fl-Boron Effect Enables Regioselective and Stereospecific Electrophilic Addition to Alkenes

Electrophilic addition to alkenes is a textbook-taught reaction, yet it is not always possible to control the regioselectivity of addition to unsymmetrical 1,2-disubstituted substrates. We report the observation and applications of the beta-boron effect that accounts for high regioselectivity in electrophilic addition reactions to allylic MIDA (N-methyliminodiacetic acid) boronates. While the well-established beta-silicon effect bears partial resemblance to the observed reactivity, the silyl group is typically lost during functionalization. In contrast, the boryl moiety is retained in the product when B(MIDA) is used as the nucleophilic stabilizer. Mechanistic studies elucidate the origin of this effect and demonstrate how sigma(C-B) hyper-conjugation helps stabilize the incipient carbocation. This trans-formation represents a rare example of the stereospecific hydro-halogenation of secondary allyl MIDA-boronates that proceeds in a syn-fashion.

Automated summary

We report the observation and applications of the beta-boron effect that provides high regioselectivity in electrophilic addition reactions to allylic MIDA boronates. The boryl moiety is retained in the product, unlike the silyl group in the beta-silicon effect. Mechanistic studies reveal the role of sigma(C-B) hyper-conjugation in stabilizing the carbocation.