Chiral Unsymmetrically Substituted Bipyridine N,N′‐Dioxides as Catalysts for the Allylation of Aldehydes

A series of unsymmetrically substituted diastereoisomeric (R-a,R) and (S-a,R) bipyridine N,N-dioxides was synthesized by using oxidative coupling of the corresponding metallated tetrahydroisoquinoline N-oxides in the presence of iodine. The N,N-dioxides contained substituted aryl groups with electron-donating or electron-accepting groups in the near vicinity of the N,N-dioxide moiety. Their catalytic activity was tested in a series of reactions of allyltrichlorosilane with various substituted benzaldehydes, thiophenecarbaldehyde, and cinnamaldehyde. The reactions proceeded with high enantioselectivities (up to 98% ee) with catalyst loadings as low as 0.5 mol-%. Furthermore, allylation reactions of (E)-3-iodomethacrylaldehyde were also carried out to give chiral (E)-1-iodo-2-methylpenta-1,4-dien-3-ol, a convenient building block for the synthesis of natural products. The allylations proceeded with high enantioselectivities (up to ca. 99% ee) with a catalyst loading of 2.5 mol-%.