Synthesis of cyanohydrin trimethylsilyl ethers of acylferrocenes

Article abstract of DOI:10.1016/S0277-5387(03)00262-6

The addition of trimethylsilyl cyanide to acylferrocenes FcCOR (R = Me, Et, ⁿPr, ⁱPr, Ph, p-MeOC₆H₄, o-ClC₆H₄, m-ClC₆H₄, p-ClC₆H₄, Fc; Fc = C

Full text of DOI:10.1016/S0277-5387(03)00262-6

Polyhedron 22 (2003) 1523ꢃ1528
/
www.elsevier.com/locate/poly
Synthesis of cyanohydrin trimethylsilyl ethers of acylferrocenes
Zhan-Xi Bian *, Hai-Ying Zhao, Bao-Guo Li
College of Chemistry and Chemical Engineering, Inner Mongolia University, Huhhot 010021, China
Received 6 January 2003; accepted 13 March 2003
Abstract
The addition of trimethylsilyl cyanide to acylferrocenes FcCOR (Rꢀ
/
Me, Et, nPr, iPr, Ph, p-MeOC6H4, o-ClC6H4, m-ClC6H4, p-
ClC6H4, Fc; FcꢀC5H4FeC5H5) catalyzed by zinc iodide in methylene chloride provided the corresponding cyanohydrin
/
trimethylsilyl ethers in moderate to high yields. Factors affecting the reaction and yields of adducts were investigated. All new
compounds were characterized by IR and 1H NMR spectroscopies. In addition, the adducts of p-chlorobenzoylferrocene and
diferrocenyl ketone were structurally determined by X-ray crystallography.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Acylferrocene; Trimethylsilyl cyanide; Cyanohydrin trimethylsilyl ether
1. Introduction
reports on the preparation of cyanohydrin trimethylsilyl
ethers of acylferrocenes except for formylferrocene [11].
In this paper we describe the synthesis of cyanohydrin
trimethylsilyl ethers of acylferrocenes via the addition of
TMSCN to various acylferrocenes in the presence of
ZnI2 in CH2Cl2 (Scheme 1). Factors affecting the
reaction and yields of adducts were investigated. The
crystal structures of compounds 3i and 3j were deter-
mined by X-ray diffraction analysis.
Cyanohydrin trimethylsilyl ethers are important in
organic synthesis since they serve not only for the
protection of carbonyl groups [1] but also as versatile
intermediates [2] for the synthesis of cyanohydrins, a,b-
unsaturated nitriles and b-aminoalcohols. The general
method for the preparation of cyanohydrin trimethylsi-
lyl ethers is the addition of trimethylsilyl cyanide
(TMSCN) to carbonyl compounds with the aid of a
catalyst including Lewis acids, such as ZnI2 [2a,2c,3] and
AlCl3 [4], as well as solubilized anionic species, such as
KꢁCNꢂ/18-Crown-6 and nBu4NꢁCNꢂ [5]. Other
catalysts like Lewis bases, inorganic solid acids and
bases [6], copper triflate (Cu(OTf)2) [7], tetracyanoethy-
lene [8] and some chiral catalysts [9] have been used in
this reaction. In addition, one-pot cyanosilylation of
carbonyl compounds has also been carried out by the
combined use of Me3SiCl and KCN (or NaCN) [10].
In view of their synthetic potential and ready pre-
paration method, a wide variety of cyanohydrin tri-
methylsilyl ethers of aldehydes, aliphatic ketones and
conjugated aromatic ketones and their derivatives have
2. Experimental
2.1. General
Melting points were obtained by the XT7-4 apparatus
and are uncorrected. 1H NMR spectra were recorded in
CDCl3 solutions on a Bruker DRX-500 spectrometer
with Me4Si as internal standard. IR spectra were
recorded on a NEXUS-670FT-IR spectrophotometer
using KBr pellets. Elemental analysis were carried out
on a Perkinꢃ
collected on a Bruker Smart 1000 diffractometer. Silica
gel (60H) plates and alkaline Al2O3 (100ꢃ200 mesh)
/Elmer-2400 apparatus. X-ray data was
been synthesized [3ꢃ10]. Nevertheless, there are few
/
/
were used for analytical TLC and column chromato-
graphy, respectively. TMSCN [12] and acylferrocenes
[13] were prepared according to literature methods. ZnI2
was oven-dried at 120 8C overnight. Dry CH2Cl2 was
* Corresponding author. Tel.: ꢁ
499-2984.
