Aromatic π-Components for Enantioselective Heck Reactions and Heck/Anion-Capture Domino Sequences.

Abstract

Olefin functionalization represents one of the most important synthetic transformations in organic synthesis. Over the past decades, palladium-catalyzed enantioselective Heck reactions, and Heck/anion-capture domino sequences through olefin carbopalladation followed by termination of the resulting alkyl-Pd species have been extensively developed. Extension of the coupling partners from classical olefins to other π-components would enable further advances and open new space in this field. Aromatics are important and easily available bulk chemicals. Dearomative transformation of endocyclic aromatic π-bonds via the Heck reaction pathway provides an efficient and straightforward route to structurally diverse alicyclic compounds. Nevertheless, major challenges for this transformation include aromaticity breaking and reactivity and selectivity issues.Recently, we have engaged in developing catalytic enantioselective dearomative Heck reactions and related domino reactions. A range of heteroarenes and naphthalenes have been employed as novel π-coupling partners in these reactions. Through dearomative migratory insertion of endocyclic aromatic C-C π-bonds followed by interception of the transient alkyl-Pd species, enantioselective Heck reactions, reductive Heck reactions, Heck/anion-capture difunctionalization reactions, and heteroarenyne cycloisomerization reactions have been established. Relying on β-H elimination of the alkyl-Pd intermediate, we realized enantioselective dearomative Heck reactions with a range of aromatic partners, including heterocyclic indoles, pyrroles, furans, benzofurans, and more challenging carbocyclic naphthalenes. In order to avoid the utilization of organohalide electrophiles, heteroarenyne cycloisomerization reaction was developed by merging intermolecular alkyne hydropalladation with intramolecular dearomative Heck reaction. Cycloisomerization of alkyne-tethered indoles delivered chiral indolines in excellent enantioselectivities with 100% atom economy. On the other hand, Heck/anion-capture domino sequences were established through nucleophilic trapping of the alkyl-Pd intermediate. When HCO2Na was employed as a capturing reagent, the enantioselective dearomative reductive Heck reaction of indoles was realized. By employing other nucleophiles, including alkynes, N-sulfonylhydrazones, and organoboron reagents, we developed a series of dearomative difunctionalization reactions. Two vicinal stereocenters with excellent enantio- and diastereoselectivities were constructed in the corresponding Heck/Sonogashira, Heck/vinylation, and Heck/borylation reactions. Moreover, dearomative 1,4-diarylation of naphthalenes was developed through Heck/Suzuki domino reactions, in which competitive C-H arylation and the direct Suzuki reaction were almost fully inhibited in the presence of a spiro-phosphoramidite ligand.In this Account, we provide a panoramic view of our results since 2015 on enantioselective Heck reactions and related domino sequences by extending the coupling partners from classical olefins to aromatic systems. Investigations outlined in this Account established straightforward and efficient access to a variety of structurally diverse chiral heteropolycyclic molecules starting from simple and planar aromatic compounds.