Lewis acids performance effects

Similar additions may be performed with the enamine 13. However, with 3-buten-2-one or methyl 2-propenoate Lewis acid catalysis is needed to activate the Michael acceptor chloro-trimethylsilane proved to be best suited for this purpose. A remarkable solvent effect is seen in these reactions. A change from THF to HMPA/toluene (1 1) results in a reversal of the absolute configuration of the product 14, presumably due to a ligand effect of HMPA235. 

Asymmetric reactions using chiral copper Lewis acids are also performed in aqueous media. It has been reported that an asymmetric Diels-Alder reaction proceeds smoothly in water using Cu(OTf)2 and abrine as a chiral ligand (Scheme 49).214 The Cu -bis(oxazoline) system is effective in asymmetric aldol reactions in an aqueous solvent such as water/ethanol and even in pure water.215... 

Corma and Renz6 developed an effective heterogeneous catalyst system. Incorporation of tin into a beta zeolite network (Sn-Beta) gave a catalyst that was used to convert citronellal 5 to racemic isopulegol (6, Equation (3)) with 85% diastereoselectivity. It was calculated that each metal site performed 11,500 reaction cycles. No leaching of the tin was detected. This catalyst system is advantageous over normal Lewis acids, since precautions against humidity are not required, and it is suitable for use in a fixed bed continuous reactor. 

Yang12 has effected an intramolecular asymmetric carbonyl-ene reaction between an alkene and an a-keto ester. Reaction optimization studies were performed by changing the Lewis acid, solvent, and chiral ligand. Ligand-accelerated catalysis was observed for Sc(OTf)3, Cu(OTf)2, and Zn(OTf)2 (Equation (6)). The resulting optically active m-l-hydroxyl-2-allyl esters provide an entry into multiple natural products. 

Alkylation with Carbonyl Compounds The Prins Reaction. Carbonyl compounds react with alkenes in the presence of Brpnsted acids to form a complex mixture of products known as the Prins reaction. The use of appropriate reaction conditions, solvents, and catalysts allows one to perform selective syntheses. Characteristically formaldehyde is the principal aldehyde used. Mineral acids (sulfuric acid, phosphoric acid), p-toluenesulfonic acid, and ion exchange resins are the most frequent catalysts. Certain Lewis acids (BF3, ZnCl2, SnCl4) are, however, also effective. 

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