Electrocyclic reactions of siloles: A combined experimental and theoretical study

Article abstract of DOI:10.1002/chem.201302544

The reaction of 4-chloro-1,2-dimethyl-4-supersilylsila-1-cyclopentene (2 a) with Li[NiPr₂] at -78 °C results in the formation of the formal 1,4-addition product of the silacyclopentadiene derivative 3,4-dimethyl-1- supersilylsila-1,3-cyclopentadiene (4 a) with 2,3-dimethyl-4-supersilylsila-1,3- cyclopentadiene (5 a). In addition the respective adducts of the Diels-Alder reactions of 4 a+4 a and 4 a+5 a were obtained. Compound 4 a, which displays an s-cis-silacyclopentadiene configuration, reacts with cyclohexene to form the racemate of the [4+2] cycloadduct of 4 a and cyclohexene (9). In the reaction between 4 a and 2,3-dimethylbutadiene, however, 4 a acted as silene as well as silacyclopentadiene to yield the [2+4] and [4+2] cycloadducts 10 and 11, respectively. The constitutions of 9, 10, and 11 were confirmed by NMR spectroscopy and their crystal structures were determined. Reaction of 4-chloro-1,2-dimethyl-4-tert-butyl-4-silacyclopent-1-ene (2 c) with KC ₈ yielded the corresponding disilane (12), which was characterized by X-ray crystal structure analysis (triclinic, Pβar 1$). DFT calculations are used to unveil the mechanistic scenario underlying the observed reactivity. Added value: The intermediately formed 2 H-silole 4 a dimerizes to three different products in a 1:1:1 ratio (see scheme): the formal 1,4-addition product of 4 a and 5 a and the Diels-Alder products of 4 a with 4 a and 4 a with 5 a. Compound 4 a undergoes Diels-Alder reactions with cyclohexene and dimethylbutadiene. DFT calculations elucidate the underlying reaction mechanisms.

Full text of DOI:10.1002/chem.201302544

DOI: 10.1002/chem.201302544
Full Paper
&
Siloles
Electrocyclic Reactions of Siloles: A Combined Experimental and
Theoretical Study
Frank Meyer-Wegner, Josef H. Wender, Konstantin Falahati, Timo Porsch, Tanja Sinke,
Michael Bolte, Matthias Wagner, Max C. Holthausen,* and Hans-Wolfram Lerner*[a]
Dedicated to Professor Werner Uhl on the occasion of his 60th birthday
Abstract: The reaction of 4-chloro-1,2-dimethyl-4-supersilyl-
sila-1-cyclopentene (2a) with Li[NiPr2] at À788C results in
the formation of the formal 1,4-addition product of the sila-
cyclopentadiene derivative 3,4-dimethyl-1-supersilylsila-1,3-
cyclopentadiene (4a) with 2,3-dimethyl-4-supersilylsila-1,3-
cyclopentadiene (5a). In addition the respective adducts of
the Diels–Alder reactions of 4a+4a and 4a+5a were ob-
tained. Compound 4a, which displays an s-cis-silacyclopenta-
diene configuration, reacts with cyclohexene to form the
racemate of the [4+2] cycloadduct of 4a and cyclohexene
(9). In the reaction between 4a and 2,3-dimethylbutadiene,
however, 4a acted as silene as well as silacyclopentadiene to
yield the [2+4] and [4+2] cycloadducts 10 and 11, respec-
tively. The constitutions of 9, 10, and 11 were confirmed by
NMR spectroscopy and their crystal structures were deter-
mined. Reaction of 4-chloro-1,2-dimethyl-4-tert-butyl-4-sila-
cyclopent-1-ene (2c) with KC8 yielded the corresponding dis-
ilane (12), which was characterized by X-ray crystal structure
¯
analysis (triclinic, P1). DFT calculations are used to unveil the
mechanistic scenario underlying the observed reactivity.
Introduction
(OSiMe3) is significantly elongated. Silenes of the Brook type
possess a less polar double bond and are less reactive than si-
lenes of the Wiberg type. In keeping with this general notion,
theoretical studies by Apeloig et al. have shown that the
former exhibit low or even negative activation energies (À3–
8 kcalmolÀ1) for the nucleophilic addition of water, whereas
a larger activation barrier of 16 kcalmolÀ1 was found for the
same reaction of a Brook-type silene.[10]
The reactivity of unsaturated silicon compounds has been
studied extensively.[1,2] In this context, stable examples with
Si=C double bonds, for example, silenes,[2–5] silaallenes,[2,6] and
silaaromatics[2,7] such as silabenzene or silole anions and dia-
nions,[2,8] have been synthesized and structurally characterized.
In the 1980s, the metastable silenes (Me3Si)2Si=C(1-adamantyl)-
(OSiMe3)[3] and Me2Si=C(SiMe3)(SiMetBu2)[4] were described by
Brook et al. and Wiberg et al. and the X-ray structure determi-
nations of these silenes provided for the first time a clear char-
acterization of the Si=C double bond.
