Chirality Control in Planar Chiral Cobalt Oxazoline Palladacycles

Palladation of (η5-(S)-2-(4-R-substituted)­oxazolinylcyclopentadienyl)­(η4-tetraphenylcyclobutadiene)­cobalt­(I) with Pd­(OAc)2 (R = Me, CH2Cy, CH2i-Pr, CH2Ph) resulted exclusively in formation of the (S,Sp)-palladacycle with R = CH2Cy (acetic acid solvent) and up to 13:1 selectivity in favor of the (S,Sp)-palladacycle with R = CH2i-Pr (CH2Cl2 solvent). In the latter case diastereomeric purity was achieved by recrystallization, and the absolute configuration of the new element of planar chirality was established by X-ray crystallography. These outcomes contrast with the opposite sense of diastereoselectivity observed for the corresponding R = i-Pr-substituted oxazoline, which results in the palladacycle (S,Rp)-COP-OAc (acetic acid solvent), these palladation reactions being under either kinetic (S,Sp) or thermodynamic (S,Rp) control. Following conversion to the chloride-bridged (S,Sp)-palladacycle dimer (R = CH2Cy), this was employed as a catalyst for the rearrangement of allylic trichloroacetimidates (0.25 mol % catalyst loading, up to 80% ee).