E-mail address: fxby@sina.com (Z.-X. Bian).
/
86-471-499-2981; fax: ꢁ86-471-
/
0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0277-5387(03)00262-6

1524
Z.-X. Bian et al. / Polyhedron 22 (2003) 1523ꢃ
/1528
Scheme 1.
distilled from P2O5. All other solvents and regents were
obtained from commercial sources and used without
further purification.
1.97 (s, 3H, CCH3), 4.28, 4.32, 4.49 (3s, together 9H,
ferrocenyl-H). Calc. for C16H21FeNOSi: C, 58.72; H,
6.47; N, 4.28. Anal. Found: C, 58.13; H, 7.03; N, 3.99%.
3b. Yellow crystal, m.p.: 46ꢃ
(Fc, CH), 2230 (CÅ
N), 1253, 833 (SiCH3) cmꢂ1
1H NMR (CDCl3, 500 MHz) d 0.12 (s, 9H, Si(CH3)3),
1.15 (t, 3H, Jꢀ7.3 Hz, CH2CH3), 2.09 (m, 1H, CHH),
2.21 (m, 1H, CHH), 4.27, 4.30, 4.48 (3s, together 9H,
ferrocenyl-H). Calc. for C17H23FeNOSi: C, 59.8; H,
6.79; N, 4.10. Anal. Found: C, 59.91; H, 7.02; N, 4.24%.
/
48 8C. IR (KBr) n 3097
/
;
2.2. General procedure for the synthesis of 3aꢃ3j
/
/
Into a 100 ml 3-neck round-bottomed flask equipped
with magnetic stirring bar, reflux conditioner and CaCl2
drying tube was placed acylferrocene (10 mmol) in dry
CH2Cl2 and ZnI2 (0.5ꢃ/1 mmol). After stirring for 20-
3c. Orange crystal, m.p.: 60ꢃ
(Fc, CH), 2233 (CÅ
N), 1252, 843 (SiCH3) cmꢂ1
1H NMR (CDCl3, 500 MHz) d 0.12 (s, 9H, Si(CH3)3),
1.01 (t, Jꢀ7.4 Hz, 3H, CH2CH3), 1.61 (sext, Jꢀ7.8 Hz,
/
61 8C. IR (KBr) n 3092
min, TMSCN (20ꢃ80 mmol) was added. The resulting
/
/
;
solution was stirred under the conditions given in Table
1. The progress of the reaction was monitored by TLC.
On completion, the solvent was evaporated under
reduced pressure and the residue obtained was extracted
with pentane. The solution was washed with saturated
cold aqueous NaHSO3 and dried over Na2SO4. Re-
moval of the solvent under reduced pressure yielded the
crude product that was purified by recrystallization
/
/
2H, CH2CH3), 2.03 (m, 1H, CHHCH2CH3), 2.14 (m,
1H, CHHCH2CH3), 4.26, 4.29, 4.46 (3s, together 9H,
ferrocenyl-H). Calc. for C18H25FeNOSi: C, 60.84; H,
7.09; N, 3.94. Anal. Found: C, 60.74; H, 7.59; N, 3.94%.
3d. Orange crystal, m.p.: 25ꢃ
(Fc, CH), 2235 (CÅ
N), 1252, 839 (SiCH3) cmꢂ1; H
NMR (CDCl3, 500 MHz) d 0.37 (s, 9H, Si(CH3)3), 0.87
(dd, Jꢀ6.7 Hz, 6H, CH(CH3)2), 2.19 (sept, Jꢀ6.7 Hz,
/
27 8C. IR (KBr) n 3106
1
/
from pentaneꢃ
pound 3j was purified by alkaline Al2O3 column
chromatography using 10:1 petroleum ether (60ꢃ
90 8C)*ethylether as eluent followed by recrystalliza-
tion.)
3a. Yellow crystal, m.p.: 55ꢃ
/
ethylether. (The crude product of com-
/
/
/
1H, CH(CH3)2), 4.17, 4.20, 4.26, 4.29 (4s, together 9H,
ferrocenyl-H). Calc. for C18H25FeNOSi: C, 60.84; H,
7.09; N, 3.94. Anal. Found: C, 61.63; H, 7.64; N, 3.49%.
