New bicyclic sila-heterocycles: Syntheses and crystal structure analyses of rac-7-ethoxy-2,2-diorganyl-2,3,5,7a-tetrahydro-1H-3a,6-diaza-2-sila-inden-4-ones

Article abstract of DOI:10.1016/S0022-328X(00)00461-7

Metalation of 3,6-diethoxy-2,5-dihydropyrazine with one molar equivalent of n-BuLi and subsequent treatment with one molar equivalent of R₂Si(CH₂Cl)₂ (R = Me, Ph) yielded the racemic 2-R₂Si(CH₂Cl)CH

Full text of DOI:10.1016/S0022-328X(00)00461-7

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 613 (2000) 19–25
New bicyclic sila-heterocycles: syntheses and crystal structure
analyses of rac-7-ethoxy-2,2-diorganyl-2,3,5,7a-
tetrahydro-1H-3a,6-diaza-2-sila-inden-4-ones
Vera Iris Handmann, Ru¨diger Bertermann, Christian Burschka, Reinhold Tacke *
Institut fu¨r Anorganische Chemie, Uni6ersita¨t Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany
Received 7 April 2000
Abstract
Metalation of 3,6-diethoxy-2,5-dihydropyrazine with one molar equivalent of n-BuLi and subsequent treatment with one molar
equivalent of R2Si(CH2Cl)2 (R=Me, Ph) yielded the racemic 2-R2Si(CH2Cl)CH2-substituted 3,6-diethoxy-2,5-dihydropyrazines
rac-2a (R=Me) and rac-2b (R=Ph). Heating of rac-2a and rac-2b led almost quantitatively (along with the formation of EtCl)
to the new bicyclic sila-heterocycles rac-7-ethoxy-2,2-dimethyl-2,3,5,7a-tetrahydro-1H-3a,6-diaza-2-sila-inden-4-one (rac-3a) and
rac-7-ethoxy-2,2-diphenyl-2,3,5,7a-tetrahydro-1H-3a,6-diaza-2-sila-inden-4-one (rac-3b), respectively. Compounds rac-2b, rac-3a,
and rac-3b were structurally characterized by NMR studies and crystal structure analyses. In addition, the kinetics of the
cyclization reaction rac-2a“rac-3a were investigated. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Bicyclic sila-heterocycles; Kinetics; Syntheses; Crystal structures; NMR spectroscopy
1. Introduction
new type of sila-heterocycle (for recent reviews on
sila-heterocycles, see refs. [5–7]). The studies presented
here were carried out in the context of our systematic
investigations on biologically active organosilicon com-
pounds. In general, sila-heterocycles are promising
compounds for the development of silicon-based drugs
and agrochemicals.
Recently, we have reported on the synthesis of rac-
2 - {[(chloromethyl)dimethylsilyl]methyl} - 3,6 - diethoxy-
2,5-dihydropyrazine (rac-2a) which served as a precur-
sor for the preparation of rac-4,4-dimethyl-4-sila-pro-
line ethyl ester (rac-1) [1]. The sila-proline derivative
rac-1 was synthesized in context with our studies on
silicon-containing a-amino acids and peptides [1–4]. As
already reported briefly (without any experimental de-
tails), compound rac-2a undergoes a thermally induced
cyclization reaction to give rac-7-ethoxy-2,2-dimethyl-
2,3,5,7a-tetrahydro-1H-3a,6-diaza-2-sila-inden-4-one
(rac-3a) [1]. We now could demonstrate that the deriva-
tive rac-3b can be obtained analogously, starting from
rac-2b. We report here on the syntheses of the bicyclic
sila-heterocycles rac-3a and rac-3b on a preparative
scale, including kinetic studies of the cyclization reac-
tion rac-2a“rac-3a. In addition, the crystal structures
of rac-2b, rac-3a, and rac-3b are described. Com-
pounds rac-3a and rac-3b represent derivatives of a
2. Results and discussion
2.1. Syntheses
* Corresponding author. Tel.: +49-931-888-5250; fax: +49-931-
888-4609.
