An Investigation into the Allylic Imidate Rearrangement of Trichloroacetimidates Catalysed by Cobalt Oxazoline Palladacycles

Abstract

Dimeric palladacycles, di-μ-X-bis[{η⁵-(S)-(_pR)-2-[2′-(4′-methylethyl)oxazolinyl]cyclopentadienyl,1-C,3′-N}(η⁴-tetraphenylcyclobutadiene)cobalt]dipalladium (COP-X), containing bridging groups X=OAc, Cl, Br, I, O₂CCF₃, p-O₂CC₆H₄F, were synthesised and compared as catalysts for the asymmetric allylic imidate rearrangement of (E)-Cl₃CC(NH)OCH₂CHCHR with R=nPr. The enantiomeric excess of the product (S)-Cl₃CC(O)NHCHRCHCH₂ was essentially invariant of X (93–96 %) and the yield increased in the sequence I<p-O₂CC₆H₄F<OAc<O₂CCF₃≈Br≈Cl. With X=Cl (COP-Cl), the catalyst loading was reduced to 0.25 mol % (CH₃CN/70 °C/48 h) and these conditions applied to various trichloroacetimidates (R=nPr, Me, CH₂Ph, CH₂CHCH₂, CH₂OTBDMS) to give the corresponding (S)-trichloroacetamides (68–88 % yield, 84–94 % ee; ee=enantiomeric excess). Addition of COP-Cl to triphenylphosphinobenzoyl NovaGel AM resin gave a recyclable catalyst in which the ee was maintained over three cycles (89–94 %). Catalysis with COP-OAc displayed a small positive non-linear effect. The factors responsible for the activity of COP-X are discussed.