Ligand, Chiral bipyridine

These authors further described the synthesis and resolution (by chiral HPLC) of a new C2-symmetric planar-chiral bipyridine ligand [43] (see structure 35 in Scheme 18). They obtained an X-ray crystal structure of the corresponding copper complex proving a bidentate complexation. This system led to high diastereo- (up to 94%) and enantioselectivity (up to 94%) in the... 

Helquist et al. [129] have reported molecular mechanics calculations to predict the suitability of a number of chiral-substituted phenanthrolines and their corresponding palladium-complexes for use in asymmetric nucleophilic substitutions of allylic acetates. Good correlation was obtained with experimental results, the highest levels of asymmetric induction being predicted and obtained with a readily available 2-(2-bornyl)-phenanthroline ligand (90 in Scheme 50). Kocovsky et al. [130] prepared a series of chiral bipyridines, also derived from monoterpene (namely pinocarvone or myrtenal). They synthesized and characterized corresponding Mo complexes, which were found to be moderately enantioselective in allylic substitution (up to 22%). 

A heterobimetallic BINOL-Ga/Li complex 53 has been developed for the enantioselective ARO of meso-cpoxides (BINOL = l,T-bi(2-naphthol)).278 Using />-methoxyphenol as the nucleophile, this etherification reaction was observed to take place with a high level of asymmetric induction. An improved catalyst 54 has also been reported that exhibits greater stability under the reaction conditions and delivers higher yields and ee s (Equation (78)).279 A simple catalyst derived from Sc(OTf)3 and the chiral bipyridine ligand 52 has been shown to be effective for the ARO of aryl-substituted /// -epoxides with aliphatic alcohols to give high ee s (Equation (79)).280... 

Ito and Katsuki (55) examined the use of chiral bipyridine (bpy) compounds as ligands in the asymmetric cyclopropanation of alkenes. Moderate diastereoselectivities and excellent enantioselectivities were observed in the cyclopropanation of vinyl arenes, Eq. 38. This catalyst system afforded very high ee values of the cis isomer. 

Asymmetric induction in the ylide formation/[l,2]-shift has also been studied with chiral metal complexes. Katsuki and co-workers examined the reaction of ( )-2-phenyloxetane with 0.5 equiv. of /< //-butyl diazoacetate in the presence of Gu(i) catalyst. With chiral bipyridine ligand 53, trans- and m-tetrahydrofurans 54 and 55 are obtained with 75% and 81% ee, respectively (Equation (6)). This asymmetric ring expansion was applied by the same group to their enantioselective synthesis of translactone. 

Copper-catalyzed allylic oxidation allows the functionalization of unactivated alkenes into chiral allylic carboxylates.1347 The use of oxazoline-containing ligands give good enantioselectivities, but the reaction is extremely slow.1348-1351 Chiral bipyridine complexes, in turn, are much more active and give products in good yields and enantioselectivities up to 70% when applied in benzoyloxylation of cycloalkenes with rm-butyl perbenzoate.1352,1353... 

Worsdorfer U, Vogtle F, Nieger M, Waletzke M, Grimme S, Glorius F, Pfaltz A (1999) A new planar chiral bipyridine ligand. Synthesis 597-602... 

The reaction of stilbene oxide with a variety of aniline derivatives in the presence of indium triflate and the chiral bipyridine ligand 22 provided aminoalcohols in generally good yields and enantioselectivity <07SL2136>. 

In recent years, many chiral catalysts for the enantioselective synthesis of optical active 1,5-dicarbonyl compounds have been developed, such as chiral crown ethers with potassium salt bases and chiral palladium complexes, including bimetallic systems. Nakajima and coworkers reported on enantioselective Michael reactions of S-keto esters to a,/3-unsaturated carbonyl compounds in the presence of a chiral biquinoline N,N dioxide-scandium complex, which catalyzed the additions in high yields and with enan-tioselectivities up to 84% ee . Kobayashi and coworkers found that the combination of Sc(OTf)3 with the chiral bipyridine ligand 149 (equation 41) was also effective as a chiral catalyst for asymmetric Michael additions of 1,3-dicarbonyl compounds 147 to a,/3-unsaturated ketones 148. The corresponding Michael adducts 150 were obtained in good to high yields with excellent enantiomeric excesses in most cases (Table 10). 

