Developmentof Enantiospecific Coupling of Secondaryand Tertiary Boronic Esters with Aromatic Compounds

摘要

The stereospecific cross-coupling of secondary boronic esters with sp² electrophiles (Suzuki–Miyaura reaction) is a long-standing problem in synthesis, but progress has been achieved in specific cases using palladium catalysis. However, related couplings with tertiary boronic esters are not currently achievable. To address this general problem, we have focused on an alternative method exploiting the reactivity of a boronate complex formed between an aryl lithium and a boronic ester. We reasoned that subsequent addition of an oxidant or an electrophile would remove an electron from the aromatic ring or react in a Friedel–Crafts-type manner, respectively, generating a cationic species, which would trigger 1,2-migration of the boron substituent, creating the new C–C bond. Elimination (preceded by further oxidation in the former case) would result in rearomatization giving the coupled product stereospecifically. Initial work was examined with 2-furyllithium. Although the oxidants tested were unsuccessful, electrophiles, particularly NBS, enabled the coupling reaction tooccur in good yield with a broad range of secondary and tertiary boronicesters, bearing different steric demands and functional groups (esters,azides, nitriles, alcohols, and ethers). The reaction also workedwell with other electron-rich heteroaromatics and 6-membered ringaromatics provided they had donor groups in the meta position. Conditionswere also found under which the B(pin)- moiety could be retained inthe product, ortho to the boron substituent. This protocol, whichcreated a new C(sp²)–C(sp³) and an adjacentC–B bond, was again applicable to a range of secondary andtertiary boronic esters. In all cases, the coupling reaction occurredwith complete stereospecificity. Computational studies verified thecompeting processes involved and were in close agreement with theexperimental observations.