Reversible insertion of unactivated alkenes into silicon(II)-tin bonds

Article abstract of DOI:10.1002/anie.201303705

Insertion without assertion: Alkenes reacted reversibly under mild conditions (25-85 °C) with phosphine-silylene complexes containing a Si ^II-Sn bond to give alkyl silylene complexes (see scheme). Theoretical studies indicated that the insertion reaction proceeds through oxidative addition and migratory insertion in a two-step process and thus revealed that silicon(II)-phosphine complexes can behave like transition-metal complexes. Copyright

Full text of DOI:10.1002/anie.201303705

Angewandte
Chemie
Communications
Alkene Insertion
Insertion without assertion: Alkenes
reacted reversibly under mild conditions
R. Rodriguez, Y. Contie, D. Gau,
N. Saffon-Merceron, K. Miqueu,
J.-M. Sotiropoulos, A. Baceiredo,*
(25–858C) with phosphine–silylene com-
II
À
plexes containing a Si Sn bond to give
alkyl silylene complexes (see scheme).
Theoretical studies indicated that the
insertion reaction proceeds through oxi-
dative addition and migratory insertion in
a two-step process and thus revealed that
silicon(II)–phosphine complexes can
behave like transition-metal complexes.
T. Kato*
&&&& — &&&&
Reversible Insertion of Unactivated
Alkenes into Silicon(II)–Tin Bonds
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DOI: 10.1002/anie.201303705
Alkene Insertion
Reversible Insertion of Unactivated Alkenes into Silicon(II)–Tin
Bonds**
Ricardo Rodriguez, Yohan Contie, David Gau, Nathalie Saffon-Merceron, Karinne Miqueu,
Jean-Marc Sotiropoulos, Antoine Baceiredo,* and Tsuyoshi Kato*
The migratory insertion of alkenes is one of the most
fundamental processes in organometallic chemistry and thus
plays a crucial role in many catalytic processes, such as
polymerization,[1] 1,2-addition reactions,[2,3] and Heck-type
reactions.[4] In many cases, the insertion reaction is reversible,
and a key step in catalysis is the formation of alkenes through
a b-hydride elimination from alkyl transition-metal com-
plexes.
regenerate a reactive SiII species.[17] This reaction can be
regarded as a transition-metal-catalyst-free hydrosilylation
process.
Many other alkene-insertion reactions into a silicon–
heteroatom bond, such as silylstannylation,[3b] have also been
described as synthetically useful transformations, although
such reactions generally require transition-metal catalysts.
Thus, we were interested in extrapolating our system to other
In the case of nonmetallic systems, the olefin-insertion
reaction is long-known in the chemistry of Group 13 ele-
ments, as in the case of alkene hydroboration.[5,6] Olefin
polymerization promoted by aluminum-based catalysts is
silylene–phosphine complexes featuring a silicon(II)–hetero-
II
À
atom bond, such as a Si Sn bond. Herein, we report that
II
À
alkene insertion into the Si Sn bond of trimethylstannyl-
substituted silylene–phosphine complexes 1-Sn takes place
without any catalyst and under very mild conditions. Of
particular interest is the reversibility of the insertion reaction
even at room temperature.
