Thermal and photochemical isomerization of exocyclic 1,3-dienes: 1,1-Diphenyl-3,4-bis(trimethylsilylmethylene)-1-silacyclopentane s-cis(E,E)

Article abstract of DOI:10.1016/j.jorganchem.2004.11.013

This paper describes the photochemical and the thermal isomerization of s-cis(E,E) 1,1-diphenly-3,4-bis(trimethylsilylmethylene)-1-silacyclopentane (1a). Under thermal conditions a 1,3-sigmatropic of the methylene hydrogen occurs, yielding the s-trans iso

Full text of DOI:10.1016/j.jorganchem.2004.11.013

Journal of Organometallic Chemistry 690 (2005) 1028–1032
www.elsevier.com/locate/jorganchem
Thermal and photochemical isomerization of exocyclic 1,3-dienes:
1,1-diphenyl-3,4-bis(trimethylsilylmethylene)-1-silacyclopentane
s-cis(E,E)
a,
b
c
c
Maryam Mirza-Aghayan *, Rabah Boukherroub , Georges Manuel , Max Koenig
a
Chemistry and Chemical Engineering Research Center of Iran (CCERCI), P.O. Box 14335-186, Tehran, Iran
b
´
Interdisciplinary Research Institute, IRI – IEMN, Avenue Poincare – B.P 69, 59652 Villeneuve d’Ascq, France
´
c
´ ´ ´
Laboratoire d’Heterochimie Fondamentale et Appliquee, UMR-CNRS No. 5069, Universite Paul-Sabatier, 118 Route de Narbonne,
31062 Toulouse Cedex 04, France
Received 9 August 2004; accepted 9 November 2004
Available online 15 December 2004
Abstract
This paper describes the photochemical and the thermal isomerization of s-cis(E,E) 1,1-diphenly-3,4-bis(trimethylsilylmethylene)-
1-silacyclopentane (1a). Under thermal conditions a 1,3-sigmatropic of the methylene hydrogen occurs, yielding the s-trans isomer
(1b). The photochemical irradiation of (1a) at 300 nm for 1 h in deoxygenated benzene gives the corresponding s-cis(E,Z) isomer (1c)
and then the s-cis(Z,Z) isomer (1d) after prolonged irradiation (3 h). There was no evidence for the formation of the corresponding
cyclobutene resulting from the ring closure of the exocylic diene.
Ó 2004 Elsevier B.V. All rights reserved.
Keywords: 1,1-Diphenly-3,4-bis(trimethylsilylmethylene)-1-silacyclopentane; s-cis(E,E); s-trans(E); s-cis(E,Z); s-cis(Z,Z) isomers; Thermal isomer-
ization; Photochemical isomerization
1. Introduction
ported to yield cis-3,4-dimethyl-cyclobutene stereospe-
cifically, albeit in low quantum yield, owing to
competing cis M trans isomerization, upon direct pho-
tolysis in solution [10]. Several reports have demon-
strated that direct photolysis of alkyl-substituted
cyclobutenes in solution results predominantly in non-
stereospecific ring opening to the corresponding conju-
gated dienes, in competition with fragmentation to the
corresponding alkene and alkyne.
It is recognized that most acyclic 1,3-dienes exist as a
mixture of two conformers [11]. In most cases the more
stable conformation is planar and s-trans about the cen-
tral bond [12], while the minor form has an s-cis config-
uration. Although much of the thermal chemistry and
photochemistry of 1,3-dienes proceeds through this min-
or conformer, direct examination of s-cis chemistry has
been limited because of its high energy relative to the
The photochemistry of exocyclic 1,3-dienes has been
extensively investigated [1–3]. It has been found that di-
rect irradiation of these 1,3-dienes in dilute solution at
room temperature results mostly in cyclobutene forma-
tion [4,5] and double bond isomerization [6,7]. There
are examples that illustrate the preferred disrotatory ste-
reochemistry of the photochemical ring closure of 1,3-
butadiene derivatives to the corresponding cyclobutene
[8–10]. For example, (E,E)-2,4-hexadiene has been re-
*
Corresponding author. Tel.: +98 21 8036144-5; fax: +98 21
8037185.
