Synthesis of <i>tert</i>-Leucine-Derived Cobalt Oxazoline Palladacycles. Reversal of Palladation Diastereoselectivity and Application to the Asymmetric Rearrangement of <i>N</i>-Aryl Trifluoroacetimidates

Starting from (η5-carboxycyclopentadienyl)(η4-tetraphenylcyclobutadiene)cobalt, (η5-(S)-2-(4-tert-butyl)oxazolinylcyclopentadienyl)(η4-tetraphenylcyclobutadiene)cobalt and (η5-(S)2-(4-methyl)oxazolinylcyclopentadienyl)(η4-tetraphenylcyclobutadiene)cobalt were synthesized in 70 and 79% yield, respectively. On heating with palladium acetate, only the former oxazoline resulted in the formation of a palladacycle (79% yield) that was determined to have an (S)-(pS)-configuration. Following ligand exchange (acetate to chloride to trifluoroacetate), the tert-butyl palladacycle was applied as a catalyst for the rearrangement of N-(4-methoxyphenyl)trifluoroacetimidates.