À
believed to proceed through olefin insertion into an Al C
bond,[7] although the mechanism and active catalytic species
are still controversial.[8] The thermal decomposition of
trisopropylaluminum through b elimination is a well-known
synthetic method involving diisobutylaluminum hydride
(DIBAL).[9] In marked contrast, similar chemistry of
Group 14 elements is underdeveloped. Only a few hydro-
stannylation[10] and hydrogermylation[11] reactions of unsatu-
rated compounds with tin(II) or germanium(II) hydrides[12]
have recently been reported. It was also reported that the
hydrogermylation reaction of a phosphaalkyne is reversi-
ble.[13]
The trimethylstannyl-substituted silylene complexes 1-Sn
can be prepared readily from the corresponding chlorosily-
lene[18] derivatives by treatment with one equivalent of
trimethylstannyllithium. Products 1a-Sn (81% yield) and
1b-Sn (85% yield) could both be isolated in the solid state
and were characterized in solution as well as by X-ray
diffraction analysis (Figure 1, top).[19] Complex 1a-Sn slowly
reacted with ethylene gas (10 bar) at room temperature to
We recently developed stable silylene–phosphine com-
plexes, which showed high reactivity as silylenes[14] and as
phosphonium silaylides.[15] In particular, we reported that they
react reversibly with unactivated alkenes, such as ethylene, in
a [2+1] cycloaddition process.[16] Furthermore, in the case of
the silicon(II) hydride derivative 1a-H (named in analogy
with 1a-Sn in Scheme 1), the oxidative addition was followed
À
by a migratory insertion of the alkene into the Si H bond to
[*] Dr. R. Rodriguez, Dr. Y. Contie, Dr. D. Gau, Dr. A. Baceiredo,
Dr. T. Kato
Universitꢀ de Toulouse, UPS, and CNRS, LHFA UMR 5069
31062 Toulouse Cedex 9 (France)
E-mail: kato@chimie.ups-tlse.fr
À
Scheme 1. Reversible alkene-insertion reactions into the Si Sn bond of
trimethylstannyl-substituted silylene–phosphine complexes 1-Sn.
Homepage: http://hfa.ups-tlse.fr
Dr. N. Saffon-Merceron
Universitꢀ de Toulouse, UPS, and CNRS, ICT FR2599
31062 Toulouse (France)
give the corresponding alkyl-substituted silylene 2a-Sn
(Scheme 1). When the reaction was monitored by 31P NMR
spectroscopy, total conversion was observed after 24 h, and
none of the [2+1] cycloadduct intermediate was detected. The
new silylene 2a-Sn was stable at room temperature under
inert conditions and could be isolated as pale-yellow crystals
in high yield (87%).
Dr. K. Miqueu, Dr. J.-M. Sotiropoulos
Universitꢀ de Pau et des Pays de l’Adour, IPREM (UMR CNRS 5254)
Technopꢁle Hꢀlioparc, 64053 Pau Cedex 09 (France)
[**] We are grateful to the CNRS, the ANR (NOPROBLEM), and the
European Research Council (ERC Starting Grant agreement no.
306658) for financial support of this research.
31P NMR spectroscopy of 2a-Sn revealed two singlets (d =
82.2 and 80.1 ppm, 84:16) in agreement with the presence of
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303705.
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Figure 2. 31P{1H} NMR spectroscopy of isolated 2a-Sn at different
temperatures (25–1358C) in C6D6 under Ar (4 bar).
Table 1: Proportions of 1-Sn and 2-Sn in the presence of ethylene (4 bar)
in C6D6 at different temperatures.
T [8C]
25
100:0
85
85:15
90
81:19
105
71:29
120
53:47
135
28:72
2a-Sn/1a-Sn[a]
Figure 1. Molecular structure of 1a-Sn (top) and 2a-Sn (bottom).
Thermal ellipsoids represent 30% probability. H atoms and disordered
atoms are omitted for clarity. Selected bond lengths [ꢂ] and angles [8]:
1a-Sn: Si1–P 2.305(2), Si1–N1 1.848(2), Si1–Sn 2.612 (1), N1-Si1-P
88.80(7), N1-Si1-Sn 112.76(7), P-Si1-Sn 105.00(3); 2a-Sn: Si1–P
2.322(1), Si1–N1 1.850(2), Si1–C3 1.891(3), Sn–C4 2.145(2), C3–C4
1.542(3), N1-Si1-C3 105.25(10), N1-Si1-P 88.32(6), C3-Si1-P 105.56(9),
Si1-C3-C4 109.45(18), C3-C4-Sn 114.29(17).
