Gold-catalyzed dehydrogenative cycloaddition of tethered 1,n-dihydrodisilanes to alkynes

Article abstract of DOI:10.1021/om3011678

Gold nanoparticles supported on TiO₂ (0.1-2% mol) catalyze at mild conditions the dehydrogenative addition of tethered 1,n-dihydrodisilanes, such as 1,1,3,3-tetramethyldisiloxane (1), 1,1,3,3-tetraphenyldisiloxane (2), 1,1,1,3,5,7,7,7-octamethyltetrasiloxane (3), 1,1,3,3,5,5-hexamethyltrisiloxane (4), and 1,2-bis(dimethylsilyl)benzene (5), to alkynes, forming cycloadducts and releasing H₂. Under the same conditions, polymeric methylhydrosiloxane is completely unreactive. For the majority of terminal alkynes and 1,n-dihydrodisilanes the yields are excellent (up to 99%). In general, terminal alkynes are more reactive as compared to internal. The reaction tolerates several functional groups and can be performed in a variety of solvents. In the case of 1,1,3,3-tetramethyldisiloxane, it is proposed that gold nanoparticles form intermediate cyclo-gold-tetramethyldisiloxane via a dehydrogenative pathway, which undergoes a formal [3+2] cycloaddition to alkynes.

Full text of DOI:10.1021/om3011678

Article
pubs.acs.org/Organometallics
Gold-Catalyzed Dehydrogenative Cycloaddition of Tethered
1,n‑Dihydrodisilanes to Alkynes
Vasiliki Kotzabasaki,† Ioannis N. Lykakis,‡ Charis Gryparis,† Androniki Psyllaki,† Eleni Vasilikogiannaki,†
and Manolis Stratakis*,†
†Department of Chemistry, University of Crete, 71003 Voutes, Iraklion, Greece
‡Department of Chemistry, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
S
* Supporting Information
ABSTRACT: Gold nanoparticles supported on TiO2 (0.1−2% mol)
catalyze at mild conditions the dehydrogenative addition of tethered
1,n-dihydrodisilanes, such as 1,1,3,3-tetramethyldisiloxane (1), 1,1,3,3-
tetraphenyldisiloxane (2), 1,1,1,3,5,7,7,7-octamethyltetrasiloxane (3),
1,1,3,3,5,5-hexamethyltrisiloxane (4), and 1,2-bis(dimethylsilyl)benzene
(5), to alkynes, forming cycloadducts and releasing H2. Under the same
conditions, polymeric methylhydrosiloxane is completely unreactive.
For the majority of terminal alkynes and 1,n-dihydrodisilanes the yields
are excellent (up to 99%). In general, terminal alkynes are more reactive
as compared to internal. The reaction tolerates several functional groups
and can be performed in a variety of solvents. In the case of 1,1,3,3-
tetramethyldisiloxane, it is proposed that gold nanoparticles form
intermediate cyclo-gold-tetramethyldisiloxane via a dehydrogenative
pathway, which undergoes a formal [3+2] cycloaddition to alkynes.
Dehydrogenative cycloaddition of tethered 1,n-dihydrodisi-
lanes to alkynes is known in a few cases, under catalysis by Pt,
Ni, and Pd,9 often competing with the hydrosilylation pathway
(Scheme 1). TMDS for instance has been reported to afford
INTRODUCTION
■
The gold-catalyzed reaction of hydrosilanes with alkynes has
attracted the interest of organic chemists over the past few
years, especially under heterogeneous conditions using nano
gold substances.1 It has been recognized that the hydro-
silylation pathway is predominant, as occurs under catalysis by a
variety of metals including Pt, Pd, Rh, or Ru. Among the
different variants of gold catalysts, acetone-solvated gold
nanoparticles supported on carbon or γ-alumina2 and Au/
Scheme 1. Possible Pathways in the Metal-Catalyzed
Reaction of a Terminal Alkyne with Tethered 1,n-
Dihydrodisilanes
3
CeO2 afford exclusively hydrosilylation products with β-(E)
selectivity. On the contrary, a homogeneous Au(III) catalyst3
provides unselectively products of β-(E) and β-(Z) addition. A
thin gold film on the surface of boronsilicate capillaries was also
proven as an active catalyst for the hydrosilylation of terminal
alkynes in a continuous-flow reactor under microwave
irradiation.4 Additionally, we have recently shown that Au/
TiO2 catalyzes the β-(E)-selective hydrosilylation of terminal
alkynes, forming minor cis-disilyl alkenes as side-products.5 On
the other hand, in the presence of an unsupported nanoporous
gold catalyst6 the hydrosilylation pathway does not takes place.
Instead, facile semihydrogenation of alkynes into alkenes
occurs, with one hydrogen atom provided by the hydrosilane,
while the second arises from a water molecule. A surprising
differentiation takes place upon using 1,1,3,3-tetramethyldisi-
loxane (TMDS) as a reactant. Although under Pt(0) catalysis
conditions TMDS affords the typical double hydrosilylation
products with terminal alkynes,7 in the presence of Au/TiO2
dehydrogenative cycloaddition occurs readily, forming sub-
stituted 2,5-dihydro-1,2,5-oxadisiloles in excellent yields.8
the dehydrogenation cycloadduct with an alkyne (in a single
example) as a minor byproduct, while the typical hydro-
silylation products prevail.9b Formally, 1,2-disilyl addition
products to alkynes are derived via the oxidative insertion of
metals (e.g., Pd, Pt) into the Si−Si bond of 1,2-disilanes
followed by double addition to the triple bond10 and by other
means.11
Received: December 3, 2012
© XXXX American Chemical Society
A
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In this article we present a full account of our studies
regarding the oxidative cycloaddition of 1,1,3,3-tetramethyldi-
siloxane, TMDS (1), to alkynes,8 including recent studies that
shed light on the reaction mechanism. Additionally, herein we
show that a series of tethered 1,n-dihydrodisilanes (Chart 1)
and 1,2-bis(dimethylsilyl)benzene (5) follow the same trend as
TMDS does, providing in the presence of Au/TiO2 mainly or
exclusively the dehydrogenative cycloaddition pathway with a
series of alkynes.
