Alkenes bipyridine ligands

The use of a,P-unsaturated 2-acylimidazoles [151] as the dienophile increased the practicality of the approach, since the imidazole group can be removed readily from the product. The substrate scope is quite broad substituents at the alkene moiety (e.g., alkyl, aryl and 2-furanyl) were very well tolerated, hi all cases good to excellent enantioselectivies and diastereoselectivities were obtained, i.e., 80-98% ee for the endo isomer and endo. exo > 94 6, in the case of the 4,4 -dimethyl-2,2 -bipyridine ligand [152]. Moreover, the D-A reaction was successfully performed on a preparative scale (1 mmol), making it attractive from a synthetic point of view. 

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. 

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... 

Asymmetric cyclopropanations of alkenes and alkynes with a-diazocarbonyl compounds have been extensively explored in recent years and a number of very effective chiral catalysts have been developed2. Copper complexes modified with such chiral ligands as salicy-laldimines 38202,203, semicorrins 39204 208, bis(oxazolines) 40209-2" and bipyridines 41212 have... 

On the other hand, Takacs and coworkers added organometallic reducing agents to the reaction mixture and promoted the formation of low-valent iron(O) bipyridine complexes. The mechanism of the low-valent iron-catalyzed Alder-ene reaction involves coordination of the two starting materials within the ligand sphere of the iron, which makes the Woodward-Hoffmann rules for such reactions obsolete [11]. Thereby, the scope of the reactions was broadened so that alkenes and 1,3-dienes could also be used as educts in a formal [4 + 4]-cycloisomerization (Scheme 9.3) [12]. Intriguingly, the diastereoselectivity of the cydopentane products can be influenced by either the application of the 2Z-isomer 3 or the 2 E-isomer 4. Especially the E-isomers 4 gave almost exclusive cis selectivity [13]. 

Apart from the catalytic properties of the Mn-porphyrin and Mn-phthalo-cyanine complexes, there is a rich catalytic chemistry of Mn with other ligands. This chemistry is largely bioinspired, and it involves mononuclear as well as bi- or oligonuclear complexes. For instance, in Photosystem II, a nonheme coordinated multinuclear Mn redox center oxidizes water the active center of catalase is a dinuclear manganese complex (75, 76). Models for these biological redox centers include ligands such as 2,2 -bipyridine (BPY), triaza- and tetraazacycloalkanes, and Schiff bases. Many Mn complexes are capable of heterolytically activating peroxides, with oxidations such as Mn(II) -> Mn(IV) or Mn(III) -> Mn(V). This chemistry opens some perspectives for alkene epoxidation. 

Knifton has also shown (36 - 38,40) that nitrogen- or phosphorus-ligand modified ruthenium complexes, in a phosphonium salt matrix, can conveniently catalyze the hydroformylation of terminal alkenes with high selec-tivities in linear oxo products. Usually selectivities better than 80% were achieved. In the best case (160°C, 95 bar. CO/H2= 1/2) a linearity in nonanol of 94% was obtained starting from [Ru3(CO),2], 2,2 -bipyridine. and [PBu4]Br. The main products were alcohols and not aldehydes. However, it is often difficult to reduce the isomerization of oct-l-ene as well as its hydrogenation. The [Ru3(CO),2l/2,2 -bipyridine (bipy) system has been extensively explored. Two equilibria have been proposed to account for the infrared data and the effects of the bipy ligand [eqs. (8) and (9)]. 

Planar coordination compounds with aromatic ligands (Fig. 42), especially those having extended 77 systems, show 77-77 interaction in solid states. In alkene or alkyne 77-bonded Cud) complexes, in-plane coordination ofaC = CorC = C bond to trigonal planar Cu(I) centers often leads to planar molecular conformations (167-171). The infinite 77-77 stacking columns are confirmed in the 2,2 -bipyridine (L55) com-... 

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