The title compounds rac-3a and rac-3b were synthe-
E-mail address: r.tacke@mail.uni-wuerzburg.de (R. Tacke).
sized according to Scheme 1, starting from 3,6-di-
0022-328X/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00461-7

20
V.I. Handmann et al. / Journal of Organometallic Chemistry 613 (2000) 19–25
ethoxy-2,5-dihydropyrazine (4). Metalation of 4 with
one molar equivalent of n-butyllithium and subsequent
treatment with one molar equivalent of the bis-
(chloromethyl)silane Me2Si(CH2Cl)2 (5a) and Ph2Si-
(CH2Cl)2 (5b), respectively, gave compounds rac-2a
(yield 50%) and rac-2b (yield 53%). The products were
isolated as a colorless liquid (rac-2a) or crystalline solid
(rac-2b). Heating of neat rac-2a and rac-2b at 120°C
gave the crystalline title compounds rac-3a and rac-3b
in almost quantitative yield. Their identities were estab-
Fig. 3. Molecular structure of one of the two enantiomers in the
crystal of rac-3b (probability level of displacement ellipsoids 50%),
showing the atomic numbering scheme.
lished by elemental analyses (C, H, N), NMR-spectro-
scopic investigations (1H, 13C, 29Si), and crystal
structure analyses.
Scheme 1.
2.2. Crystal structure analyses
Compounds rac-2b, rac-3a, and rac-3b were struc-
turally characterized by single-crystal X-ray diffraction.
(
They crystallize in the space group P1 (rac-2b), Pbca
(rac-3a), and P21/c (rac-3b). The molecular structures
are depicted in Figs. 1–3. The crystal data and the
experimental parameters used for the crystal structure
analyses are summarized in Table 1. Selected inter-
atomic distances and angles are listed in Tables 2 and 3.
Compound rac-2b crystallizes with two molecules (A
and B) in the asymmetric unit. As can be seen from
Table 2, the interatomic distances and angles of
molecules A and B are quite similar. The respective
six-membered ring systems (C2, C3, N2, C4, C5, N1;
C32, C33, N32, C34, C35, N31) are almost planar with
,
,
mean deviations of 0.08 A (molecule A) and 0.10 A
(molecule B) from the best plane.
Fig. 1. Molecular structure of one of the two enantiomers of molecule
A in the crystal of rac-2b (probability level of displacement ellipsoids
50%), showing the atomic numbering scheme. The structure of
molecule B (not depicted) is very similar.
As can be seen from Figs. 2–4 and Table 3, the
six-membered rings of the bicyclic heterocycles rac-3a
and rac-3b are almost planar, whereas the five-mem-
bered rings adopt envelope conformations. These envel-
ope conformations differ from one another, the carbon
atom C2 (rac-3a) and the nitrogen atom N1 (rac-3b),
respectively, deviating from the plane generated by the
other four atoms of the five-membered rings (deviations
,
,
from the plane: 0.60 A, C2; 0.55 A, N1). The atoms of
the bicyclic skeletons of rac-3a and rac-3b form two
planes [plane A, with the atoms C6, N1, C2, C3, N2,
C4, and C5; plane B, with the atoms N1, C6, Si, and
C1 (rac-3a) and with the atoms C6, Si, C1, and C2
(rac-3b)], the mean deviations from these planes
Fig. 2. Molecular structure of one of the two enantiomers in the
crystal of rac-3a (probability level of displacement ellipsoids 50%),
showing the atomic numbering scheme.
,
,
amounting to 0.05 A (rac-3a, plane A), 0.01 A (rac-3a,

V.I. Handmann et al. / Journal of Organometallic Chemistry 613 (2000) 19–25
21
1
,
,
plane B), 0.03 A (rac-3b, plane A), and 0.01 A (rac-3b,
plane B). The respective dihedral angles formed by these
planes amount to −160.8° (rac-3a) and 132.4° (rac-3b),
indicating significant differences between the conforma-
tions of the bicyclic skeletons of rac-3a and rac-3b in the
crystal.
