The reductive dimerization of some 1,3-dienes and of 1,3,5-cycloheptatriene in the presence of trimethylchlorosilane: a DFT investigation

Article abstract of DOI:10.1016/j.tet.2009.02.087

Treatment of 1,3-dienes and 1,3,5-cycloheptatriene by chlorotrimethylsilane in the presence of wire of lithium led mainly to reductive dimerization with formation of bis(allylsilane) derivatives. Bis-silyl compounds obtained: from 1,3-butadiene, 1,8-bis(t

Full text of DOI:10.1016/j.tet.2009.02.087

Tetrahedron 65 (2009) 5563–5570
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The reductive dimerization of some 1,3-dienes and of 1,3,5-cycloheptatriene
in the presence of trimethylchlorosilane: a DFT investigation
,
Chahinez Aouf a, Douniazad El Abed a, Michel Giorgi b, Maurice Santelli a
*
a Laboratoire de Synthe`se Organique, Equipe Chirosciences, associe´e au CNRS nꢀ 6263, Universite´ d’Aix-Marseille, Faculte´ de St-Je´roˆme, Avenue Escadrille Normandie-Niemen,
13397 Marseille Cedex 20, France
b
ˆ
´ ˆ
´
´
´ ˆ
SpectropoledCentre Scientifique St-Jerome, Universite d’Aix-Marseille, Faculte de St-Jerome, Avenue Escadrille Normandie-Niemen, 13397 Marseille Cedex 20, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2008
Received in revised form 16 February 2009
Accepted 17 February 2009
Available online 14 April 2009
Treatment of 1,3-dienes and 1,3,5-cycloheptatriene by chlorotrimethylsilane in the presence of wire of
lithium led mainly to reductive dimerization with formation of bis(allylsilane) derivatives. Bis-silyl
compounds obtained: from 1,3-butadiene, 1,8-bis(trimethylsilyl)-2,6-octadiene (70%); from isoprene,
(Z,Z)-2,7-dimethyl-1,8-bis(trimethylsilyl)-2,6-octadiene (44%) and 2,6-dimethyl-1,8-bis(trimethylsilyl)-
2,6-octadiene (19%); from butadiene–isoprene mixture (1:1), 3-methyl-1,8-bis(trimethylsilyl)-2,6-octa-
diene (55%); from 2,3-dimethylbutadiene, (E,E)-2,3,6,7-tetramethyl-1,8-bis(trimethylsilyl)-2,6-octadiene
(36%), from 1,3-cyclohexadiene, 4,40-bis(trimethylsilyl)-bicyclohexyl-2,20-diene (48%); from 1,3,5-cyclo-
Keywords:
Alkadiene
Lithium
heptatriene, 1,10-bi[(S
*
,S )-6-(trimethylsilyl)cyclohepta-2,4-dien-1-yl] (53%). The structure of the various
*
intermediates (radical anion, dianion, silylated radical, silylated anion) has been established by calcu-
lations at the B3LYP/6-311þþG(d,p) level of theory with zero-point energy correction. These results are
in accordance with a pathway including the formation of a radical anion, its silylation furnishing to a
Dimerization
Electron transfer
Density functional calculations
g
-silylated allylic radical followed by a dimerization reaction in the head to head manner.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
In order to end-capping the polymerization of butadiene with
alkali metals, Nelson and his co-workers used dispersed lithium
(lithium sand) or pieces of sodium in THF in the presence of tri-
methylchlorosilane.6 Major products were 1,4-bis(trimethylsilyl)-2-
butene cis 1 and trans 2 in quite different isomers ratios (Scheme 1).
All conditions other than lithium metal in THF gave a cis-1,4-addi-
tion (except for lithium metal in diethyl ether and lithium
naphthalenide in THF, which gave mainly a cis-1,4-addition).
In connection with the production of synthetic rubber, metal-
ations of 1,3-dienes (butadiene and isoprene) with sodium metal
are known as far back as 1912.1 Anionic polymerization of butadi-
ene by alkali metal bulk was described by Ziegler in 1938 (Buna
rubbers).2 Since the polymerization of butadiene by sodium is so
rapid, its mechanism has been studied by analogy by replacing
butadiene by substituted dienes, which react more slowly and by
replacing sodium by lithium.3 The industrial interest of the poly-
merization of butadiene in the presence of lithium is due to the
structure of polymer (92.8% cis-1,4), which then vulcanized in ei-
ther gum or a reinforced stock gives tensile strengths comparable to
Hevea rubber.4
Frank and Foster have shown that it is possible to block the
sodium-diene reaction at the dimer stage by using sodium dis-
persion in diglyme at ꢁ30 ꢀC with p-terphenyl as sodium ‘carrier’
followed by carboxylation. Hydrogenation of the resultant un-
saturated diacid mixture yielded three major products, sebacic
acid, 2-ethylsuberic acid and 2,5-diethyladipic acid in the ratio
3.5:5:1.5
SiMe3
Li or Na
+
Me3Si
SiMe3
Me3SiCl
THF
Me3Si
1
2
<10 %
>90 %
>90 %
<10 %
Li
Na
Scheme 1.
To explain the dramatic effects of metal and solvents on the
course of the reaction, Nelson proposed a sequence of reactions
involving the initial formation of the cis-anion radical 3, which
would allow the gegenion to neutralize the partial charge on both
the C(1) and C(4) atoms (Scheme 2).7
According to Nelson, in the presence of lithium metal in a polar
solvent as THF, the radical anion 3cis is quickly reduced (kbut.1>kbut.2
in dianion 4trans, which is trapped by trimethylchlorosilane to give
)
* Corresponding author. Fax: þ33 4 91289112.
