Direct Syn Addition of Two Silicon Atoms to a C≡C Triple Bond by Si?Si Bond Activation: Access to Reactive Disilylated Olefins

Article abstract of DOI:10.1002/anie.201611719

A catalytic intramolecular silapalladation of alkynes affords, in good yields and stereoselectively, syn-disilylated heterocycles of different chemical structure and size. When applied to silylethers, this reaction leads to vinylic silanols that undergo a rhodium-catalyzed addition to activated olefins, providing the oxa-Heck or oxa-Michael products, depending on the reaction conditions.

Full text of DOI:10.1002/anie.201611719

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International Edition: DOI: 10.1002/anie.201611719
German Edition: DOI: 10.1002/ange.201611719
Silylation Reactions
ꢀ
Direct Syn Addition of Two Silicon Atoms to a C C Triple Bond by
Si Si Bond Activation: Access to Reactive Disilylated Olefins
ꢁ
Maha Ahmad, Annie-Claude Gaumont, Muriel Durandetti,* and Jacques Maddaluno*
Abstract: A catalytic intramolecular silapalladation of alkynes
affords, in good yields and stereoselectively, syn-disilylated
heterocycles of different chemical structure and size. When
applied to silylethers, this reaction leads to vinylic silanols that
undergo a rhodium-catalyzed addition to activated olefins,
providing the oxa-Heck or oxa-Michael products, depending
on the reaction conditions.
possibly, in a stereocontrolled fashion. The relatively weak
ꢁ
ꢁ1
Si Si bond (ca. 54 kcalmol )[13] suggests that disilanes can be
an appropriate source of silicon and precedents in the
literature hint that a metal-induced dissociation is conceiv-
able. However, except in a few particular cases, only one
silicon is usually transferred.[14] A first exception was evi-
denced by Sakurai et al. who showed, in 1975, that a strained
cyclic disilane reacts with three activated alkynes to provide
disilyla-cycloheptenes (Figure 1a).[15] Subsequently, Ito and
Silicon is a fascinating element to organic chemists, and is
a kind of “big brother” of carbon but with very different
character. These two Group 14 elements share fundamental
properties; for instance, a strong tendency to establish four
covalent bonds, but differ also in crucial characteristics, such
as covalent radius or electronegativity. When introduced into
organic molecules, silicon transforms the chemical and
physicochemical characteristics and adds unique properties
to the resulting structure. In particular, a carbon–silicon swap
increases the chemical diversity in drug design[1] since it alters
both the physiological and biological properties of usual
active principles.[2] Beyond pharmacy,[3a–d] fragrances that
include silicon atoms have recently been synthesized and
evaluated.[3e–f] In the field of materials science, the heat-
resistance, light-stability, and light-transparency often dis-
played by organosilicon materials make them attractive
targets.[4] All these applications infer that simple accesses to
original silylated synthons through reliable and cheap meth-
ods are sought. Recently, many silylation procedures have
appeared that give access to new organosilicon building
blocks.[5] Among those, the attractive vinylsilanes[6] are
available by methods such as hydrosilylation,[7] Heck cou-
pling,[8] or “soft” silylmetalations (zincation,[9] boration,[10] or
stannation[11]).
Figure 1. Inter- and intramolecular metallo-catalyzed disilylation of
alkynes.
colleagues described a palladium-catalyzed intramolecular
syn bis-silylation of the triple bond of activated propargylic
oxydisilanes, furnishing tetrasubstituted (Z)-olefins (Fig-
ure 1b).[16] Very recently, Matsuda and Ichioka have observed
an intramolecular RhI-catalyzed transformation of aryldi-
silanes into silylbenzosilanes (Figure 1c).[17] Intriguingly, the
authors showed that when rhodium is replaced by palladium,
the reaction derails and affords a fused four-member sila
heterocycle (benzosiletane). In a footnote, they also mention
that the “trans bis-silylation failed to occur when the tether
link was increased by one carbon”, limiting the scope of the
method. Worthy of note is the result reported recently by
Stratakis and colleagues, who have shown that gold nano-
Within this context, molecules juxtaposing two vicinal
silicon atoms with different functionalities can become
particularly advanced substrates.[12] Preparing such com-
pounds by a “silyl-silylation” of alkynes is appealing since
the two silicon atoms would be delivered synchronously and,
[*] Dr. M. Ahmad, Dr. M. Durandetti, Dr. J. Maddaluno
Normandie Universitꢀ
UNIROUEN, INSA Rouen, CNRS, COBRA (UMR 6014 & FR 3038)
76000 Rouen (France)
E-mail: muriel.durandetti@univ-rouen.fr
jmaddalu@crihan.fr
ꢁ
particles supported on metal oxides activate the Si Si bond.
