Bisoxazoline iron complexes

Iron complexes can also be employed for ene cyclization of triene systems (Scheme 26). Though a chiral bisoxazoline complex exhibits not only higher 1,3-stereoinduction but also diastereoselectivity, no asymmetric induction was observed by the use of chiral bisoxazoline iron complex [70]. 

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. 

Takacs has reported the cyclization of tu-enedienes catalyzed by an iron(0) complex that is generated in situ through the reduction of iron(m) tris(acetylacetonate) with triethylaluminum in the presence of a ligand such as 2,2 -bipyridine or bisoxazoline (Scheme 100). The 1,4-diene cycloadducts are obtained in very good yields.366... 

This explanation is described as a mnemonic rule [228], which can only be taken as a first approximation of reality. The same rule can be used to rationalize the topicity of other asymmetric Diels-Alder reactions, such as those employing titanium BINOLate catalysts (Figure 6.18i, [230]), or iron bisoxazoline catalysts (Figure 6.18j,k [206,215]). Although the explanation seems reasonable, the picture is not complete, since it does not account for a number of observations, including the fact that the dioxolane substituents exert an extraordinary effect on catalyst efficiency (c/ Table 6.6, entries 2 and 5). Additionally, both titanium TADDOLate [228] and BINOLate [230] complexes show a nonlinear relationship between enantiomeric purity of the catalyst and that of the product, which suggests that some sort of dimerization phenomenon is involved. 

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