Enantiopure Planar Chiral and Chiral-at-Metal Iridacycles Derived from Bulky Cobalt Sandwich Complexes
journal contributionposted on 12.11.2018, 15:21 by Ross A. Arthurs, Christopher C. Prior, David L. Hughes, Vasily S. Oganesyan, Christopher J. Richards
Reaction of (η5-(S)-2-(4-methylethyl)oxazolinylcyclopentadienyl)-(η4-tetraphenylcyclobutadiene)cobalt with [IrCp*Cl2]2 in acetonitrile with KPF6 and KOt-Bu resulted in S,Sp,SIr and S,Rp,RIr configured acetonitrile and Cp* coordinated cationic iridacycles (d.r. up to 4.8:1 – kinetic control), the planar chiral configuration dictating the configuration of the pseudo-tetrahedral iridium-based stereogenic center. Addition of water to the cycloiridation reaction resulted in an increase in yield (up to 78%) at the cost of diastereoselectivity. Use of the corresponding substrate containing a t-Bu rather than an i-Pr substituted oxazoline gave exclusively the S,Sp,SIr diastereoisomer, and under the same conditions, (S)-2-ferrocenyl-4-(1,1-dimethylethyl)oxazoline cycloiridated to give only the S,Sp,SIr diastereoisomer. Substitution reactions of the title complexes at iridium proceeded with retention of configuration, a computational study revealing the proposed coordinatively unsaturated intermediate of a dissociative mechanism to display a relatively weak Co–Ir interaction, and a pronounced steric effect as the basis of stereocontrol.