Rh‐Catalyzed Coupling of Aldehydes with Allylboronates Enables Facile Access to Ketones

Abstract

We present herein a novel strategy for the preparation of ketones from aldehydes and allylic boronic esters. This reaction involves the allylation of aldehydes with allylic boronic esters and the Rh‐catalyzed chain‐walking of homoallylic alcohols. The key to this successful development is the protodeboronation of alkenyl borylether intermediate via a tetravalent borate anion species in the presence of KHF2 and MeOH. This approach features mild reaction conditions, broad substrate scope, and excellent functional group tolerance. Mechanistic studies also supported that the tandem allylation and chain‐walking process were involved.

Aldehyde Olefination with Arylboroxines Enabled by Binary Rhodium Catalysis

“…In terms of the reaction mechanism, an alkoxy–metal species generated via 1,2-addition was proposed as a pivotal intermediate in the catalytic cycle . In general, this intermediate was inclined to undergo protonation to produce the corresponding alcohols (Scheme Aa) . Alternatively, the alkoxy–metal intermediate can experience β-hydride elimination to yield ketone products in the presence of a suitable oxidant (Scheme Ab) .…”

“…On the basis of the above preliminary mechanistic investigations and prior studies, , a plausible reaction mechanism is proposed in Scheme . Initially, the alcohol–boron adduct A is generated through the Rh­(I)-catalyzed 1,2-addition of arylboroxine to aliphatic aldehyde, which is a fast process and finishes within about 2 min.…”

Catalytic Contra-Thermodynamic Isomerization–Asymmetric Hydroboration of Alkenyl Alcohols and Amines

“…In view of our long-standing interest in organoboron chemistry and our successes in the deconjugative isomerization of silyl enol ethers and the long-distance isomerization of alkenyl alcohols as well as the cobalt-catalyzed asymmetric hydroboration, we wondered whether the enantioselective contra-thermodynamic isomerization of (homo)­allylic alcohols or amines (i.e., isomerizing astern the hydroxyl or amine group) would be achieved via controlling the regioselectivity of M–H insertion (intermediate I vs intermediate II ) and applying an irreversible terminal borylation, which is extensively applied in catalytic isomerization/hydroboration of alkenes including the lone example of asymmetric version developed by Lu et al (Figure b). If realized, this approach will open a new avenue for the development of asymmetric contra-thermodynamic isomerization and offer an unprecedented reaction mode of alkenyl alcohols/amines.…”