As monitored by H-NMR spectroscopy (Fig. 5), the
transformation rac-2a“rac-3a is characterized by first-
order kinetics, with a rate constant k=1.3×10−5 s−1
at 80°C (solvent C6D6; Fig. 6). This result is in accor-
dance with an intramolecular cyclization process.
2.3. Kinetic studies
3. Experimental
The 2-{[(chloromethyl)diorganylsilyl]methyl}-3,6-di-
ethoxy-2,5-dihydropyrazines rac-2a and rac-2b (neat
compounds or solutions in diethyl ether) were found to
be thermally stable at −20°C. No significant changes
could be observed over a period of 2 weeks (1H-NMR
studies). However, upon heating both compounds un-
dergo a cyclization reaction to yield the bicyclic products
rac-3a and rac-3b, respectively, along with ethyl chloride
3.1. Syntheses
3.1.1. General procedures
The syntheses of rac-2a, rac-2b, and 5b were carried
out under dry nitrogen. Tetrahydrofuran (THF) was
dried and purified according to standard procedures and
stored under nitrogen. Melting points were determined
with a Bu¨chi Melting Point apparatus. The 1H-, 13C-, and
29Si-NMR spectra were recorded at 22°C on a Bruker
1
(isolated by condensation and identified by H-NMR
spectroscopy).
Table 1
Crystal data and experimental parameters for the crystal structure analyses of rac-2b, rac-3a, and rac-3b
rac-2b
rac-3a
rac-3b
Empirical formula
Formula mass (g mol−1
Collection T (K)
C22H27ClN2O2Si
415.00
173(2)
C10H18N2O2Si
226.35
173(2)
C20H22N2O2Si
350.49
173(2)
)
,
u(Mo–Ka) (A)
Crystal system
0.71073
Triclinic
0.71073
Orthorhombic
Pbca (61)
0.71073
Monoclinic
P21/c (14)
(
Space group (no.)
Unit cell parameters
P1 (2)
,
a (A)
9.0176(12)
13.762(2)
18.149(3)
84.184(17)
82.431(16)
79.866(16)
2190.7(5)
4
15.8531(12)
6.9484(6)
22.044(2)
90
90
90
2428.3(4)
8
8.0321(16)
31.250(6)
7.1882(14)
90
96.03(3)
90
1794.3(6)
4
,
b (A)
,
c (A)
h (°)
i (°)
k (°)
V (A )
3
,
Z
Dcalc (g cm−3
)
1.258
0.249
880
1.238
0.178
976
1.297
0.147
744
v (mm−1
F(000)
)
Crystal dimensions (mm)
2q Range (°)
Index ranges
0.5×0.4×0.3
4.54–54.10
−115h511, −175k517,
−235l523
32039
0.6×0.3×0.3
4.50–51.82
−195h518, −85k58,
−265l527
12731
0.7×0.5×0.4
5.10–54.10
−105h510, −395k539,
−95l59
23502
Collected reflections
Independent reflections
Rint
8888
0.0338
2360
0.0769
3855
0.0507
Reflections used
Parameters
S
8888
509
1.055
0.0526/0.4712
0.0352
2360
139
0.897
0.0705/0.0000
0.0420
3855
230
1.051
0.0491/0.6323
0.0377
a
b
Weight parameters a/b
c
R1 [I\2|(I)]
d
wR2 (all data)
0.0978
0.1146
0.0981
Maximum/minimum residual electron density
+0.261/−0.330
+0.591/−0.338
+0.281/−0.313
−3
,
(e A
)
a S={ꢀ[w(Fo2−Fc2)2]/(n−p)}0.5; n=no. of reflections; p=no. of parameters.
b w−1=|2(Fo2)+(aP)2+bP, with P=(Max Fo2,0+2F2c)/3.
c R1=ꢀꢁꢁFoꢁ−ꢁFcꢁꢁ/ꢀꢁFoꢁ.
d wR2={ꢀ[w(Fo2−Fc2)2]/ꢀ[w(F2o)2]}0.5
.