The [2+2] cycloreversion of silacyclobutane derivatives at
high temperatures has evolved as one of the most important
methods to generate transient silenes in the gas phase.[11,12]
Other examples of thermally driven pericyclic routes to silene
formation are the [4+2] cycloreversion of silabicyclo-
[2.2.2]octadiene derivatives or the retro-ene fragmentation of
allylsilanes.[13] Further, a [1,5] sigmatropic silyl shift has been
discussed as the key elementary step involved in the formation
of a transient silene that has been suggested as intermediate
in the thermolysis of silyl-substituted 4-silacyclopentadiene de-
rivatives.[14] Most interesting for reactivity studies, however, are
processes in which silenes are generated in solution at low
temperatures. This includes 1,2-eliminations of MX (M=alkali
metal; X=halogens or other good leaving groups), which
occur often at temperatures below À1008C.[2,15] Accordingly,
1,2-eliminations of MX at low temperatures have preferably
been used for reactivity studies of Wiberg-type silenes.
The structural features of Wiberg’s silene Me2Si=C(SiMe3)-
(SiMetBu2) agree closely with those of H2Si=CH2,[9] whereas the
double bond in Brook’s silene (Me3Si)2Si=C(1-adamantyl)-
[a] F. Meyer-Wegner, J. H. Wender, K. Falahati, T. Porsch, T. Sinke, Dr. M. Bolte,
Prof. Dr. M. Wagner, Prof. Dr. M. C. Holthausen, Dr. H.-W. Lerner
Institut fꢀr Anorganische und Analytische Chemie, Goethe-Universitꢁt
Max-von-Laue-Strasse 7, 60438 Frankfurt am Main (Germany)
Fax: (+49)69-798-29260
E-mail: max.holthausen@chemie.uni-frankfurt.de
lerner@chemie.uni-frankfurt.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201302544. It contains synthesis of silacy-
clopentenes 2b, 2c, 3d, 3e, 3g, and 3h; detailed discussion of the struc-
tures of 11, 3g, 3h, 13, 14 (monoclinic P21/c), and 14 (monoclinic P21/n);
NMR spectra of 9; assignment of H and 13C NMR signals of 10 and 11;
X-ray crystallographic parameters; computational results for the thermo-
chemistry of conceivable silole dimers; potential energy surface scans of
the dimerizations and reaction with DMB; and HOMO–LUMO gaps.
Overall, the reactivity of Wiberg-type silenes embraces the
following reaction types (Scheme 1): i) adduct formation, ii) di-
merization, iii) insertion reactions, iv) [2+2] cycloadditions,
v) [2+3] cycloadditions, vi) [2+4] cycloadditions, and vii) ene re-
actions. Further electrocyclic reactions of disilenes, such as the
1
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Scheme 2. Synthesis of 1,2-dimethyl-4-silacyclopentene derivatives. i)+NMe3
or NMe2Et in 2,3-dimethyl-1,3-butadiene at RT; ii) 2a–c: +1 equivalent of
Na[SitBu3] (R=SitBu3, 2a), Na[SiPhtBu2] (R=SiPhtBu2, 2b), or LitBu (R=tBu,
2c) in hexane at À788C; iii) for 3e–g: +2 equivalents of LiMe (R=Me, 3e),
LiMes (R=Mes, 3 f), or LiPh (R=Ph, 3g) in Et2O, benzene, or Bu2O at À788C.
Scheme 1. Reactivity of Wiberg-type silenes.[17]
However, the reaction of 1 with an excess of LitBu or
Na[SitBu3][21,22] and Na[SiPhtBu2][21,23] took another course.
When 1 was treated with two molar equivalents of Na[SitBu3],
no supersilyl derivative of 3 was formed but instead the NMR
signals of compounds 6, 7, and 8 (Scheme 3) were found in
a 1:1:1 ratio; the same products resulted from the reactions of
2b and 2c with two equivalents of Na[SiPhtBu2] and LitBu, re-
spectively.
[2+4] cycloaddition with 1,3-dienes, have also been report-
ed.[16]
Herein, we report the pericyclic reactions of 2,3-dimethylbu-
tadiene (DMB) and cyclohexene with 3,4-dimethyl-1-supersilyl-
1-silacyclopenta-1,3-diene (4a), which was formed quantita-
tively by treatment of 4-chloro-1,2-dimethyl-4-supersilyl-4-sila-
cyclopent-1-ene (2a) with Li[NiPr2] (LDA) at À788C. The struc-
tural features of trapping products of this 1-silacyclopenta-1,3-
diene derivative with cyclohexene and DMB have been investi-
gated. In addition we present the structure of a novel disilane,
which was obtained from the reaction of 1,2-dimethyl-4-tert-
butyl-4-silacyclopent-1-ene with KC8. Furthermore, the funda-
mental reaction mechanisms of the experimentally observed
reaction patterns were explored with DFT methods.
Results and Discussion
Experimental studies
Over the past few decades, the disproportionation reaction of
Si2Cl6 leading to the perchlorinated neopentasilane Si(SiCl3)4
and SiCl4 has been the subject of numerous studies.[18] Yet, al-
though this reaction has been known for 60 years and al-
though several tentative mechanistic proposals have been put
forward, no solid evidence for the nature of the intermediates
involved has been reported. Recently, we have repeated the re-
action of Si2Cl6 with NR3 (R=Me, Et) and we have been able to
show that amine-complexed dichlorosilylenes represent the
key intermediates in this disproportionation reaction.[19,20] In
the course of these studies we isolated the [4+1] cycloadduct
of dichlorosilylene SiCl2 with DMB in good yield upon treat-
ment of Si2Cl6 with catalytic amounts of NMe3 or NMe2Et in
neat DMB. Reaction of 1 with one equivalent of LitBu, Na-
[SitBu3],[21,22] and Na[SiPhtBu2][21,23] exclusively led to the corre-
sponding monosubstituted derivatives shown in Scheme 2.
