Allylation Reactions Using Allylsilanes and Allylstannanes

Allylsilanes and allylstannanes are known to undergo the allylation reactions of aldehydes and ketones in the presence of Lewis acids (the most commonly used are TiCU and SnCU). Zirconium Lewis acids have often been compared with the titanium analogs. For most allylation reactions using allylsilanes the best results were obtained when TiCU or SnCU was used. For allylation reactions using allylstannanes, as illustrated in Eq. (15), however, ZrCU was used as the Lewis acid to obtain the bicyclic compound 36 via the intramolecular allylation of stannylaldehyde (35) [16]. 

Although various Lewis acid-catalyzed or -mediated reactions of aldehydes have been developed, less progress has been made in the reactions of imines using Lewis acids, probably because Lewis acids are often deactivated or decomposed by basic imines. Imines (37) have, however, been efficiently allylated by Zr(OTf)4 or Hf(OTf)4 catalyst in CH3CN to give homoallylamines (38) (Eq. 16) under similar conditions with using AICI3 or SnCU as catalyst low yields of allylated products were obtained [17]. 

Asymmetric intramolecular allylation of imine 39, with a chiral auxiliary on the imino nitrogen, was attempted with 2 equiv. Lewis acids (Eq. 17) [18]. When ZrCU was employed as a Lewis acid, of four possible diastereomers of /3-aminotetrahydro-furan (40), the only isomer obtained was that with trans substituents on the tetrahy-dropyran ring. 

When catalytic asymmetric allylation was attempted with a catalytic amount of chiral titanium complexes, BINOL-TiCl2 or BIN0L-Ti(0-/-Pr)2 the reaction was found to be slow. The reaction was performed satisfactorily when BIN0L-Zr(0-/-Pr)2 was employed as catalyst in the presence of molecular sieves (Eq. 18) [19a]. 

This asymmetric reaction was recently improved by using a Lewis acid system of BINOL, ZrCl4(THF)2 and 4-tert-calix[4]arene prepared in situ [19bj. The modified method required only 2 % BINOL and zirconium to obtain up to 96 % ee of the homoallylalcohols. 

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