/
/
56 8C. IR (KBr) n 3082
(Fc, CH), 2230 (CÅ
/
N), 1254, 835 (SiCH3) cmꢂ1
1H NMR (CDCl3, 500 MHz) d 0.12 (s, 9H, Si(CH3)3),
;
3e. Yellow crystal, m.p.: 43ꢃ
/
44 8C. IR (KBr) n 3095
(Fc, CH), 3060, 3026 (Ar, CH), 2243 (CÅ/N), 1253, 851
Table 1
Influence of substrates and reaction conditions on the addition
Substrate 1aꢃ
/j
R
Reaction conditions
Ratio (2:1aꢃj)
Yield a (%) 3aꢃ
j
/
/
Time (h)
Temperature (8C)
a
b
c
CH3
C2H5
CH2CH2CH3
2:1
9
5
r.t.
r.t.
r.t.
95.6
96.8
93.0
2.2:1
2.2:1
13
d
e
f
CH(CH3)2
Ph
2.2:1
3.6:1
16
16
r.t.
r.t.
80.5
99
ꢀ
/
3:1
4:1
21.5
21.5
r.t.
r.t.
65.0
99
ꢀ
/
p-CH3OPh
3:1
5.5:1
23
23
r.t.
r.t.
52.3
71.5
g
h
i
o-ClPh
m-ClPh
p-ClPh
3:1
3:1
3:1
48
23
23
reflux
reflux
r.t.
32.1
60.6
70.8
j
Fc
5:1
8:1
46
46
reflux
reflux
72.4
88.5
a
Yield of isolated product.

Z.-X. Bian et al. / Polyhedron 22 (2003) 1523ꢃ
/1528
1525
1
(SiCH3) cmꢂ1; H NMR (CDCl3, 500 MHz) d 0.15 (s,
9H, Si(CH3)3), 4.08, 4.20, 4.26, 4.29, 4.53 (5s, together
Table 2
Crystal data and structure refinement for compounds 3i and 3j
9H, ferrocenyl-H), 7.35ꢃ7.64 (m, 5H, Ar). Calc. for
/
Compound
3i
3j
C21H23FeNOSi: C, 64.78; H, 5.95; N, 3.60. Anal.
Found: C, 64.77; H, 6.38; N, 3.52%.
Empirical formula
Formula weight
Crystal system
Space group
C21H22ClFeNOSi C25H27Fe2NOSi
423.79
monoclinic
P21/c
497.27
monoclinic
P21/c
3f. Orange crystal, m.p.: 91.5ꢃ
/
92.5 8C. IR (KBr) n
N), 1250,
3108 (Fc, CH), 3080, 3050 (Ar, CH), 2231 (CÅ
/
847 (SiCH3) cmꢂ1; 1H NMR (CDCl3, 500 MHz) d 0.14
(s, 9H, Si(CH3)3), 3.84 (s, 3H, OCH3), 4.06, 4.18, 4.24,
˚
a (A)
18.420(2)
7.3401(7)
16.401(2)
90
13.820(1)
20.094(2)
17.127(2)
90
˚
b (A)
˚
c (A)
4.28, 4.50 (5s, together 9H, ferrocenyl-H), 6.90 (d, Jꢀ
/
a (8)
b (8)
8.7 Hz, 2H, Ar), 7.53 (d, Jꢀ8.7 Hz, 2H, Ar). Calc. for
/
109.878(2)
90
105.114(2)
90
C22H25FeNO2Si: C, 63.01; H, 6.01; N, 3.34. Anal.