The use of less bulky nucleophiles, such as LiMe, LiPh, or LiMes
(Mes=mesityl),[24] in excess resulted in nearly quantitative
yields of the related literature-known, disubstituted deriva-
tives 3.
Scheme 3. Reaction of 2a with Na[SitBu3] or Li[NiPr2] (LDA). i) 2 equivalents
of Na[SitBu3] or LDA.
We isolated 6, 7, and 8 after HPLC separation of the reaction
mixture and assigned their structures based on NMR spectros-
copy. Compound 6 was further characterized by X-ray structure
analysis (see below). After HPLC separation, a few single crys-
tals of another minor product were grown from the eluate
(3h, R=SitBu3, R’=H, see the Supporting Information). Howev-
er, only the signals of 6, 7, and 8, but no resonances of 3h
were present in the NMR spectra of the reaction solution.
Formally, compounds 6, 7, and 8 all represent products of
a dimerization between silacyclopentadiene isomers 4a and
5a formed as reactive intermediates after deprotonation of 2a
and MCl elimination (M=Li, Na) from the resulting transient
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carbanion. Such a scenario is in line with findings of Barton
et al. in earlier work.[14] To explain the nature of a dimeric
Diels–Alder product obtained by reaction of silole 5d in
a sealed tube, the authors postulated the intermediacy of the
isomeric silole 4d formed through 1,5-trimethylsilyl migration.
The rearranged isomer 4d was suggested to act as a dienophile
for a subsequent [4+2] cycloaddition with 5d. The observation
of the related Diels–Alder product for the reaction of 5d with
diphenylacetylene has lent further support to the suggested
mechanism (Scheme 4).
As alluded to above, silenes that are generated by thermal
salt elimination tend to react with dienes. Accordingly, reac-
tions of silenes with DMB give rise to the corresponding [2+4]
cycloadducts. However, the reaction of 4a, generated in situ
from 2a and LDA, with a 20-fold excess of DMB at À788C af-
forded a 1:1 mixture of the [2+4] cycloadduct 10 and the
[4+2] cycloadduct 11 (Scheme 5).
Clearly, in contrast to the reaction of 4a with cyclohexene,
in which 4a acts as the (silabuta)diene partner, 4a reacts with
DMB as a silene as well as a silacyclopentadiene. In the reac-
tion of 4a with DMB, a pair of enantiomers of 10 is formed,
both of which possess a boat conformation of the C6 ring
(Figure 3). Apparently, the addition of 4a with DMB proceeds
in a concerted fashion.
Notably, the reaction of 2c with the intercalation complex
KC8 proceeds with dehalogenation of 2c, whereas a deprotona-
tion of 2c was not observed. When 2c was treated with KC8,
no silacyclopentadiene derivative was formed, but disilane 12
was isolated in 81% yield (Scheme 6). X-ray quality crystals of
12 were obtained from hexane at room temperature (see
Figure 4).
Scheme 4. Isomers of 5-trimethylsilyl-1-sila-1,3-cyclopentadiene derivatives
(1508C).
To confirm an occurrence of the postulated intermediate 4a
in the course of the reaction studied here, we conducted trap-
ping studies with cyclohexene and DMB as both compounds
have long been known to be excellent trapping agents for
silicon–carbon double bonds. Treatment of 2a with LDA in the
presence of a 20-fold excess of cyclohexene at À788C afforded
a 1:1 mixture of two enantiomers of the [4+2] cycloadduct 9
in quantitative yield (Scheme 5). We identified 9 by its spectral
characteristics and by X-ray structure analysis of single crystals,
which were obtained as a racemate. The [4+2] cycloadduct 9
has four chiral centers; however, only two diastereomers (i.e.,
the R,S,R,R and S,R,S,S enantiomers) with boat conformation of
the C6 ring are formed (Figure 2). This corresponds to addition
of cyclohexene in a concerted reaction to both faces of the si-
lacyclopentadiene scaffold of 4a.
Scheme 6. Synthesis of disilane 12: i)+2KC8 in THF at RT.
The molecular structures of the silacyclopentadiene dimer 6,
the [4+2] cycloadduct 9, the [2+4] cycloadduct 10, and the
disilane 12 are shown in Figures 1–4. A cyclotrisiloxane (13)
and a cyclotetrasiloxane (14) (see Figures S4 and S5 in the Sup-
porting Information) were obtained upon hydrolysis of 3 f (the
mesityl derivative of 3). We found that decomposition of 3 f
occurs during column chromatography. The structures of the
silacyclopentene 3g (R=Ph, Scheme 2) and 3h (R=SitBu3, R’=
H), the [4+2] cycloadduct 11 (see Figure S1 in the Supporting
Information), the cyclotrisiloxane (C6H10SiO)3 (13), and the cy-
clotetrasiloxane (C6H10SiO)4 (14) are presented in the Support-
ing Information together with the crystal data and refinement
details of all compounds (Table S1).
Silole dimer 6 (Figure 1), crystallizes in the orthorhombic
space group P212121. It comprises a silacyclopentadiene and
a silacyclopentene ring, both connected by a central SiÀSi
bond.