22
V.I. Handmann et al. / Journal of Organometallic Chemistry 613 (2000) 19–25
Table 2
,
Selected interatomic distances (A) and angles (°) for rac-2b
Molecule A
Molecule B
Si1–C1
Si1–C6
Si1–C7
Si1–C8
C1–C2
C2–C3
C4–C5
N1–C2
N1–C5
N2–C3
N2–C4
O1–C3
O2–C5
1.8875(15)
1.8938(17)
1.8767(15)
1.8659(16)
1.538(2)
1.5084(19)
1.498(2)
1.458(2)
1.2666(19)
1.267(2)
1.449(2)
1.348(2)
1.347(2)
Si2–C31
Si2–C36
Si2–C38
Si2–C37
C31–C32
C32–C33
C34–C35
N31–C32
N31–C35
N32–C33
N32–C34
O31–C33
O32–C35
1.8892(14)
1.8895(16)
1.8738(15)
1.8670(16)
1.538(2)
1.5148(19)
1.502(2)
1.460(2)
1.2681(19)
1.2656(19)
1.450(2)
1.345(2)
1.344(2)
Fig. 4. Superposition of the bicyclic skeletons of one of the two
enantiomers each of rac-3a and rac-3b, indicating their different
conformations in the crystal. The hydrogen atoms (except for C2–H)
are omitted for clarity.
C1–Si1–C6
C1–Si1–C7
C1–Si1–C8
C6–Si1–C7
C6–Si1–C8
C7–Si1–C8
Si1–C1–C2
C1–C2–C3
N1–C2–C1
N1–C2–C3
N2–C3–C2
N2–C3–O1
O1–C3–C2
N2–C4–C5
N1–C5–C4
N1–C5–O2
O2–C5–C4
C2–N1–C5
C3–N2–C4
106.63(7)
112.18(7)
112.51(7)
109.92(7)
103.91(7)
111.24(7)
114.32(10)
111.61(12)
109.23(12)
113.55(12)
127.42(15)
121.85(13)
110.73(12)
115.05(13)
126.76(15)
122.14(13)
111.11(13)
117.10(12)
116.41(13)
C31–Si2–C36
C31–Si2–C38
C31–Si2–C37
C36–Si2–C38
C36–Si2–C37
C37–Si2–C38
Si2–C31–C32
C31–C32–C33
N31–C32–C31
N31–C32–C33
N32–C33–C32
N32–C33–O31
O31–C33–C32
N32–C34–C35
N31–C35–C34
N31–C35–O32
O32–C35–C34
C32–N31–C35
C33–N32–C34
106.45(7)
112.84(7)
112.76(7)
109.61(7)
106.58(7)
108.37(7)
115.66(10)
111.15(12)
109.92(12)
112.95(12)
126.87(14)
121.89(13)
111.24(12)
114.61(13)
126.35(15)
122.45(13)
111.20(13)
116.74(12)
116.27(13)
DRX-300 (1H, 300.1 MHz; 13C, 75.5 MHz; 29Si, 59.6
MHz) or Bruker DMX-600 NMR spectrometer (1H,
600.1 MHz). CDCl3 was used as solvent. Chemical
shifts (ppm) were determined relative to internal CHCl3
(1H, l=7.24), CDCl3 (13C, l=77.0), and external
1
TMS (29Si, l=0). Assignment of the H-NMR data of
rac-2b, rac-3a, and rac-3b was supported by 1H,1H
COSY experiments, and for rac-3a and rac-3b addi-
1
1
tionally by H,1H NOESY experiments. The H spin
systems of rac-2a, rac-2b, rac-3a, and rac-3b were
analyzed by simulations using the Bruker software pro-
gram WIN-
DAISY 4.0 [8]. Assignment of the 13C-NMR
data was supported by DEPT 135 experiments, and for
rac-3a and rac-3b additionally by 13C,1H HMBC
experiments.
