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

Starting from (η 5 -carboxycyclopentadienyl)(η 4 -tetraphenylcyclobutadiene)cobalt, (η 5 -( S )-2-(4- tert -butyl)oxazolinylcyclopentadienyl)(η 4 -tetraphenylcyclobutadiene)cobalt and (η 5 -( S )2-(4-methyl)oxazolinylcyclopentadienyl)(η 4 -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 )-( p S )-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.