E-mail address: m.mirzaaghayan@ccerci.ac.ir (M. Mirza-
Aghayan).
0022-328X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.11.013

M. Mirza-Aghayan et al. / Journal of Organometallic Chemistry 690 (2005) 1028–1032
1029
s-trans isomer and the small barrier separating the s-cis
from the s-trans isomer.
6.25) for the vinylic protons and a doublet at d 2.25
for the two hydrogens in the a position to Si with a cou-
pling constant of 1.7 Hz. The Me3Si groups are seen as a
singulet at 0.19 ppm. There is a full symmetry in the
molecule. This structure was confirmed by 13C NMR
spectrum of the s-cis(E,E). In fact, the intracyclic (C2/
C5, C3/C4) [18] and the exocyclic carbons (including
the TMS group) are magnetically equivalent, which is
in agreement with the existence of a symmetry in the
molecule (Fig. 1). A previous example based on tita-
nium-mediated stereoselective cyclization of diynes to
E,E-exocyclic dienes have been reported by Nugent
et al. [19].
In the last few years, we have been interested in the
synthesis of heterocyclic compounds containing silicon,
because of their potential as building blocks for the
preparation of functional macromolecules and poly-
mers. These compounds are easily obtained by the reac-
tion of (di) allyl and (di) propargylsilanes with reactive
zirconocene ‘‘Cp2Zr’’ generated in situ by addition of
butyllithium to Cp2ZrCl2. This versatile method is easy
to carry out and allows the synthesis of various func-
tionalized main group heterocylopentanes and cyclo-
pentadienes (Si, Ge, P) starting from bis allylic and
propargylic derivatives [13–17]. Direct reaction of the
reactive zirconocene ‘‘Cp2Zr’’ with dipropargylic deriva-
tives of Si, Ge, Sn and P yields bicyclocomplexe interme-
diates, which can be easily converted to the corresponding
exo dienic metallacyclopentanes by reaction with differ-
ent electrophiles. While, the cyclozirconation of the dial-
lylic derivatives leads to the corresponding saturated
metallacyclopentanes, after exposure to different electro-
philes. This way, it was possible to prepare racemic phos-
phines and diphosphines.
In a similar way, electrophilic addition of bromine
(Br2) at low temperature to the zirconium intermediates
obtained by reaction of ‘‘Cp2Zr’’ with the above diprop-
argylsilanes (and germanes) afforded the correspond-
ing 3,4-bis(bromo-trimethysilyl-methylene)1-sila (and
germa)cyclopentane in 35% and 52% yield, respectively
1
[14]. Comparison of H and 13C NMR spectra of 1a
with the brominated product 2, obtained by electrophilic
displacement of the same zirconium intermediate with
Br2 (Scheme 1), is interesting from a stereochemical
point of view. The two hydrogen atoms of the methylene
groups in a position to the silicon atom in the bromi-
nated molecule display an AB system (JAB = 14 Hz).
The two bulky bromine atoms in the endo position in-
duce a noticeable and stable twisting of the silacyclopen-
tane ring. So, the two hydrogen atoms of the methylene
groups in a to silicon are always magnetically equivalent
in 1a, whereas they remain an AB system in the bromi-
nated compound 2.
In this paper, we investigate the thermal and the photo-
chemical behavior of the exo diene: s-cis(E,E) 1,1-diph-
enly-3,4-bis(trimethylsilylmethylene)-1-silacyclopentane
(1a). It is the first example of a diene bearing two bulky
trimethylsilyl groups reported in the literature, so far.
The different steps and the resulting products were char-
acterized using 1H and 13C NMR.
2. Results
The product s-cis(E,E) 1a is stable at room tempera-
ture in solution in hexane, benzene and diethyl ether.
However, heating the s-cis(E,E) compound 1a in ben-
zene solution at 50 °C led to the formation of s-trans(E)
isomer 1b after one week in 70% yield. The thermal
isomerization of 1a compound is a consequence of a
1,3-sigmatropic rearrangement of a methylene hydrogen
(Scheme 2).
