Kinetic Resolution of Planar‐Chiral (η⁶‐Arene)Chromium Complexes by Molybdenum‐Catalyzed Asymmetric Ring‐Closing Metathesis

Abstract

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...

Simultaneous Induction of Axial and Planar Chirality in Arene–Chromium Complexes by Molybdenum‐Catalyzed Enantioselective Ring‐Closing Metathesis

“…The reactions were carried out in benzene at 40 °C with an appropriate chiral molybdenum catalyst (10 mol %) that was generated in situ from [(pyrrolyl) 2 Mo(CHCMe 2 Ph)(NC 6 H 3 ‐2,6‐ i Pr 2 )] and an axially chiral biphenol derivative 9. Under these conditions, the Mo/( R )‐ L1 species,10a which was the best catalyst for the kinetic resolution of the planar‐chiral (π‐arene)chromium complexes,7a showed good enantioselectivity (97 % ee ) but insufficient activity (Table 1, entry 1). The Mo/( R )‐ L2 species10b was not effective at all (entry 2).…”

“…Plausible stereochemical pathways of the ARCM of 1 are shown in Scheme . Based on our previous studies,5a, c, 7a it is postulated that the initial metathesis between the ( R )‐Mo‐alkylidene catalyst and 1 takes place at the P ‐allyl group, which is the sterically least hindered olefin in 1 , to form intermediate A . The more electron‐donating PR 2 group in 1 (and in A as well) increases electron density at the two isopropenyl groups through the chromium atom and the π‐arene moiety, which eventually facilitates the second metathesis in A .…”

Theoretical investigations of Rh‐catalyzed asymmetric 1,4‐addition to enones using planar‐chiral phosphine‐olefin ligands

“…Among various phosphine‐olefin ligands described in the previous literatures, we have developed planar‐chiral phosphine‐olefin ligands based on ( η 6 ‐arene)chromium(0), which is known as “first‐generation.” The S type first‐generation ligand ( S )‐ 1 gives the R type 1,4‐adducts from cyclic enones and the S type 1,4‐adducts from acyclic enones, respectively (Schemes and ). When ( R )‐1 ligand is employed, the enantiomers of the adducts are obtained.…”

Enantioselective Synthesis of Macrocyclic Heterobiaryl Derivatives of Molecular Asymmetry by Molybdenum‐Catalyzed Asymmetric Ring‐Closing Metathesis

“…[9] Although an asymmetric metathesis reaction itself does not form as tereogenic carbon atom directly,i th as been demonstrated to control peripheral chirality by kinetic resolution of racemic chiral molecules or desymmetrization of prochiral substrates by the use of ac hiral metathesis catalyst. [11][12][13] Although we have shown how to successfully control the planar chirality in ferrocenes [12] as well as in (p-arene)chromium complexes by asymmetric metathesis reactions, [13] we were challenged to apply the metathesis protocol to the enantioselective synthesis of the winding-vine-shapedc om-pounds.H erein, we report the results of our studies for preparing bisimidazole derivatives 1 in high enantiomeric purity by the molybdenum-catalyzed asymmetric ring-closing metathesis (ARCM). [11][12][13] Although we have shown how to successfully control the planar chirality in ferrocenes [12] as well as in (p-arene)chromium complexes by asymmetric metathesis reactions, [13] we were challenged to apply the metathesis protocol to the enantioselective synthesis of the winding-vine-shapedc om-pounds.H erein, we report the results of our studies for preparing bisimidazole derivatives 1 in high enantiomeric purity by the molybdenum-catalyzed asymmetric ring-closing metathesis (ARCM).…”

“…However,l ess studies have shown how to control noncarbon centrostereogenicities [10] and non-centrochirality. [11][12][13] Although we have shown how to successfully control the planar chirality in ferrocenes [12] as well as in (p-arene)chromium complexes by asymmetric metathesis reactions, [13] we were challenged to apply the metathesis protocol to the enantioselective synthesis of the winding-vine-shapedc om-pounds.H erein, we report the results of our studies for preparing bisimidazole derivatives 1 in high enantiomeric purity by the molybdenum-catalyzed asymmetric ring-closing metathesis (ARCM).…”