Abstract
Here we report that Morita–Baylis–Hillman carbonates from diverse aldehydes and methyl vinyl ketones can be directly utilised as palladium-trimethylenemethane 1,4-carbodipole-type precursors, and both reactivity and enantioselectivity are finely regulated by adding a chiral ammonium halide as the ion-pair catalyst. The newly assembled intermediates, proposed to contain an electronically neutral π-allylpalladium halide complex and a reactive compact ion pair, efficiently undergo asymmetric [4 + 2] annulations with diverse activated alkenes or isatins, generally with high regio-, diastereo- and enantio-selectivity, and even switchable regiodivergent or diastereodivergent annulations can be well realised by tuning the substrate or catalyst assemblies. An array of control experiments, including UV/Vis absorption study and density functional theory calculations, are conducted to rationalise this new double activation mode combining a palladium complex and an ammonium halide as an ion-pair catalyst.
Highlights
With the development of transition metal catalysis, zwitterionic species possessing a p-allylmetal complex moiety, which could be generated from allylic alcohol derivatives in situ, have been extensively employed for the construction of enantioenriched cyclic frameworks.[1]
Typical MBH adducts from b-keto esters have been recently employed as Pd-TMM-type 1,4-carbodipole precursors in asymmetric [4 + 2] annulations via the deprotonation strategy;6e the common analogues derived from simple enones, such as methyl vinyl ketone (MVK), and with broader substitution patterns (Scheme 1c), are much more challenging for similar transformations, and no success has been achieved yet
The combination of a bisphosphine ligand L1 or L2 and C4 proved to be unsuccessful. These results indicated that adding an ammonium halide would apparently change the previously formed Pd(II) species of 1a
Summary
With the development of transition metal catalysis, zwitterionic species possessing a p-allylmetal complex moiety, which could be generated from allylic alcohol derivatives in situ, have been extensively employed for the construction of enantioenriched cyclic frameworks.[1]. Morita–Baylis–Hillman (MBH) adducts, accessible by condensation between activated alkenes and carbonyl compounds, have been explored in asymmetric allylic alkylation reactions by forming p-allylpalladium intermediates.[9] Typical MBH adducts from b-keto esters have been recently employed as Pd-TMM-type 1,4-carbodipole precursors in asymmetric [4 + 2] annulations via the deprotonation strategy;6e the common analogues derived from simple enones, such as methyl vinyl ketone (MVK), and with broader substitution patterns (Scheme 1c), are much more challenging for similar transformations, and no success has been achieved yet.
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