This process, which applies to terminal alkynes only, leads to
Z-isomers, which gradually isomerize into the corresponding
E-isomers under the reaction conditions.[18]
Prof. A.-C. Gaumont
Normandie Universitꢀ
UNICAEN, ENSICAEN, CNRS, LCMT (UMR 6507 & FR 3038)
14000 Caen (France)
These promising results, while restricted to specific cases,
highlight the potential synthetic value of fragmentation of the
ꢁ
Si Si bond of non-activated disilanes. Herein, we show that
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201611719.
ꢁ
the palladium-catalyzed activation of the Si Si bond suffices
to trigger a direct intramolecular disilylation of alkynes,
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providing straightforward access to a large set of syn-
disilylated olefins.
Inspired by Matsudaꢀs works, we prepared substrate 1a in
92% yield from the known o-disylanylphenol[19] and
1-bromobut-2-yne. Our study began using Itoꢀs catalytic
conditions, based on the Pd-tOcNC system that afforded the
expected disilylated product 2a with 35% conversion after
24 h at reflux in toluene. Optimizing[20] the source of Pd0, the
ligand, the solvent, and the temperature, led to a remarkably
simple system (5% Pd(dba)2 + 10% P(OEt)3), which was
able to perform this same transformation with a high 97%
isolated yield—even in relatively harsh conditions (3 h at
reflux in xylene, Figure 1d).
With this tool in hand, we first examined its sensitivity to
the alkyne terminal substituent. Results in Table 1 show that
the cyclization takes place in good yields and with total
stereo- and regioselectivities, except for the terminal alkyne
1b that gives a 9:1 mixture of Z/E isomers (isomerization
Table 1: Intramolecular disilylation of alkynes. Influence of the alkyne
terminal substituent.[a]
Figure 2. Generalization of the intramolecular disilylation of alkynes.
tetrahydronaphthalene 3 and quinoline 4 were obtained in
the same experimental conditions, from the corresponding
all-carbon and nitrogenated alkynes. Electron-donating and
-accepting groups on the aromatic ring were tolerated, as
shown by the meta (to silicon) methyl (5a: 81%) or
trifluoromethyl (5b: 67%) substituents. A free aldehyde is
also fully compatible with the cyclization, even if it seems to
interfere with the catalytic process and decelerates the
reaction (5c: 24% after 24 h, 100% based on starting
material recovery).[21] A substituent on the propargylic
position is equally compatible with the silapalladation pro-
cess, as shown in the case of 6, which was recovered in
excellent yield. This reaction is respectful of a propargylic
asymmetric center, as demonstrated using the chiral non-
racemic substrate (ee > 99%), leading to (S)-6 with a similarly
high ee. Varying the trimethylsilyl group has little impact: an
alkyl, aryl, or alkoxy substituent (see 13 and 14, Scheme 1)
were introduced on the terminal silicon with a minor effect on
the yield. Finally, if the tether cannot be lengthened[22] it can
be shortened, as shown in the case of compounds 9 and 10.
Thus, benzosiloles are within reach of this method—even
Entry
R
Product
Yield [%][b]
1
2
3
4
5
6
7
8
Me (1a)
H (1b)
Ph (1c)
2a
2b
2c
2d
2e
2 f
2g
2h
2i
97
72[c]
88
93
53
86
84
74
20
4-F-C6H4 (1d)
4-Cl-C6H4 (1e)
3-Pyr (1 f)
SMe (1g)
GeMe3 (1h)
SiMe3 (1i)
PPh2 (1j)
Cl (1k)
9
[d]
10
11
12
2j
2k
2l
-
[d]
-
diyne (1l)
84[e]
[a] Typical procedure: aryl disilane 1a–k (0.5 mmol), Pd(dba)2
(0.025 mmol), P(OEt)3 (0.05 mmol), xylenes (4 mL) under argon at
1308C for 3 h. [b] Isolated yields, based on aryl disilane. All products gave
satisfactory analytical data. [c] Z/E=9:1 after purification. [d] Degrada-
tion. [e] See product 2l in Figure 2.