RESULTS AND DISCUSSION
■
Chart 1. Structures of Tethered 1,n-Dihydrodisilanes 1−5
a. Au/TiO2-Catalyzed Reaction of Alkynes with
1,1,3,3-Tetramethyldisiloxane (1), 1,1,3,3-Tetraphenyldi-
siloxane (2), and 1,1,1,3,5,7,7,7-Octamethyltetrasiloxane
(3). A series of terminal alkynes (1.0 equiv) react smoothly at
room temperature with 1,1,3,3-tetramethyldisiloxane (1.0−1.1
equiv only if dry solvent is used) in the presence of 0.3 mol %
Au/TiO2 to afford substituted 2,5-dihydro-1,2,5-oxadisiloles12
(Table 1) in up to 99% isolated yield. For some of the
substrates, minor side-products (up to 10% relative yield)
resulting from the competing hydrosilylation and/or semi-
hydrogenation of the triple bond were seen. Only in the case of
ethyl propiolate (23) do the byproducts become substantial
such as 1,1,3,3-tetraphenyldisiloxane (2), 1,1,1,3,5,7,7,7-octa-
methyltetrasiloxane (3), 1,1,3,3,5,5-hexamethyltrisiloxane (4),
Table 1. Dehydrogenative Cycloaddition of 1,1,3,3-Tetramethyldisiloxane, 1,1,3,3-Tetraphenyldisiloxane, and 1,1,1,3,5,7,7,7-
a
Octamethyltetrasiloxane to Alkynes Catalyzed by Au/TiO2
a
Reaction conditions for 1,1,3,3-tetramethyldisiloxane (1): 1.1 equiv, 25 °C, dry CH2Cl2, Au/TiO2 (0.3 mol %); reaction conditions for 1,1,3,3-
tetraphenyldisiloxane (2): 2.0 equiv, 25 °C, dry CH2Cl2, Au/TiO2 (1.5 mol %); reaction conditions for 1,1,1,3,5,7,7,7-octamethyltetrasiloxane (3):
1.2 equiv, 80 °C, dry 1,2-dichloroethane, Au/TiO2 (1.2 mol %). In general, if nondried solvent is used, a moisture-dependent excess of hydrosilane
b
should be added to compensate its hydrolysis. Isolated yields after column chromatography. Alkyne conversions were >99% by GC-MS analysis in
c
all cases. 2.2 molar equiv of TMDS was used.
B
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and reduce the isolated yield of cycloadduct 23a. Even 0.1 mol
% of the catalyst can drive the reaction to completion, yet
longer reaction times are necessary (>3 h). With 1−2 mol %
catalyst loading, the reaction occurs instantaneously with
violent evolution of H2. As for the reaction solvent, dichloro-
methane, 1,2-dichloroethane, hexane, toluene, diethyl ether, or
ethyl acetate can be used with a narrow variation in reaction
rate, isolated yield, and product selectivity. Nondried solvents
can also be used, simplifying the process. In such case, excess
TMDS should be used to compensate its partial hydrolysis
from the moisture present in the solvent. GC-MS analysis
revealed that in the presence of moisture 1,1,3,3-tetramethyldi-
siloxane transforms primarily at the initial stages of the reaction
into 1,1,3,3,5,5,7,7-octamethyltetrasiloxane (dimer) and minor
amounts of trimer, while at a later stage, the dimeric
tetrasiloxane transforms into octamethylcyclotetrasiloxane
(cyclic dimer) and higher oligomers. The hydrolysis of
TMDS by moisture is rapidly catalyzed by Au/TiO2, as proven
by independent experiments in the absence of the alkyne. It is
well established in recent years that silanes undergo hydrolytic
oxidation catalyzed by gold nanoparticles13 or nanoporous
gold.14 Byproduct oligosiloxanes are highly nonpolar and
therefore can be easily removed by column chromatography,
eluting the crude reaction mixture with hexane. Alkynes are
completely unreactive to TMDS in the absence of the catalyst,
as well as in the presence of unsupported TiO2 (anatase or
rutile). Therefore, the active catalytic sites are undoubtedly
provided by the gold nanoparticles. It is also important to
emphasize that a series of homogeneous Au(I) catalysts do not
provide any product (of either oxidative cycloaddition or
hydrosilylation). In the presence of 3 mol % AuCl3 on the other
hand (refluxing DCE, 5 h), alkynes such as 6−8 were reduced
by TMDS to the corresponding styrenes (GC-MS) and
gradually to aryl alkanes, while cycloadducts 6a−8a were
detected in the crude reaction mixture in up to 10% relative
yield. As seen in Table 1, the reaction tolerates a variety of
functional groups bound on either the aryl ring or alkyl chains.
Notably, in the case of 3-butyn-1-ol (16), by using 2.2 equiv of
TMDS, 16a was isolated in almost quantitative yield. Upon
using <1.0 equiv of TMDS, the analysis of the crude reaction
(2,5-dihydro-1,2,5-oxadisiloles), typical β-(E)-hydrosilylation
adducts in relative yields of 35−45% were also identified.
1,1,3,3-Tetraphenyldisiloxane has a high tendency to hydrolyze
in the presence of Au/TiO2; thus in the case of using nondried
solvent up to 3−4 equiv of 2 is necessary to drive the reaction
of alkynes to completion. 1,1,1,3,5,7,7,7-Octamethyltetrasilox-
ane (3) is even more slowly reacting with terminal alkynes at
ambient conditions and requires refluxing DCE to go to
completion (typically 8−12 h). Yet, the reactions are very clean
and the dehydrogenative cycloaddition products were formed
in 78−96% isolated yields (Table 1) as an equimolar mixture of
two diastereomers, shown in Scheme 2. 13C NMR spectroscopy
Scheme 2. Diastereomeric Products from the Reaction of
1,1,1,3,5,7,7,7-Octamethyltetrasiloxane (3) with Terminal
Alkynes in the Presence of Au/TiO2
revealed that 3 also appears as an equimolar mixture of two
diastereomers (meso and racemic). In addition, polymethylhy-
drosiloxane (PMHS), which is a highly reactive polymeric
hydrosilane (an extended analogue of TMDS) and has
exhibited remarkable enhancement of reaction rates in several
metal-catalyzed reductive processes,20 is surprisingly completely
unreactive in our case. It is even almost inert toward hydrolysis
catalyzed by Au/TiO2, in contrast to dimeric TMDS, which is
extremely sensitive. Being highly hydrophobic, it is likely that
PMHS has a low affinity to the hydrophilic surface of the
catalyst, preventing thus its proximal and product-forming
interaction with the gold nanoparticles.
