Magnesium-based chiral catalysts

Enantioselective conjugate addition [40] has become truly useful with the aid of dialkylzinc, cationic copper catalyst, and a chiral ligand (Eq. 1, see also Chapt. 7) [41]. Magnesium-based reagents have found use in quantitative fivefold arylation of Cgo (Eq. 10.2) [42] and threefold arylation of C70 [43], paving ways to new classes of cyclopentadienyl and indenyl ligands with unusual chemical properties. 

Several metallic species other than titanium have been reported. Kobayashi showed that the tin(II) complex (16, Fig. 4) modified by cinchonine, a natural alkaloid, catalyzed the reaction to give the chiral cyanohydrin of cyclohexanecar-baldehyde in 90% ee [54]. Corey applied a magnesium complex of chiral bisox-azoline (17, Fig. 4) to the asymmetric silylcyanation. High selectivity of up to 95% ee was observed in the reactions of aliphatic aldehydes compared with benzaldehyde (52% ee) [55]. An aluminum complex of a peptide containing the phenolic Schiff base (7) was shown by Mori and Inoue to be an efficient catalyst for the addition of TMSCN to aldehydes [56,57]. 

At the same time, Antilla et al. developed a vaulted biphenanthrol (VAPOL)-based magnesium phosphate 20b mediated asymmetric aza-Darzens reaction for the synthesis of chiral aziridine derivatives. The catalyst was prepared in an identical procedure to the previously described process with VAPOL-derived phosphate and magnesium fert-butoxide, and applied in the enantioselective aza-Darzens reaction of N-benzoyl imines 23 and ot-chloro-1,3-diketone 24. The process formed a series of substituted aziridines 25 bearing various substituents at the aromatic ring, with good... 

---