E-mail address: m.santelli@univ-cezanne.fr (M. Santelli).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.02.087

5564
C. Aouf et al. / Tetrahedron 65 (2009) 5563–5570
Table 1
Metal
kbut.1
(2 −)
(−)
2 M(+)
Calculated total energya of alkadienes, corresponding radical anions, silylated radi-
Metal
THF
2 Me3SiCl
2
cals and silylated anions at the B3LYP/6-311þþG(d,p) level of theory, with zero-
M(+)
3cis
point energy (ZPE) correctionb
4trans
Entry
Compound
Total energy
cis–trans energy
kbut.2
Me3SiCl
Metal
with ZPE corr. (hartree)
difference (kcal/mol)
1
2
3
4
5
6
7
8
s-cis 1,3-Butadiene
s-trans 1,3-Butadiene
3cis
3trans
4cis
4trans
5cis
5trans
6cis
6trans
s-cis Isoprene
s-trans Isoprene
12cis
12trans
13cis
13trans
14cis
14trans
15cis
15trans
16cis
16trans
17cis
17trans
21cis
21trans
24cis
24trans
25cis
25trans
26cis
26trans
27cis
27trans
1,3,5-Cycloheptatriene
32
33
34
35
1,3-Cyclohexadiene
39
40
41
42
ꢁ155.951 024
ꢁ155.957 050
ꢁ155.942 042
ꢁ155.942 256
ꢁ155.804 066
ꢁ155.811 388
ꢁ565.173 080
ꢁ565.174 212
ꢁ565.193 782
ꢁ565.190 450
ꢁ195.251 812
ꢁ195.257 151
ꢁ195.242 132
ꢁ195.246 357
ꢁ195.112 933
ꢁ195.119 245
ꢁ604.471 731
ꢁ604.471 858
ꢁ604.473 320
ꢁ604.473 283
ꢁ604.493 264
ꢁ604.486 452
ꢁ604.491 595
ꢁ604.490 348
ꢁ234.548 140
ꢁ234.555 083
ꢁ234.537 103
ꢁ234.543 756
ꢁ234.421 655
ꢁ234.423 283
ꢁ643.768 473
ꢁ643.768 513
ꢁ643.786 478
ꢁ643.785 359
ꢁ271.458 573
ꢁ271.458 151
ꢁ680.673 774
ꢁ680.705 128
ꢁ271.326 088
ꢁ233.363 520
ꢁ233.344 607
ꢁ642.576 710
ꢁ642.590 642
ꢁ233.229 543
(−)
ꢁ3.78
ꢁ0.13
ꢁ4.60
ꢁ0.71
2.09
Me3SiCl
1
Me3Si
Me3Si
M(+)
6cis
5cis
SiMe3
+
SiMe3
Me3Si
Me3Si
9
7, 1.5:1 (Metal: Li)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
45
45
Scheme 2.
ꢁ3.35
ꢁ2.65
ꢁ3.96
ꢁ0.08
0.02
2.8 With sodium metal, the corresponding very reactive radical anion
3cis rapidly reacts with chlorosilane (at the beginning of the reaction,
theconcentrationoftrimethylchlorosilaneisabout4 M)(kbut.2>kbut.1
)
to give the radical 5cis, which is further reduced in anion 6cis neu-
tralized by chlorosilane.9
This bis-silylation of dienic or trienic hydrocarbons constitutes
a convenient route to obtain bis(silyl) unsaturated compounds that
can represent useful intermediates in organic chemistry or can be
used as building blocks for the organic synthesis.10–12
4.27
0.79
Some years ago, we have observed that by decreasing the
lithium metal surface using pieces of lithium (8 mm each side) or
better 3 mm wire of lithium, the rate of reduction of 3 and 5
decreases (kbut.2>kbut.1) making easier the dimerization of radical
5cis into 1,8-bis(trimethylsilyl)-2,6-octadiene (Bistro) 7. Fractional
distillation gave 1, 2 (1:1 mixture, 20%) and 7 obtained in 68–72%
yield, which appeared as a mixture of (Z,Z)-isomer (ca. 50%), (Z,E)-
isomer (ca. 40%).13 The formation of 7 involved about 74% of
radical 5cis. In contrast, the use of pieces of sodium leads mainly
to the (Z,Z)-isomer (up to 80%). The heavy fraction of the distil-
lation contains bis(trimethylsilyl)dodecatriene 8 (ca. 8–10%). To
confirm its structure, this bis-allylsilane was added to succinic
ꢁ4.36
ꢁ4.17
ꢁ1.02
ꢁ0.02
0.70
anhydride giving rise to the spiro
g-lactone 9 (33%, mixture of
isomers) (Scheme 3). Taking into account the (Z)-geometry of the
cyclic double bond, we conclude that the internal double bond of
8 is mainly similar.
a
Hartrees.
b
ZPEs are scaled by a factor of 0.989, as recommended for calculation at the
Becke3LYP/6-311þþG(3df,2p) level of theory.34
Me3Si
SiMe3
O
TiCl4
O
8
+
O
MeNO2
CH2Cl2
O
the case of the dianion 4, and as predicted by Nelson, the trans-
O
isomer is the most stable (
DE¼4.60 kcal/mol) (Table 1, entry 6).
9, 33%
Interestingly, the cis-isomer 4cis is not plane and the structure of
the two isomers looks like two vinyl anions linked by a single bond
indicating that the interconversion barrier cis–trans is very low (see
Supplementary data). However, it is well known that the stereo-
chemical preference is the reverse in the presence of two lithium
counter-ion adducts of TMEDA. These complexes prefer to adopt
dibridging structures involving a delocalization of charge.15 Finally,
Scheme 3.
Now we report the results concerning the reductive dimerization
of isoprene, 2,3-dimethyl-1,3-butadiene, 1,3-cyclohexadiene, 1,3,5-
cycloheptatriene, cyclopentadiene and 1,3-cyclooctadiene in the
presence of chlorosilane.
if the two silylated radicals
E¼ꢁ0.71 kcal/mol) (Table 1, entry 8), it is not the case for their
corresponding anions 6, the cis-isomer being the most stable
5 have comparable energies
2. Results and discussion
(D
We have investigated the structure of the cis and trans radical
anion 3, the cis and trans dianion 4 and the cis and trans radical 5 at
the B3LYP/6-311þþG(d,p) level of theory with zero-point energy
correction (Table 1, entries 1–10, and Supplementary data).14
These calculations indicate that for the radical anion 3, the cis
(
D
E¼2.09 kcal/mol) (Table 1, entry 10). For silylated compounds, in
each case, the C–Si bond is almost perpendicular to the allylic
moiety revealing a hyperconjugation with the unsaturated system
(see Supplementary data).
As shown by Nelson,6 the disilylation reaction of isoprene in the
presence of dispersed lithium metal leads mainly to the trans-iso-
mer 11 (Scheme 4).
and the trans-isomers have similar energies (DE¼ꢁ0.13 kcal/mol),
even in the absence of counter-ion (Table 1, entry 4). In contrast, in

C. Aouf et al. / Tetrahedron 65 (2009) 5563–5570
5565
Li or Na
Me3SiCl
7 (ZZ:ZE = 1:1), 10%
Me
SiMe3
Li
+
+
1, 2
+
THF
Me
Me3SiCl
THF
Me3Si
SiMe3 Me3Si
Me
(E:Z = 1:1)
Me3Si
SiMe3
10
11
30%
20, 55%
15%
95%
81%
85%
5%
19%
Li
Na
Scheme 6.