1H NMR displays a doublet at d 2.26 with a coupling
constant of 1.6 Hz and a singulet (d 2.09) for the meth-
ylene protons on the C2 and C7, respectively. The vinylic
Compound (1a) was prepared by reaction of
a,x-(trimethylsilyl) dipropargyldiphenyl silane with
‘‘Cp2Zr’’ followed by protonation of the zirconium com-
plex intermediate, in 72% yield (Scheme 1).
The cyclozirconation reaction of bis-trimethylsilyl-
propargyl-1,1-diphenylsilane followed by the electro-
philic addition of H+ leads to the stereoselective
formation of (1a) in the s-cis(E,E) exocyclic configura-
1
tion. H NMR spectrum (Table 1) exhibits a triplet (d
SiMe3
SiMe3
Ph
Ph
Ph
E
E
i) Cp2ZrCl2, n-BuLi, THF
ii)
Si
Si
Ph
SiMe3
SiMe3
1a, E= H (ii: H+)
2, E= Br (ii: Br2 /CCl4, H+ )
Scheme 1. Synthesis of s-cis(E,E) isomers 1a and 2.

1030
M. Mirza-Aghayan et al. / Journal of Organometallic Chemistry 690 (2005) 1028–1032
Table 1
13C and 1H NMR characteristics of the 1a–d isomers
SiMe3
SiMe3
SiMe3
Ph
SiMe3
SiMe3
Si
Ph
Ph
Ph
Ph
Si
Si
Si
SiMe3
Ph
Ph
Ph
SiMe3
SiMe3
1a:s-cis(E,E)
1b:s-trans(E)
1c:s-cis(E,Z)
1d:s-cis(Z,Z)
13C NMR
d
13C
C2, C5
C3, C4
C6, C7
SiMe3
21.25
127.11
19.98
26.44
22.70
163.93
160.12
124.99
124.52
0.96
29.57
159.80
124.53
0.45
160.56
122.89
À0.07
166.11
159.25
124.11
24.78
À0.09
À0.94
À0.08
1H NMR
d
1H
CH2Si (4JHH Hz)
2.25 (d) (1.7)
2.26 (d) (1.6)
2.09 (s)
2.25 (d) (1.6)
2.21 (d) (1.6)
d
d
B = 2.20
A = 2.25
4JAB = 14 (2.1)
5.35 (d) (1.6)
CH = (4JHH Hz)
6.25 (t) (1.7)
0.19
6.06 (s)
5.92 (t) (1.6)
5.49 (t) (1.6)
0.24 (s)
5.88 (t) (1.6)
0.21 (s)
0.03 (s)
CH3Si
0.21 (s)
0.19 (s)
SiMe3
SiMe3
Ph
Ph
benzene
Si
Si
∆
Ph
Ph
SiMe3
SiMe3
1b
1a
s-trans(E)
s-cis(E,E)
Scheme 2. Thermal isomerization of s-cis(E,E) isomer 1a.
Photochemical irradiation of the s-cis(E,E) isomer 1a
at 254 nm in deoxygenated benzene solution after 13 h
led to the formation of a mixture of isomers: 20% s-ci-
s(E,E), 30% s-trans(E) 1b and 50% of unknown prod-
ucts. The s-trans(E) resulted more likely from the
thermal isomerization of 1a as shown in the above sec-
tion. Indeed, the prolonged irradiation (13 h) can cause
an heating effect responsible of the observed reaction
pathway. However, irradiation for 1 h of the s-cis(E,E)
isomer at a higher wavelength (300 nm) instead of 254
nm led to the quantitative formation of s-cis(E,Z) iso-
mer. This isomer was formed by single double bond
photochemical isomerization of the diene1,3-dienes [20]
(Scheme 3).
Fig. 1. 1H NMR spectra of the 1a–d isomers.
protons on C5 and C6 appear as a triplet (4J = 1.6 Hz)
and a singulet, respectively. All the carbons are magnet-
ically non equivalent (Table 1 and Fig. 1), which is in
agreement with a symmetry loss induced by the thermal
rearrangement.