Found: C, 62.45; H, 6.12; N, 3.04%.
g (8)
3
V (A )
˚
2085.4(4)
4
1.350
4591.4(7)
8
1.439
3g. Orange crystal, m.p.: 78ꢃ
/
79 8C. IR (KBr) n 3105
Z
Calculated density (Mg mꢂ3
)
(Fc, CH), 3064 (Ar, CH), 2238 (CÅ
/
N), 1256, 848
Absorption coefficient (mmꢂ1
F(0 0 0)
)
0.918
880
1.332
2064
(SiCH3) cmꢂ1; H NMR (CDCl3, 500 MHz) d 0.23 (s,
9H, Si(CH3)3), 4.24, 4.33, 4.56 (3s, together 9H,
1
u range for data collection (8) 2.35ꢃ
Limiting indices
/
28.27
245h 5
95k 59,
205l 521
11 854/4773
1.53ꢃ
135
255
225
27 943/10 698
/
28.32
h 5
k 5
l 517
ferrocenyl-H), 7.32ꢃ7.84 (m, 4H, Ar). Calc. for
/
ꢂ
/
/
/16,
ꢂ
/
/
/
18,
C21H22ClFeNOSi: C, 59.52; H, 5.23; N, 3.31. Anal.
Found: C, 58.95; H, 5.54; N, 3.67%.
ꢂ
/
/
/
ꢂ
/
/
/
26,
ꢂ/
/
/
ꢂ/
/
/
Reflection collection/unique
Max. and min. transmission
3h. Orange crystal, m.p.: 68ꢃ
/
69 8C. IR (KBr) n 3090
1.0000 and 0.6334 1.0000 and
0.7690
(Fc, CH), 3024 (Ar, CH), 2232 (CÅ
/
N), 1253, 847
(SiCH3) cmꢂ1; H NMR (CDCl3, 500 MHz) d 0.16 (s,
9H, Si(CH3)3), 4.09, 4.24, 4.31, 4.52 (4d, together 9H,
1
Data/restraints/parameters
Goodness-of-fit on F2
4773/0/255
0.882
10 698/0/743
0.871
Final R indices [I ꢀ
wR2
/
2s(I)] R1, 0.0418, 0.0926
0.0408, 0.0700
ferrocenyl-H), 7.29ꢃ7.62 (m, 4H, Ar). Calc. for
C21H22ClFeNOSi: C, 59.52; H, 5.23; N, 3.31. Anal.
Found: C, 59.45; H, 5.69; N, 2.99%.
/
R indices (all data) R1, wR2
Largest difference peak and
0.0667, 0.0985
0.667, ꢂ0.276
0.0750, 0.0783
/
0.483, ꢂ0.249
/
ꢂ3
3i. Red Orange crystal, m.p.: 115ꢃ
/
116 8C. IR (KBr) n
N), 1253, 847
˚
hole (e A
)
3108 (Fc, CH), 3042 (Ar, CH), 2236 (CÅ
/
1
(SiCH3) cmꢂ1; H NMR (CDCl3, 500 MHz) d 0.17 (s,
9H, Si(CH3)3), 4.02, 4.19, 4.25, 4.26, 4.47 (5s, together
3. Results and discussion
9H, ferrocenyl-H), 7.37 (d, Jꢀ
/
8.5 Hz, 2H, Ar), 7.56 (d,
Jꢀ8.5 Hz, 2H, Ar). Calc. for C21H22ClFeNOSi: C,
59.52; H, 5.23; N, 3.31. Anal. Found: C, 59.62; H, 5.41;
N, 3.13%.
/
3.1. Factors affecting the reaction
Although the addition reaction of carbonyl com-
pounds with TMSCN in the presence of ZnI2 usually
can be carried out under mild conditions to give the
cyanohydrin trimethylsilyl ethers in good yields, we have
found that the reaction is still influenced by some factors
such as steric effect, electronic effect and TMSCN/
substrate ratio when the substrates are acylferrocenes.