Chromium(salen) catalysts are excellent reagents for the desymmetrization of OT to-epoxides. Thus, tfr-stilbene oxide is converted to the (3, 3 )-aminoalcohol in the presence of catalytic quantities of chromium-salen complex in methylene chloride solution open to the atmosphere. The addition of small quantities of triethylamine was found to dramatically increase enantioselectivities (by almost 25%). This catalytic system also promotes an efficient aminolytic kinetic resolution (AKR) of racemic epoxides with 2-type symmetry (Equation 18) <20040L2173, 1999TL7303>. W fo-Epoxides can be opened with aromatic amines in water in the presence of 1 mol% of an Sc(ni) catalyst ligated to 1.2mol% of a chiral bipyridine ligand <2005OL4593>. 

Moreover, catalytic asymmetric ring-opening reactions of me o-epoxides with indoles, alcohols, and thiols proceeded smoothly in the presence of catalytic amounts of Sc(DS)3 and chiral bipyridine ligand 3 in water to afford (3-amino alcohols in high yield and enantioselectivity (Scheme 15.19 and Scheme 15.20)." Note that an excellent hydrophobic, asymmetric environment has been created in water. 

Self-Assembled Multinudear Coordination Species with Chiral Bipyridine Ligand... 

In 2007, Fu described the first examples of diastereo- and enantioselective Cu-catalyzed [4+1] cycloadditions of enones 180 with diazo compounds 181 using the planar chiral bipyridine ligand 179. This new method furnishes synthetically useful, highly substituted 2,3-dihydrofuran derivatives 183 with good efficiency and stereoselection (Scheme 11.39) [60]. 

Scandium trifiate was found to be an effective catalyst for the aldol reactions of silyl enol ethers with aldehydes in aqueous solvent/micellar systems (205). While the reactions proceeded sluggishly in water, remarkable enhancement of the reactivity was observed in the presence of a small amount of a surfactant (206). In related asymmetric version, scandium trifiate (Sc(OTf)3) catalyzed asymmetric aldol of formaldehyde (hydroxymethylation) could be conducted with highly enantioselectively in the presence of chiral bipyridine based ligand (Scheme 53) (207). A significant progress was also made by Feng and co-workers recently a C-2-symmetric iV,iV -dioxide-Sc(III) complex has been developed to asymmetric catalytic aldol reaction of a-ketoesters and diazoacetate... 

Reversal of enantioselectivity in Cu-chiral bipyridine-catalyzed asymmetric ring-opening reactions of meso-epoxides with indole and aniline derivatives is observed compared to Sc-chiral bipyridine-catalyzed reactions, where the same chiral ligand is used (Scheme 32). " It was revealed from X-ray crystal structural analysis that a square pyramidal structure for the Cu° complex and a pentagonal bipyramidal... 

Silver. The complexation reactions between Ag+ and a number of chiral bipyridine ligands have been examined quantitatively by and Ag NMR spectroscopy. NMR spectroscopy has been used to examine Ag+... 

Using a chiral bipyridine ligand, an asymmetric ring opening of meso epoxides has been achieved successfully in water giving P-amino alcohols with enantioselec-tivities up to 94% (Equation 39) [82]. 

The LACS system has been successfully applied to asymmetric catalysis. Catalytic asymmetric ring-opening reactions of meso epoxides with aromatic amines proceeded smoothly in the presence of 1 mol% of Sc(DS)3 and 1.2 mol% of chiral bipyridine ligand (3) in water to give P-amino alcohols in high yields with excellent enantioselectivities (Scheme 12.75) [172]. It has been noted that excellent chiral hydrophobic environments have been created in water. 

Related Sc(OTf)3/chiral bipyridine ligand complex-catalyzed asymmetric aldol reaction is also developed by Kobayashi and co-workers. Ishikawa, S., Hamada,... 

Moreover, catalytic asymmetric ring-opening reactions of meso-epoxides with aromatic amines also proceeded in the presence of a catalytic amount of bismuth triflate (Bi(OTf)3), chiral bipyridine ligand 7, and sodium dodecylbenzene sulfonate (SDBS) in pure water. The corresponding /3 -amino alcohols were obtained in good yields with high enantioselectivities (Scheme 3.57). ... 

---