Science of Synthesis

Wang, P.-S.; Sayed, M.; Gong, L.-Z., Science of Synthesis Knowledge Updates, (2023) 1, 217.

The direct borylation of C(sp3)—H bonds has emerged as a useful synthetic method for the preparation of organoboronates. In 2014, Szabó and co-workers described a one-pot sequential reaction involving palladium-catalyzed allylic C—H borylation and allylation of aldehydes, enabling the synthesis of stereodefined homoallylic alcohols (e.g., ‌149‌) from exocyclic alkenes and aldehydes (‌Scheme 32‌).[‌140‌] Later, a similar reaction of allylarenes and aldehydes was accessed by using a Brønsted acid as co-catalyst and N-fluorobenzenesulfonimide (NFSI) as oxidant, giving, for example, (1R*,2S*)-1,2-diphenylbut-3-en-1-ol (‌150‌) in 67% yield (‌Scheme 32‌).[‌141‌] In particular, an enantioselective version of this reaction is achieved by using a chiral Brønsted acid as co-catalyst.[‌141‌] In addition, the double chiral induction allowed the reaction to give even higher levels of stereoselectivity.[‌142‌] Interestingly, Szabó, Marder, and co-workers found that the palladium pincer complex ‌148‌ promotes allylic C—H borylations using 1-(chloromethyl)-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (Selectfluor) as oxidant (‌Scheme 32‌).[‌143‌] Mechanistic studies indicated that the C—H bond cleavage proceeds via a palladium(IV) pincer intermediate. In addition to allylic C—H borylation, palladium catalysis is also applicable to allylic C—H silylation using hexamethyldisilane as nucleophile and molecular oxygen as the sole oxidant (‌Scheme 32‌).[‌144‌]

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