Li(+) naphthalenide(−)
Scheme 4.
envisaged by Nelson,6 the cis configuration of 12cis would allow
a better neutralization of the charge than the trans one, 12trans. As
regards 3-neopentylallyllithium, a dimeric form has been envi-
sioned by Glaze and co-workers to explain the lithium exchange
By using pieces of lithium, isoprene reacts to yield 10 (9%), 11
(14%) and the disilyloctadienes, which are isolated as an in-
separable mixture (63%, 2.4:1) of (Z,Z)-isomer 18 (2,7-dimethyl)
and of 19 (2,6-dimethyl) (Scheme 5).16 In 1984, Sakurai showed that
the palladium complex-catalyzed reaction of hexamethyldisilane
with isoprene gave the symmetrical (E,E)-3,6-dimethyl-1,8-bis-
(trimethylsilyl)-2,6-octadiene.17 Titanium tetrachloride-mediated
reactions of 18 and 19 with various electrophiles gave rise to
products confirming to these structures.18
between the
a and g
positions.9
It was very interesting to study the reductive dimerization of an
equimolar mixture of 1,3-butadiene and isoprene in the presence of
lithium and chlorotrimethylsilane (Scheme 6). The distillation of
reaction mixture followed by GC analysis revealed that the first
fraction contained only 1,4-bis(trimethylsilyl)-2-butene cis 1 and
trans 2 (30%) in equal proportions. In the second fraction, we found
7 (10%) and 20 as a mixture of inseparable isomers (55%).
The disilyloctadienes 18 and 19 resulting from the reductive
dimerization of isoprene are not formed during this reaction.
The (Z)-configuration of the trisubstituted double bound of 20
was established by NOE experiment that showed cross-peaks be-
tween the signal of allylic protons and the vinylic one. The structure
of 20 has been confirmed after its TiCl4-mediated addition reaction
to the benzoyl chloride.20
(−)
(2 −)
Me
2 Li(+)
Li
Li
2 Me3SiCl
Me3SiCl
11
Li(+)
kiso.1
THF
Me
13trans
Me
12cis
kiso.2
Me3SiCl
(−)
Me
Me
From this experience, we can conclude that the butadiene is
faster reduced than the isoprene. This relative fast reduction is
confirmed by theoretical calculations (vide infra and Table 2, en-
tries 1 and 3).
Li
10
Me3Si
Me3Si
Li
Li(+)
14cis
16cis
+
Me3SiCl
The disilylation of 2,3-dimethyl-1,3-butadiene 21 is a practical
reaction as only few products have been obtained after distillation,
an inseparable mixture of 22 (19%) and 23 (28%), and then a mix-
ture of 28 (36%) and 29 (7.5%) (Fig. 1). The latter mixture was stored
at ꢁ20 ꢀC and 28 slowly crystallized. Its trans–trans structure was
confirmed by X-ray crystallographic analysis (Fig. 2). We have re-
cently described titanium tetrachloride-mediated stereoselective
reactions of 28 with aldehydes, anhydrides and acyl chlorides.21 As
for isoprene, when lithium sand is used, the trans-isomer 23 is
obtained in larger proportion (96%) comparatively to the cis-isomer
22 (4%).6 By addition of metal-free trimethylsilyl anion to 21,
Hiyama and his co-workers have obtained only 23.22
(−)
Me
Me
Me3Si
Me3Si
Li(+)
15cis
Me
17cis
Me
Me
Me
+
Me3Si
SiMe3
18, 45% yield
Me3Si
SiMe3
19, 18.5% yield
Scheme 5.
Thus, the use of lithium wire induces a modification of the ratio
of cis–trans isomers for the disilylbutenes 10–11 to the detriment of
the trans-isomer 11. Decreasing the surface of the lithium metal
(kiso.2>kiso.1) would favour the route via the silylated anions 16cis or
17cis to give a more large proportion of 10.
As regards isoprene, the overall mechanism is more complex
than the butadiene one since two isomeric silylated radicals 14 and
15 can be formed from the radical anion 12 and chlorosilane.
Structure determination of various intermediates by calcula-
tions at the B3LYP/6-311þþG(d,p) level of theory reveals that the
relative stabilities of the cis–trans isomers are variable:
12trans>12cis; 13trans[13cis; 14transw14cis; 15cisw15trans;
16cis[16trans; 17cisw17trans (Table 1, entries 13–24). In-
teresting result is the great stability of the silylated anion 16cis
Then, we studied the disilylation of the cycloheptatriene. This is
an interesting case because an unusual base-promoted [
p6sþp8s]
cyclodimerization can occur by treatment with potassium amide in
liquid ammonia. In addition ditropyl was subjected to lithium–
ammonia reduction to give the same cyclodimers.23 Likewise, the
oxidative dimerization of heptafulvenes yields bicycloheptatrienyl
derivatives according to a single electron transfer reactions.24
Treatment of a THF solution of cycloheptatriene with lithium wire
in the presence of chlorotrimethylsilane gave mainly a monocyclic
Table 2
Total energy differences between alkadienes and corresponding radical anions at the
B3LYP/6-311þþG(d,p) level of theory, with zero-point energy (ZPE) correction
against 16trans (
17trans (
E¼1.83 kcal/mol) and the relative stability of the sily-
lated radical 15 in comparison with 14 (15cis–14cis:
E¼1.0 kcal/
mol; 15trans–14trans:
D
E¼4.27 kcal/mol), 17cis (
DE¼1.05 kcal/mol), or
D
Entry
1,3-Diene (or 1,3,5-heptatriene)þe/radical anion
DE (kcal/mol)
D
1
2
3
4
5
6
7
8
(s-cis)-1,3-Butadiene/3cis
(s-trans)-1,3-Butadiene/3trans
(s-cis)-Isoprene/12cis
(s-trans)-Isoprene/12trans
(s-cis)-21/24cis
(s-trans)-21/24trans
1,3,5-Cycloheptatriene/32
1,3-Cyclohexadiene/39
5.63
9.28
6.07
6.77
6.93
7.11
D
E¼0.89 kcal/mol). The dimerization of
15cis in the head to head manner19 giving 18 as major product is in
accordance with the steric hindrance and the Mulliken atomic spin
density (C(1)¼0.685; C(3)¼0.650) (see Supplementary data).
However, theoretical results do not explain the formation of the
(Z,Z)-isomer. 15cis and 15trans have similar energies, consequently,
the importance of the counter-ion lithium will be confirmed. As
0.26
11.87

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C. Aouf et al. / Tetrahedron 65 (2009) 5563–5570
Me
Me
SiMe3
Me3Si
Me
Me
Me
SiMe3
H
Si
H
3H
Me3Si
Me
SiMe3
Me3Si
Me
4
l-approach
H
H
2
or
( )-31
21
1
22
23
H
5
Si
H
Me3Si
H
Me3Si
(−)
(2−)
Me
Me
A
B
33 + 33
Re
Me
25
24
ul-approach
(−)
Me3Si
H
Me
Me
38
Me
Me
H
SiMe3
Me3Si
Si
Me3Si
40 + 40
26
27
Scheme 8.