The [4+2] cycloadduct 9 crystallizes in the triclinic space
¯
group P1 as a racemate. Figure 2 represents the structure of
one of two enantiomers of 9 (selected bond lengths are re-
ported in the caption of Figure 2). The crystal structure of 9
contains two enantiomeric molecules, which are disordered
over two almost equally occupied positions (site occupation
factor 0.510(5) for the major occupied site). X-ray structure
analysis of 9 indicates a silanorbornene and an annelated six-
membered ring. This tricycle is formed of one Si and ten
C atoms. In both isomers the C6 ring of this tricycle possesses
Scheme 5. Reactions of 4a with cyclohexene and DMB. i)+cyclohexene at
À788C; ii)+DMB at À788C.
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Figure 3. Solid-state structure of both enantiomers of 10 (monoclinic, P21/n).
Displacement ellipsoids are drawn at the 50% probability level. The H atoms
are omitted for clarity. Selected bond lengths [ꢁ] and angles [8] of the left
enantiomer: C1ÀC2 1.499(5), C2ÀC3 1.329(5), C3ÀC4 1.526(5), C4ÀSi1
1.904(4), Si1ÀC1 1.881(4), C4ÀC5 1.551(5), C5ÀC6 1.503(5), C6ÀC7 1.341(5),
C7ÀC8 1.510(5), C8ÀSi1 1.889(3), Si1ÀSi2 2.3772(13); C1-C2-C3 119.5(3),
C2-C3-C4 116.8(3), C3-C4-Si1 104.7(2), C4-Si1-C1 93.59(16), Si1-C1-C2 105.1(2),
C4-C5-C6 111.1(3), C5-C6-C7 119.8(3), C6-C7-C8 120.4(3), C7-C8-Si1 108.3(2),
C8-Si1-C4 102.53(17), Si1-C4-C5 109.0(2), C1-Si1-C8 107.32(16), C1-Si1-Si2
117.64(12), C4-Si1-Si2 117.68(11), C8-Si1-Si2 115.15(13).
Figure 1. Solid-state structure of the silacyclopentadiene dimer 6 (ortho-
rhombic, P212121). Displacement ellipsoids are drawn at the 50% probability
level. H atoms of methyl groups are omitted for clarity. Selected bond
lengths [ꢁ], bond angles [8], and torsion angle [8]: C1ÀC2 1.343(4), C2ÀC3
1.483(5), C3ÀC4 1.366(5), C4ÀSi2 1.880(3), Si2ÀC1 1.865(3), C5ÀC6 1.509(5),
C6ÀC7 1.527(6), C7ÀC8 1.318(5), C8ÀSi3 1.867(3), Si3ÀC5 1.893(3), Si1ÀSi2
2.4206(11), Si2ÀSi3 2.4187(11), Si3ÀSi4 2.4279(11); C1-C2-C3 116.1(3),
C2-C3-C4 114.9(3), C3-C4-Si2 109.0(3), C4-Si2-C1 90.04(15), Si2-C1-C2
109.7(3), C1-Si2-Si1 108.88(9), C1-Si2-Si3 111.89(10), C4-Si2-Si1 111.54(11),
C4-Si2-Si3 103.40(10), Si1-Si2-Si3 125.28(4), C5-C6-C7 109.6(3), C6-C7-C8
118.1(3), C7-C8-Si3 112.4(3), C8-Si3-C5 90.39(15), Si3-C5-C6 108.7(3),
C5-Si3-Si2 105.96(11), C5-Si3-Si4 111.52(11), C8-Si3-Si2 111.63(10), C8-Si3-Si4
110.59(10), Si2-Si3-Si4 122.08(4); Si1-Si2-Si3-Si4 À138.80(5).
a boat conformation. The distance of 2.3643(8) ꢁ in 9 is of
a characteristic length for SiÀSi bonds.[25]
Crystals of the [2+4] cycloadduct 10 (see Figure 3) were
grown from benzene at room temperature, and belong to the
monoclinic, P21/n space group as a racemate. Both enantio-
mers feature a bicyclic structural motif composed of a five-
and a six-membered ring annelated at the former Si=C bond.
In both structures, the C6 rings adopt a boat conformation; all
CÀC distances are in the expected range and comparable to
those in the X-ray structure of 9.
¯
Disilane 12 crystallizes in the triclinic space group P1. As
shown in Figure 4, its structure features two symmetry-equiva-
lent silapentene moieties in a staggered configuration. The
SiÀSi distance of 2.415(4) ꢁ in 12 is somewhat longer than the
typical SiÀSi bond length in disilanes (mean length of SiÀSi
bonds: 2.37 ꢁ[25]) but the presence of equal dihedral angles as
found in 12 indicates that the disilane is not sterically over-
crowded.[26] All other structural parameters are in the expected
ranges.[25]
Quantum-chemical studies
We have investigated the reaction mechanism underlying the
observed reaction patterns by using double-hybrid density
functional theory (RI-B2GP-PLYP-D3/def2-QZVPP//RI-B97-D/SVP
level; see Computational Details section). Unless noted other-
wise, relative energies discussed in the following refer to Gibbs
energy differences computed at T=À788C. To avoid ambigui-
ties with respect to the numbering scheme used in the experi-
mental part, we use Latin capitals for denoting minimum struc-
tures identified along the reaction pathways throughout the
theoretical sections of this paper.
¯
Figure 2. Solid-state structure of one enantiomer of 9 (triclinic, P1). Displace-
ment ellipsoids are drawn at the 50% probability level. The H atoms with
the exception of those of the silacyclopentene ring are omitted for clarity.
