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.

Synthesis and characterization of difunctionalized derivatives of the cyclobutadiene linked dimeric cobalt sandwich compound [(η5-Cp)Co(η4-C4Ph3)]2

“…Some hydrogen atoms have been omitted for clarity. Selected bond lengths ( A) and bond angles ( ): Co(1)eC(1) 2.081(3), Co(1)eC(7) 1.974(3), C(1)eC(2) 1.418(5), C(1)eC(10) 1.501(5), C(6)eC(6) 1.453(6), C(6)eC(7)1.468(4), C(7)eC(11) 1.475(4), C(8)eC(17) …”

Chemistry of the highly stable hindered cobalt sandwich compound (η5-Cp)Co(η4-C4Ph4) and its derivatives

“…Sutherland and coworkers have also used COP-Cl catalyst for the stereoselective synthesis of hydroxylated 3-aminoazepanes, polyhydroxylated and Richards provides consolidated information on the allylic imidate rearrangements catalyzed by planar chiral palladacycles [222]. A resin bound recyclable variant of the COP catalyst in which the ee was maintained over three cycles was also reported by Nomura and Richards [223].…”

Metallocyclic Ferrocenyl Ligands

“…The catalyst loading may be reduced 10-fold from 5 mol% of COP-Cl (a dimer) to 1 mol% by the use of the monomeric complex (S,R p )-124 (COP-hfacac) in acetonitrile at 50 C (entry 2) [81]. Alternatively, just 0.25 mol% of (S,R p ) 2 -90 (COP-Cl) may be used in acetonitrile at 70 C with little or no erosion of enantioselectivity (entry 3) [82]. Activation of (S,R p ) 2 -90 with 1 equiv per palladium of silver trifluoroacetate gives catalyst (S,R p ) 2 -125 [82] for the rearrangement of (allyloxy) iminodiazaphospholidines 126.…”

“…Alternatively, just 0.25 mol% of (S,R p ) 2 -90 (COP-Cl) may be used in acetonitrile at 70 C with little or no erosion of enantioselectivity (entry 3) [82]. Activation of (S,R p ) 2 -90 with 1 equiv per palladium of silver trifluoroacetate gives catalyst (S,R p ) 2 -125 [82] for the rearrangement of (allyloxy) iminodiazaphospholidines 126. Both (E)-and (Z)-isomers of 126 react (entries 4 and 5), the latter result in a higher yield and enantiomeric excess of the product phosphoramide [83].…”