T [8C]
25
99:1
40
97:3
50
89:11
60
79:21
70
51:49
80
13:87
2b-Sn/1b-Sn[a]
[a] The ratio was determined by 31P NMR and H NMR spectroscopy.
1
smaller substituents reacted with ethylene much faster, and
the reaction was complete within 15 min at room temperature
(Scheme 1). More interestingly, in this case, the equilibrium
was observed at room temperature. As expected, the Gibbs
a pair of diastereomers, although the starting silylene 1a-Sn
was present as a single diastereomer (d = 87.1 ppm). The
free energies for these reactions are quite small (DG858C
=
major isomer was fully characterized by NMR spectroscopy,
À5.21 kcalmolÀ1 for 1a-Sn and DG258C = À6.43 kcalmolÀ1 for
II
À
and the insertion of ethylene into the Si Sn bond was clearly
1b-Sn).
indicated by the typical 119Sn NMR chemical shift at d =
3.8 ppm (4JSn,P = 3.8 Hz) for a tetralkylstannane fragment.
The silylene complex 1a-Sn also reacted with styrene at
458C to afford the corresponding alkene-inserted silylene 3a-
Sn-X (X = H; Figure 3). The reaction proceeded regioselec-
tively in an anti-Markovnikov manner and was also an
equilibrium process. Indeed, the ratio 3a-Sn-H/1a-Sn at 458C
depended strongly on the relative amount of styrene. At this
temperature, the equilibrium is slightly shifted toward 1a-Sn,
and therefore an excess amount of styrene (36 equiv) was
required to reach 90% conversion at 458C. The Gibbs free
This signal differs significantly from that observed for 1a-Sn,
2
À
which features
a
Si Sn bond (d = À104.7 ppm, JSn,P
=
81.2 Hz).[20] The inserted ethylene fragment exhibited two
CH2 signals in the 13C NMR spectrum, at d = 5.9 ppm (3JC,P
=
7.3 Hz) and at d = 12.7 ppm. The 29Si NMR spectrum of
silylene 2a-Sn showed a doublet at d = À1.1 ppm with
a typical large coupling constant 1JSi,P = 186.0 Hz;[15] this
signal is very similar to that observed for 1a-Sn (d =
À26.0 ppm, 1JSi,P = 190.4 Hz). The structure of 2a-Sn was
unambiguously confirmed by X-ray diffraction analysis
(Figure 1, bottom).[19]
energy at 458C is only slightly negative (DG458C
=
À0.680 kcalmolÀ1). This reaction of 1a-Sn was also tested
with various p-substituted styrenes. In all cases, the reaction
proceeded in an anti-Markovnikov manner. In the case of the
reaction with p-fluorostyrene, the structure of product 3a-Sn-
F was confirmed by X-ray diffraction analysis.[19,21] The
reversibility of the process was observed at 458C with
electron-poor styrenes (X = H, F, Cl), whereas slight thermal
activation was required to reach the equilibrium (608C) in the
case of more electron rich styrenes (X = Me, OMe; Figure 3).
Thus, the equilibrium constant is strongly related to the
nature of the styrene substituent (Figure 3A), and the
reaction follows the Hammett correlation (Figure 3B).[22]
The reaction of 1a-Sn with ethylene was reversible. The
evolution of a pure sample of 2a-Sn in solution at different
temperatures was monitored by 31P NMR spectroscopy, which
indicated that the regeneration of 1a-Sn starts at 858C (2a-
Sn/1a-Sn 85:15), and that the proportion of the starting
silylene 1a-Sn increases with temperature (Figure 2). Above
858C, the ratio 2a-Sn/1a-Sn is temperature-dependent
(Table 1), which indicates that the reaction is at equilibrium
under these conditions. The silylene complex 1b-Sn with
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Figure 3. a) Equilibrium constant (Keq708C) and Gibbs free energy
(DG608C) and b) Hammett plot for the reaction of 1a-Sn with several
p-substituted styrene derivatives.
