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

Abstract

Dimeric palladacycles, di-mu-X-bis[{eta(5)-(S)-((p)R)-2-[2'-(4'-methylethyl)oxazolinyl]cyclopentadienyl,1-C,3'-N}(eta(4)-tetraphenylcyclobutadiene)cobalt]dipalladium (COP-X), containing bridging groups X=OAc, Cl, Br, I, O(2)CCF(3), p-O(2)CC(6)H(4)F, were synthesised and compared as catalysts for the asymmetric allylic imidate rearrangement of (E)-Cl(3)CC(=NH)OCH(2)CH=CHR with R=nPr. The enantiomeric excess of the product (S)-Cl(3)CC(=O)NHCHRCH=CH(2) was essentially invariant of X (93-96%) and the yield increased in the sequence I<p-O(2)CC(6)H(4)F<OAc<O(2)CCF(3) approximately Br approximately Cl. With X=Cl (COP-Cl), the catalyst loading was reduced to 0.25 mol% (CH(3)CN/70 degrees C/48 h) and these conditions applied to various trichloroacetimidates (R=nPr, Me, CH(2)Ph, CH(2)CH=CH(2), CH(2)OTBDMS) 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.