Selected bond lengths [ꢁ] and angles [8]: C1ÀC2 1.282(10), C2ÀC3 1.538(9),
C3ÀC4 1.602(8), C4ÀSi1 1.868(3), Si1ÀC1 1.865(2), Si1ÀC7 1.917(4), C7ÀC8
1.581(6), C8ÀC3 1.571(8), C8ÀC9 1.425(5), C9ÀC10 1.509(4), C10ÀC11
1.539(8), C11ÀC12 1.524(8), C12ÀC7 1.520(7), Si2ÀSi1 2.364(1); C1-C2-C3
114.2(8), C2-C3-C4 108.4(7), C3-C4-Si1 93.4(3), C4-Si1-C1 90.8(1), Si1-C1-C2
108.0(4), Si1-C7-C8 102.3(3), C7-C8-C3 109.1(4), C7-C8-C9 113.0(4), C8-C9-C10
117.1(3), C9-C10-C11 113.1(3), C10-C11-C12 112.9(5), C11-C12-C7 112.2(5),
C1-Si1-C7 103.2(2), C4-Si1-C7 86.5(2), C1-Si1-Si2 121.2(1), C4-Si1-Si2 121.7(1),
C7-Si1-Si2 124.3(1).
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¯
Figure 4. Solid-state structure of the disilane 12 (triclinic, P1). Only minor de-
viations are detected for the corresponding bond lengths and angles of the
two crystallographically independent molecules in the asymmetric unit. Dis-
placement ellipsoids are drawn at the 50% probability level. The H atoms
with the exception of those of the silacyclopentene ring are omitted for
clarity. Selected bond lengths [ꢁ] and angles [8] of the displayed molecule:
C1ÀC2 1.525(9), C2ÀC3 1.351(10), C3ÀC4 1.509(9), C4ÀSi1 1.899(6), Si1ÀC1
1.879(7), Si1ÀC7 1.910(7), Si1ÀSi1B 2.415(4); C1-C2-C3 116.5(6), C2-C3-C4
119.2(6), C3-C4-Si1 102.3(4), C4-Si1-C1 94.3(3), Si1-C1-C2 103.6(5), C1-Si1-C7
113.1(3), C1-Si1-Si1B 110.4(3), C4-Si1-C7 112.7(3), C4-Si1-Si1B 110.9(3),
C7-Si1-Si1B 113.9(3). Symmetry transformation used to generate equivalent
atoms: B: Àx+1, Ày, Àz+1.
Scheme 7. Computed isomerization pathways of the 1-silacyclopenta-1,3-
diene A (DG195 in kcalmolÀ1 with respect to isomer A).
Assuming that the reaction sequence leading to 6, 7, and 8
commences with the initial formation of A (corresponding to
4a in the discussion above), we first set out to explore viable
isomerization pathways of this species. With a moderate activa-
tion barrier of 14 kcalmolÀ1 the isomeric silole B (correspond-
ing to 5a) is readily accessible by [1,5] hydrogen shift from A.
We find B to be 19 kcalmolÀ1 more stable than A, which is
fully in line with expectations based on the fact that the
ly formed silole A can isomerize efficiently to build up a signifi-
cant stationary concentration of B. This step is driven by the
strong inclination of A to abolish the intrinsically less stable
Si=C double bond by formation of a thermodynamically pre-
ferred C=C bond. With a moderate computed activation barrier
of DG° =14 kcalmolÀ1 this is well feasible even at low temper-
atures, so that B, once formed, can indeed be seen as a rele-
vant reaction intermediate along the pathway to product for-
mation. The significant exoergicity renders this step irreversi-
ble.
former isomer contains
a conjugated C=C double-bond
system, whereas the latter features a Si=C bond in conjugation
with a C=C bond. A [1,3] hydrogen shift connects B with C, an
only slightly less stable, transoid isomer. Bꢂteille et al.[27] have
observed the formation of a similar isomer in small yields upon
flash vacuum pyrolysis of 1-methyl-1-allylsilacyclopent-3-ene.
However, with an activation barrier exceeding 82 kcalmolÀ1 for
this step, we can safely neglect it here as irrelevant at the tem-
perature (À788C) applied in the experiments (Scheme 7).
As an alternative to the formation of B, [1,5] hydrogen shift
along the CÀC framework in A leads to D. With its Si=C double
bond, this isomer is also substantially less stable than B and
the activation barrier for its formation (DG° =28 kcalmolÀ1) is
too high for this process to compete with the formation of B.
Finally, A can also undergo a ring contraction to yield the bicy-
clic isomer E. Although such a species has been postulated to
play a role in the course of stereospecific additions to siloles,[14]
we conclude here that this isomer is unlikely to contribute sig-
nificantly to the overall reactivity given its high relative energy
(21 kcalmolÀ1 above A) and the large activation barrier separat-
ing A and E (DG° =34 kcalmolÀ1).
Proceeding further along the reaction sequence to product
formation sketched in Scheme 3, we investigated the [4+2] cy-
cloadditions leading to G and H (corresponding to 7 and 8 in
the experimental part) through dimerization of A and reaction
of A and B, respectively (Scheme 8). For both steps we were
unable to localize transition states, and systematic reaction-co-
ordinate scans on smaller molecular models confirmed that
both reactions are barrierless (Figures S15 and S16 in the Sup-
porting Information).
