Non-racemic, planar chiral 1 ,Zdisubstituted [Cr(tf-arene)(CO),] complexes were obtained via external chiral ligand-controlled nucleophilic addition of alkyl-, vinyl-, and aryllithium reagents to monosubstituted complexes followed by an endo-hydride abstraction with trityl cation. The reactions with [Cr(CO),(rf-phenyloxazoline)], [Cr(C0),(q6-phenylmethaneimine)J, and [Cr(CO),(q6-phenylmethanehydrazone)] took place with complete orrhoselectivity and a high degree of enantioselectivity (up to 98% ee).Unsymmetrically, 1,2-disubstituted [Cr(rf-arene)(CO),] complexes are chiral molecules. They have emerged as important intermediates in asymmetric organic synthesis, the presence of the metal fragment facilitating regio-and diastereoselective reactions on or adjacent to the aromatic ring (for reviews, see [l]). Recent attention has focused on the preparation of complexes in enantiomerically enriched form. Available methods include chromatography on chiral supports [2], kinetic resolution of racemic complexes [3], diastereoselective complexation of arenes bearing chiral auxiliaries [4], and diastereoselective reactions on prochiral complexes. The most widely used reaction in the last category is the auxiliary directed ortho-lithiation followed by trapping with electrophiles [5]. A complementary approach, developed in this laboratory, and which is pertinent to this paper, is the diastereoselective nucleophilic additionthydride abstraction with a SAMP-hydrazone complex [6].Enantioselective approaches are scarce. Modest enantioselectivities (up to 69 YO ee) were reported in a Suzuki coupling reaction with a chiral Pd and a [Cr(CO),(l,2-dichlorobenzene)] complex [7] while higher selectivities (up to 90% ee) were realized in enantioselective deprotonations of prochiral complexes [8]. In related studies high enantioselectivities were also achieved in benzylic deprotonations [9].In this paper, we detail our first results of a new enantioselective route to non-racemic [Cr(q6-arene)(CO),] complexes. The first step of the one-pot procedure is an external chiral-ligand-controlled nucleophilic addition to mono-substituted arene complexes, a step that we have used previously in the transformation of arenes into enantiomerically enriched substituted cyclohexadienes [lo]. The second step is an endo-hydride abstraction from the intermediate anionic cyclohexadienyl complex (Scheme I).The reaction sequence was applied to the three [Cr($-arene)(CO),] complexes 1-3. Alkyl-, vinyl-, and phenyllithium reagents were used in the nucleophilic addition. Based

A new room-temperature chromium tricarbonyl-mediated cycloaromatization of enediynes is reported. The reaction occurs with both cyclic and acyclic enediynes in the presence of [Cr(CO)3(η6-naphthalene)] and both a coordinating solvent and a hydrogen atom source, providing chromium–arene complexes in reasonable yield and good diastereocontrol. The mechanism of the reaction has been probed through DFT computational and spectroscopic methods. These studies suggest that direct C1–C6 bond formation from an η6-enediyne complex is the lowest-energy path, forming a metal-bound p-benzyne biradical. NMR spectroscopy suggests that enediyne alkene coordination occurs in preference to alkyne coordination, forming a THF-stabilized olefin intermediate; subsequent alkyne coordination leads to cyclization. While biradical quenching occurs rapidly and primarily via the singlet biradical, the triplet state biradical is detectable by EPR spectroscopy, suggesting intersystem crossing to a triplet ground state

The surprising answer to the question “Are the structural characteristics of the product of the nucleophilic addition to complex 1 those of a cyclohexadienyl complex (2 a) or those of a diene aza‐enolate complex (2 b)?” is: both. This is indicated by the X‐ray structure determination of the complex with R=naphthyl. An unprecedented endo‐hydride abstraction concludes the sequence that provides a new route to 1,2‐substituted planar chiral [(η6‐arene)Cr(CO)3] compounds.

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1998 organo-chromium compounds organo-chromium compounds S 5500 -1821,2-Disubstituted [(η 6 -Arene)Cr(CO) 3 ] Complexes by Sequential Nucleophilic Addition/endo-Hydride Abstraction.-The title compounds (III), (V), and (VII) are synthesized by a new regioselective addition of organo-Li compounds at the o-position of substituted (π 6 -arene)tricarbonyl Cr complexes via anionic (π 5 -cyclohexadienyl)Cr(CO) 3 intermediates.-(FRETZEN, A.; RIPA, A.; LIU, R.; BERNARDINELLI, G.; KUENDIG, E. P.; Chem.