Tandem Reactions Involving 1-Silyl-3-Boryl-2-Alkenes. New Access to (Z)-1-Fluoro-1-Alkenes, Allyl Fluorides, and Diversely α-Substituted Allylboronates

Abstract

The reactions of 1-silyl-3-boryl-2-alkenes with various electrophilic reagents (Selectfluor, N-halosuccinimides, benzhydryl, and propargylic alcohols in the presence of a Lewis acid, N-alkoxycarbonyliminium ion) have been investigated as new routes to α-substituted allylboronates. Further functional transformations, including allylboration, Suzuki coupling, protodeboronation, and cycloisomerization, have been carried out to illustrate the synthetic potential of these γ-borylallylsilanes.

Synthesis of Monofluoroalkenes: A Leap Forward

“…Starting from the (E)-3-borylallylsilane, a series of fluorinated homoallylic alcohols were prepared through a tandem reaction in moderate to good yields with high Z selectivity (Scheme 37). 47 This selectivity results from the allylboration reaction occurring via a Zimmerman-Traxler chairlike transition state where the fluorine substituent lies in the axial position. In one example where the transformation was carried out with (Z)-3-borylallylsilane isomer of the starting material, a similar yield as well as Z/E selectivity were observed in the product.…”

“…Carboni and co-workers reported the synthesis of a terminal trisubstituted monofluoroalkene in 2 steps (see Scheme 37, Section 2.2.3.1 and Scheme 160, Section 3.6.1). 47 A fluorodesilylation/allylboration sequence was achieved on a 1-silyl-3-borylalk-2-ene (see Scheme 160, Section 3.6.1), followed by a borono-Mannich condensation in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP) and esterification to afford the corresponding amino ester in moderate yield (Scheme 120).…”

SelectFluor and Its Analogs Fluorination for Preparing Alkenyl Fluorides

“…In addition, Z-fluorinated alkenols were prepared with excellent stereocontrol starting from g-boryl allylsilanes (Scheme 4c) [11]. This threecomponent reaction started with the fluorination of the allylsilane moiety to form the corresponding allylboronate, which reacted in situ with an aldehyde to provide the final fluorinated homoallylic alcohols.…”

Recent Developments in the Chiral Brønsted Acid‐catalyzed Allylboration Reaction with Polyfunctionalized Substrates

“…As expected, complete E/Z selectivity was achieved, although with a minor loss of optical purity (Scheme ). As far as we know, this is the first available methodology for the asymmetric synthesis of enantioenriched fluorinated allylic alcohols of this kind 37…”