Me
SiMe3
Me
SiMe3
Me
Me
Me
Me
largest coefficient of the singly occupied MO (SOMO) is at the C(1) of
the pentadienyl radical [SOMO, 46th MO, atom. coef. (2s, 2p): C(1),
S
Me3Si
Me
Me3Si
Me
0.231; C(2), 0.032; C(3), ꢁ0.255; C(4), 0.017; C(5), 0.224] and, thus,
steric effects might well dominate over frontier molecular orbital
control.29 Interestingly, the steric approach A induces a minimiza-
tion of the dipole moment along the reaction coordinate.30
We also investigated the reductive dimerization of 1,3-cyclo-
hexadiene.31 Three disilanes were obtained and mainly the 4,40-
bis(trimethylsilyl)-bicyclohexyl-2,20-diene 38 (48% yield), an
interesting intermediate in organic synthesis (Scheme 9). This
compound appeared as a mixture of three isomers (1.5:1:1) but its
TiCl4-mediated reaction with 2-naphthaldehyde gave rise to one
crystallized tricyclic hydrocarbon (62% yield) corresponding to
a double alkylation. Its structure has been achieved by an X-ray
diffraction analysis. This structure determination confirms the
meso configuration of the two stereogenic centres of the bond
linkage.32
28
29
Figure 1.
Figure 2. ORTEP drawing for 28. Non-hydrogen atoms are drawn with 50% probability
thermal ellipsoids.
Contrary to the 31 formation, the stereoselectivity of the di-
merization of 40 comes from a transition state corresponding to an
unlike relative approach with an anti geometry indifferent to the
uncontrolled centres bearing trimethylsilyl groups (Scheme 8).
Interestingly, in both cases, formation of 31 or 38, a transition state
involving a large molecular orbitals overlap did not occur. The di-
merization of 33 to give 31 is weakly exothermic (28.72 kcal/mol) at
the B3LYP/6-311þþG(d,p) level of theory in contrast to the di-
merization of 40 (66.29 kcal/mol). This comparison underlines the
stability of the silylated radical 31.
Finally, we are interested in the cyclopentadiene reductive di-
merization. The major product is 1,1-bis(trimethylsilyl)cyclo-2,4-
diene 44 coming from the disilylation of the cyclopentadienyl anion
(Scheme 10). Dimerization product was not observed.
It is the same for 1,3-cyclooctadiene, which gave rise only to
a mixture of meso and DL (ꢂ)-1,4-bis(trimethylsilyl)cyclooct-2-ene
45 (Scheme 10).
disilane 30 (21%) and the 1,10-bi[(S
*
,S )-6-trimethylsilylcyclohepta-
*
2,4-dien-1-yl] (ꢂ)-31 (53%) (Scheme 7).25
Compound 31, which appeared as one isomer with a C2 axis
should be the result of a C–C bond formation exclusively from the
less hindered face of the rings of 33 and with a like relative topicity.26
Consequently, transition state corresponding to an unlike relative
approach with a possibility of a large molecular orbitals overlap did
not occur (Scheme 8). In contrast, the dimerization of the methyl
2,4,6-cycloheptatriencarboxylate radical cation yields to the unlike
(meso) dimer.24 This stereoselectivity is remarkable since the di-
merization of two organic radicals is known to take place non-
stereoselectively27 (dimerization of secondary and tertiary benzyl
radicals gave a 1:1 ratio of diastereoisomeric products).28 The rela-
tive high yield of dimeric product 31 confirms the stability of the
radical 33. The hyperconjugation of the C–Si bond with the penta-
trienyl radical moiety is confirmed by calculations: the C–Si bond is
nearly perpendicular to the medium plane of the pentadienyl radical
moiety (see Supplementary data). The stereo- and regioselectivityof
coupling of 33 is controlled only by steric effects (Scheme 8, A). The
Li(+)
Me3Si
Me3SiCl
Li
Li
(−)
(−)
Me3Si
Li(+)
Li
40
39
41
Li(+)
Li(+)
(−)
(−)
2 Li(+)
Me3SiCl
Li
Li
Me3SiCl
(2 −)
Me3Si
Me3Si
H H
H H
33
H H
34
42
SiMe3
H
32
Li
Me3SiCl
SiMe3
SiMe3
SiMe3
Me3Si
H
H
SiMe3
H
(2 −)
H
2 Li(+)
Me3Si
( )-31, 53%
Scheme 7.
SiMe3
H H
H
38, 48%
37, 22%
36, 8%
Me3Si
SiMe3
35
30, 21%
Scheme 9.

C. Aouf et al. / Tetrahedron 65 (2009) 5563–5570
5567
Table 4
SiMe3
SiMe3
H
Li
+
+
Total energy differences between silylated radicals and corresponding silylated
anions and comparison with isoprenyl derivatives 46 and 47 at the B3LYP/6-
311þþG(d,p) level of theory with zero-point energy (ZPE) correction
SiMe3
43b, 15%
Me3SiCl
SiMe3
43a, 35%
44, 43%
Entry
Silylated radicalþe/silylated anion
DE (kcal/mol)
SiMe3
1
2
3
4
5
6
7
8
5cis/6cis
ꢁ12.99
ꢁ10.19
ꢁ13.51
ꢁ9.16
5trans/6trans
14cis/16cis
14trans/16trans
15cis/17cis
15trans/17trans
26cis/27cis
26trans/27trans
33/34
Li
Me3SiCl
ꢁ11.47
ꢁ10.71
ꢁ11.30
ꢁ10.57
ꢁ19.67
ꢁ8.74
SiMe3
45, 52% (mixture of isomers 65:35)
Scheme 10.
9
10
11
40/41
46/47
ꢁ11.64
3. Conclusion
+
electron
Major steps of the reductive dimerization of alkadienes corre-
spond to the capture of one electron coming from lithium metal.
These reactions are moderately endothermic for the formation
(−)
46
47
Scheme 11.
of radical anion
(
D
E¼5.63–11.87 kcal/mol; 1,3,5-heptatriene,
DE¼0.26 kcal/mol) (Table 2). As regards butadiene and isoprene, the
reaction is less endothermic for the Z-isomers than the E-one (Table
2, entries 1, 2 and 3, 4), justifying the easy formation of the cis-de-
rivatives and particularly 3cis. In contrast, the obtaining of corre-
sponding dianions is highly unlikely even in the presence of lithium
interest because of the synthetic potential of the resulting
diallysilane.
counter-ion (
Interestingly, the reduction of silylated unsaturated radical is an
exothermic reaction (
establish the importance of the trimethylsilyl group for the stabi-
lization of the negative charge, we have calculated the energy of
reduction of allylic radical 46 to the corresponding allylic anion 47
(Scheme 11) and we have compared it to the reaction energy
14cis/16cis (one hydrogen atom is substituted by the trime-
D
E¼72.20–86.58 kcal/mol) (Table 3).