Internal alkynes are less reactive against TMDS (1% mol of
Au/TiO2 was used), and the product selectivity is lower (Table
1). Generally, in the entries where the isolated yield of 2,5-
dihydro-1,2,5-oxadisiloles is moderate, typical hydrosilylation
products are formed via a competing pathway. Surprisingly,
while diphenylacetylene (24) provided 24a in high yield,
internal alkynes 28 and 29 are completely unreactive. Dimethyl
acetylenedicarboxylate (27) did not yield either cycloadduct or
hydrosilylation product, but was slowly reduced within 24 h to
a mixture of dimethyl maleate (∼90%) and dimethyl succinate
(∼10%). It is possible that the stereoselective reduction
proceeds through a gold-catalyzed semihydrogenation7 by the
slow oxidative hydrolysis of TMDS, as the reaction was
performed within an open-air flask. The dihydrosilanes 2 and 3
are either completely unreactive with internal alkynes (e.g., with
28, 29), or react extremely slowly (with 24). A notable
exemption is unsaturated ester 26, which provides with
1,1,1,3,5,7,7,7-octamethyltetrasiloxane the dehydrogenation
cycloadduct 26c in high yield.
1
mixture by H NMR revealed that protection of the hydroxyl
group is faster than the addition on the triple bond, which
indicates that TMDS rapidly protects alcohols as silyl ethers in
a dehydrogenative manner.15 Additionally, for substrates 19 and
21, which possess an additional C−C double bond, the
cycloaddition proceeds selectively on the triple bond in good
yield. Regarding 1,6-enyne 21 no intramolecular cyclization
products16 were detected, nor in the case of aryl propargyl ether
20.17 The cyclopropyl-substituted alkyne 22, which has been
used as a highly sensitive probe to study the mechanism of
addition on a C−C triple bond,18 cleanly afforded the
cycloadduct 22a without leading to any rearranged products.
This result implies that there is lack of development of a charge
or radical (partial or full) on the internal sp-C atom of the
alkyne during the transition state of the reaction.
The studies with terminal alkynes were extended to 1,1,3,3-
tetraphenyldisiloxane (2) and 1,1,1,3,5,7,7,7-octamethyltetrasi-
loxane (3). 1,1,3,3-Tetraphenyldisiloxane19 was easily prepared
from the basic hydrolysis of chlorodiphenylsilane. In general,
the reactions of terminal alkynes with 2 proceed much more
slowly as compared to TMDS and require increased loading of
catalyst (see Table 1). Apart from the mainly formed products
of dehydrogenative cycloaddition in relative yield of 55−65%
b. Au/TiO2-Catalyzed Reaction of Alkynes with
1,1,3,3,5,5-Hexamethyltrisiloxane (4) and 1,2-Bis-
(dimethylsilyl)benzene (5). The facile formation of the
five-membered ring of 2,5-dihydro-1,2,5-oxadisiloles from the
gold-catalyzed reaction among alkynes and TMDS or the
structurally similar 2 and 3 (Table 1) possessing two Si−H
functionalities at the 1,3-position urged us to examine whether
a more remote position of the Si−H groups within the same
molecule would lead to the same dehydrogenative cyclo-
addition pathway. For this purpose we studied the Au/TiO2-
catalyzed reaction of terminal alkynes with 1,1,3,3,5,5-
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Scheme 3. Products from the Au/TiO2-Catalyzed Oxidative Cycloaddition of 1,1,3,3,5,5-Hexamethyltrisiloxane (4) and 1,2-
a
Bis(dimethylsilyl)benzene (5) to Alkynes
a
Reaction conditions for 1,1,3,3,5,5-hexamethyltrisiloxane (4): 1.2 equiv, 25 °C, dry CH2Cl2, 3 h, Au/TiO2 (1.2 mol %); reaction conditions for 1,2-
bis(dimethylsilyl)benzene (5): 1.2 equiv, 25 °C, dry CH2Cl2, 6 h, Au/TiO2 (1.0 mol %). If nondried CH2Cl2 is used, a moisture-dependent excess of
b
4 or 5 should be added to compensate their competing hydrolyses. Isolated overall yields after column chromatography. Alkyne conversions were
>99% by GC-MS analysis in all cases.
hexamethyltrisiloxane (4) and 1,2-bis(dimethylsilyl)benzene
(5). 1,1,3,3,5,5-Hexamethyltrisiloxane, the higher analogue of
TMDS bearing the Si−H functionalities at the 1,5-position,
reacts smoothly at ambient conditions with the alkynes,
affording the seven-membered ring 4,7-dihydro-1,3,2,4,7-
dioxatrisilepines in 44−92% isolated yields (Scheme 3). The
reaction rate as compared to TMDS is approximately 20−30
times lower, possibly due to the higher negative entropy of
activation during the formation of the seven-membered ring.
This is the first time that the 1,3,2,4,7-dioxatrisilepanic
heterocyclic ring appears in the literature. Internal alkynes
such as 24, 28, and 29 tested in their reaction with 4 proved
unreactive, yet 26 provided 26d in moderate yield. 1,2-
Bis(dimethylsilyl)benzene (5) is already known to undergo
oxidative cycloaddition with alkynes catalyzed by (ethylene)-
bis(triphenylphosphine)Pt(0).9b Under catalysis by Au/TiO2,
adducts bearing the 1,1,4,4-tetramethyl-1,4-dihydrobenzo[b]-
[1,4]disiline core skeleton (Scheme 3) were also smoothly
formed in yields varying from 73% to 93%. While internal
alkynes are in general unreactive or provide hydrosilylation
products (e.g., reaction of 5 with 3-hexyne), unsaturated ester
26 gave adduct 26e in excellent yield. The reactivity of this
specific substrate not only with 1,2-bis(dimethylsilyl)benzene
but with other 1,n-dihydro disilanes (1, 3 and 4) might be
related to the intrinsic low energy of its LUMO orbital, as it will
be analyzed in the accompanying mechanistic discussion.
