Ferrocenyloxazoline-Derived Planar Chiral Palladacycles: C–H Activation, Transmetalation, and Reversal of Diastereoselectivity

Reinvestigation of the palladation of (S)-2-ferrocenyl-4-(methylethyl)­oxazoline with Pd­(OAc)2 in CH2Cl2 was found to proceed with a dr of 3.6:1 in favor of the resulting S,S p palladacycle. A similar 4:1 dr was obtained using Na2PdCl4 in MeOH. As an alternative approach, highly diastereoselective lithiation (dr >100:1) and transmetalation were investigated. Addition of PdX2(COD) (X = Cl, Br) to (S,R p)-2-lithio-1-(2′-(4′-methylethyl)­oxazolinyl)­ferrocene resulted in double halide substitution and formation of cis-(S,S,S p,S p)-bis­[2-(2′-(4′-methylethyl)­oxazolinyl)­ferrocene-1-C,3′-N]­palladium­(II) (42% from X = Cl, 50% from X = Br). Selective monoprotodepalladation with HCl gave an S,S p palladacycle containing a removable ferrocenyloxazoline ligand. Addition of PdCl2(MeCN)2 to mercuracycles in acetonitrile, themselves generated from Li–Hg transmetalation, followed by a brine wash gave (S,S p)-di-μ-chlorobis­[2-(2′-(4′-methylethyl)­oxazolinyl)­ferrocene-1-C,3′-N]­dipalladium­(II) as a single diastereoisomer in high yield. The alternative S,R p diastereoisomer was obtained in the same way by use of a deuterium blocking group to reverse lithiation diastereoselectivity.