3.1. X-ray crystallography
D
E¼ꢁ8.74 to ꢁ19.67 kcal/mol) (Table 4). To
CCDC-629277 (for 28) contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from the
Cambridge Crystallographic Data Centre,12 Union Road, Cambridge
CB2 1EZ, UK; fax: (internat.) þ44-1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk].
thylsilyl group). The reaction is not as exothermic (
D
E¼ꢁ11.64 kcal/
Crystallographic data: C18H38Si2, Mw¼310.66, monoclinic, col-
ourless crystal (0.5ꢃ0.5ꢃ0.1 mm3), a¼13.484(1) Å, b¼11.9711(8) Å,
mol) as the 14cis/16cis one (
D
E¼ꢁ13.51 kcal/mol). So, the pres-
c¼7.1118(6) Å,
b
¼104.434(5)ꢀ, V¼1111.74(15) Å3, space group C2,
ence of the trimethylsilyl group stabilizes the negative charge by
1.87 kcal/mol at this level of computation. We also note that the
1,3,5-cycloheptatriene is very easily reduced (Table 2, entry 7) as its
silylated radical 33 (Table 4, entry 9).
Z¼2,
r
¼0.928 g cmꢁ3
,
m
(Mo K
a
)¼1.53 cmꢁ1, 1118 unique reflections,
118 parameters refined on F2 [Shelxl] to final indices
R[F2>4
s
F2]¼0.0492 (951 reflections), wR[w¼1/[
s
2(Fo2)]¼0.138 (all
Strangely, excepting butadiene derivatives, the silylation re-
action of radical anions is more exothermic for the cis-isomers than
for the trans one (Table 5, entries 4, 7, and 10) (see Supplementary
data).
The high endothermic character of the reduction of radical anions
in contrast with the exothermic formation of the silylated anions al-
lows to consider that the most likely way for the reductive
dimerization of alkadienes is the following: diene/radical
anion/silylated radical/bis(silyl)-diene and concerning the for-
mation of monomers: silylated radical/silylated anion/bis(silyl)-
alkene.
reflections). The last residual Fourier positive and negative peaks
were equal to 0.115 and ꢁ0.131, respectively (Table 6).
3.2. Computational methods
Calculations by using Gaussian 03, revision D.02.33 For the open-
shell species (radicals and radical anions), the CS2D values of cal-
culations were less than 0.7862 (see, Table S8 in Supplementary
data).
Even in the case of moderate yields, the very easy and cheap
reductive dimerization of dienes or trienes is of considerable
Table 5
Calculations of relative energy of silylation of radical anions to give corresponding
silylated radicals at the DFT/6-311þþG(d,p) level of theory with correction of the
zero-point energy
Table 3
Entry
Radical anion
Corresponding silylated radical
Relative energy
reaction (kcal/mol)
Total energy differences between radical anions and corresponding dianions at the
B3LYP/6-311þþG(d,p) level of theory with zero-point energy (ZPE) correction
1
2
3
4
5
6
7
8
9
10
3cis
5cis
0.67
0.09
1.57
0.57
4.14
3.25
0.46
4.61
10.34
0.00
Entry
Radical anionþe/dianion
3cis/4cis
DE (kcal/mol)
3trans
12cis
12cis
12trans
12trans
24cis
24trans
32
5trans
14cis
15cis
14trans
15trans
26cis
26trans
33
1
2
3
4
5
6
7
8
86.58
82.12
81.07
79.76
72.44
75.60
82.87
72.20
3trans/4trans
12cis/13cis
12trans/13trans
24cis/25cis
24trans/25trans
32/35
39/42
39
40

5568
C. Aouf et al. / Tetrahedron 65 (2009) 5563–5570
Table 6
bis(trimethylsilyl)-2,7-octadiene and 2% of (2E)-1,6-bis(trimethyl-
silyl)-2,7-octadiene. IR nmax/cmꢁ1 3015, 2962, 1255, 1159, 852, (Z)-
isomers nmax/cmꢁ1 695, (E)-isomers, /cmꢁ1 964; 1H NMR (CDCl3,
Crystal data and structure refinement for 28
n
Crystallographic data: C18H38Si2, Mw¼310.66, monoclinic, colourless crystal
(0.5ꢃ0.5ꢃ0.1 mm3), a¼13.484(1) Å, b¼11.9711(8) Å, c¼7.1118(6) Å,
300 MHz)
d
¼0.07 (s, 9H), 0.09 (s, 9H), 1.47–1.57 (m, 4H), 2.12 (br d,
b
m
¼104.434(5)ꢀ, V¼1111.74(15) Å3, space group C2, Z¼2,
r
¼0.928 g cmꢁ3
,
J¼7.9 Hz, 4H), 5.30–5.50 (m, 4H); 13C NMR (CDCl3, 75 MHz) (Z,Z)-
(MoK
a
)¼1.53 cmꢁ1,1118 unique reflections,118 parameters refined on F2 [Shelxl]
isomer:
d
¼ꢁ1.6 (q), 18.6 (t), 21.5 (t), 125.7 (d), 127.4 (d); (Z,E)-iso-
to final indices R[F2>4
s
F2]¼0.0492 (951 reflections), wR[w¼1/[
s
2(Fo2)]¼0.138 (all
mer, ꢁ1.8 (q), ꢁ1.6 (q), 18.6 (t), 22.8 (t), 29.1 (t), 33.2 (t), 125.5 (d),
reflections). The last residual Fourier positive and negative peaks were equal to
0.115 and ꢁ0.131, respectively
125.7 (d), 127.4 (d), 128.7 (d). 29Si NMR
d/TMS: (Z,Z)-isomer, 1.20,
(Z,E)-isomer, 0.44, 1.25.