occurring during column purification). Alkyl-, aryl-, hetero-
aryl, or alkynes substituted with a haloaryl group are
tolerated, (Table 1, entries 4 and 5). Thio-substituted alkyne
1g transforms efficiently into the matching enol thioether
(Table 1, entry 7). The germanylated alkyne 1h reacts well
and affords disilylated-germanylated olefin 2h exclusively
while its trimethylsilyl analogue 1i gives trisilylated olefin 2i,
albeit with poor conversion (Table 1, entries 8 and 9). Note
that the diphenylphosphine substituted alkyne 1j and
chloroalkyne 1k did not deliver any identifiable product
(Table 1, entries 10 and 11). Finally, diyne 1l was easily
converted into the unusual tetrasilylated conjugated diene 2l.
Other substitution patterns have also been studied
(Figure 2). First, the tether linking the aryl to the triple
bond was revealed to be properly flexible; the sila
Scheme 1. Synthesis of silanols 15. Reaction conditions: a) Ref [23];
b) nBuLi (R=Me) or nBuMgBr (R=Ph) then ClSiMe2-SiMe2Cl;
c) iPrOH, DMAP, imidazole; d) Pd(OAc)2, PCy3, xylenes, 1308C; e) ace-
tate buffer (pH 5.0), MeCN, 508C.
2
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when the propargylic position is functionalized (9b). The
four-membered benzosiletane 10, prepared previously by
Matsuda, is also obtained in good yield.
were able to favor the reductive conjugate addition by
running the reaction in 5:1 dioxane/water mixture
(Scheme 2; Supporting Information, Table S4). The data
show that, in these conditions, the reaction is efficient (a
conversion of ca. 100%) but not as selective as above.
a
To further increase the synthetic potential of these
disilylated synthons, we decided to alter the oxidation state
of the exocyclic silicon atom and to evaluate the correspond-
ing silanols. With this aim in mind, we prepared the new
disilanes monoether 13 through a revised procedure in which
o-iodophenol is first O-propargylated into 11,[23] reacted with
commercially available ClSiMe2-SiMe2Cl and then treated
with isopropanol in basic conditions (Scheme 1). Regrettably,
the conditions described above for the cyclization did not
apply to this substrate and we had to reoptimize the catalytic
system to access 14 in 78% yield.[24] A pH-controlled
hydrolysis of 14 ultimately afforded silanols 15 in 80% yield.
To evaluate substrates 15 in non-fluorinated, non-basic
conditions,[25] we turned our attention to the rhodium-
catalyzed Heck-type coupling of arylsilanediols reported by
Mori.[26] Applied to 15, this reaction was expected to provide
attractive new dienes (Table 2). To our surprise, adjusting
conditions led instead, and for the first time, to a fully
selective oxa-Heck coupling, affording the corresponding
silylenol ether (Table 2).
Scheme 2. Rhodium-catalyzed oxa-Michael reaction with silanol 15a.
Repeating the reaction with silanol 15b did not improve the
ratios significantly and decreased the reactivity. This drop in
selectivity is probably due to water that triggers a competition
between the C(sp2)-Si oxidative addition of RhI and rhodium
ꢁ
alkoxide formation, leading to C C bond formation through
a conjugate addition in the first case (providing 19) or oxa-
Michael product in the second case (providing 18). In all cases
these two additions clearly disfavor oxa-Heck coupling
(providing 17).
From a mechanistic point of view, we propose that RhOH
first deprotonates 15 to provide rhodium silanolate A which
adds to the acrylate to afford a s-alkylrhodium B (Scheme 3).
Table 2: Rhodium-catalyzed oxa-Heck reaction with silanol 15a.
Entry
EWG
Product
Yield [%]
1
2
3
4
5
6
CO2Bu
CO2Bn
CO2Ph
CONMe2
CN[b]
17a
17b
17c
84
76
65
85
57
57
17d
17e
COEt
18a[c]
[a] Electron-withdrawing group (EWG). [b] Reaction run at 708C for 24 h.
