Improved enantioselectivity by using novel bulk bases in chiral lithium amide catalysed deprotonations: mixed dimers as reagents and catalysts

Novel bulk bases have been developed yielding improved enantioselectivity of chiral lithium amide catalysed deprotonations as compared to using the bulk base lithium diisopropylamide (LDA). The new bulk bases are 2-lithio-1-methylimidazole, 2-(lithiomethyl)-1-methylimidazole, 2-lithio-furan and 1,8-diazabicyclo-6-lithio[5.4.0]undec-7-ene which have been used together with chiral lithium amides in deprotonations of cyclohexene oxide. Using the chiral lithium amides enhanced stereoselectivities (96% ee) have been reached. The reactivity change has been traced to the formation of novel reagents—mixed dimers—formed from a bulk base molecule and a molecule of a chiral lithium amide. The results also show that DBU, which has commonly been used as an additive to alter reactivity and enantioselectivity in deprotonations, has another important role. DBU is lithiated under the conditions used and becomes a bulk base, which forms catalytic mixed dimers with the chiral lithium amides.