M. Mirza-Aghayan et al. / Journal of Organometallic Chemistry 690 (2005) 1028–1032
1031
SiMe3
SiMe3
Ph
Ph
Ph
SiMe3
SiMe3
hν
hν
Si
Si
Si
SiMe3
Ph
Ph
Ph
SiMe3
1a
1c
1d
s-cis(Z,Z)
s-cis(E,E)
s-cis(E,Z)
Scheme 3. Photochemical isomerization of s-cis(E,E) isomer 1a.
1
From H NMR spectrum of 1c one can see two dou-
blets (d 2.21 and 2.25) for the methylene protons and
two triplets (d 5.49 and 5.92) for the vinylic protons with
a coupling constant of 1.6 Hz. The TMS groups display
two singulets at d 0.19 and 0.24 ppm. 13C NMR con-
of the rules proposed by Woodward and Hoffmann
would predict that the ring closure to a cyclobutene in
the electronically excited state is disrotatory. This will
lead to the formation of a cis bicyclobutene (where the
TMS groups are in a cis position). In accordance to
the orbital symmetry, the conrotatory thermal ring
opening of the cyclobutene will give the s-cis(Z,E)
isomer.
Srinivasan [21] have studied the thermal decomposi-
tion of cis-3,4-dimethylcyclobutene. At high tempera-
tures (420–450 K), the (Z,E) 2,4-hexadiene was
obtained quantitatively in a conrotatory process. How-
ever, more recent reports have shown that the photo-
chemistry (193 or 214 nm) of constrained cyclobutenes
results in the formation of (E,E) and (E,Z) isomers.
The photochemical ring opening reaction is not stereo-
specific even though that partial orbital symmetry con-
trols the process.
1
firms the H NMR results and shows features that are
compatible with a disrupted symmetry of the s-cis(E,Z)
molecule (Table 1 and Fig. 1). Indeed, all the carbons
are magnetically non equivalent.
When the irradiation at 300 nm in deoxygenated ben-
zene solution was prolonged the s-cis(E,Z) isomer 1c
slowly disappears to give the s-cis(Z,Z) isomer 1d. The
reaction conversion is quantitative after 3 h irradiation.
NMR spectra analysis indicates the recovery of the sym-
metry of the molecule, but the two bulky trimethylsilyl
groups in the endo position induce a noticeable and sta-
ble twisting of the silacyclopentane ring. The two hydro-
gen atoms of the methylene groups in the a position to
the silicon are clearly seen as an AB system (2JAB = 14
The
photolysis
of
1,1-diphenyl-3,4-bis(tri-
Hz) and the vinylic protons as
a
doublet
methylsilylmethylene)-1-silacyclopentane s-cis(E,E) 1a
can lead directly to the formation of the (E,Z) isomer
1c (photoisomerization reaction: E/Z) [20,22]. The same
product can be obtained in a stepwise process: disrota-
tory photochemical cyclization of (1a) to the corre-
sponding cis-bicyclobutene, followed by a thermal ring
opening reaction. However, the latter step, according
to the orbital symmetry control requires heating of the
mixture. The heat generated in the photochemical reac-
tor at 300 nm is not high enough to cause the ring open-
ing reaction (the rise of the temperature in the
‘‘Rayonet’’ would not exceed 40 °C). We believe that
the most probable reaction mechanism is the direct
Z/E isomerization. Similarly, a further photochemical
isomerization of s-cis(E,Z) isomer 1c leads to the
s-cis(Z,Z) isomer 1d. As previously, one can assume a
stepwise mechanism: disrotatory photoprocess leading
to the formation of a trans-bicyclobutene intermediate,
accompanied with a thermal ring opening. However
without an extra heating source, the photoisomerization
is believed to be the dominant pathway. Moreover, the
AB system observed for the methylene protons
(4JAB = 14 Hz) in the 1d s-cis(Z,Z) is comparable to
(4JAX = 4JBX = 2.1 Hz) [14] (Table 1). This transforma-
tion is easily characterized by 13C NMR. The high sym-
metry of the s-cis(Z,Z) isomer 1d involves an
equivalence of C2/C5, C3/C4 and C6/C7 carbons and
the TMS groups, while these carbons are diastereotopic
in the s-cis(E,Z) form.