Turning to the mechanism of the formation of dimer F (cor-
responding to 6 in the experimental part), we identified a reac-
tion sequence that starts with the dimerization of two mole-
cules of A by direct SiÀSi bond formation leading to a formally
zwitterionic intermediate I (Scheme 9). This linear head-to-head
approach of the two siloles occurs without reaction barrier and
the formation of I is exergonic (DGR =À17 kcalmolÀ1).
Yet, I does not represent a stable intermediate but under-
goes subsequently an essentially barrierless rotation about the
SiÀSi bond from its initial anti to a gauche conformation,
We conclude this section by noting that, consistent with
tentative mechanistic suggestions by Barton et al.,[14] the initial-
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the preferred conformation of 6 found in the X-ray analysis,
see above).
As an alternative to the singlet pathway, a second route in-
volving the triplet species is also conceivable; such species
have been described as intermediates for reactions of related
silenes.[28,29] Here, the triplet biradical form IT is indeed 5.8 kcal
molÀ1 (RI-B97-D/SVP) more stable than the corresponding sin-
glet. Starting from the triplet biradical species, two pathways
can either lead to the linear head-to-head dimer[28] F or to
a 1,2-disilacyclobutane[29] through CÀC coupling. We identified
a singlet–triplet curve crossing on both reaction paths. Com-
parison of the particular minimum-energy crossing points
(MECPs)[30] reveals that the formation of F is favored. Hence,
the potential occurrence of triplet biradical intermediates leads
to the same product as the reaction on the singlet surface re-
gardless of whether surface crossing is efficient or not. Further-
more, the actual hydrogen shift occurs on the singlet potential
energy surface (see the Supporting Information for a presenta-
tion of results).
Scheme 8. [4+2] cycloadditions leading to the dimers G and H (DG1R95 in kcal
molÀ1 with respect to the reactants).
To investigate the trapping reaction of A with cyclohexene
used in excess, we carried out calculations for the [4+2] cyclo-
addition of A with cyclohexene (Scheme 10). With a minute ac-
tivation barrier of DG° =7 kcalmolÀ1 the most favorable transi-
Scheme 10. [4+2] cycloaddition of the 1-silacyclopenta-1,3-diene A with
cyclohexene (DG195 in kcalmolÀ1). Conformational isomers are characterized
by the indices hc (half-chair), c (chair), and b (boat).
tion state leads to the endo product, in which the cyclohexane
moiety assumes a thermodynamically favored chair conforma-
tion (DG=À38 kcal molÀ1). The corresponding addition from
the exo face has a slightly higher barrier with DG° =9 kcal
molÀ1. For the various stereochemically relevant combinations
of reactants we localized transition-state structures that all fea-
ture closer SiÀC than CÀC contacts for the bonds being
formed (an example is displayed in Figure 5). In other words,
the formation of J (corresponding to product 9) from A and
Scheme 9. Computed reaction path for the formation of F in the right ste-
reochemistry (DG1R95 in kcalmolÀ1 with respect to the reactants). The values
are calculated with RI-B2GP-PLYP-D3/def2-QZVPP//RI-B97-D/SVP and RI-B97-
D/SVP (in parentheses). S/T denotes a transition from the singlet to triplet
state for which the MECPs have been calculated (Supporting Information).
during which a concomitant hydrogen exchange among the
two silolyl moieties occurs and a gauche conformer of product
F is formed in a highly exergonic step. Reverse rotation about
the SiÀSi bond interconverts it to the thermodynamically
slightly more stable anti conformer of F (which corresponds to
Figure 5. Computed RI-B97-D/SVP structures of the endo transition state
(left) and product (right) of the [4+2] cycloaddition of silacyclopentadiene A
and cyclohexene in boat conformation (selected bond lengths in ꢁ; H atoms
not located at the silacyclopentene ring are omitted for clarity).
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cyclohexene occurs in a concerted, but highly asynchronous
step.[31]
The relative free energies of conformers resulting for the re-
spective products vary within 3 kcalmolÀ1 (see Scheme 10,
which lists free energies computed at À788C). Upon use of
thermal corrections to free energies computed for 258C, the
endo product with its cyclohexyl moiety in a boat conforma-
tion represents the most stable isomer (see Table S2 in the
Supporting Information), perfectly in line with the crystal struc-
ture (Figure 2). We note in passing that the Diels–Alder reac-
tion is kinetically/thermodynamically favored over the compet-
ing H migrations, which is in agreement with former theoreti-
cal work on the corresponding unsubstituted systems.[31b]
We studied two pathways relevant for the [4+2] cycloaddi-
tion of DMB. Both routes, that is, A acting as silene to yield K
(corresponding to 10) and A acting as diene to yield L (corre-
sponding to 11), are exergonic and we identified the endo
isomer of K as the most stable product (DG=À49 kcalmolÀ1 at
À788C). Again, we were unable to localize transition states for
either step, and a systematic reaction-coordinate scan using
a smaller molecular model (see Figures S23 and S24 in the
Supporting Information) corroborates our interpretation that
both products are formed in barrierless reaction steps. We
identified a transition structure connecting the endo isomers of
L and K; with an activation barrier of DG° =20 kcalmolÀ1 com-
puted for the interconversion of L to the thermodynamically
favored K, this path might well contribute to the overall reac-
tivity at least at elevated temperatures (Scheme 11).
