6.12 20.09, 21.70
37.35
49.82, 49.94, CH=CH C(O)NCH2=CH2COOH 50.54,
50.66, 51.85 128.03, 165.74, 165.98, 166.31, 52.39
131.95, 166.48, 166.72, 166.83,
134.17, 167.16, 168.11
135.08
The neutral reaction of the reaction mixture in-
dicates that the carboxy groups of the formed amido-
acids react with residual amino groups to form both
intra- and intermolecular ammonium salts.
is acylation of the secondary amino group by maleic
anhydride with subsequent formation of an am-
monium salt with a primary amino group. The shapes
of the SiCH2 (two quartets or, probably, overlap-
ping triplets) and C CH2 C (complex multiplets)
signals suggest cis trans isomerism about the amide
bond in the reaction products.
1
Analysis of the H NMR spectra does not allow
determining the number of isomers. However, the
splitting patterns and chemical shifts of the proton
signals of the trimethylene fragment attest the pres-
ence of both the amide and ammonium forms of N1.
The formation of the cisoid and transoid isomers
1
about the amide bond is confirmed by the H and 13C
1
In the H NMR spectrum (Fig. 4), the SiCH2 and
NMR spectra of the reaction product of the diamine
with two molecules of maleic anhydride. The reaction
mixture can contain four isomers, because the product
has two centers of isomerism about the amide bond.
C CH2 C signals are doubled ( 0.49, 0.61 and
1.56, 1.72 ppm, respectively). The higher intensity of
the downfield signals related to amide N1 provide
evidence to show that the predominant reaction route
O
(MeO)3Si(CH2)3NH(CH2)2NH2 + 2
O
O
(MeO)3SiCH2CH2CH2NCH2CH2NHC(O)CH=CHCOOH
C(O)CH=CHCOOH
VIIIc
The presence of all possible isomers is confirmed
Thus, unlike (aminopropyl)trialkoxysilanes Ia and
Ib, diamine Ic reacts with acetic and maleic anhyd-
rides to form a mixture of cisoid and transoid isomers
by the observation of eight carbonyl carbon signals in
the 13C NMR spectrum of compound VIIIc (Table 3).
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54
KOVYAZIN et al.
2.0
1.6
1.2
0.8
0.4
0
, ppm
1
Fig. 4. Fragment H NMR spectrum of compound VIIIc.
about both amide bonds. The resulting data are rather
unexpected, since, according to published data, isomer
formation is more probable with primary amines Ia
and Ib.
5. Zhdanov, A.A., Pakhomov, V.I., and Shaldo, N.I., Izv.
Akad. Nauk SSSR, 1970, no. 11, p. 2538.
6. US Patent 3576031, Ref. Zh. Khim., 1972, 3C414P.
7. US Patent 3325450, Ref. Zh. Khim., 1968, 19C293P.
8. Czech Inventor’s Certificate 221786, Ref. Zh. Khim.,
EXPERIMENTAL
1986, 4N77P.
1
The H and 13C NMR spectra were registered on a
9. US Patent 4800125, Ref. Zh. Khim., 1989, 23T 246P.
Bruker AM-360 instrument (360 MHz) at 300 K,
solvent CDCl3. Syntheses were performed according
to the procedures described in [15], solvent CCl4.
10. Czech Inventor’s Certificate 215229, Ref. Zh. Khim.,
1985, 17N176P.
11. US Patent 5254621, Chem. Abstr., 1994, vol. 120,
N10310m.
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