Mechanistic Investigations of the Rhodium Catalyzed Propargylic CH Activation

Previously we reported the redox-neutral
atom economic rhodium
catalyzed coupling of terminal alkynes with carboxylic acids using
the DPEphos ligand. We herein present a thorough mechanistic investigation
applying various spectroscopic and spectrometric methods (NMR, in situ-IR, ESI-MS) in combination with DFT calculations.
Our findings show that in contrast to the originally proposed mechanism,
the catalytic cycle involves an intramolecular protonation and not
an oxidative insertion of rhodium in the OH bond of the carboxylic
acid. A σ-allyl complex was identified as the resting state
of the catalytic transformation and characterized by X-ray crystallographic
analysis. By means of ESI-MS investigations we were able to detect
a reactive intermediate of the catalytic cycle.