4. Experimental section
4.1. General
4.1.4. 1,12-Bis(trimethylsilyl)dodeca-2,6,10-triene (8)
Colourless oil, mixture of isomers, bp 130–140 ꢀC, 0.2 Torr,
61.6 g, 0.2 mol, 10% yield. The major isomer 1H NMR (CDCl3,
300 MHz)
d
¼ꢁ0.04 (s, 9H), ꢁ0.03 (s, 9H), 1.37–1.47 (m, 4H), 2.01 (br
All reactions were run under argon in oven-dried glassware. TLC
was performed on silica gel 60 F254. Flash chromatography was
performed on silica gel (230–400 mesh) obtained from Macherey-
Nagel & Co. CH2Cl2 was distilled before use from calcium hydride
and THF was distilled from sodium-benzophenone. 1H and 13C NMR
spectra were recorded at 25 ꢀC in CDCl3 solutions at 300, and
75 MHz, respectively, using a Bruker AC300 spectrometer. Chemical
shift is reported in parts per million relative to CDCl3 (signals for
residual CDCl3 in the CDCl3: 7.24 for 1H NMR and 77.16 (central) for
13C NMR). Carbon–proton couplings were determined by DEPT
sequence experiments. High resolution ESI-MS analyses were per-
formed using a Qstar Elite (Applied Biosystems SCIEX) mass
spectrometer.
s, 8H), 5.24–5.41 (m, 6H); 13C NMR (CDCl3, 75 MHz)
d
¼ꢁ1.8 (q),
ꢁ1.6 (q), 18.6 (t), 22.8 (t), 27.4 (t), 32.6 (t), 34.3 (t), 38.6 (t), 126.7 (d),
127.1 (d), 127.7 (d), 128.5 (d), 129.7 (d), 129.9 (d).
4.1.5. General procedure for the preparation of the other
bis-silylated products
The previous procedure was employed with 250 mL of THF, 9 g
(1.28 g atom) of pieces of lithium, 156 mL (1.22 mol) of chloro-
trimethylsilane and 1.22 mol of diene.
4.1.6. 2-Methyl-1,4-bis(trimethylsilyl)but-2-ene (10 and 11), (Z,Z)-
2,7-dimethyl-1,8-bis(trimethylsilyl)octa-2,6-diene (18), 2,6-
dimethyl-1,8-bis(trimethylsilyl)octa-2,6-diene (19)
From isoprene, 10 and 11, colourless oil, bp 32–35 ꢀC, 0.2 Torr,
4.1.1. 1,4-Bis(trimethylsilyl)but-2-ene (1 and 2) and 1,8-
30 g, 0.14 mol, 23% overall yield. 1H NMR (CDCl3, 300 MHz)
bis(trimethylsilyl)octa-2,6-diene (7)
d
¼ꢁ0.03 (s, 18H), 1.35 (dd, J¼8.7, 8.9 Hz, 2H), 1.48 (d, J¼4.4 Hz, 2H),
A 3-L three-necked flask equipped with a thermometer, a drop-
ping funnel, a refluxcondenser connected with a stopcock to a rubber
balloon filled with argon and a magnetic stirring bar was charged
with 600 mL of anhydrous tetrahydrofuran. The solution was cooled
to 0 ꢀC with an ice bath and lithium metal (3 mm wires cut as pieces
of 1.5 cm long, 28 g, 4 atoms) was added. The stopper of the addition
funnel is removed under a slight positive flow of argon and 434 g
(507 mL, 4 mol) of chlorotrimethylsilane is poured into the addition
funnel. The stopper is put in place and chlorotrimethylsilane is added
neat over 20 min. The U-shaped double-tipped needle connected to
the flask containing neat cold liquid 1,3-butadiene is introduced
through the rubber septum of the dropping funnel and butadiene
(450 mL, ca. 280 g, ca. 5.18 mol) is transferred. The reactants are
vigorously stirred and butadiene is slowly added over approximately
a 1.5 h period. The reaction mixture is stirred at 0 ꢀC for 6 h and
overnight at room temperature. Then pentane (or light petroleum) is
added to fill in the flask, and the possibly small remaining pieces of
lithium are removed with tweezers. The milked solution is poured
onto 1 kg of crushed ice in a 6-L Erlenmeyer, after stirring, the layers
are separated. The organic one is washed with chilled water
(6ꢃ500 mL) and then dried over anhydrous MgSO4. After filtration
and concentration in vacuo, the colourless residue was distilled
through 12-cm Vigreux column.
1.53 (s, 3H), 4.99 (dd, J¼8.7, 8.9 Hz, 1H); 13C NMR (CDCl3, 75 MHz).
(Z)-isomer (10) (40%),
(d), 131.0 (s). (E)-isomer (11) (60%),
d
¼ꢁ1.0 (q), 18.7 (q), 18.8 (t), 22.9 (t), 117.3
d
¼ꢁ1.5 (q), 18.9 (t), 26.4 (q),
30.2 (t), 118.0 (d), 130.1 (s). 18 and 19 (2.4:1), colourless oil, bp 85–
90 ꢀC, 0.2 Torr, 73.3 g, 0.26 mol, 63% overall yield. 18, 1H NMR
(CDCl3, 300 MHz)
d
¼ꢁ0.02 (s, 18H), 1.50 (s, 4H), 1.65 (s, 6H), 1.91–
1.93 (m, 2H), 1.97 (br d, J¼7.0 Hz, 2H), 5.00 (br s, 2H); 13C NMR
(CDCl3, 75 MHz)
d
¼ꢁ0.85 (q), 23.4 (t), 26.4 (q), 29.1 (t), 122.4 (d),
133.1 (s). 19, 1H NMR (CDCl3, 300 MHz)
d
¼ꢁ0.02 (s, 18H), 1.41 (d,
J¼8.3 Hz, 2H), 5.13 (t, J¼8.5 Hz, 1H), 5.16 (t, J¼8.4 Hz, 1H); 13C NMR
(CDCl3, 75 MHz)
133.1 (s).
d
¼ꢁ0.85 (q), 18.5 (t), 23.6 (q), 27.1 (t), 120.4 (d),
4.1.7. 3-Methyl-1,8-bis(trimethylsilyl)octa-2,6-diene (20)
From 1,3-butadiene–isoprene (1:1), colourless oil, mixture of
isomers, bp 95–98 ꢀC, 0.2 Torr, 107.2 g, 0.4 mol, 65% yield; 1H NMR
(CDCl3, 300 MHz)
d¼ꢁ0.01 (s, 18H), 1.37–1.53 (m, 4H), 1.66 (s, 3H),
1.92–2.02 (m, 4H), 5.34–5.36 (m, 3H); 13C NMR (CDCl3, 75 MHz)
d
¼ꢁ1.6 (q), ꢁ1.0 (q), 18.6 (t), 22.8 (t), 23.4 (t), 26.4 (d), 27.5 (t), 125.6
(d), 127.3 (d), 128.7 (d), 133.3 (s).
