Letter
Ruthenium-Catalyzed Brook Rearrangement Involved Domino
Sequence Enabled by Acylsilane−Aldehyde Corporation
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ABSTRACT: A ruthenium-catalyzed [1,2]-Brook rearrangement
involved domino sequence is presented to prepare highly
functionalized silyloxy indenes with atomic- and step-economy.
This domino reaction is triggered by acylsilane-directed C−H
activation, and the aldehyde controlled the subsequent enol
cyclization/Brook Rearrangement other than β−H elimination.
The protocol tolerates a broad substitution pattern, and the further
synthetic elaboration of silyloxy indenes allows access to a diverse
range of interesting indene and indanone derivatives.
cylsilane represents a fascinating class of organosilicon
compounds, exhibiting a wide variety of synthetic
transformations, so the synthesis of silyloxy indenes seems
particularly attractive. Recently, rhodium(III)-catalyzed ortho-
olefination of aroylsilanes followed by light-induced intra-
molecular cyclization via siloxycarbenes represents an efficient
two-step access to silyloxy indenes (Scheme 1g).3e
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applications, including nucleophilic addition, cross-coupling,
radical cyclization, and aldol reaction.1 Notably, acylsilanes are
intriguing for potential [1,2]-anion transposition, defined as
the Brook rearrangement, and there are many reports on
nucleophilic addition/Brook rearrangement reactions using a
quantitative amount of organometallic reagents (Scheme
1a).1a,2 Acylsilane also undergoes a photochemical or thermal
[1,2]-Brook rearrangement to provide the siloxycarbene
intermediate for further insertion of C−H, B−H, C−B, and
unsaturated C−C bonds (Scheme 1b).3 Furthermore, there
have been several exciting organo-catalytic sequences,4
including thiazolium-catalyzed acylsilane addition to unsatu-
rated esters/ketone (Scheme 1c),4a cyanide-catalyzed silyl
benzoin reaction (Scheme 1d),4b phosphine-promoted se-
quential Brook/Wittig reactions (Scheme 1e),4c and enantio-
selective bisguanidinium-catalyzed anionotropic rearrangement
(Scheme 1f).4d To the best of our knowledge, transition-metal-
catalyzed Brook rearrangement of acylsilanes still remains
elusive,5 although there are several Cu- or Pd-catalyzed
examples using other different carbonyl or organosilicon
substrates.6
Indenes are widely utilized as building blocks for the
synthesis of natural products and pharmaceutical molecules as
well as ligands in various transition-metal-catalyzed reactions.7
Consequently, many efforts have been devoted to the
preparation of indene frameworks, but these conventional
approaches usually require multiple steps and/or suffer from
limited substrate scopes.8 In recent years, tremendous
advances have been made in transition-metal-catalyzed C−H
activations9 as well as the C−H activation/carbocyclization
sequence to construct indenes, indenones, and hetero-
cycles.10−12 Silyl enol ethers are recognized as versatile
functionalities that have been utilized in numerous synthetic
Domino-type reactions by careful design of a multistep
reaction in one-pot sequence provide efficient and step-
economical approaches using much simpler raw chemicals.
Despite the wide application of acylsilanes in synthetic organic
chemistry, there is still no report on tandem bond formation
integrating directed C−H activation9 and Brook rearrange-
ment to produce valuable organosilicon compounds. With our
ongoing interest in directed C−H activation,9f−h,13j herein, we
report the first ruthenium-catalyzed C−H functionalization/
cyclization/[1,2[-Brook rearrangement sequence, and such a
one-pot/cascade transformation provides one atom/step-
economic access toward silyloxy indenes from readily available
aroylsilanes and acroleins. Cooperation of the acylsilane and
aldehyde controls the selectivity and sequence of the domino
reaction efficiently (Scheme 1h).
Control of chemoselectivity is highly sought after in organic
synthesis. In the realm of directed C−H functionalization,
chemoselectivity can be controlled by change of catalyst,
directing group, electron-withdrawing group, or additive.13 We
postulated that the C−H functionalization of aroylsilanes using
acroleins or vinyl ketones could facilitate the carbocyclization-
involved cascade sequence due to a key metallo−enol
Received: June 13, 2020
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
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