Halogenation Hetero Diels-Alder

Catalysts (25) are the Lewis acid-Lewis base bifunctional catalysts in which Lewis acid-Al(III) moiety activates acyl iminium ion and the Lewis base (oxygen of phosphine oxide) does TMSCN, simultaneously (Scheme 5.7). Halogen atoms at the 6-position enhanced both yields and enantioselectivity in Reissert-type cyanation of the imino part of 26. However, the order for the activation is not parallel to the electronegativity of the halogen atoms and, moreover, the strong electron-withdrawing trifluoromethyl group provided unexpectedly the worst result for the activation [13]. It is not simple to explain this phenomenon only in terms of the increased Lewis acidity of the metal center. Trifluoromethylated BINOL-zirconium catalysts (28) for asymmetric hetero Diels-Alder reaction (Scheme 5.8) [14], trifluoromethylated arylphosphine-palladium catalyst (32) for asymmetric hydrosilylation (Scheme 5.9) [15], and fluorinated BINOL-zinc catalyst (35) for asymmetric phenylation (Scheme 5.10) [16] are known. 

Asymmetric Lewis acid-catalyzed hetero Diels-Alder reactions of 2-aza-1,3-dienes have been studied for isolated cases. Highly selective thermal cycloadditions have been observed in the reactions of chiral, halogenated 2-aza-1,3-dienes with acyclic diazo compounds conducted at 80°C. ... 

Cuellar MA, Alegria LK, Prieto YA, Cortes MJ, Tapia RA, Preite MD (2002) Hetero-Diels-Alder reaction of halogenated quinones with a polygodial-derived azadiene. Tetrahedron Lett 43 (12) 2127-2131. doi 10.1016/S0040-4039(02)00246-0... 

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