3j. Red Orange crystal, m.p.: 120ꢃ
3088 (Fc, CH), 2239 (CÅ
N), 1251, 843 (SiCH3) cmꢂ1
1H NMR (CDCl3, 500 MHz) d 0.05 (s, 9H, Si(CH3)3),
4.24, 4.28, 4.30, 4.47 (4s, together 18H, 2ꢄferrocenyl-
H). Calc. for C25H27Fe2NOSi: C, 60.38; H, 5.47; N,
2.82. Anal. Found: C, 59.55; H, 5.62; N, 2.57%.
/
121 8C. IR (KBr) n
/
;
/
2.3. Single crystal X-ray diffraction analysis of 3i and 3j
Data of 3i and 3j were collected on a Bruker Smart
3.1.1. Influence of steric effect
We investigated the influence of steric effect of a
substrate and found that the reaction was very sensitive
to the steric hindrance near the carbonyl group. As is
shown in Table 1, the reaction activity and yield were
high when the substrate bears a less bulky group as in 1a
1000 4-circle diffractometer with Mo Ka radiation (lꢀ
/
˚
0.71073 A) at 293 K in the u ꢃ2u scan mode, and
/
corrected for absorption semiempirically. The two
structures were solved by direct methods (SHELXL-97)
and refined by full-matrix least-squares. The non-
hydrogen atoms were refined anisotropically, whereas
the hydrogen atoms were placed in calculated positions
and refined isotropically. All calculations were per-
formed using the SHELXTL program system. Crystal
data and structure refinement parameters are listed in
Table 2.
(Rꢀ
1d (Rꢀi
r.t.) and the yield of 3d decreased to 80.5%. Similarly,
the steric hindrance increases in the sequence: 1gꢀ1hꢀ
/
Me) and 1b (Rꢀ/Et), while in the case of hindered
/
Pr) the reaction rate was relatively slow (16 h at
/
/
1i, but there is inversion of the order of reaction activity.
For example, treatment of 1i at room temperature for 23
h gave 3i in 70.8% yield, but when 1g was treated at high

1526
Z.-X. Bian et al. / Polyhedron 22 (2003) 1523ꢃ1528
/
Table 3
˚
Selected bond distances (A) and angles (8) of 3i and 3j
3i
3j
Bond distances
OÃ
C(10)Ã
C(11)Ã
C(11)Ã
NÃ
/
C(11)
1.413(3)
1.507(3)
1.528(3)
1.500(3)
1.131(3)
SiÃ
SiÃ
SiÃ
SiÃ
/
O
1.655(2)
1.858(3)
1.844(3)
1.857(4)
1.740(2)
O(1)Ã
C(10)Ã
C(11)Ã
C(11)Ã
N(1)Ã
/
C(11)
1.420(3)
Si(1)Ã
Si(1)Ã
Si(1)Ã
Si(1)Ã
/
O(1)
1.642(2)
1.842(4)
1.855(5)
1.832(5)
/
C(11)
C(12)
C(21)
/
C(18)
C(19)
C(20)
/
C(11)
C(12)
C(13)
1.514(3)
1.490(3)
1.518(3)
1.137(3)
/
C(23)
C(24)
C(25)
/
/
/
/
/
/
/
/
/
C(21)
C(15)Ã
/Cl
/
C(12)
Bond angles
OÃ
OÃ
OÃ
C(10)Ã
C(21)Ã
C(21)Ã
C(11)Ã
/
C(11)Ã
C(11)Ã
C(11)Ã
/
C(10)
C(12)
C(21)
106.8(2)
110.1(2)
110.2(2)
112.1(2)
108.5(2)
109.0(2)
132.7(1)
OÃ
OÃ
OÃ
C(19)Ã
C(19)Ã
C(20)Ã
NÃC(21)Ã
/
SiÃ
SiÃ
SiÃ
/
C(18)
C(19)
C(20)
108.7(1)
102.8(1)
109.9(1)
110.8(2)
112.5(2)
111.8(2)
179.6(3)
O(1)Ã
O(1)Ã
O(1)Ã
C(12)Ã
C(10)Ã
C(12)Ã
C(11)Ã
/
C(11)Ã
C(11)Ã
C(11)Ã
C(11)Ã
C(11)Ã
C(11)Ã
O(1)Ã
/
C(10)
C(12)
C(13)
108.0(2)
106.6(2)
112.0(2)
109.1(2)
112.2(2)
108.7(2)
138.2(2)
O(1)Ã
O(1)Ã
O(1)Ã
C(23)Ã
C(25)Ã
C(25)Ã
N(1)ÃC(12)Ã
/
Si(1)Ã
Si(1)Ã
Si(1)Ã
Si(1)Ã
Si(1)Ã
Si(1)Ã
/
C(23)
C(24)
C(25)
102.9(2)
112.8(2)
110.9(2)
109.4(2)
111.8(3)
109.0(4)
179.4(3)
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
C(11)Ã
C(11)Ã
C(11)Ã
OÃSi
/
C(12)
C(10)
C(12)
/
SiÃ
SiÃ
SiÃ
/
C(18)
C(20)
C(18)
C(11)
/
/
C(10)
C(13)
C(13)
/
/
C(24)
C(23)
C(24)
C(11)
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/
/Si(1)
/
/
temperature (reflux) for 48 h compound 3g was obtained
only in 32.1% yield. With two bulky ferrocenyls near the
carbonyl group, compound 1j was treated with a large
amount of TMSCN (8 equiv.) to give 3j in 88.5% yields.
