Features:

• Highly efficient
• Excellent enantioselectivities
• Available in both enantiomers
• Operationally simple transformations

Catalyst for the enantioselective Weitz-Scheffer epoxidation of α,β-unsaturated aldehydes.

Condensation of this secondary amine with an aldehyde reversibly generates a β-fluoroiminium species where the fluorine atom is positioned gauche to the electropositive nitrogen centre.

Consequently, the phenyl substituents on the fluorine bearing carbon are placed in a predictable region of space. One of the groups effectively shields the upper face of the pendant iminium chain.

Addition of hydrogen peroxide occurs in a highly selective manner to furnish optically enriched epoxides with excellent levels of enantiocontrol (Scheme 1).

Simple α,β-disubstituted enals such as trans-cinnamaldehyde are smoothly converted to the corresponding epoxy aldehydes (up to 96% ee) as are their aliphatic analogues. The enantioselective, catalytic epoxidation of more challenging cyclic α,α,β-trisubstituted enals, and even an example of a α,α,β,β-tetrasubstituted enal, proceed with good yields and excellent enantioselectivities (up to 98% ee) (Scheme 2).

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