4.1.8. 1,6-Bis(trimethylsilyl)cyclohepta-2,4-diene (30)
From 1,3,5-cycloheptatriene, yellow oil, one isomer, bp 55–
60 ꢀC, 0.2 Torr, 31 g, 0.13 mol, 21% yield; 1H NMR (CDCl3, 300 MHz)
4.1.2. (Z)-1,4-Bis(trimethylsilyl)but-2-ene (1) and (E)-1,4-
bis(trimethylsilyl)but-2-ene (2)
d
¼0.00 (s, 9H), 0.05 (s, 9H), 2.10–2.19 (m, 2H), 2.25 (t, J¼6.1 Hz, 1H),
2.50–2.65 (m, 1H), 5.29 (dd, J¼4.3, 11.9 Hz, 1H), 5.51–5.57 (m, 1H),
5.64–5.74 (m, 2H); 13C NMR (CDCl3, 75 MHz)
¼ꢁ1.91 (q), ꢁ1.66 (q),
29.9 (d), 31.6 (d), 39.1 (t), 124.1 (d), 127.3 (d), 129.6 (d), 131.8 (d).
Colourless oil, bp 40 ꢀC, 0.2 Torr, 36 g, 0.18 mol, 18% overall yield
d
(1:1).1,1HNMR (CDCl3, 300 MHz)
d
¼0.00 (s, 9H),1.40(br s, 2H), 5.27–
5.31 (m,1H); 13C NMR (CDCl3, 75 MHz)
ꢁ1.5 (q),18.0 (t),123.3 (d). 2,
1H NMR (300 MHz, CDCl3):
¼ꢁ0.01 (s, 9H),1.37 (br s, 2H), 5.18–5.22
(m, 1H); 13C NMR (CDCl3, 75 MHz)
ꢁ1.8 (q), 23.0 (t), 124.5.
d
d
4.1.9. 1,1-Bi[(S
Yellow oil, bp 120–122 ꢀC, 0.2 Torr, 105.6 g, 0.32 mol, 53% yield;
1H NMR (CDCl3, 300 MHz)
¼0.05 (s, 18H), 1.24–1.53 (m, 2H), 2.27–
2.30 (m, 4H), 2.53–2.57 (m, 2H), 5.43 (br s, 4H), 5.64–5.69 (m, 4H);
13C NMR (CDCl3, 75 MHz)
* *
,S )-6-(trimethylsilyl)cyclohepta-2,4-diene-1-yl] (31)
d
d
4.1.3. 1,8-Bis(trimethylsilyl)octa-2,6-diene (7)
Colourless oil, bp 87–95 ꢀC, 0.2 Torr, 355.6 g, 1.4 mol, 70% yield.
Compound 7 is a mixture of (Z,Z)-isomer (50%), (Z,E)-isomer (40%)
and (E,E)-isomer (4%) contaminated with 4% of (2Z)-1,6-
d
¼ꢁ1.5 (q), 31.0 (t), 39.0 (d), 44.8 (d),
128.0 (d), 128.1 (d), 131.7 (d), 132.6 (d). C20H34Si2, MS: m/z (%)¼330
(Mþ, 12%), 91 (52), 73 (100), 45 (25).

C. Aouf et al. / Tetrahedron 65 (2009) 5563–5570
5569
4.1.10. 3,4-Bis(trimethylsilyl)cyclohexene (36) and 3,6-
bis(trimethylsilyl)cyclohexene (37)
(m, 1H), 2.43–2.53 (m, 3H), 4.86–4.93 (m, 2H), 5.08–5.16 (m, 4H),
5.63–5.72 (m, 2H); 13C NMR (CDCl3, 75 MHz), major isomer,
(t), 27.5 (t), 28.7 (t), 28.9 (t), 29.2 (t), 30.5 (t), 43.0 (d), 46.6 (d), 87.7
(s), 114.0 (t), 118.1 (t), 136.0 (d), 137.7 (d), 141.0 (d), 143.3 (d), 177.4
(s). High resolution ESI-MS calcd for C16H22O2 [MþH]þ 247.1692;
found 247.1687.
d
¼25.1
From 1,3-cyclohexadiene. Colourless oil, bp 50 ꢀC, 0.2 Torr,
40.7 g, 0.18 mol, 30% overall yield. 37, (70%), 1H NMR (CDCl3,
300 MHz)
(CDCl3, 75 MHz)
d
¼0.02 (s, 9H), 1.33–1.82 (m, 3H), 5.60 (s, 1H); 13C NMR
d
¼ꢁ2.5 (ꢁ3.2) (q), 23.5 (24.6) (t), 26.6 (26.1) (d),
126.0 (126.5) (d). C12H26Si2, MS: m/z 226 (Mþ 80%), 211 (50), 152
(40), 138 (38), 78 (41), 73 (100), 45 (30). 36, (30%), 1H NMR (CDCl3,
Acknowledgements
300 MHz)
d
¼0.01 (s, 18H), 1.33–1.82 (m, 6H), 5.60 (br s, 2H); 13C
NMR (CDCl3, 75 MHz)
d
¼ꢁ2.1 (q), 20.7 (d), 22.1 (t), 23.8 (t), 26.9 (d),
We thank the CNRS for financial support and C.A. is grateful to
124.3 (d), 128.2 (d). C12H26Si2, MS: m/z 226 (Mþ 28%), 211 (20), 152
´
´
the ‘Association des Femmes Diplomees de l’Universite’ for a grant
(35), 137 (18), 78 (45), 73 (100), 45 (30).
´ `
´
(‘bourse Helene Delavaud’). Dr. Nicolas Ferre is kindly acknowl-
edged for their help.
4.1.11. 4,40-Bis(trimethylsilyl)bicyclohexyl-2,20-diene (38)
Colourless oil, mixture of isomers, bp 135–145 ꢀC, 0.2 Torr,
Supplementary data
91.8 g, 0.3 mol, 48% yield; 1H NMR (CDCl3, 300 MHz)
d
¼0.01–0.03
(m, 18H), 1.30–1.43 (m, 8H), 1.55–1.80 (m, 2H), 2.06–2.07 (m, 2H),
5.50–5.68 (m, 4H); 13C NMR (CDCl3, 75 MHz)
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2009.02.087.
d¼ꢁ3.3 (q), 23.7 (t),
26.4 (d), 26.8 (t), 40.3 (d), 128.1 (d), 128.4 (d). C18H34Si2, MS: m/z
306 (Mþ 4%), 153 (25), 79 (28), 73 (100), 45 (20).
References and notes
4.1.12. 1-Trimethylsilylcyclopenta-2,4-diene (43)
1. (a) Perkin, W. H., Jr. J. Soc. Chem. Ind. 1912, 31, 616–624; (b) Harries, C. Angew.
Chem. 1920, 33, 226–227; (c) Konrad, E. Angew. Chem. 1936, 49, 799–801; (d)
Campbell, K. N.; Campbell, B. K. Chem. Rev. 1942, 31, 77–175; (e) Saltman, W. M.