Table 3
,
Selected interatomic distances (A) and angles (°) for rac-3a and rac-3b
rac-3a
rac-3b
rac-3a
rac-3b
Si–C1
Si–C6
Si–C7
Si–C8
1.883(2)
1.890(2)
1.852(3)
1.853(3)
1.512(3)
1.502(3)
1.498(3)
1.479(3)
1.339(3)
1.469(3)
1.250(3)
1.450(3)
1.359(3)
1.227(3)
92.87(10)
115.30(12)
111.25(12)
112.46(13)
1.8831(13)
1.8903(15)
1.8696(14)
1.8691(14)
1.5480(18)
1.5017(18)
1.506(2)
1.4683(17)
1.3460(19)
1.4650(17)
1.2612(18)
1.4562(19)
1.3520(16)
1.2303(18)
94.48(6)
C6–Si–C8
C7–Si–C8
Si–C1–C2
C1–C2–C3
N1–C2–C1
N1–C2–C3
N2–C3–C2
N2–C3–O1
O1–C3–C2
N2–C4–C5
N1–C5–C4
O2–C5–C4
N1–C5–O2
Si–C6–N1
C2–N1–C5
C2–N1–C6
C5–N1–C6
C3–N2–C4
111.86(12)
111.80(12)
102.83(15)
116.4(2)
107.24(19)
109.72(18)
128.5(2)
111.90(6)
108.76(6)
104.15(9)
114.13(11)
109.12(10)
110.91(11)
128.64(12)
121.95(12)
109.39(11)
119.30(12)
117.44(12)
119.94(13)
122.62(13)
102.03(8)
125.34(11)
112.59(11)
121.54(12)
118.35(12)
C1–C2
C2–C3
C4–C5
N1–C2
N1–C5
N1–C6
N2–C3
N2–C4
O1–C3
O2–C5
C1–Si–C6
C1–Si–C7
C1–Si–C8
C6–Si–C7
121.8(2)
109.50(18)
118.99(18)
117.78(18)
120.2(2)
122.0(2)
104.96(15)
124.41(18)
113.11(18)
121.74(18)
118.59(18)
113.81(6)
115.14(6)
112.24(7)

V.I. Handmann et al. / Journal of Organometallic Chemistry 613 (2000) 19–25
23
3.1.2. rac-2-{[(Chloromethyl)dimethylsilyl]methyl}-3,6-
diethoxy-2,5-dihydropyrazine (rac-2a)
This compound was synthesized according to ref.
[1].
up to room temperature within 2 h, followed by addi-
tion of diethyl ether (100 ml) and water (100 ml). The
organic phase was separated and the aqueous layer
extracted with diethyl ether (3×100 ml), and the com-
bined organic extracts were dried over anhydrous
Na2SO4. The solvent was removed under reduced pres-
sure and the oily residue purified by column chro-
matography on silica gel 60 (particle size 0.015–0.040
mm) using n-hexane–diethyl ether (2:1, v/v) as eluent.
