Chemo- and Stereoselective Crotylation of Aldehydes and Cyclic Aldimines with Allylic gem-Diboronate Ester

Abstract

We report a highly chemo- and stereoselective crotylation of aldehydes and cyclic aldimines with allylic-gem-diboronate ester as a new type of organoboron reagent. The allylic-gem-diboronate ester undergoes the crotylation with aldehydes and cyclic aldimines in excellent stereoselectivity, forming anti-5,6-disubstituted oxaborinin-2-ols or (E)-δ-boryl-anti-homoallylic amines in high efficiency. The reaction shows a wide range of substrate scope and excellent functional group tolerance. The synthetic applications of the obtained products, including stereospecific C-C, C-O, and C-Cl bond formation, are also demonstrated.

Recent developments in the asymmetric synthesis and functionalization of symmetrical and unsymmetricalgem-diborylalkanes

“…The synthesis and reactivity of allylic gem-bis[( pinacolato)boryl]alkanes were initially reported by the Cho group. 39 They reported a successful racemic crotylation of aldehydes and cyclic aldimines using allylic gem-bis[( pinacolato)boryl] alkanes. In 2020, the Meek group reported the addition of a boron-stabilized chiral allylic copper species to a variety of aryl aldimines, 40 generating enantioenriched homoallylic amines 60 (Scheme 10a).…”

“…In the same year, the Cho group reported another protocol for the enantioselective allyl transfer of aldehydes using the allylic gem-diboronate ester (Scheme 18a). 57 The reaction engaged (R)-TRIP catalysis proceeds through the intermediate 99. Chen and his group later extended the enantioselective functionalization of α,α-disubstituted crotylboronate reagents 100 with an aldehyde in the presence of chiral phosphoric acid (Scheme 18b).…”

Reactivity Trends with Borylalkyl Copper(I) Species

“…Cho and co‐workers reported that allylic‐ gem ‐diboronates undergo the crotylation with cyclic aldimines forming ( E )‐δ‐boryl‐ anti ‐homoallylic amines in high efficiency, despite the fact that copper(I) complexes were not catalyzing the reaction [19] . However, Meek and co‐workers found that a (phosphoramidite)‐Cu complex facilitates the catalytic reaction between allyldiboronates and aldimines towards the enantio‐ and diastereoselective formation of homoallylic amines.…”

Diastereo- and Enantioselective Synthesis of Homoallylic Amines Bearing Quaternary Carbon Centers

“…Subsequently, Cho et al reported the diastereoselective addition of isolated allyldiboron ester 5 to a variety of cyclic sulfonyl imines, although the report is limited to the E -crotyl reagent (eq ). …”