Figure 6. HOMO and LUMO representations (left: RI-B97-D/SVP, right: sche-
matic) of the two isomeric siloles A and B, cyclopentadiene (CpH), cyclohex-
ene (CHE), and (Z)-dimethylbutadiene (DMB) (orbital energies are given in
eV, 0.05 au isodensity surfaces).
Consistent with earlier findings,[32] the HOMOs and LUMOs of B
and cyclopentadiene are very similar in shape. Yet, the ex-
change of a CH2 fragment in cyclopentadiene by SiH2 in B re-
sults in a significant drop in the LUMO energy level and the
frontier-orbital energy gap of the latter is smaller by 0.45 eV.
Consequently, silacyclopentadienes can dimerize through cy-
cloaddition reactions much more easily than the correspond-
ing cyclopentadienes, which is in keeping with our computa-
tional evidence for a barrierless dimerization of B. For the sys-
tems and methods under study here it appears, in fact, that cy-
cloadditions with SiÀC bond formation are barrierless if the
correlating frontier orbitals of the reaction partners exhibit an
energy difference below 3 eV (see Table S3 in the Supporting
Information).
To qualitatively rationalize the computational results, we per-
formed a frontier orbital analysis for reactants involved in the
[4+2] cycloadditions discussed above. Figure 6 displays the
molecular orbitals of A and B together with those of both trap-
ping agents and, for comparison, those of cyclopentadiene.
Conclusion
The starting point of the present study was the observation
that the reaction of 4-chloro-1,2-dimethyl-4-supersilylsilacyclo-
pent-1-ene (2a) with LDA at À788C yields products that imply
the formation of transient 1-silabutadiene analogues, such as
the 1-silacyclopentadiene derivative 4a, in the course of the re-
action. We were able to obtain crystallographic data for three
corresponding products, namely the formal 1,4-addition prod-
uct of 4a with 4-silacyclopenta-1,3-diene 5a as well as the re-
spective adducts of the Diels–Alder reactions of 4a with 4a
and 4a with 5a. In addition, we have investigated the pericy-
clic reactions of DMB and cyclohexene with 4a. Interestingly,
in the reaction between 4a and DMB, the former clearly acted
as a silene as well as a 1-silabutadiene partner to yield the
[2+4] and [4+2] cycloadducts 10 and 11, respectively.
The underlying reaction mechanisms have been investigated
based on the suggested intermediacy of the two silacyclopen-
Scheme 11. [4+2] cycloadditions of the 1-silacyclopenta-1,3-diene A with
(Z)-DMB (DG195 in kcalmolÀ1 at À788C with respect to the reactants).
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Na[SitBu3] (1.2 mmol) in THF (3 mL) was added at À788C. The NMR
spectra of the reaction mixture revealed that in this reaction the
same products were formed as were obtained from 2a and LDA (6
(35%), 7 (35%), and 8 (30%)). The NMR data for 6–8 are given in
the Supporting Information. Compound 6: elemental analysis calcd
(%) for C36H72Si4 (617.30): C 70.04, H 11.76; found: C 69.87, H 11.43.
Compound 7: elemental analysis calcd (%) for C36H72Si4 (617.30): C
70.04, H 11.76; found: C 70.24, H 11.96. Compound 8: elemental
analysis calcd (%) for C36H72Si4 (617.30): C 70.04, H 11.76; found: C
70.02, H 11.48.
tadiene isomers 4a and 5a and our computational exploration
of conceivable alternative intermediates and interconnecting
reaction paths corroborates this view. The results of our inves-
tigations on the fate of the two key intermediates revealed
that [4+2] cycloadditions of 4a+4a and 4a+5a as well as
a reaction sequence commencing with direct SiÀSi bond for-
mation between two molecules of 4a lead to the formation of
the experimentally characterized products 9, 10, and 11. All of
these subsequent pathways are highly exergonic and occur
without activation barriers. The latter finding can be rational-
ized on the grounds of a qualitative picture derived from fron-
tier-orbital analyses. We have thereby established a mechanistic
scenario that straightforwardly explains all experimental obser-
vations.
Reaction of cyclohexene with 4a, generated from 2a and
LDA
A solution of LDA (130 mg, 1.18 mmol) in THF (10 mL) was drop-
ped into a cooled (À788C) mixture of 2a (410 mg, 1.18 mmol) and
cyclohexene (2.38 mL, 1930 mg, 23.53 mmol). The reaction solution
was allowed to warm up to room temperature overnight. The NMR
spectra of the reaction mixture revealed that the [4+2] cycload-
duct 9 was obtained nearly quantitatively. After aqueous workup
the organic layer (20 mL pentane) was dried with MgSO4. The
[4+2] cycloadduct 9 was finally purified by HPLC (Reprosil-pur
C18-AQ, flow rate: 3 mLminÀ1, two-solvent system (MeOH/TBME,
2:1), t=29 min (9)). Yield: 170 mg (38%). The NMR data of 9 are
given in the Supporting Information; elemental analysis calcd (%)
for C24H64Si2 (390.79): C 73.76, H 11.86; found: C 73.24, H 11.96.
Overall, our study sheds light on fundamental mechanistic
aspects underlying the rich chemistry of silole reagents and
helps to fill some voids in the detailed understanding of the
reactivity of unsaturated silicon compounds.
Experimental Section
All experiments were carried out under dry argon or nitrogen by
using standard Schlenk and glovebox techniques. Alkane solvents
were dried over sodium and freshly distilled prior to use. Benzene,
toluene, and THF were distilled from sodium/benzophenone.