Continuation of the photochemical irradiation of the
s-cis(Z,Z) isomer 1d under the same conditions did not
show any changes in the reaction mixture composition
1
as revealed by H and 13C NMR. This result indicates
that this isomer corresponds to the final stage of the
isomerization. It should also be noted that the s-cis(E,Z)
isomer and s-cis(Z,Z) isomer had been observed previ-
ously by irradiation of the s-cis(E,E) isomer at 254 nm
with the yield 32% and 28% yields, respectively.
3. Discussion
Direct photochemical irradiation of conjugated
dienes is known to give rise to both double bond iso-
merization and cyclobutene formation. The application

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M. Mirza-Aghayan et al. / Journal of Organometallic Chemistry 690 (2005) 1028–1032
SiMe3
SiMe3
4.2. Photochemical isomers
Ph
Ph
Ph
Ph
In a quartz tube, a 100 mg of the 1a isomer in deox-
ygenated C6D6 solution was irradiated at 300 nm for the
Si
Si
1
time indicated. As shown by H and 13C NMR spectra,
SiMe3
4
3
the 1c isomer was only obtained after 1 h photochemical
irradiation of 1a isomer. Similarly, the 1d isomer was
obtained after 3 h irradiation of the 1c isomer, was only
Scheme 4.
1
characterized by H and 13C NMR (see Fig. 1).
the one obtained with the brominated compound (2)
implying a twisted form induced by the two bulky
TMS in the endo position.
The accuracy of the percentages of the different iso-
mers, estimated from NMR, do not allow us to conclude
that the described photoisomerization at 300 nm are
univocal reactions. Consequently, we can not assert that
a ‘‘one way’’ mechanism, as described by Tokumaru and
Arai [22], is involved.
In conclusion, we have shown that the thermal treat-
ment of the exo diene s-cis(E,E) (1a) leads stereospecif-
ically to s-cis/s-trans isomerization, whereas the
photochemical irradation at 300 nm of (1a) results in
the selective isomerization of one double bond to yield
the isomer s-cis(E,Z) 1c. The prolonged irradiation
causes the isomerization of the second double bond to
produce the constrained s-cis(Z,Z) isomer 1d. Whereas
the bicyclo[3.2.0]hept-1(7)ene 3 was recently described
[17], we had no evidence for cyclobutene formation of
4 (Scheme 4).
References
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4. Experimental
[16] V. Dejean, H. Gornitzka, G. Oba, G. Manuel, M. Koenig,
Organometallics 19 (2000) 711.
All manipulations were carried out under an argon
atmosphere. The product s-cis(E,E) 1a was synthesized
by cyclozirconation of diyne correspondent [14]. The
photochemical irradiation was effected with a Rayonet
RPR 100 at 254 or 300 nm. All NMR spectra were re-
corded at 25 °C on Bruker AC 80 and 250 MHz in
[17] G. Oba, G. Moreira, G. Manuel, M. Koenig, J. Organomet.
Chem. 643-644 (2002) 324.
[18] We adopted the unique numeration for the common sila cyclic
skeleton:
SiMe3
2
3
Ph
6
1
CDCl3. The H and 13C chemical shifts were referenced
Si
Ph
relative to Me4Si. Coupling constants are given in Hertz.
7
4
5
SiMe3
4.1. Thermal isomer
1
.
[19] W.A. Nugent, J.C. Calabrese, J. Am. Chem. Soc. 106 (1984) 6422.
[20] M.E. Squillacote, T.C. Semel, J. Am. Chem. Soc. 112 (1990) 5546.
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[22] K. Tokumaru, T. Arai, J. Photochem. Photobiol. A: Chem. 65
(1992) 1.
The s-trans(E) 1b isomer was obtained by heating 100
mg of the 1a isomer at 50 °C for 1 week in deoxygenated
C6D6 solution in 70% yield or for 10 days in deoxygen-
ated CDCl3 solution in 100% yield.