Diversely substituted arylsilyl triflates, as aryne precursors for aryne cycloaddition reactions, were accessed from benzodioxasilines. Catalytic reductive CH ortho-silylation of phenols with traceless acetal directing groups was exploited to prepare benzodioxasilines. Sequential addition of MeLi and then trifluoromethanesulfonic anhydride to benzodioxasilines provided arylsilyl triflates in a single pot. Notably, this approach was successfully utilized to prepare sterically hindered 1,2,3-trisubstituted arylsilyl triflates, which ultimately underwent fluoride-mediated aryne cycloaddition.Download high-res image (80KB)Download full-size image

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 ortho-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 ortho-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.

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 (E)-vinylsilanes or β-alkylsilanes with high regio- and stereocontrol. cis,cis-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.

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 R3Si–H across the π-bond of a Ru-benzylidene.Keywords: alkene hydrosilylation; metathesis; ruthenium; silane; σ-bond metathesis

This work describes the design and application of a single-pot, reductive arene C–H silanolization of aromatic esters for synthesis of ortho-formyl arylsilanols. This strategy involves a sequence of two transition metal (Ir and Rh)-catalyzed reactions for reductive arene ortho-silylation directed by hydridosilyl acetals and hydrolysis.

A Lewis base promoted deprotonative pronucleophile addition to silyl acetals has been developed and applied to the iridium-catalyzed reductive Horner–Wadsworth–Emmons (HWE) olefination of esters and the chemoselective reduction of the resulting enoates. Lewis base activation of silyl acetals generates putative pentacoordinate silicate acetals, which fragment into aldehydes, silanes, and alkoxides in situ. Subsequent deprotonative metalation of phosphonate esters followed by HWE with aldehydes furnishes enoates. This operationally convenient, mechanistically unique protocol converts the traditionally challenging aryl, alkenyl, and alkynyl esters to homologated enoates at room temperature within a single vessel.

We report a modular approach to catalytic reductive Csp2–H and Csp3–H silylation of carboxylic acid derivatives encompassing esters, ketones, and aldehydes. Choice of either an Ir(I)/Rh(I) or Rh(I)/Rh(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 Csp2–H and benzylic Csp3–H silylation in a single vessel.

Ligand-controlled, norbornene-mediated, regio- and diastereoselective rhodium-catalyzed intramolecular alkene hydrosilylation of homoallyl silyl ethers (1) exploiting either BINAP or 1,6-bis(diphenylphosphino)hexane (dpph) has been developed. This method permits selective access to either trans-oxasilacyclopentanes (trans-2) or oxasilacyclohexanes (3) at will. A substoichiometric amount of norbornene markedly increased both yield and selectivity. A norbornene-mediated hydride shuttle process is discussed.

This work describes the design and application of a single-pot, reductive arene C–H silanolization of aromatic esters for synthesis of ortho-formyl arylsilanols. This strategy involves a sequence of two transition metal (Ir and Rh)-catalyzed reactions for reductive arene ortho-silylation directed by hydridosilyl acetals and hydrolysis.