Chiral metal complexes rhenium

In conclusion, it can be noted that high valent transition metals seem perfectly capable of serving as effective Lewis acids. Many of the systems discussed here exhibit exceptional robusmess, stability and a propensity to form crystalline complexes. This would facilitate the task of crystallization and structural analysis, and one can imagine that transition metal complexes can be used as structural probes of Lewis acid-carbonyl interactions. In this vein, the first glimpse of the origins of Cram selectivity in a-chiral aldehydes may have been obtained from the crystal structure of a rhenium aldehyde complex. Lastly, the... 

Other chiral metal catalysts such as iridium, osmium, and rhenium catalysts have also been utilized for asymmetric cyclopropanation. In particular, iridium catalyst gave exceptional enantio- and diastereoselectivities. Chiral Ir(III) salen complexes, which contain a-coordinated aryl ligands, were found to catalyze the cyclopropanation of st3rrene derivatives with TDA. Higher yields (99%), higher diastereoselectivity t/c = 1 99), and enantioselectivity (99% ee) were observed when the reactions were conducted at lower temperature (—78°C) (98). For osmium, an Schiff-based complex of [Os(fBu-salch)(OH)Cl] (H2tBu-salch... 

Hydridotris(3,5-dimethyl-l-pyrazolyl)borate]molybdenum-(i72-acyl) complexes, such as 1, are deprotonated by butyllithium or potassium hydride to generate enolate species, such as 488.8> jjie overa]] structure of these chiral complexes is similar to that of the iron and rhenium complexes discussed earlier the hydridotris(3,5-dimethyl-l-pyrazolyl)borate ligand is iso valent to the cyclopentadienyl ligand, occupying three metal coordination sites. However, several important differences must be taken into account when a detailed examination of the stereochemical outcome of deprotonation-alkylation processes is undertaken. 

In principle, steric control may be exercised by a chiral transition metal catalyst in which the metal center is the asymmetry origin rather than the phosphine or its substituents. Gladysz and coworkers have separated the enantiomers of rhenium complexes using HPLC. 

One means of stereoselective cleavage of biaryl lactones [53] is activation of the carbonyl group with a Lewis acid and subsequent attack with a chiral nucleophile. Conversely, activation can be effected with a chiral Lewis acid followed by attack of an achiral nucleophile. Complexation of a biaryl lactone to the chiral fragment [CpRe (NO)(PPh3)j then reduction with K(s-Bu)3BH (K-selectride) and ring opening of the intermediate rhenium lactolate gives the metalated aldehyde (dr = 75 25) which is converted to the alcohol without essential loss of optical purity (Sch. 6) [54]. 

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