Planar chiral arenes nucleophiles

An asymmetric variant makes use of a planar chiral complex. This arene to substituted cyclohexenone conversion has been elegantly used in a synthesis of (-F)-ptilocaulin. (Scheme 5 see also Chap. 8) [22]. The sequence is unusual in that after nucleophilic addition, acid free ClSiMe3 is added. When a strong acid... 

Finally, methods to control the stereochemistry of the additions have been developed by a number of strategies. For example, chiral auxiliaries have been placed on the arene system,chiral nucleophiles, such as Evans enolates - or amino acids, have been added, or chiral ligands have been added to the organoUthium reagent. Planar chiral chromium compounds have also been resolved and then used as a single enantiomer. - No general solution has been developed, but acceptable diastereoselectivities have been obtained in many cases. 

In an Sj Ar process, a nucleophile reacts stereoselectively with an electron-deficient arene activated by one or more EWGs located ortho and/or para to a leaving group (L), creating a new C—C, C—O, or C—N bond (Fig. 8.1). The reaction provides access to a variety of aromatic derivatives with axial, central, and planar chirality. 

Planar chiral orf/to-substituted (arene)Cr(CO)3-stabilized a-propargylic cations (67) may be generated from (66) in 5n1 fashion mediated by Lewis acid. They are in principle ambident electrophiles, susceptible to nucleophilic attack at the propar-gyl or allenyl positions. However, they react with 5-, N-, and jr-nucleophiles to give regioselectively the propargyl derivatives (68) in good yields and excellent diastereoselectivities, overall a process of retention by double inversion. Without the orf/to-substituent nucleophilic attack at both possible positions occurs. 

The preparation of enantiopure or enriched complexes possessing planar chirality has been accomplished either by resolution of racemic mixtures or by asymmetric syntheses. Reported methods for the resolution of planar chirality include both chemical and kinetic resolution procedures, whilst reported asymmetric syntheses of enantiomerically pure or enriched benchrotrenic complexes include enantioselective ort/io-deprotonations with chiral lithium amide bases, and the transfer of side chain chirality onto the arene ring mediated by diastereoselective orf/io-nucleophilic additions and o/tfeo-metalations. 

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