Structure cobalt enolates

The high levels of, sy -diastereoselectivity suggest aldolization through a closed Zimmerman-Traxler-type transition structure via intermediacy of the Z-enolate. When the transformation is performed using PhSiDj, a single deuterium is incorporated at the /3-position of the product as an equimolar mixture of epimers, inferring rapid isomerization of the kinetically formed cobalt enolate prior to cyclization or reversible aldol addition. The stereochemistry of the deuterated product was established by single crystal neutron diffraction analysis (Scheme 44). 

Some very interesting structural features exist for neutral jS-keto-enolate complexes (18) of the type M(AA)nLm. Adducts with nitrogenous bases or water have yielded a 5-coordinate irregular Zn(AA)2 H20 complex and trans 6-coordinate cobalt(II) and nickel (II) complexes M(AA)2 2B, with B = H2O or pyridine. The vanadyl complex, VO(AA)2, contains a square pyramid of oxygens about the metal, while U02(AA)2-H2O shows a pentagonal bipyramidal structure. The anhydrous material is dimeric. 

Enantioselection can be controlled much more effectively with the appropriate chiral copper, rhodium, and cobalt catalyst.The first major breakthrough in this area was achieved by copper complexes with chiral salicylaldimine ligands that were obtained from salicylaldehyde and amino alcohols derived from a-amino acids (Aratani catalysts ). With bulky diazo esters, both the diastereoselectivity (transicis ratio) and the enantioselectivity can be increased. These facts have been used, inter alia, for the diastereo- and enantioselective synthesis of chrysan-themic and permethrinic acids which are components of pyrethroid insecticides (Table 10). 0-Trimethylsilyl enols can also be cyclopropanated enantioselectively with alkyl diazoacetates in the presence of Aratani catalysts. In detailed studies,the influence of various parameters, such as metal ligands in the catalyst, catalyst concentration, solvent, and alkene structure, on the enantioselectivity has been recorded. Enantiomeric excesses of up to 88% were obtained with catalyst 7 (R = Bz = 2-MeOCgH4). 

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