c. Mechanistic Studies Regarding the Au-Catalyzed
Oxidative Cycloaddition of TMDS to Alkynes. To draw
mechanistic conclusions on this quite general cycloaddition, we
focused on 1,1,3,3-tetramethyldisiloxane (1). A postulated
rational mechanism to account for the dehydrogenative
cycloaddition of TMDS to alkynes has been already presented
earlier by us,8 through a modification of the Crabtree−Ojima
mechanism, as shown in Scheme 4. This mechanism involves
insertion of the metal (nanoparticle) on the Si−H bond
(intermediate I), silylmetalation to the alkyne to form
intermediate II, followed by an intramolecular elimination of
H2 and [Au]. While this proposal seems reasonable, we went
forward examining other alternatives. For example, terminal
alkynes that react at a remarkable rate could possibly undergo
dehydrogenative silylation, forming silyl alkynes (intermediate
III), followed by a gold-catalyzed intramolecular hydro-
D
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Scheme 4. Postulated Mechanism Regarding the Au-
Catalyzed Oxidative Cycloaddition of TMDS to Alkynes
through a Modification of the Crabtree−Ojima Mechanism
Scheme 6. Proposal Regarding the Intermediacy of a Cyclo-
gold-disiloxane IV in the Au/TiO2-Catalyzed Hydrolysis of
TMDS and in Its Cycloaddition with Alkynes
silylation3−5 (Scheme 5). The dehydrogenative silylation
pathway among terminal alkynes and hydrosilanes is well
Scheme 5. Unlikeliness of a Possible Mechanism of
Cycloaddition of TMDS to Terminal Alkynes, Involving
Sequential Dehydrogenative Silylation and Hydrosilylation
compounds, and their formation arises from the dehydrogen-
ative reaction (elimination of H2) among 1,ω-dihydro tethered
oligosilanes and the metal.24 It is possible that cyclo-gold-
disiloxane IV is formed through an intramolecular elimination
of H2 in intermediate I (Scheme 6), through a synergistic effect
of the two proximal Si−H bonds and the gold nanoparticle.25
The existence of gold disilyl species such as IV was recently
documented from the oxidative insertion of Au(I) into a Si−Si
bond.26 The possibility that cyclo-gold-disiloxane IV decom-
poses into tetramethylsilylene oxide (1c) cannot be excluded.
Silylene oxides27 are labile compounds and could reasonably
undergo a formal [3+2] cycloaddition to the alkynes, forming
2,5-dihydro-1,2,5-oxadisiloles.
established in the literature under Lewis acid catalysis
conditions21 or in the presence of iridium catalysts.22 We
studied the reaction between TMDS and labeled alkyne 8-d,23
which provided 2,5-dihydro-1,2,5-oxadisilole 8a-d with >97% D
content (see the Supporting Information (SI)). If the
mechanism appearing in Scheme 5 was taking place, no D
atom should be seen on the cycloadduct due to the anticipated
elimination of HD in the initial dehydrogenative silylation step.
A clue for the possible intermediates was acquired by taking a
closer look at the hydrolysis side-products of TMDS. As
commented earlier, if traces of water are present in the reaction
mixture, at the initial stages of the reaction TMDS undergoes
oxidative hydrolysis to the corresponding monosilanol 1a,
which condenses into dimeric 1,1,3,3,5,5,7,7-octamethyltetrasi-
loxane (1b, Scheme 6). At the latter stages, 1b either
oligomerizes or forms octamethylcyclotetrasiloxane (cyclic
dimer). In the case where traces of D2O were added into the
reaction mixture, GC-MS analysis revealed the incorporation of
two deuterium atoms in 1b (compound 1b-d2 of Scheme 6).
The mass spectrum of 1b shows the typical M+−Me
fragmentation at m/z = 267, while we observed in the presence
of deuterium oxide the same fragmentation at m/z = 269,
indicative for the formation of 1b-d2 (see SI). On the basis of
the mechanism presented in Scheme 4, no D incorporation in
dimeric side-product 1b should be expected. The formation of
1,1,3,3,5,5,7,7-octamethyltetrasiloxane-1,7-d2 (1b-d2) could be
rationalized upon considering the cyclo-disilylmetalla inter-
mediate IV shown in Scheme 6. Cyclo-metalla-oligosiloxanes
and cyclo-metalladisilabutanes of Pt, Pd, Ir, and Rh are known
A kinetic study regarding the competitive cycloaddition of
aryl acetylenes with TMDS was also performed. It was found
that electron-withdrawing substituents on the aryl ring
accelerate the reaction rate relative to electron-donating ones
(Table 2). The competitions were carried out upon reacting an
Table 2. Hammett-Type Kinetics in the Competing
Oxidative Cycloaddition of TMDS to Aryl Acetylenes
entry
X
kX/kH
1
2
3
4
5
p-Me
0.74
0.88
0.68
1.39
4.72
p-MeO
p-NMe2
p-F
p-CF3
E
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each, corresponding to the two diastereomers), 0.36, 0.32, 0.30, and
0.25 (four singlets, 3H each, corresponding to the two diastereomers),
0.14, 0.12, 0.11, and 0.09 (four singlets, 9H each, corresponding to the
two diastereomers). 13C NMR (75 MHz, CDCl3, equimolar mixture of
two diastereomers): 162.9, 162.8, 140.6, 140.5, 137.9, 137.9, 137.0,
136.9, 129.3, 129.3, 126.6, 126.6, 21.2, 21.2, 1.9, 1.8, 1.8, 1.7, −0.2,
−0.4, −0.5, −0.7. HRMS: calcd for C17H32O3Si4 + H, 397.1507; found
397.1502.
3-(2-Methoxyphenyl)-2,2,5,5-tetraphenyl-2,5-dihydro-1,2,5-oxa-
disilole (8b). 1H NMR (300 MHz, CDCl3): 7.85 (s, 1H), 7.72 (dd, J1
= 7.5 Hz, J2 = 1.5 Hz, 1H), 7.67−7.59 (m, 8H), 7.45−7.28 (m, 12H),
7.23 (dt, J1 = 7.5 Hz, J2 = 1.5 Hz, 1H), 6.96 (t, J = 7.5 Hz, 1H), 6.76
(d, J = 7.5 Hz, 1H), 3.11 (s, 3H). 13C NMR (75 MHz, CDCl3): 157.2,
155.7, 143.7, 136.1, 134.9, 134.9, 134.5, 130.1, 129.7, 129.3, 127.8,
127.6, 127.0, 123.7, 121.0, 110.6, 53.4. HRMS: calcd for C33H28O2Si2
+ H, 513.1706; found 513.1704.
equimolar mixture of the co-reacting acetylenes with a limited
amount of TMDS, at 10−40% conversion.28 Then the crude
reaction mixture was analyzed by GC-MS and 1H NMR.
