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20.5.1.3.1.2.2 Method 2: Intramolecular Hydroacylation Reactions

DOI: 10.1055/sos-SD-020-00673

Yan, L.; Lin, S.; Liu, P.Science of Synthesis, (200720749.

A novel rhodium-catalyzed redox hydroacylation process, which stereoselectively transforms alk-4-ynals into cis-alk-4-enoates was discovered in 2002.[‌96‌] In this reaction, the initial insertion of a rhodium catalyst {[Rh(dppe)]2(BF4)2} into the CH bond of the aldehyde gives a cyclic intermediate, which subsequently leads to either a six-membered metallacycle 75 (path A) or a five-membered metallacycle 77 (path B) (Scheme 24). When the reaction is carried out in a polar solvent (e.g., acetone) the six-membered metallacycle 75 is generated exclusively and this leads to the production of a cyclopent-2-enone 76 via reductive elimination in a good yield.[‌97‌] In contrast, when a nonpolar solvent (e.g., dichloromethane) is employed, a five-membered metallacycle 77 is formed exclusively via a stereospecific transfer of the aldehydic hydrogen atom (as hydride) to what becomes the exocyclic methylene carbon atom of the product. The reaction of the five-membered adduct 77 with a nucleophile, such as a phenol (Ar1OH), gives a phenyl cis-alk-4-enoate 78 (Nu=OAr1) such that the original phenolic proton is transferred cleanly to the γ-position of the ester. This mechanism is consistent with an isotope labeling experiment.

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