The cooperative action
of the acetate ligand, the 2-pyridyl sulfonyl
(SO
2
Py) directing group on the alkyne substrate, and the
palladium catalyst has been shown to be crucial for controlling reactivity,
regioselectivity, and stereoselectivity in the acetoxylation of unsymmetrical
internal alkynes under mild reaction conditions. The corresponding
alkenyl acetates were obtained in good yields with complete levels
of β-regioselectivity and
anti
-acetoxypalladation
stereocontrol. Experimental and computational analyses provide insight
into the reasons behind this delicate interplay between the ligand,
directing group, and the metal in the reaction mechanism. In fact,
these studies unveil the multiple important roles of the acetate ligand
in the coordination sphere at the Pd center: (i) it brings the acetic
acid reagent into close proximity to the metal to allow the simultaneous
activation of the alkyne and the acetic acid, (ii) it serves as an
inner-sphere base while enhancing the nucleophilicity of the acid,
and (iii) it acts as an intramolecular acid to facilitate protodemetalation
and regeneration of the catalyst. Further insight into the origin
of the observed regiocontrol is provided by the mapping of potential
energy profiles and distortion–interaction analysis.
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