Borane catalysed ring opening and closing cascades of furans leading to silicon functionalized synthetic intermediates

Abstract

The conversion of renewable biomass resources to synthetically valuable chemicals is highly desirable, but remains a formidable challenge in regards to the substrate scope and reaction conditions. Here we present the development of tris(pentafluorophenyl)borane–catalysed conversion of furans via ring-opening and closing cascade processes to afford silicon-functionalized synthetic chemicals under transition metal-free conditions. The furan ring-opening with hydrosilanes is highly efficient (TON up to 2,000) and atom-economical without forming any byproduct to give rise to α-silyloxy-(Z)-alkenyl silanes. Additional equivalents of silane smoothly induce a subsequent B(C6F5)3-catalysed cyclization of initially formed olefinic silane compounds to produce anti-(2-alkyl)cyclopropyl silanes, another versatile synthon being potentially applicable in the synthesis of natural products and pharmacophores.

Modern Developments in the Chemistry of Homoenolates

“…In 1997, Pietruzska reported a protocol to convert tartrate‐bound boronic esters 69 to the corresponding cyclopropanols in modest to good yields and good enantiomeric excesses (Scheme a) . In 2016, Chang and co‐workers showed that cyclopropylsilane 71 could undergo a Tamao–Fleming oxidation to deliver the corresponding cyclopropanol 72 (Scheme b) …”

Catalytic Reduction of Cyclic Ethers with Hydrosilanes

“…In 2016, Chang and co‐workers discovered the borane‐catalyzed hydrosilylative ring opening and closing cascades of furans, which enabled the chemoselective synthesis of α‐silyloxy homoallylsilanes and silylated cyclopropanes with excellent chemo‐, regio‐, and stereoselectivities . Opening of the furan ring with hydrosilanes was highly efficient (TON up to 2000) at 25 °C, forming a new sp 3 ‐C−Si bond α to the oxygen atom.…”

Reductive Carbocyclization of Homoallylic Alcohols to syn‐Cyclobutanes by a Boron‐Catalyzed Dual Ring‐Closing Pathway

“…[7] Recently,w ereported aB (C 6 F 5 ) 3 -catalyzed cascade conversion of furans providing silicon-functionalized compounds, in which ahomoallylic intermediate was assumed to undergo ac oncerted S N 2'-type ring-closing process to furnish cyclopropanes with exclusive trans-selectivity (Scheme 1b). [8] GagnØ and co-workers found that B(C 6 F 5 ) 3 promotes areductive cyclization of silyl-protected unsaturated polyols to give cyclopropanes and cyclopentanes depending on the substituents adjacent to the alkenyl moiety (Scheme 1c). [9] Unlike in our previous work, [8] GagnØ suggested as tepwise pathway involving an intramolecular attack of an eighboring alkenyl group at the activated CÀObond of the silaoxonium ion.…”