Attempts to fit the kinetic data (kX/kH) in a linear Hammett
plot versus either σ or σ+ values gave a poor outcome (R2 =
0.68 and 0.64, respectively; see SI), indicative of a nonionic or
radical-type transition state, as also exemplified by the lack of a
rearrangement pathway in the reaction of cyclopropyl alkyne 22
with TMDS (Table 1). The current reactivity trend might be
attributed to a LUMO-lowering effect of the electron-
withdrawing substituents in reacting alkynes. Goddard and
co-workers have calculated and interpreted a similar trend in
the Huisgen cycloaddition of some aryl-fused cyclooctynes with
alkyl azides.29
As supported gold nanoparticles on metal oxide surfaces
contain, apart from Au(0) nanoclusters, ionic gold species,1a,30
we postulate Au(I) bearing NPs (presumably cationic gold
3-(4-Fluorophenyl)-2,2,5,5-tetraphenyl-2,5-dihydro-1,2,5-oxadisi-
lole (11b). 1H NMR (300 MHz, CDCl3): 7.72−7.65 (m, 8H), 7.62 (s,
1H), 7.48−7.36 (m, 14H), 6.97 (t, J = 7.5 Hz, 2H). 13C NMR (75
MHz, CDCl3): 163.3 (d, JC−F = 245.0 Hz), 162.1, 143.5 (d, JC−F = 2.0
+
clusters Aun stabilized by the support) as the most reasonable
Hz), 135.1, 134.9, 134.8, 130.5, 130.4, 128.8, 128.7, 128.4 (d, JC−F
=
active catalytic sites. Thus, the oxidative insertion of a Au(I) site
into the σ Si−H bonds31 of the hydrosilane will lead to the
formation of Au(III) metallacycle IV. Following the formal
[3+2] cycloaddition of IV to the alkyne, the Au(I)-catalytic site
will be regenerated. A Au(I)−Au(III) redox sequence involving
intramolecular oxidative addition of a Si−Si bond followed by
the insertion of an oxygen atom has been recently
authenticated.26
8.0 Hz), 128.1, 128.0, 115.5 (d, JC−F = 21.0 Hz). HRMS: calcd for
C32H25FOSi2 + H, 501.1506; found 501.1502.
3-(4-Fluorophenyl)-2,5-dimethyl-2,5-bis((trimethylsilyl)oxy)-2,5-
dihydro-1,2,5-oxadisilole (11c). 1H NMR (300 MHz, CDCl3): 7.51−
7.44 (m, 2H), 7.04 (t, J = 8.5 Hz, 2H), 6.96 and 6.94 (two singlets, 1H
each, corresponding to the two diastereomers), 0.34, 0.30, 0.29, and
0.25 (four singlets, 3H each, corresponding to the two diastereomers),
0.14, 0.11, 0.09, and 0.08 (four singlets, 9H each, corresponding to the
two diastereomers). 13C NMR (75 MHz, CDCl3, equimolar mixture of
two diastereomers): 162.6 (d, JC−F = 245.0 Hz), 162.6 (d, JC−F = 245.0
Hz), 161.8, 161.8, 141.6 (d, JC−F = 2.0 Hz), 141.5 (d, JC−F = 2.0 Hz),
135.9, 135.9, 128.2 (d, JC−F = 8.0 Hz), 128.1 (d, JC−F = 8.0 Hz), 115.4
(d, JC−F = 21.0 Hz), 115.4 (d, JC−F = 21.0 Hz), 1.8, 1.7, 1.7, 1.6, −0.4,
−0.6, −0.6, −0.8. HRMS: calcd for C16H29FO3Si4 + H, 401.1256;
found 401.1252.
CONCLUSIONS
■
In conclusion, we have shown that Au/TiO2 is a highly efficient
catalyst for the dehydrogenative cycloaddition of several 1,n-
dihydro tethered disilanes to alkynes under mild conditions.
Depending on the remoteness of the Si−H functionalities in
reacting disilane and the tether as well, this simple protocol
provides access to the five-membered ring of 2,5-dihydro-1,2,5-
oxadisiloles, to the six-membered ring of 1,1,4,4-tetramethyl-
1,4-dihydrobenzo[b][1,4]disilines, and to the seven-membered
ring of 4,7-dihydro-1,3,2,4,7-dioxatrisilepines. Terminal alkynes
are significantly more reactive as compared to internal and
provide the cycloadducts in good to excellent yields. It is
proposed that ionic gold sites on Au NPs form with 1,1,3,3-
tetramethyldisiloxane intermediate cyclo-gold-disiloxane via H2
elimination, which consequently undergoes a formal [3+2]
cycloaddition to alkynes.
3-Pentyl-2,2,5,5-tetraphenyl-2,5-dihydro-1,2,5-oxadisilole (13b).
1H NMR (300 MHz, CDCl3): 7.68−7.62 (m, 8H), 7.46−7.35 (m,
12H), 7.09 (s, 1H), 2.48 (t, J = 7.5 Hz, 2H), 1.52−1.42 (m, 2H),
1.25−1.17 (m, 4H), 0.80 (t, J = 7.5 Hz, 3H). 13C NMR (75 MHz,
CDCl3): 168.5, 141.4, 135.1, 134.9, 134.8, 134.1, 130.2, 130.1, 127.9,
127.9, 36.1, 31.6, 27.8, 22.4, 13.9. HRMS: calcd for C31H32OSi2 + H,
477.2070; found 477.2063.
2,5-Dimethyl-3-pentyl-2,5-bis((trimethylsilyl)oxy)-2,5-dihydro-
1
1,2,5-oxadisilole (13c). H NMR (300 MHz, CDCl3): 6.39 and 6.38
(two broad singlets, 1H each, corresponding to the two
diastereomers), 2.25 (t, J = 7.5 Hz, 2H), 1.52−1.44 (m, 2H), 1.34−
1.25 (m, 4H), 0.90 (t, J = 7.5 Hz, 3H), 0.21, 0.20, 0.16, and 0.15 (four
singlets, 3H each, corresponding to the two diastereomers), 0.11, 0.10,
0.09, and 0.08 (four singlets, 9H each, corresponding to the two
diastereomers). 13C NMR (75 MHz, CDCl3, equimolar mixture of two
diastereomers): 168.1, 168.0, 142.0, 142.0, 36.1, 36.1, 31.7, 31.7, 28.1,
28.1, 22.5, 22.5, 14.0, 14.0, 1.8, 1.8, 1.8, 1.8, −0.6, −0.8, −0.9, −1.2.