Compound rac-2b was isolated in 53% yield (767 mg,
3.1.3. rac-2-{[(Chloromethyl)diphenylsilyl]methyl}-3,6-
diethoxy-2,5-dihydropyrazine (rac-2b)
A 1.6 M solution of n-butyllithium in n-hexane (500
ml, 800 mmol n-BuLi) was added dropwise at −10°C
within 5 min to a stirred solution of 4 (605 mg, 3.55
mmol) in THF (20 ml). After cooling the reaction
mixture to −70°C, again a 1.6 M solution of n-butyl-
lithium in n-hexane (1.70 ml, 2.72 mmol n-BuLi) was
added dropwise over a period of 20 min. The mixture
was stirred at −70°C for 15 min, warmed up to
−40°C, and then added dropwise at −5°C within 4 h
to a stirred solution of 5b (1.00 g, 3.56 mmol) in THF
(20 ml). The resulting mixture was allowed to warm
1
1.85 mmol) as a crystalline solid; m.p. 49–52°C. H-
NMR (600.1 MHz): l 1.14 (lX), 3.61 (lA), and 3.92
3
3
(lB) [2J(AB)=10.5 Hz, J(AX)=7.1 Hz, J(BX)=7.1
Hz, 5H; O–CHAHB–CHX3], 1.21 (lX), 3.94 (lA), and
4.02 (lB) [2J(AB)=10.5 Hz, 3J(AX)=7.1 Hz,
3J(BX)=7.1 Hz, 5H; O–CHAHB–CHX3], 1.55 (lA),
1.90 (lB), 3.92 (lK), 3.95 (lL), and 4.12 (lX) [2J(AB)=
3
3
14.8 Hz, Jtrans(AX)=11.1 Hz, Jcis(BX)=4.7 Hz,
5
5
2J(KL)=20.2 Hz, J(KX)=4.3 Hz, J(LX)=3.5 Hz,
5H; Si–CHAHB–CHX–NꢀC–CHKHL] [9], 3.42 (s, 2H;
SiCH2Cl), 7.32–7.44 and 7.55–7.64 (m, 10H; SiC6H5).
13C-NMR: l 14.2 (2C, OCH2CH3), 18.0 (SiCH2CH),
28.9 (SiCH2Cl), 46.5 (CCH2N), 52.5 (SiCH2CH), 60.7
(OCH2CH3), 61.0 (OCH2CH3), 127.9 (C-2/C-6 or C-3/
C-5, SiC6H5), 129.8 (2C, C-4, SiC6H5), 133.3 (C-1,
SiC6H5), 133.5 (C-1, SiC6H5), 135.1 (C-2/C-6 or C-3/C-
5, SiC6H5), 161.6 (CꢀN), 165.4 (CꢀN). 29Si-NMR: l
−9.5. Anal. Calc. for C22H27ClN2O2Si (415.0): C,
63.67; H, 6.56; N, 6.75. Found: C, 63.4; H, 6.4; N,
6.7%.
3.1.4. rac-7-Ethoxy-2,2-dimethyl-2,3,5,7a-tetrahydro-
1H-3a,6-diaza-2-sila-inden-4-one (rac-3a)
Compound rac-2a (1.50 g, 5.16 mmol) was heated at
120°C in vacuo (300 Torr) for 6 h and the resulting
product was then distilled in a Kugelrohr apparatus
(oven temperature 130°C, 0.01 Torr). After crystalliza-
tion from n-hexane at −20°C, compound rac-3a was
isolated in quantitative yield (1.17 g, 5.16 mmol) as a
Fig. 5. Selected 1H-NMR partial spectra showing the kinetics of the
transformation rac-2a“rac-3a at 80°C in C6D6 (SiCH2Cl protons of
rac-2a and SiCH2N protons of rac-3a; 300.1 MHz, 10°C, C6D6; for
details, see Section 3).
1
crystalline solid; m.p. 45°C. H-NMR (600.1 MHz): l
0.24 (s, 3H; SiCH3), 0.26 (s, 3H; SiCH3), 0.92 (lA), 1.48
(lB), 2.32 (lK), 3.54 (lL), 3.96 (lX), 4.08 (lY), and 4.17
(lZ) [2J(AB)=14.5 Hz, 3Jtrans(AX)=12.7 Hz,
3Jcis(BX)=5.5 Hz, 2J(KL)=15.6 Hz, 5J(KY)=1.0
Hz, 5J(KZ)=0.6 Hz, 5J(LY)=0.6 Hz, 5J(XY)=
4.0 Hz, 5J(XZ)=2.6 Hz, 2J(YZ)=20.9 Hz, 7H;
Si–CHAHB–CHX–CꢀN–CHYHZ–(CꢀO)–N–CHKHL]
[10], 1.26 (lX), 4.08 (lA), and 4.10 (lB) [2J(AB)=10.6
Hz, 3J(AX)=7.1 Hz, 3J(BX)=7.1 Hz, 5H; O–
CHAHB–CHX3]. 13C-NMR: l −3.1 (SiCH3), −3.0
(SiCH3), 14.2 (OCH2CH3), 19.5 (SiCH2CH), 33.1
(SiCH2N), 51.0 (CCH2N), 56.9 (SiCH2CH), 61.3
(OCH2CH3), 160.5 (CꢀN), 166.6 (CꢀO). 29Si-NMR: l
11.5. Anal. Calc. for C10H18N2O2Si (226.4): C, 53.06;
H, 8.02; N, 12.38. Found: C, 53.0; H, 8.0; N, 12.6%.