Encycl. Polym. Sci. Technol. 1964, 2, 678–754; (f) Morawetz, H. Rubber Chem.
Technol. 2000, 73, 405–426.
2. (a) Ziegler, K. Angew. Chem. 1936, 49, 499–502; (b) Ziegler, K. Chem. Ztg. 1938,
62, 125–127; (c) Ziegler, K.; Jakob, L.; Wollthan, H.; Wenz, A. Liebigs Ann. Chem.
1934, 511, 64–88.
3. Midgley, T.; Henne, A. L. J. Am. Chem. Soc. 1929, 51, 1294–1296.
4. Polymerization of butadiene in the presence of lithium (Firestone Tire & Rubber
Co.) 1959, GB 817695 19590806 Patent; Chem. Abstr. 1960, 54, 59545.
5. Frank, C. E.; Foster, W. E. J. Org. Chem. 1961, 26, 303–307.
6. Weyenberg, D. R.; Toporcer, L. H.; Nelson, L. E. J. Org. Chem. 1968, 33, 1975–1982.
7. The empirical ‘cis rule’ concerning the allyl anion has previously been estab-
lished by some results, see: (a) Bauld, N. L. J. Am. Chem. Soc. 1962, 84, 4347–
4348; (b) Fujita, K.; Ohuma, Y.; Yasuda, H.; Tani, H. J. Organomet. Chem. 1976, 113,
201–213; (c) Cram, D. J. Fundamentals of Carbanion Chemistry; Academic: New
York, NY, 1965; p 193.
From cyclopentadiene, yellow oil, two isomers, bp 28–29 ꢀC,
0.2 Torr, 84.2 g, 0.61 mol, 50% overall yield; 1H NMR (CDCl3,
300 MHz)
d
¼ꢁ0.02 (s, 9H), 0.18 (s, 9H), 3.03–3.39 (m, 3H), 6.50–
6.67 (m, 5H); 13C NMR (CDCl3, 75 MHz)
d
¼ꢁ1.91 (q), ꢁ0.61 (d),
130.2 (d) (2C), 133.4 (d) (2C). 43b (30%):
(d), 133.2 (d), 137.9 (d), 141.5 (s).
d
¼ꢁ1.90 (q), 45.2 (t), 132.4
4.1.13. 1,1-Bis(trimethylsilyl)cyclopenta-2,4-diene (44)
Yellow oil, one isomer, bp 50–52 ꢀC, 0.2 Torr, 54.6 g, 0.26 mol,
43% yield; 1H NMR (CDCl3, 300 MHz)
d
¼ꢁ0.06 (s, 18H), 6.50 (d,
J¼5.7 Hz, 2H), 6.70 (d, J¼5.7 Hz, 2H); 13C NMR (CDCl3, 75 MHz)
d
¼ꢁ0.65 (q), 57.0 (s), 130.6 (d), 136.2 (d). High resolution ESI-MS
calcd for C11H22Si2 [MþH]þ 211.1332; found 211.1328.
8. Lithium is a better reducing agent than sodium (lithium, Eꢀ¼ꢁ3.04 V in water,
and ꢁ2.99 V in liquid ammonia at ꢁ50 ꢀC; sodium, respectively, Eꢀ¼ꢁ2.71 V
and ꢁ2.59 V), see: Wilds, A. L. N.; Nelson, A. J. Am. Chem. Soc. 1953, 75, 5360–
5365; Lithium is more reactive than sodium in the Birch reaction, see: Rabid-
eau, P. W. Tetrahedron 1989, 45, 1579–1603.
9. The 1,4-addition of tert-butyllithium to 1,3-butadiene led to neo-
pentylallyllithium of similar structure to 6. cis–trans isomerization occurs
slowly in THF. By heating at 30 ꢀC and then lowered to ꢁ18 ꢀC, extensive
cis–trans isomerization occurred in favor of the cis-isomer, see: Glaze, W. H.;
Hanicak, J. E.; Chaudhuri, J.; Moore, M. L.; Duncan, D. P. J. Organomet. Chem.
1973, 51, 13–21.
4.1.14. 1,4-Bis(trimethylsilyl)cyclooctene (45)
From 1,3-cyclooctadiene, colourless oil, mixture of isomers, bp
95 ꢀC, 0.2 Torr, 76.2 g, 0.3 mol, 52% yield; 1H NMR (CDCl3, 300 MHz)
d
¼ꢁ0.04 (s, 9H), 1.47–1.86 (m, 5H), 5.35 (d, J¼4.7 Hz, 1H); 13C NMR
(CDCl3, 75 MHz)
d
¼ꢁ3.0 (q), 28.3 (t), 28.4 (t), 29.5 (d), 129.0 (d).
C14H30Si2, MS: m/z 254 (Mþ 50%), 239 (50), 180 (51), 165 (100), 151
(40), 73 (40), 45 (10).
`
10. (a) Fleming, I.; Dunogues, J.; Smithers, R. Org. React. 1989, 37, 57–575; (b) Masse,
4.1.15. Reaction of 8 with succinic anhydride
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2004, 3173–3199.
A 100-mL three-necked flask equipped with a thermometer,
septum cap, magnetic stirring bar and an argon outlet was charged
with anhydrous CH2Cl2 (15 mL) and anhydrous nitromethane
(1 mL, 19.5 mmol). The solution was cooled to ꢁ60 ꢀC; TiCl4 (0.93 g,
0.5 mL, 4.9 mmol) was added, followed by the slow addition of
succinic anhydride (0.5 g, 4.9 mmol). After 1 h, the mixture was
cooled to ꢁ90 ꢀC and 8 (3 g, 9.7 mmol) in CH2Cl2 (5 mL) was added.
The mixture was stirred for 2 h at ꢁ90 ꢀC and then slowly warmed
to ꢁ60 ꢀC and stirred for 24 h. The reaction was quenched by the
addition of a saturated aqueous solution of NH4Cl (50 mL), and the
aqueous phase was extracted with CH2Cl2 (3ꢃ20 mL). The organic
phase was washed with a saturated aqueous solution of HNaCO3,
brine and water. The solution was dried with MgSO4, filtered and
concentrated in vacuo, and the residue was purified by flash
chromatography (petroleum ether–diethyl ether 70:30) on silica
gel to give 9.
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4.1.16. 1-Oxa-6,13-divinylspiro [4.8]tridec-9-en-2-one (9)
Colourless oil, mixture of isomers, 0.4 g, 1.6 mmol, 33% yield; 1H
NMR (CDCl3, 300 MHz)
d¼1.15–1.25 (m, 1H), 1.39–1.53 (m, 3H),
1.67–1.77 (m, 3H), 1.78–1.80 (m, 1H), 1.90–2.01 (m, 2H), 2.13–2.22

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`
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