4 research outputs found
Regio- and Stereoselective Dehydrogenative Silylation and Hydrosilylation of Vinylarenes Catalyzed by Ruthenium Alkylidenes
Development
of regio- and stereoselective dehydrogenative silylation
and hydrosilylation of vinylarenes with alkoxysilanes, catalyzed by
ruthenium alkylidenes, is described. Varying L- and X-type ligands
on ruthenium alkylidenes permits selective access to either (<i>E</i>)-vinylsilanes or β-alkylsilanes with high regio-
and stereocontrol. <i>cis,cis</i>-1,5-Cyclooctadiene was
identified as the most effective sacrificial hydrogen acceptor for
the dehydrogenative silylation of vinylarenes, which allows use of
a nearly equimolar ratio of alkenes and silanes
Modular Approach to Reductive C<sub>sp2</sub>–H and C<sub>sp3</sub>–H Silylation of Carboxylic Acid Derivatives through Single-Pot, Sequential Transition Metal Catalysis
We
report a modular approach to catalytic reductive C<sub>sp2</sub>–H
and C<sub>sp3</sub>–H silylation of carboxylic acid
derivatives encompassing esters, ketones, and aldehydes. Choice of
either an IrÂ(I)/RhÂ(I) or RhÂ(I)/RhÂ(I) sequence leads to either exhaustive
reductive ester or reductive ketone/aldehyde silylation, respectively.
Notably, a catalyst-controlled direct formation of doubly reduced
silyl ethers is presented, specifically via Ir-catalyzed exhaustive
hydrosilylation. The resulting silyl ethers undergo C<sub>sp2</sub>–H and benzylic C<sub>sp3</sub>–H silylation in a single
vessel
Catalytic Reductive <i>ortho</i>-C–H Silylation of Phenols with Traceless, Versatile Acetal Directing Groups and Synthetic Applications of Dioxasilines
A new,
highly selective, bond functionalization strategy, achieved
via relay of two transition metal catalysts and the use of traceless
acetal directing groups, has been employed to provide facile formation
of C–Si bonds and concomitant functionalization of a silicon
group in a single vessel. Specifically, this approach involves the
relay of Ir-catalyzed hydrosilylation of inexpensive and readily available
phenyl acetates, exploiting disubstituted silyl synthons to afford
silyl acetals and Rh-catalyzed <i>ortho</i>-C–H silylation
to provide dioxasilines. A subsequent nucleophilic addition to silicon
removes the acetal directing groups and directly provides unmasked
phenol products and, thus, useful functional groups at silicon achieved
in a single vessel. This traceless acetal directing group strategy
for catalytic <i>ortho-</i>C–H silylation of phenols
was also successfully applied to preparation of multisubstituted arenes.
Remarkably, a new formal α-chloroacetyl directing group has
been developed that allows catalytic reductive C–H silylation
of sterically hindered phenols. In particular, this new method permits
access to highly versatile and nicely differentiated 1,2,3-trisubstituted
arenes that are difficult to access by other catalytic routes. In
addition, the resulting dioxasilines can serve as chromatographically
stable halosilane equivalents, which allow not only removal of acetal
directing groups but also introduce useful functional groups leading
to silicon-bridged biaryls. We demonstrated that this catalytic C–H
bond silylation strategy has powerful synthetic potential by creating
direct applications of dioxasilines to other important transformations,
examples of which include aryne chemistry, Au-catalyzed direct arylation,
sequential orthogonal cross-couplings, and late-stage silylation of
phenolic bioactive molecules and BINOL scaffolds
Mechanistic Insights into Grubbs-Type Ruthenium-Complex-Catalyzed Intramolecular Alkene Hydrosilylation: Direct σ‑Bond Metathesis in the Initial Stage of Hydrosilylation
Grubbs-type
ruthenium-complex-mediated intramolecular alkene hydrosilylation
of alkenylsilyl ethers has been developed to provide cyclic silyl
ethers with high regioselectivity. This non-metathetical use of such
ruthenium complexes for alkene hydrosilylation via preferential Si–H
bond activation over alkene activation is notable, where the competing
alkene metathesis dimerization was not detected. In addition to the
synthesis of organosilicon heterocycles from readily available olefins,
this study provides fundamental mechanistic insights into the non-metathetical
function of Grubbs-type ruthenium catalysts. In the initial stage
of hydrosilylation within a ruthenium coordination sphere, evidence
for activation of a ruthenium complex by direct σ-bond metathesis
between Si–H and Ru–Cl via a four-centered transition
state is presented. This study counters the traditionally accepted
Chauvin-type mechanism, specifically the addition of R<sub>3</sub>Si–H across the π-bond of a Ru-benzylidene