HRMS: calcd for C15H36O3Si4 + H, 377.1820; found 377.1814.
3-Cyclopropyl-2,2,5,5-tetraphenyl-2,5-dihydro-1,2,5-oxadisilole
(15b). 1H NMR (300 MHz, CDCl3): 7.73 (d, J = 7.0 Hz, 4H), 7.66 (d,
J = 7.0 Hz, 4H), 7.51−7.36 (m, 12H), 6.87 (s, 1H), 1.88−1.82 (m,
1H), 0.87−0.80 (m, 2H), 0.68−0.62 (m, 2H). 13C NMR (75 MHz,
CDCl3): 170.4, 136.4, 135.1, 135.0, 134.8, 134.2, 130.3, 130.2, 128.0,
127.9, 16.4, 9.7. HRMS: calcd for C29H26OSi2 + H, 447.1600; found
447.1602.
EXPERIMENTAL SECTION
■
General Procedure for the Au/TiO2-Catalyzed Dehydrogen-
ative Cycloaddition Reaction. To a vial containing the alkyne (1.0
mmol) and a suitable amount of the 1,n-dihydrodisilane in 0.5 mL of
solvent (see Table 1 and Scheme 3) was added Au/TiO2 (concerning
the loading levels of catalyst see Table 1 and Scheme 3). The reaction
was monitored by TLC and GC-MS. After the disappearance of the
starting material, the slurry was filtered with the aid of 3 mL of solvent
through a short pad of silica gel. The filtrate was evaporated under
vacuum, and the residue was chromatographed (if necessary).
Characterization of Products. The spectroscopic data of 2,5-
dihydro-1,2,5-oxadisiloles 6a−16a, 18a−21a, and 23a−27a have been
previously reported.8
3-Cyclopropyl-2,5-dimethyl-2,5-bis((trimethylsilyl)oxy)-2,5-dihy-
1
2,2,3,5,5-Pentaphenyl-2,5-dihydro-1,2,5-oxadisilole (6b). 1H
NMR (300 MHz, CDCl3): 7.70−7.66 (m, 8H), 7.49−7.33 (m,
17H), 7.28 (s, 1H). 13C NMR (75 MHz, CDCl3): 163.7, 143.7, 140.6,
135.2, 134.7, 134.5, 134.2, 130.4, 130.4, 128.6, 128.0, 128.0, 127.1,
126.8. HRMS: calcd for C32H26OSi2 + H, 483.1600; found 483.1595.
2,5-Dimethyl-3-(p-tolyl)-2,5-bis((trimethylsilyl)oxy)-2,5-dihydro-
dro-1,2,5-oxadisilole (15c). H NMR (300 MHz, CDCl3): 6.32 and
6.30 (two broad singlets, 1H each, corresponding to the two
diastereomers), 1.66−1.47 (m, 1H), 0.81−0.75 (m, 1H), 0.66−0.55
(m, 3H), 0.21, 0.19, 0.17, and 0.15 (four singlets, 3H each,
corresponding to the two diastereomers), 0.13, 0.10, 0.10, and 0.08
(four singlets, 9H each, corresponding to the two diastereomers). 13C
NMR (75 MHz, CDCl3, equimolar mixture of two diastereomers):
170.0, 169.9, 138.2, 138.1, 16.8, 18.8, 8.2, 8.2, 8.0, 7.9, 1.8, 1.8, 1.8, 1.7,
1
1,2,5-oxadisilole (7c). H NMR (300 MHz, CDCl3): 7.43 (d, J = 8.0
Hz, 2H), 7.18 (d, J = 8.0 Hz, 2H), 7.00 and 6.99 (two singlets, 1H
F
dx.doi.org/10.1021/om3011678 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
−0.3, −0.6, −0.6, −0.9. HRMS: calcd for C13H30O3Si4 + H, 347.1350;
found 347.1346.
(75 MHz, CDCl3): 162.2, 147.2, 145.8, 145.0, 144.2, 133.4, 133.1,
128.3, 128.2, 128.1, 126.5, 126.3, 0.0, −0.5.
3-(Cyclohexylmethyl)-2,2,5,5-tetramethyl-2,5-dihydro-1,2,5-oxa-
disilole (17a). 1H NMR (300 MHz, CDCl3): 6.48 (s, 1H), 2.18 (br d,
J = 7.0 Hz, 2H), 1.75−1.62 (m, 5H), 1.45−1.35 (m, 1H), 1.29−1.11
(m, 3H), 0.92−0.78 (m, 2H), 0.21 (s, 6H), 0.19 (s, 6H). 13C NMR
(75 MHz, CDCl3): 167.6, 144.7, 45.0, 36.9, 33.5, 26.6, 26.3, 1.0, 0.5.
MS (EI): 254 (M+, 2), 239 (M+ − Me, 16), 172 (22), 157 (46), 133
(33), 83 (45), 73 (34), 55 (100).
1,1,4,4-Tetramethyl-2-(p-tolyl)-1,4-dihydrobenzo[b][1,4]disiline
1
(7e). H NMR (300 MHz, CDCl3): 7.64−7.58 (m, 2H), 7.43−7.36
(m, 2H), 7.17 (d, J = 7.0 Hz, 2H), 7.15 (d, J = 7.0 Hz, 2H), 6.84 (s,
1H), 2.37 (s, 3H), 0.38 (s, 6H), 0.36 (s, 6H). 13C NMR (75 MHz,
CDCl3): 161.9, 145.1, 145.1, 144.3, 144.2, 136.1, 133.3, 133.1, 128.9,
128.3, 128.1, 126.2, 21.1, 0.1, −0.5. HRMS: calcd for C19H24Si2 + H,
309.1495; found 309.1490.