[c] See structure of 18 in Scheme 2.
Scheme 3. Proposed catalytic cycles for the reactivity of 15.
This reaction was successfully extended to more sensitive
substrates, such as benzyl or phenyl acrylates, as well as to
dimethylacrylamide and acrylonitrile. The yields are decent to
good considering the products sensitive functions. Note that
the reaction does not work with electron-rich olefins (sty-
rene), or with a- or b-substituted esters such as ethyl
methacrylate or crotonate. The particular case of ethyl vinyl
ketone also deserves comment since a selective oxa-Michael
addition was observed this time, leading to silylether 18 in
moderate yield. When it comes to these competing reactions,
the diverging behavior of methyl vinyl ketone (or ethyl vinyl
ketone) with respect to acrylates has been reported before by
Mori[26] and others,[27] and can be understood on the basis of
the mechanism proposed below.
At this point B can undergo b-elimination of rhodium
hydride, leading to the oxa-Heck product 17. The catalytic
cycle continues with the Rh(ꢁ1)!Rh(+ 1) oxidation trig-
gered by a second molecule of acrylate. Alternatively, in the
presence of water B tautomerizes into the rhodium enolate C
from which protonation affords the oxa-Michael product 18
and regenerates RhOH.
In conclusion, this paper describes access to original di-
and poly-silylated structures. We first report on a catalytic
intramolecular silapalladation of alkynes that affords a set of
silylated heterocycles in good yields. This transformation adds
ꢀ
two silicon atoms on the C C triple bond in one single syn-
selective step. Pushing the investigation further revealed that
the resulting vinylic silanols react with activated olefins to
provide the oxa-Heck or oxa-Michael products, depending on
the reaction conditions. Note that the oxa-Heck sequence
This observation suggested that the reaction could be
tuned toward an oxa-Heck or oxa-Michael addition by
varying the conditions. Inspired again by Moriꢀs work, we
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pull building blocks.
Experimental Section
To a solution of aryldisilane 1 (0.5–1 mmol, 1 equiv) in anhydrous
xylenes (5 mL) under argon atmosphere was added Pd(dba)2
(0.05 equiv) and triethylphosphite (0.1 equiv). The mixture was
stirred for 3 h at 1308C then hydrolyzed with water. The aqueous
layer was extracted with diethyl ether and combined organic layers
were washed with brine, dried over anhydrous MgSO4, and concen-
trated to afford the crude product, which was purified by flash
chromatography to give product 2.
Acknowledgements
M.A. is grateful to the Rꢁgion Normandie (CRUNCh
program) and Labex SynOrg (ANR-11-LABX-0029) for
a PhD stipend.
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Conflict of interest
The authors declare no conflict of interest.
Keywords: cyclization · disilane · heterocycles · palladium ·
vinylsilane
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[20] See Tables S1–S2 in the Supporting Information.
[21] The low yield of 5c is due to the sluggishness of the
silapalladation step, since 76% starting material is recovered
after 24 h.
[25] In basic media, the only product isolated results from a CAr-Si
ꢁ
cleavage and Si O bond formation, leading, in 60–90% yield, to
oxadisilole 16.
[22] Only the starting material is recovered in the case of 1,1,1,2,2-
pentamethyl-2-(2-(pent-3-yn-1-yloxy)phenyl)disilane SI11 (Sup-
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Manuscript received: December 1, 2016
Revised: December 21, 2016
[24] See Table S3 in the Supporting Information.
Final Article published: && &&, &&&&
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Silylation Reactions
M. Ahmad, A.-C. Gaumont,
M. Durandetti,*
J. Maddaluno*
&&& — &&&
Direct Syn Addition of Two Silicon Atoms
Disilylated olefins were obtained by
vinylic silanols that engage in rhodium-
catalyzed oxa-Heck or oxa-Michael addi-
tions. Tuning of the experimental condi-
tions allows promotion of the preferred
addition reaction.
ꢀ
ꢁ
ꢁ
to a C C Triple Bond by Si Si Bond
Activation: Access to Reactive Disilylated
Olefins
a selective Si Si bond palladium activa-
tion followed by intramolecular addition
ꢀ
of two silicon atoms to the C C triple
bond, thereby accessing a family of new
silaheterocycles. The reaction affords
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!