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Updated: Dec 6, 2025

Facile Preparation of 2Z,4E-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
Published on: June 21, 2017
Chromium-Catalyzed Selective Dimerization/Hydroboration of Allenes to Access Boryl-Functionalized Skipped
1Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore, 117543, Singapore.
Abstract:
A chromium-catalyzed dimerization/hydroboration of allenes is developed to access synthetically versatile boryl-functionalized skipped dienes with a catalyst generated in situ from CrCl2 and a pyridine-2,6-diimine ligand mes PDI. A variety of allenes reacted with pinacolborane (HBpin) to afford the corresponding boryl-functionalized (E,Z)-1,4-dienes in high yields and with excellent selectivity. Electron paramagnetic resonance (EPR) spectroscopic studies suggest that this chromium-catalyzed reaction probably proceeds through a chromium(I) hydride intermediate.
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One of the convenient methods for the preparation of aldehydes and ketones is via hydration of alkynes. Hydroboration-oxidation of alkynes is an indirect hydration reaction in which an alkyne is treated with borane followed by oxidation with alkaline peroxide to form an enol that rapidly converts into an aldehyde or a ketone. Terminal alkynes form aldehydes, whereas internal alkynes give ketones as the final product.
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Hydroboration proceeds in a concerted fashion with the attack of borane on the π bond, giving a cyclic four-centered transition state. The –BH2 group is bonded to the less substituted carbon and –H to the more substituted carbon. The concerted nature requires the simultaneous addition of –H and –BH2 across the same face of the alkene giving syn stereochemistry.
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Like alkenes, alkynes can be reduced to alkanes in the presence of transition metal catalysts such as Pt, Pd, or Ni. The reaction involves two sequential syn additions of hydrogen via a cis-alkene intermediate.
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