2-(2-Methoxyphenyl)-1,1,4,4-tetramethyl-1,4-dihydrobenzo[b]-
2,2,5,5-Tetraphenyl-3-(prop-1-en-2-yl)-2,5-dihydro-1,2,5-oxadisi-
1
1
[1,4]disiline (8e). H NMR (300 MHz, CDCl3): 7.63−7.57 (m, 2H),
lole (19b). H NMR (300 MHz, CDCl3): 7.69 (d, J = 7.0 Hz, 4H),
7.44−7.36 (m, 2H), 7.22 (dd, J1 = 7.5 Hz, J2 = 2.0 Hz, 1H), 7.04 (dd,
J1 = 7.5 Hz, J2 = 2.0 Hz, 1H), 6.93 (t, J = 7.5 Hz, 1H), 6.85 (d, J = 7.5
Hz, 1H), 6.84 (s, 1H), 3.82 (s, 3H), 0.39 (s, 6H), 0.30 (s, 6H). 13C
NMR (75 MHz, CDCl3): 161.4, 155.3, 147.0, 146.1, 144.5, 136.8,
133.0, 132.9, 128.9, 128.1, 127.9, 127.7, 121.0, 109.8, 54.7, −0.6, −0.6.
HRMS: calcd for C19H24OSi2 + H, 325.1443; found 325.1433.
2-(4-Fluorophenyl)-1,1,4,4-tetramethyl-1,4-dihydrobenzo[b][1,4]-
7.62 (d, J = 7.0 Hz, 4H), 7.49−7.35 (m, 12H), 7.28 (s, 1H), 5.19 (br s,
1H), 5.14 (br s, 1H), 2.03 (s, 3H). 13C NMR (75 MHz, CDCl3):
164.3, 144.2, 142.7, 135.2, 134.8, 134.6, 134.4, 130.3, 130.3, 127.9,
127.9, 120.1, 20.5. HRMS: calcd for C29H26OSi2 + H, 447.1600; found
447.1591.
3-(2,2-Diphenylcyclopropyl)-2,2,5,5-tetramethyl-2,5-dihydro-
1,2,5-oxadisilole (22a). 1H NMR (300 MHz, CDCl3): 7.28−7.07 (m,
10H), 5.84 (s, 1H), 2.46 (dd, J1 = 8.0 Hz, J2 = 7.5 Hz, 1H), 1.85 (dd, J1
= 8.5 Hz, J2 = 8.0 Hz, 1H), 1.59 (dd, J1 = 8.5 Hz, J2 = 7.5 Hz, 1H), 0.26
(s, 3H), 0.11 (s, 6H), −0.13 (s, 3H). 13C NMR (75 MHz, CDCl3):
163.5, 146.8, 142.6, 140.2, 131.2, 128.4, 127.9, 127.7, 126.2, 126.0,
41.0, 30.1, 20.8, 0.6, 0.4, 0.4, −0.0. MS (EI): 350 (M+, 11), 258 (12),
133 (100), 73 (44).
Ethyl 2,4,5-Trimethyl-2,5-bis((trimethylsilyl)oxy)-2,5-dihydro-
1,2,5-oxadisilole-3-carboxylate (26c). 1H NMR (300 MHz,
CDCl3): 4.27−4.14 (m, 2H), 2.22 and 2.21 (two singlets, 3H each,
corresponding to the two diastereomers), 1.31 (t, J = 7.5 Hz, 3H),
0.30, 0.26, 0.23, and 0.19 (four singlets, 3H each, corresponding to the
two diastereomers), 0.11, 0.10, 0.09, and 0.07 (four singlets, 9H each,
corresponding to the two diastereomers). 13C NMR (75 MHz, CDCl3,
equimolar mixture of two diastereomers): 172.3. 172.2, 167.2, 167.2,
144.8, 144.8, 59.7, 59.7, 17.3, 17.2, 14.3, 14.3, 1.7, 1.6, 1.6, 1.6, −0.8,
−1.2, −2.4, −2.7. HRMS: calcd for C14H32O5Si4 + H, 393.1405; found
393.1399.
2,2,4,4,7,7-Hexamethyl-5-(p-tolyl)-4,7-dihydro-1,3,2,4,7-dioxatri-
silepine (7d). 1H NMR (300 MHz, CDCl3): 7.10 (d, J = 8.0 Hz, 2H),
7.00 (d, J = 8.0 Hz, 2H), 6.33 (s, 1H), 2.34 (s, 3H), 0.26 (s, 6H), 0.22
(s, 6H), 0.16 (s, 6H). 13C NMR (75 MHz, CDCl3): 164.0, 147.1,
145.2, 135.7, 128.7, 126.3, 21.0, 1.6, 1.3, 0.7. HRMS: calcd for
C15H26O2Si3 + H, 323.1319; found 323.1314.
1
disiline (11e). H NMR (300 MHz, CDCl3): 7.64−7.58 (m, 2H),
7.43−7.37 (m, 2H), 7.23−7.18 (m, 2H), 7.02 (t, J = 8.5 Hz, 2H), 6.83
(s, 1H), 0.36 (s, 6H), 0.36 (s, 6H). 13C NMR (75 MHz, CDCl3):
161.8 (d, JC−F = 243.5 Hz), 161.1, 146.0 (d, JC−F = 1.0 Hz), 144.7,
144.0, 143.2 (d, JC−F = 3.0 Hz), 133.4, 133.1, 128.4, 128.2, 127.9 (d,
JC−F = 8.0 Hz), 115.0 (d, JC−F = 21.0 Hz), 0.0, −0.5. HRMS: calcd for
C18H21FSi2 + H, 313.1244; found 313.1235.
1,1,4,4-Tetramethyl-2-pentyl-1,4-dihydrobenzo[b][1,4]disiline
32
1
(13e) (ref ). H NMR (300 MHz, CDCl3): 7.60−7.54 (m, 2H),
7.40−7.33 (m, 2H), 6.57 (s, 1H), 2.30 (br t, J = 6.5 Hz, 2H), 1.52−
1.44 (m, 2H), 1.39−1.26 (m, 4H), 0.91 (t, J = 7.0 Hz, 3H), 0.31 (s,
6H), 0.27 (s, 6H). 13C NMR (75 MHz, CDCl3): 162.4, 145.0, 144.8,
140.4, 133.1, 133.0, 128.0, 127.9, 40.0, 31.7, 28.3, 22.6, 14.1, −0.4,
−0.8.
2-Cyclopropyl-1,1,4,4-tetramethyl-1,4-dihydrobenzo[b][1,4]-
1
disiline (15e). H NMR (300 MHz, CDCl3): 7.62−7.54 (m, 2H),
7.41−7.35 (m, 2H), 6.30 (s, 1H), 0.39 (s, 6H), 0.26 (s, 6H). 13C NMR
(75 MHz, CDCl3): 162.9, 144.8, 144.7, 134.1, 133.2, 133.1, 128.1,
128.0, 18.0, 7.0, −0.3, −0.9. HRMS: calcd for C15H22Si2 + H,
259.1338; found 259.1334.
Ethyl 1,1,4,4-Tetramethyl-1,4-dihydrobenzo[b][1,4]disiline-2-car-
1
boxylate (23e). H NMR (300 MHz, CDCl3): 8.02 (s, 1H), 7.62−
7.55 (m, 2H), 7.44−7.36 (m, 2H), 4.28 (q, J = 7.0 Hz, 2H), 1.37 (t, J =
7.0 Hz, 3H), 0.43 (s, 6H), 0.34 (s, 6H). 13C NMR (75 MHz, CDCl3):
168.4, 159.1, 150.3, 144.7, 142.6, 133.7, 133.1, 128.5, 128.2, 60.7, 14.3,
−0.2, −1.0. HRMS: calcd for C15H22O2Si2 + H, 291.1236; found
291.1231.
5-(2-Methoxyphenyl)-2,2,4,4,7,7-hexamethyl-4,7-dihydro-
1
1,3,2,4,7-dioxatrisilepine (8d). H NMR (300 MHz, CDCl3): 7.19
(dt, J1 = 7.5, J2 = 2.0 Hz, Hz, 1H), 7.04 (dd, J1 = 7.5 Hz, J2 = 2.0 Hz,
1H), 6.92 (dt, J1 = 7.5 Hz, J2 = 1.0 Hz, 1H), 6.77 (br d, J = 7.5 Hz,
1H), 6.37 (s, 1H), 3.76 (s, 3H), 0.27 (s, 6H), 0.17 (s, 6H), 0.14 (s,
6H). 13C NMR (75 MHz, CDCl3): 162.7, 155.0, 147.2, 138.2, 128.5,
127.7, 121.0, 109.6, 54.4, 1.4, 1.0, 0.8. HRMS: calcd for C15H26O3Si3 +
H, 339.1268; found 339.1262.
Ethyl 1,1,3,4,4-Pentamethyl-1,4-dihydrobenzo[b][1,4]disiline-2-
carboxylate (26e). 1H NMR (300 MHz, CDCl3): 7.60−7.53 (m,
2H), 7.41−7.36 (m, 2H), 4.28 (q, J = 7.5 Hz, 2H), 2.09 (s, 3H), 1.35
(t, J = 7.5 Hz, 3H), 0.37 (s, 6H), 0.35 (s, 6H). 13C NMR (75 MHz,
CDCl3): 170.6, 158.6, 146.3, 143.7, 143.2, 133.1, 133.0, 128.4, 128.3,
60.2, 21,0, 14.5, −0.7, −1.8. HRMS: calcd for C16H24O2Si2 + H,
305.1393; found 305.1388.
5-(4-Fluorophenyl)-2,2,4,4,7,7-hexamethyl-4,7-dihydro-1,3,2,4,7-
1
dioxatrisilepine (11d). H NMR (300 MHz, CDCl3): 7.09−6.93 (m,
4H), 6.32 (s, 1H), 0.26 (s, 6H), 0.20 (s, 6H), 0.15(s, 6H). 13C NMR
(75 MHz, CDCl3): 163.1, 161.6 (d, JC−F = 245.0 Hz), 148.0 (d, JC−F
=
ASSOCIATED CONTENT
1.0 Hz), 144.1 (d, JC−F = 3.5 Hz), 127.9 (d, JC−F = 8.0 Hz), 114.8 (d,
JC−F = 21.0 Hz), 1.5, 1.3, 0.7. HRMS: calcd for C14H23FO2Si3 + H,
327.1068; found 327.1063.
2,2,4,4,7,7-Hexamethyl-5-pentyl-4,7-dihydro-1,3,2,4,7-dioxatrisi-
lepine (13d). 1H NMR (300 MHz, CDCl3): 6.15 (s, 1H), 2.10 (t, J =
6.5 Hz, 2H), 1.52−1.35 (m, 6H), 0.89 (t, J = 7.0 Hz, 3H), 0.19 (s,
6H), 0.18 (s, 6H), 0.09 (s, 6H). 13C NMR (75 MHz, CDCl3): 163.2,
142.1, 40.3, 31.7, 29.1, 22.5, 14.0, 1.4, 1.0, 0.7. HRMS: calcd for
C13H30O2Si3 + H, 303.1632; found 303.1626.
Ethyl 2,2,4,4,6,7,7-heptamethyl-4,7-dihydro-1,3,2,4,7-dioxatrisile-
pine-5-carboxylate (26d). 1H NMR (300 MHz, CDCl3): 4.19 (q, J =
7.0 Hz, 2H), 1.80 (s, 3H), 1.30 (t, J = 7.0 Hz, 3H), 0.25 (s, 6H), 0.24
(s, 6H), 0.11 (s, 6H). 13C NMR (75 MHz, CDCl3): 172.6, 155.0,
149.1, 60.2, 21.1, 14.3, 0.9, 0.5, 0.1. HRMS: calcd for C12H26O4Si3 + H,
319.1217; found 319.1213.
1,1,4,4-Tetramethyl-2-phenyl-1,4-dihydrobenzo[b][1,4]disiline
(6e) (ref 32). 1H NMR (300 MHz, CDCl3): 7.64−7.56 (m, 2H),
7.44−7.25 (m, 7H), 6.87 (s, 1H), 0.39 (s, 6H), 0.38 (s, 6H). 13C NMR
■
S
* Supporting Information
1
Copies of H and 13C NMR spectra of all products. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: stratakis@chemistry.uoc.gr.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank ProFI (ITE, Heraklion, Greece) for obtaining the
HRMS spectra. Two of the authors (I.N.L. and M.S.) deeply
■
G
dx.doi.org/10.1021/om3011678 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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H
dx.doi.org/10.1021/om3011678 | Organometallics XXXX, XXX, XXX−XXX