Obviously, the higher steric hindrance of an acyl residue,
the more difficult is the addition reaction.
that of unsubstituted substrate 1e (yield 65.0%), while in
the case of substrate 1f bearing an electron donating
group (p-MeO) the yield of adduct decreased (52.3%).
This can be explained by the fact that an electron
donating group decreases the electrophilicity of the
CÄ
while on the contrary, an electron withdrawing group
O carbon atom
/O carbon atom and deactivates the addition reaction,
increases the electrophilicity of the CÄ
/
3.1.2. Influence of electronic effect
and facilitates the reaction.
We investigated also the dependence of yields on
benzene ring substituents by reacting three typical
acylferrocenes 1f, 1e and 1i with TMSCN under the
same reaction condition (the ratio of 2 to 1 was 3:1, r.t.,
23 h) (Table 1). It was found that the yield of adduct was
generally better in the case of 1i which contains an
electron withdrawing group (p-Cl, yield 70.8%) than
3.1.3. TMSCN/substrate ratio
In order to conduct the reaction smoothly TMSCN
must be in excess (Table 1). We found that an increased
amount of TMSCN led to a higher yield of the target
product for the same substrate. For instance, changing
the ratio of TMSCN to 1e from 3:1 to 4:1 resulted in
increasing the yield of 3e from 65 to 99%. In addition,
substrates of large steric hindrance need more TMSCN
than those of less steric hindrance.
3.2. Spectral properties
The cyanohydrin trimethylsilyl ethers of acylferro-
cenes were characterized by means of spectroscopic
methods. The IR spectra of compounds 3aꢃj showed
/
the presence of ferrocenyl, nitrile and silyl groups in
region of 3090, 2235 and 1253 cmꢂ1, respectively. In the
1H NMR spectra, nine (CH3)3Si protons appeared at d
0.37ꢃ
resonated in the region of 4.56ꢃ
the two methyl protons (CH(CH3)2) of 3d were char-
acterized by a typical double doublet at d 0.87 (Jꢀ6.7
/0.05 as a singlet, and the ferrocenyl protons
/4.06 ppm. In addition,
/
Hz) due to the influence of the chiral carbon atom
attached to the isopropyl group. Analogously, the
1H NMR spectrum of 3b showed two groups of multi-
plets for methylene protons at d 2.21 and 2.09. Because
of the deshielding effect of the phenyl ring, the chemical
Fig. 1. X-ray structure of 3i.

Z.-X. Bian et al. / Polyhedron 22 (2003) 1523ꢃ
/1528
1527
Fig. 2. X-ray structure of 3j.
shift of OCH3 protons in 3f occurred at a lower field (d
3.84) than those of alkylmethylether (d 3.20).
UK (fax: ꢁ44-1223-336033; e-mail: deposit@ccdc.cam.
ac.uk or www: http://www.ccdc.cam.ac.uk/conts/
retrieving.html).
/
3.3. X-ray structure analysis of 3i and 3j
Acknowledgements
To firmly establish the structures of the new com-
pounds 3aꢃj, X-ray crystallographic analysis of com-
/
Financial support from the National Natural Science
Foundation of China (Grant no. 20062004) is gratefully
acknowledged.
pounds 3i and 3j was carried out. Selected bond
distances and angles are given in Table 3. The molecular
structures are shown in Figs. 1 and 2. The asymmetric
unit of 3j contains two independent molecules, only one
of which is presented. The structure of 3i is very similar
to that of 3j. The central tetrahedral C11 atom is bonded
References
to CÅ
bond angles of C11Ã
3j, both close to 1808, shows sp hybridisation for the
CÅN carbon atoms. The bond angles of SiÃOÃC in 3i
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a regular tetrahedron (109.58). Moreover, because of the
influence of neighbouring Csp2 and Csp atoms, the
/
N and (CH3)3SiO groups in both compounds. The
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/
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/
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