Vinylidene Homologation of Boronic Esters and its Application to the Synthesis of the Proposed Structure of Machillene

Abstract

Alkenyl boronic esters are important reagents in organic synthesis.H erein, we report that these valuable products can be accessed by the homologation of boronic esters with lithiated epoxysilanes.A liphatic and electron-rich aromatic boronic esters provided vinylidene boronic esters in moderate to high yields,while electron-deficient aromatic and vinyl boronic esters were found to give the corresponding vinyl silane products.Through DFT calculations,this divergence in mechanistic pathway has been rationalized by considering the stabilization of negative charge in the CÀSi and CÀBb ond breaking transition states.T his vinylidene homologation was used in ashort six-step stereoselective synthesis of the proposed structure of machillene,h owever,s ynthetic and reported data were found to be inconsistent.

Orchestrating a β-Hydride Elimination Pathway in Palladium(II)-Catalyzed Arylation/Alkenylation of Cyclopropanols Using Organoboron Reagents

“…As a part of expanding the scope of boronic esters, the alkenyl variants that would be electronically different from aromatic boronic acid/ester reagents were next explored for coupling competency. While the cross-coupling of alkenyl triflates/halides with cyclopropanols has been reported, alkenylboronic esters have rarely been explored in this specific context. ,, Alkenylboronic esters have received significant attention among organic chemists in recent years because of their ready availability associated with facile synthetic methods. , These esters, which have the ability to undergo a variety of synthetic transformations to form C–C, C–O, and C–N bonds, provide ready synthetic access to tri- and tetra-substituted alkenes. We were encouraged by the initial successful attempt of alkenylation of cyclopropanol 1a upon coupling with the tri-substituted olefin-bearing cyclohexenyl boronic ester 8a , which produced the dienone 9a in 81% yield.…”

Transition Metal Catalyst Free Synthesis of Olefins from Organoboron Derivatives**

“…However, in order to achieve this lofty goal, some challenges still have to be overcome (Scheme 18B). Blakemore used benzylic, disubstituted carbenoids derived from carbamates (47), which are comparably easy to generate. However, Aggarwal has shown that non-benzylic disubstituted carbenoids are available from triisopropylbenzoates of type 52 (R 1 /R 2 = Alkyl, DG = TIB).…”

“…In the same year Aggarwal, Grayson and coworkers [47] studied the reaction of lithiated TMS-oxirane (41) with boronic esters (Scheme 14). Regioselective lithiation of 41 occures in αposition to the silyl group.…”

Stereodivergent Access to Trisubstituted Alkenylboronate Esters through Alkene Isomerization

“…In this context, our group has recently demonstrated that ω-ene alkylboronate esters can undergo long-range isomerization in the presence of a Ru–H catalyst to result in stereodefined alkenyl­boronate esters (Scheme e) . In line with our interest in the utilization of alkene isomerization in stereoselective synthesis, − we set out to explore the alkene isomerization of readily available 1,1-disubstituted alkenyl­boronates − into either ( E )- or ( Z )-trisubstituted alkenyl­boronate esters (Scheme g). Overall, this strategy would offer selective access to both stereoisomers of the target alkenyl­boronate esters from a single starting material.…”