Catalysis by Chiral Lewis Acids

Further studies with the CAB catalyst revealed that additions of crotyl tributyltin to aldehydes strongly favored the syn adduct [30]. Best results were obtained with 50 mol % catalyst and a twofold excess of (Cp3C0)20 to assist catalyst turnover (Table 16). 

Additions to the enantiomeric a-methyl-/3-ODPS propanals resulted in relatively high catalyst control compared with additions conducted with BF3 OEt as the promoter (Eqs 19 and 20). In the mismatched case (Eq. 20) the chiral catalyst effectively reverses the intrinsic substrate preference observed in the BFa-promoted addition. The matched pairing (Eq. 19) results in enhanced preference for the product of substrate control syn, syn) when the chiral catalyst is employed. 

Several other chiral Lewis acids have been developed for the addition of allyl and methallyl tributylin to aldehydes [28]. These additions usually proceed slowly with reaction times of days. Less reactive stannanes, for example crotyl tributyltin, require even longer times and diastereoselectivity is poor. The allyl and methallyl additions, however, afford products in high yield and ee. The most successful ligands are BINOL and BINAP as Ti or Zr complexes in the former case and an AgOTf complex in the latter. 

A TiCl2 complex of (P)-BINOL catalyzes the addition of cis- and traws-crotyl and 8-methylcrotyl stannanes to the reactive aldehyde, methyl glyoxylate (Table 17) [31]. The diastereoselectivities of these additions are, however, poor and the ee of the adducts is modest to poor. 

Somewhat better results with this eatalyst system are obtained in additions of allyl tributyltin to aldehydes (Table 18) [32]. Although it is also possible to catalyze the same additions with a (P)-BINOL Zr(0-/-Pr)2 complex [33], the yields and ee of the adducts are generally lower. 

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Hetero Diels-Alder reactions between prochiral aldehydes and activated dienes such as 84 are also known to be catalyzed by Lewis acids (Eq. 26) and catalysis by chiral Lewis acids supported by dimethylpolysiloxane (83a) has been investigated [100]. Enantio-selectivity up to 43 % ee was achieved by use of polymeric Lewis acids. 

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