Figure 4. Calculated reaction profiles (Gibbs energies in kcalmolÀ1) for
the ethylene-insertion reactions with silylene complexes 1a-Sn and 1a-
H at the level of M06-SDD for Sn and 6-31G(d,p) for all other atoms.
The positive value of the reaction constant (1) in the
Hammett equation [log(K/K0) = s1] indicates that the inser-
tion reaction is favored for electron-deficient styrenes,
probably owing to the strongly nucleophilic silylene character
of 1a-Sn.[23]
complexes may be related to the bond energies involved in
To gain insight into these experimental results, we
performed DFT calculations on the reaction of silylene
complexes 1a-E (E = H, SnMe3) with ethylene (Figure 4).
We reported previously that the reaction of 1a-H with
ethylene proceeds in two steps: 1) [2+1] cycloaddition at
room temperature, and 2) migratory insertion of ethylene at
808C.[17] However, no intermediate was observed experimen-
tally in the reaction between 1a-Sn and ethylene. Never-
theless, calculations predict that the reaction of 1a-Sn should
also proceed in two steps (Figure 4). In this case, the
intermediate was not observed because the rate-limiting
step is the first step, the [2+1] cycloaddition, whereas in the
case of 1a-H, the rate-determining step is the insertion of
ethylene. As expected, both steps (insertion/b elimination)
for 1a-Sn are almost thermoneutral (DG = À5.8 kcalmolÀ1)
and present relatively small energy barriers, in good agree-
ment with the experimental results (reversibility under mild
conditions). In contrast, the migratory-insertion step for 4a-H
costs more energy (DG° = 26.2 kcalmolÀ1). Besides, the
reaction is strongly exergonic (DG(2a-H–1a-H) = À15.6 kcal
molÀ1), which explains the lack of reversibility of the reaction
for 1a-H. The difference observed between the two silylene
the process. Indeed, the two bonds involved in the reaction of
À1
À
À
1a-Sn are considerably weaker (Sn Si: 45 kcalmol , Sn C:
À1
À
43 kcalmol ) than those in the case of 1a-H (H Si:
76 kcalmol , H C: 98 kcalmol );[24] thus the energy barrier
for the second migratory-insertion step and the gain in energy
are lower for the formation of 2a-Sn (formation of two
À1
À1
À
À
À
À
s bonds (Si C and E C) and loss of a s bond (E Si) and a
À1
À
C C p bond (41 kcalmol )). In contrast, the higher energy
barrier for the first step in the case of 1a-Sn could be
attributed to increased steric congestion due to the much
bulkier SnMe3 group. The reaction of 1a-Sn with styrene
should proceed through a similar two-step mechanism,
because a higher reaction rate was observed for electron-
poor styrenes (X = F, Cl), in agreement with a first rate-
limiting step involving a nucleophilic silylene.[23]
In conclusion, we have successfully demonstrated the first
II
À
reversible alkene-insertion reaction into Si Sn bonds. This
reaction takes place under mild conditions without any
transition-metal catalyst. Theoretical studies indicated that
the insertion reaction proceeds in two steps (oxidative
addition–migratory insertion) and thus revealed that sili-
con(II)–phosphine complexes can behave like transition-
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[9] K. Ziegler, Angew. Chem. 1956, 68, 721.
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the reaction is thermoneutral and proceeds with a relatively
small energy barrier, which explains the reversibility of the
process under mild conditions. This particular energy profile
in which divalent and tetravalent silicon species occur at the
same energy level is probably the consequence of a particular
phosphine-ligand effect that efficiently stabilizes SiII species
without loss of reactivity. Other systems are under active
investigation.
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[19] See the Supporting Information for the crystal data for 1a-Sn,
2a-Sn, and 3a-Sn-F. CCDC 935560 (1a-Sn), 935561 (2a-Sn), and
935562 (3a-Sn-F) contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
Received: April 30, 2013
Published online: && &&, &&&&
Keywords: cycloaddition · phosphorus · silicon · silylenes ·
.
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