[D6]benzene was distilled from sodium/benzophenone and stored
under a nitrogen atmosphere. Na[SitBu3],[21,22] Na[SiPhtBu2],[23] 1,[19]
and 2a[19] were prepared according to published procedures. All
other starting materials were purchased from commercial sources
and used without further purification. NMR spectra were recorded
on Bruker DPX 250 and Bruker Avance 300, 400, and 800 MHz
spectrometers. Elemental analyses were performed at the micro-
analytical laboratories of the Universitꢃt Frankfurt. For quantitative
separations, the dried filtrate was redissolved and separated by
HPLC (Reprosil-Pur C18-AQ, 250ꢄ20 mm, 10 mm, Maisch GmbH,
Germany; Sykam S3310 UV detector, l=254 nm; Sykam Refractive
Index Monitor RI2000) with isocratic elution. Further experimental
and crystallographic information is provided in the Supporting In-
formation.
Reaction of DMB with 4a, generated from 2a and LDA
A solution of LDA (130 mg, 1.25 mmol) in THF (5 mL) was dropped
into a mixture of 2a (430 mg, 1.25 mmol) in DMB (2.82 mL,
2050 mg, 24.92 mmol), which was cooled to À788C. The solution
was allowed to warm up to room temperature overnight. The NMR
spectra of the reaction mixture revealed that the [2+4] cycload-
duct 10 and the [4+2] cycloadduct 11 were formed in a ratio of
about 1:1. After aqueous workup the organic layer (20 mL hexane
extract) was dried with MgSO4. The cycloadducts 10 and 11 were
separated by HPLC (Reprosil-pur C18-AQ, 10 mm, 20ꢄ250 mm, flow
rate: 3 mLminÀ1, two-solvent system (MeOH/TBME, 10:1), t=86
(10) and 91 min (11)). Yield: 10: 60 mg (12%), 11: 20 mg (5%). The
NMR data for 10 and 11 are given in the Supporting Information.
Compound 10: elemental analysis calcd (%) for C24H64Si2 (390.79): C
73.76, H 11.86; found: C 73.55, H 11.44. Compound 11: elemental
analysis calcd (%) for C24H64Si2 (390.79): C 73.76, H 11.86; found: C
73.39, H 11.65.
Reaction of 2a with Li[NiPr2] (LDA)
A solution of LDA (110 mg, 0.98 mmol) in THF (15 mL) was drop-
ped into a solution of 2a (340 mg, 0.98 mmol) in THF (10 mL),
which was cooled to À788C. The mixture was allowed to warm up
to room temperature overnight. The NMR spectra of the reaction
mixture revealed that the formal 1,4-addition product of the silacy-
clopentadiene 4a with its 1,5-H migration product 5a, the formal
dimer 6 (35%), the [4+2] cycloadduct 7 (35%), and the [2+4] cy-
cloadduct 8 (30%) were formed. After aqueous workup the organic
layer (20 mL pentane extract) was dried with MgSO4. The dimer 6,
the 1,3-H migration product 7, and the 1,4-addition product 8
were separated by HPLC (Reprosil-pur C18-AQ, 10 mm, 20ꢄ
250 mm, flow rate: 3 mLminÀ1, two-solvent system (MeOH/tert-
butyl methyl ether (TBME), 1:1), t=13.8 (8), 19.2 (6), and 31.9 min
(7)). Yield: 5 mg (6, 5%); 5 mg (7, 5%); 10 mg (8, 11%). Storing of
a solution in MeOH/TBME (1:1) for 1 week at room temperature
yielded a few single crystals of 3h. Compound 3h was even
formed as the result of decomposition reactions of 7 and 8.[33]
Reaction of 2c with KC8
A solution of 2c (510 mg, 2.50 mmol) in THF (5 mL) was dropped
into a suspension of KC8 (670 mg, 5.00 mmol) in THF (15 mL) at
room temperature. The reaction mixture was stirred for 15 h. The
solvent was removed in vacuo and hexane (100 mL) was added.
After filtering, all volatiles were removed in vacuo. Recrystallization
in benzene yielded single crystals of 12. Yield: 340 mg (81%). The
NMR data of 12 are given in the Supporting Information; elemen-
tal analysis calcd (%) for C20H38Si2 (334.69): C 71.77, H 11.44; found:
C 71.75, H 11.44.
X-ray crystallography of 6, 9–12, 3g, 3h, 13, 14(monoclinic
P21/c), and 14(monoclinic P21/n)
All crystals were measured on a STOE IPDS-II diffractometer with
graphite-monochromated MoKa radiation. An empirical absorption
correction with the program PLATON was performed for all struc-
Reaction of 1 with two equivalents of Na[SitBu3] in THF: A flask
was charged with a solution of 1 (0.6 mmol) in THF (0.5 mL) and
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Acknowledgements
This work was supported by the Beilstein Institute as part of
the NanoBiC research cooperative (project eNet). Computer
time was provided by the Center for Scientific Computing
(CSC) and the LOEWE-CSC, Frankfurt.
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Keywords: cycloaddition · density functional calculations ·
electrocyclic reactions · reaction mechanisms · siloles
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Chem. Eur. J. 2014, 20, 4681 – 4690
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Full Paper
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Published online on March 11, 2014
Chem. Eur. J. 2014, 20, 4681 – 4690
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4690
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim