Journal of the American Chemical Society
COMMUNICATION
Scheme 2. Postulated Mechanism for the Catalytic Oxidative
Cycloaddition of 1,1,3,3-Tetramethyldisiloxane to Alkynes
’ ACKNOWLEDGMENT
We thank ProFI (ITE, Iraklion, Greece) for obtaining the
HRMS spectra.
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is moderate, typical hydrosilylation products are formed from a
competing pathway (see footnotes of Tables 1 and 2). Surprisingly,
internal alkynes 21 and 22 are completely unreactive, while dimethyl
acetylenedicarboxylate (20) did not yield neither cycloadduct nor
hydrosilylation product, but slowly reduced within 24 h to dimethyl
maleate (∼90%) and dimethyl succinate (∼10%). It is possible that
the reduction proceeds through a gold-catalyzed hydrogenation18
by the hydrogen gas produced from the slow oxidative hydrolysis of
TMDS, as the reaction was performed within an open air flask. It is
premature to comment on the reactivity trend and the substituent
effects, especially for the case of internal alkynes, as the reaction
mechanism (see below) is currently unclear.
Substituted 2,5-dihydro-1,2,5-oxadisiloles19 is a class of com-
pounds whose chemistry is essentially unexplored. For instance,
they could be interesting candidates for Pd-catalyzed cross
coupling reactions. Their known close analogues are the arene
fused benzobisoxadisiloles,20 which are used as synthetic equiva-
lents (precursors) of benzynes.21 Some of them were found to be
powerful musk odorants.22
From the mechanistic point of view, we do not want to
speculate at this stage what is the active catalytic species (Au0,
AuI or AuIII). For clarity we indicate those species as [Au]. One
possible rationalization (working hypothesis) for the observed
oxidative cycloaddition pathway may be invoked through a
modification of the Crabtree-Ojima23 mechanism, as shown in
Scheme 2. Thus, silylmetalation in the insertion to the alkyne
step, followed by an intramolecular elimination of H2 and [Au]
will lead to the cycloadducts. Apart of this postulation, several
other mechanistic options24 are currently under consideration.
In conclusion, we present herein an extremely simple metho-
dology for the oxidative cycloaddition of an 1,3-dihydro-1,3-
disiloxane to alkynes catalyzed by supported gold nanoparticles.
This reaction could be seen as the sila-analogue of click chemistry.
Further work is in progress to examine the generality of this
reaction with other siloxanes, to uncover the mechanistic details, as
well as to explore the chemistry of substituted 2,5-dihydro-1,2,5-
oxadisiloles, especially in Pd-catalyzed cross coupling reactions.
(17) Typical procedure for the [Au]-catalyzed oxidative cycloaddi-
tion: To a vial containing 0.38 mL (3.0 mmol) of p-tolylacetylene (1),
0.53 mL (3.0 mmol) of 1,1,3,3-tetramethyldisiloxane (TMDS), and
5 mL of dry dichloromethane were added 150 mg [Au]/TiO2 (∼0.25%
mol in Au). An evolution of hydrogen gas immediately occurred. After
30 min the reaction was complete (TLC, GC-MS). The slurry was
filtered with the aid of 5 mL dichloromethane through a short pad of
silica gel (under a low pressure) to withhold the solid catalyst. The
filtrate was evaporated under vacuum to afford 715 mg of addition
product 1a (96% yield, >99% pure).
’ ASSOCIATED CONTENT
S
Supporting Information. Spectroscopic data of all pro-
b
ducts (1H, 13C NMR and HRMS). This material is available free
’ AUTHOR INFORMATION
Corresponding Author
(18) Segura, Y.; Lopez, N.; Perez-Ramirez, J. J. Catal. 2007, 247, 383.
10428
dx.doi.org/10.1021/ja2045502 |J. Am. Chem. Soc. 2011, 133, 10426–10429