Fig. 6. First-order kinetics of the transformation rac-2a“rac-3a in
C6D6 at 80°C as monitored by 1H-NMR spectroscopy ([rac-2a]t,
concentration of rac-2a at the time t; [rac-2a]0, concentration at the
time t=0; for details, see Section 3).

24
V.I. Handmann et al. / Journal of Organometallic Chemistry 613 (2000) 19–25
3.1.5. rac-7-Ethoxy-2,2-diphenyl-2,3,5,7a-tetrahydro-
1H-3a,6-diaza-2-sila-inden-4-one (rac-3b)
(C-1, SiC6H5), 130.7 (C-4, SiC6H5), 135.2 (C-2/C-6 or
C-3/C-5, SiC6H5). 29Si-NMR: l −12.3. Anal. Calc. for
C14H14Cl2Si (281.3): C, 59.79; H, 5.02; Cl, 25.21.
Found: C, 59.8; H, 5.2; Cl, 25.2%.
Compound rac-2b (500 mg, 1.20 mmol) was heated
at 120°C in vacuo (300 Torr) for 6 h and the resulting
product was then distilled in a Kugelrohr apparatus
(oven temperature 230°C, 0.01 Torr). After crystalliza-
tion from n-hexane at −20°C, compound rac-3b was
isolated in quantitative yield (420 mg, 1.20 mmol) as a
crystalline solid; m.p. 107–109°C. 1H-NMR (600.1
MHz): l 1.27 (lX), 4.11 (lA), and 4.14 (lB) [2J(AB)=
3.2. Crystal structure analyses
Suitable single crystals of rac-2b (rac-3a) were ob-
tained by cooling a saturated solution in n-hexane
(n-hexane–diethyl ether) to −20°C (−10°C). Suitable
single crystals of rac-3b were obtained by crystallization
from n-hexane–diethyl ether by slow evaporation of
the solvent at room temperature. The crystals were
mounted in inert oil (perfluoroalkylether, ABCR) on a
glass fiber and then transferred to the cold gas stream
of the diffractometer [Stoe IPDS; graphite-monochro-
3
3
10.7 Hz, J(AX)=7.0 Hz, J(BX)=7.1 Hz, 5H; O–
CHAHB–CHX3], 1.46 (lA), 2.02 (lB), 2.90 (lK), 4.06
(lL), 4.13 (lY), 4.19 (lX), and 4.23 (lZ) [2J(AB)=14.8
Hz, 3Jtrans(AX)=12.6 Hz, 3Jcis(BX)=5.5 Hz,
5
5
2J(KL)=16.0 Hz, J(XY)=4.1 Hz, J(XZ)=2.5 Hz,
2J(YZ)=21.0 Hz, 7H; Si–CHAHB–CHX–CꢀN–
CHYHZ–(CꢀO)–N–CHKHL] [10], 7.33–7.53 and 7.57–
7.61 (m, 10H; SiC6H5). 13C-NMR: l 14.2 (OCH2CH3),
18.8 (SiCH2CH), 32.2 (SiCH2N), 51.1 (CCH2N), 57.2
(SiCH2CH), 61.5 (OCH2CH3), 128.3 (C-2/C-6 or C-3/
C-5, SiC6H5), 128.4 (C-2/C-6 or C-3/C-5, SiC6H5),
130.5 (C-4, SiC6H5), 130.6 (C-4, SiC6H5), 131.6 (C-1,
SiC6H5), 131.8 (C-1, SiC6H5), 134.8 (C-2/C-6 or C-3/C-
5, SiC6H5), 134.9 (C-2/C-6 or C-3/C-5, SiC6H5), 160.3
(CꢀN), 166.8 (CꢀO). 29Si-NMR: l 0.0. Anal. Calc. for
C20H22N2O2Si (350.5): C, 68.54; H, 6.33; N, 7.99.
