Diastereoselective Lithiation of (h6- Arene)dicarbonyltriphenylphosphane Chromium(0) Oxazoline Complexes–Direct Preparation of Enantiopure Complexes Having Planar Chiral Fragments of Either Configuration**

Planar chiral h6-arene chromium(0) complexes are finding increasing application in the synthesis of enantioenriched compounds by serving as enantiopure starting materials or ligands for catalytic asymmetric processes.[1,2] Whereas early methods of generating enantiopure 1,2-disubstituted h6-arene chromium(0) complexes relied on resolution of racemates,[3] many methods have been developed recently for asymmetric synthesis of these molecules.[1] One powerful strategy employs an enantiopure chiral lithium reagent (or an achiral lithium reagent plus a chiral additive) to elaborate a prochiral complex by either enantioselective ring lithiation followed by trapping with an electrophile,[4] or enantioselective nucleophilic addition followed by abstraction of hydride.[5] Another strategy involves diastereoselective lithiation of a monosubstituted h6-arene chromium(0) complex containing a chiral, nonracemic substituent that is capable of directing ortho lithiation.[6] We recently introduced chiral ferrocenyl oxazoline palladacycles as catalysts for the asymmetric addition of external (that is, not bound to a metal) nucleophiles to prochiral alkenes.[7] To further explore this general catalyst architecture, we became interested in the synthesis of related h6-arene chromium(0) complexes. The precursors of the ferrocenyl catalysts were prepared by diastereoselective ortho lithiation of enantiopure chiral ferrocenyl oxazolines with alkyl lithium reagents.[8] Unfortunately, this convenient approach cannot be used to prepare (h6-arene)tricarbonylchromium(0) complexes, as lithium reagents are known to add to such complexes containing strong acceptor substituents such as imines or oxazolines.[1e,5, 9] Herein we report that less electron deficient (h6-arene)dicarbonyl(triphenylphosphane) chromium(0) oxazoline complexes undergo ortho lithiation, rather than addition, upon exposure to either secor n-butyllithium, and that, depending on the presence or absence of N,N,N’,N’tetramethylethylenediamine (TMEDA), either diastereomer COMMUNICATIONS