The molybdenum-catalyzed asymmetric ring-closing metathesis of the various Cs -symmetric (π-arene)chromium substrates provides the corresponding bridged planar-chiral (π-arene)chromium complexes in excellent yields with up to >99 % ee. With a bulky and unsymmetrical substituent, such as N-indolyl or 1-naphthyl, at the 2-positions of the η(6) -1,3-diisopropenylbenzene ligands, both biaryl-based axial chirality and π-arene-based planar chirality are simultaneously induced in the products. The axial chirality is retained even after the removal of the dicarbonylchromium fragment, and the chiral biaryl/heterobiaryl compounds are obtained with complete retention of the enantiopurity.

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A new synthetic approach to novel axially chiral benzocarbazole derivatives based on the highly enantioselective intramolecular hydroarylation (94-96% ee) of linked alkyne-indole systems by using the prevalent chiral base catalyst, cinchonidine or cinchonine, under unprecedented transition-metal-free conditions is described. The process is considered to involve chiral base catalysis for enantioselective transformation of the alkyne part to a reaction intermediate with an axially chiral vinylidene o-quinone methide (VQM) functionality, which subsequently effects stereospecific cyclization with the tethered indole moiety.

Planar-chiral (h 6 -arene)chromium complexes are useful chiral scaffolds in asymmetric synthesis, and have found widespread application as chiral ligands for asymmetric catalysis, or as chiral building blocks for natural product syntheses. [1] Typical methods for the preparation of enantiomerically enriched planar-chiral (arene)chromium species are based either on the optical resolution of racemates [2] or on stereoselective transformations, which include diastereoselective complexation, [3] diastereo-or enantioselective ortho-lithiation by utilizing a chiral directing group or a chiral base, [4] and diastereo-or enantioselective nucleophilic addition/hydride abstraction. [5] Whereas these methods require a stoichiometric amount of chiral reagents or auxiliaries, asymmetric catalysis is an attractive and effective alternative for preparing optically active (h 6 -arene)chromium complexes. Since the first report on such a catalytic process by Uemura et al. in 1993, [6] only a handful of examples of the desymmetrization of prochiral (arene)chromium substrates have been reported by Gotov and Schmalz, [7] Kündig and co-workers, [8,9] and Uemura and co-workers [10,11] .Recently, we reported the preparation of phosphinechelate (h 6 -arene)chromium complexes by Ru-catalyzed ringclosing metathesis. [12] We also demonstrated that Mo-catalyzed asymmetric ring-closing metathesis (ARCM) was highly effective for the asymmetric synthesis of the various planarchiral ferrocenes. [13] Thus, we are interested in controlling the planar chirality of (h 6 -arene)chromium complexes by ARCM. [14,15] Indeed, the kinetic resolution of racemic (h 6 -1,2-disubstituted benzene)chromium complexes proceeds efficiently in the presence of a chiral Mo-alkylidene species to give the planar-chiral chromium complexes with excellent enantiomeric purity. Furthermore, a highly enantiomerically enriched (h 6 -bromoarene)chromium complex that was prepared by using the present method is an excellent precursor to various planar-chiral (arene)chromium derivatives. A (phos-phinoarene)chromium species was derived from the (h 6bromoarene)chromium complex and applied as a chiral ligand in Rh-catalyzed asymmetric reactions, which achieved excellent enantioselectivity of up to 99.5 %.The (h 6 -arene)chromium complexes (rac-1) that were used in this study contain an h 6 -(2-substituted alkenylbenzene) ligand, which constructs the planar-chiral environment upon coordination to the chromium atom, and an alkenylphosphine ligand. The chiral catalysts were screened by using racemic [(h 6 -2-methylstyrene)Cr(CO) 2 (methallyldiphenylphosphine)] (1 a) as a prototypical substrate. The asymmetric reactions were carried out in benzene at 40 8C in the presence of an appropriate chiral Mo catalyst (10 mol %), which was generated in situ from the Mo precursor [(pyrrolyl) 2 Mo( = CHCMe 2 Ph)(=NC 6 H 3 -2,6-iPr 2 )] and an axially chiral biphenol derivative (Table 1). [16] Under these conditions, the Mo catalyst that was generated with (R)-L1 [17a] gave the ARCM product 2 a...

Planar chirality: Ring-closing metathesis of 1,2-diallylmetallocenes afforded the corresponding 4,7-dihydroindenyl species in high yields. The metallocenes are planar chiral with two different allylic substituents, and kinetic resolution of the racemic 1,2-diallylmetallocene derivatives was realized by molybdenum-catalyzed asymmetric ring-closing metathesis with excellent enantioselectivity (see scheme).