Found: C, 68.2; H, 6.3; N, 8.0%.
,
mated Mo–Ka radiation (u=0.71073 A)]. The struc-
tures were solved by direct methods [12,13]. All
non-hydrogen atoms were refined anisotropically [14].
A riding model was employed in the refinement of the
hydrogen atoms.
3.3. Kinetic studies
A stirred solution of rac-2a (152 mg, 523 mmol) in
C6D6 (14 ml) was heated at 80°C for 7 days. In order to
study the kinetics of the reaction rac-2a“rac-3a, sam-
ples (0.5 ml) were taken from the reaction mixture at
t=1 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 5 h, 6 h, 8.5 h, 11 h,
14.5 h, 19.5 h, 23.5 h 27 h, 72.5 h, and 7 days and were
immediately cooled down in liquid nitrogen. These
3.1.6. 3,6-Diethoxy-2,5-dihydropyrazine (4)
This compound was synthesized according to ref.
[11].
1
samples were subsequently studied by H-NMR spec-
3.1.7. Bis(chloromethyl)dimethylsilane (5a)
This compound was purchased from Aldrich.
troscopy (300.1 MHz) at 10°C, and the kinetics were
analyzed by integration of the SiCH2Cl (rac-2a) and
SiCH2N (rac-3a) resonance signals.
3.1.8. Bis(chloromethyl)diphenylsilane (5b)
A 1.6 M solution of n-butyllithium in n-hexane (24.7
ml, 39.5 mmol n-BuLi; temperature of the solution
−75°C) was added dropwise at −75°C within 30 min
to a stirred solution of dichlorodiphenylsilane (5.00 g,
19.7 mmol) and bromochloromethane (10.2 g, 78.8
mmol) in THF (100 ml). The resulting mixture was
stirred at −70°C for 15 min and was then allowed to
warm up to room temperature within 12 h. The solvent
of the reaction mixture was removed under reduced
pressure and the residue extracted with n-pentane (3×
100 ml). The solvent of the combined organic extracts
was removed under reduced pressure and the residue
purified by column chromatography on silica gel 60
(particle size 0.015–0.040 mm) using n-hexane as elu-
ent. Compound 5b was isolated after distillation in a
Kugelrohr apparatus (oven temperature 140°C, 0.01
Torr) in 76% yield (4.23 g, 15.0 mmol) as a colorless
4. Supplementary material
Crystallographic data (excluding structure factors)
for the structures reported in this paper have been
deposited with the Cambridge Crystallographic Data
Centre as supplementary publications no. CCDC
143362 (rac-2b), CCDC 143360 (rac-3a), and CCDC
143361 (rac-3b). Copies of the data can be obtained
free of charge on application to The Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
Acknowledgements
1
liquid. H-NMR (300.1 MHz): l 3.43 (s, 4H; SiCH2Cl),
7.35–7.67 (m, 10H; SiC6H5). 13C-NMR:
l
25.9
We thank the Fonds der Chemischen Industrie for
financial support.
(SiCH2Cl), 128.2 (C-2/C-6 or C-3/C-5, SiC6H5), 130.4

V.I. Handmann et al. / Journal of Organometallic Chemistry 613 (2000) 19–25
25
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