DBFOX

DBFOX/Ph-Transition Metal Complexes and Diels-Alder Reactions... 

The cationic aqua complexes prepared from traws-chelating tridentate ligand, R,R-DBFOX/Ph, and various transition metal(II) perchlorates induce absolute enantio-selectivity in the Diels-Alder reactions of cyclopentadiene with 3-alkenoyl-2-oxazoli-dinone dienophiles. Unlike other bisoxazoline type complex catalysts [38, 43-54], the J ,J -DBFOX/Ph complex of Ni(C104)2-6H20, which has an octahedral structure with three aqua ligands, is isolable and can be stored in air for months without loss of catalytic activity. Iron(II), cobalt(II), copper(II), and zinc(II) complexes are similarly active. 

The complexation procedure included addition of an equimolar amount of R,R-DBFOX/Ph to a suspension of a metal salt in dichloromethane. A clear solution resulted after stirring for a few hours at room temperature, indicating that formation of the complex was complete. The resulting solution containing the catalyst complex was used to promote asymmetric Diels-Alder reactions between cyclopen-tadiene and 3-acryloyl-2-oxazolidinone. Both the catalytic activity of the catalysts and levels of chirality induction were evaluated on the basis of the enantio-selectivities observed for the endo cycloadduct. 

Equimolar amounts of fl,fl-DBFOX/Ph and Ni(CI04)2 6H20 are stirred in dichloromethane for a few hours at room temperature to give a clear green solution. 

The nickel and cohalt aqua complexes were even more effective, both for catalytic activity and enantioselectivity, than the corresponding anhydrous complexes (Scheme 7.5). Addition of three equivalents of water to the anhydrous nickel complex recovered the catalytic efficiency. DBFOX/Ph complexes derived from manga-nese(II), iron(II), copper(II), and zinc(II) perchlorates, both anhydrous and vef. 

Although the aqua nickel(II) complex A was confirmed to be the active catalyst in the Diels-Alder reaction, no information was available about the structure of complex catalyst in solution because of the paramagnetic character of the nickel(II) ion. Either isolation or characterization of the substrate complex, formed by the further complexation of 3-acryloyl-2-oxazolidinone on to the l ,J -DBFOX/ Ph-Ni(C104)2 complex catalyst, was unsuccessful. One possible solution to this problem could be the NMR study by use of the J ,J -DBFOX/Ph-zinc(II) complex (G and H, Scheme 7.9) [57]. 

The three water ligands located at meridional positions of the J ,J -DBFOX/Ph aqua complexes may be replaced by another molecule of DBFOX/Ph ligand if steric hindrance is negligible. Based on molecular model inspection, the hetero-chiral enantiomer S,S-DBFOX/Ph looks like a candidate to replace the water ligands to form the heterochiral meso-2 l complex J ,J -DBFOX/Ph-S,S-DBFOX/... 

It is interesting that the use of excess ligand DBFOX/Ph led to a decreased en-antioselectivity for the endo cycloadduct, especially when the enantiomeric purity of the ligand was low. This phenomenon is closely related with the chirality enrichment mechanism operating in the solution. 

The second chirality enrichment mechanism operating in the solution is most likely that some heterochiral pairs of the 1 1 complex DBFOX/Ph-Ni(C104)2 are formed or they are further associated to form relatively stable racemic aggregation [58], while weak aggregation should result in the case of enantiopure 1 1 complex... 

---