Dual-Catalytic Enantioselective Allylation of N-(Heteroaromatic-methyl)imine Derivatives

We report the dual-catalytic enantioselective allylic alkylation of 2-(pyridylmethyl)amine-derived ketimines with allylic carbonates. The reaction proceeds under mild reaction conditions to generate α-amino heteroaryl benzylamine stereocenters in good yield and enantioselectivity. Enantioselectivity is achieved through the use of a copper catalyst modified with chiral bisphosphine ligand (2S,4S)-bis(diphenylphosphino)pentane.

Aziridine Opening via a Phenonium Ion Enables Synthesis of Complex Phenethylamine Derivatives

We report that the treatment of unsymmetrical 2,3-disubstituted aziridines with TiCl₄ yields β-phenethylamine products via the intermediacy of a phenonium ion. Derivatization of the products obtained via this method is demonstrated. Computational analysis of the reaction pathway provides insight into the reaction mechanism, including the selectivity of the phenonium opening.

Homologation of Electron-Rich Benzyl Bromide Derivatives via Diazo C-C Bond Insertion

The ability to manipulate C-C bonds for selective chemical transformations is challenging and represents a growing area of research. Here, we report a formal insertion of diazo compounds into the "unactivated" C-C bond of benzyl bromide derivatives catalyzed by a simple Lewis acid. The homologation reaction proceeds via the intermediacy of a phenonium ion, and the products contain benzylic quaternary centers and an alkyl bromide amenable to further derivatization. Computational analysis provides critical insight into the reaction mechanism, in particular the key selectivity-determining step.

Revisiting the ‘Phenonium Phenomenon’: Regiodivergent Opening of Nonsymmetrical Phenonium Ions with Halide Nucleophiles

Formation of phenonium ions through anchimeric assistance (neighboring-group participation) of aryl rings has been known since 1949. Although these reactive intermediates have been studied extensively by physical organic chemists, their potential as control elements in synthesis is underutilized. Presented here are our laboratory’s recently reported first examples of regiodivergent openings of nonsymmetrical phenonium ions with chloride nucleophiles. The selectivity of these processes is under reagent control. The reactions are operationally simple and permit the stereospecific synthesis of complex chiral building blocks from readily accessible epoxide starting materials.

1 Introduction

2 Select Examples of Phenonium Ion Methodology

3 Regiodivergent Opening of Nonsymmetrical Phenonium Ions

4 Summary and Outlook

Enantioselective C2-Alkylation of Indoles through a Redox-Relay Heck Reaction of 2-Indole Triflates

A palladium-catalyzed enantioselective redox-relay Heck reaction of 2-indole triflates and disubstituted alkenes is reported. This process combines readily available indole triflates with a variety of alkenes to afford a range of indole derivatives bearing a stereocenter adjacent to C2. Enantioselectivity is achieved through use of a simple pyridine-oxazoline ligand. Tuning the electronics of the indole, through judicious choice of N-protecting group, is required to ensure selective β-hydride elimination away from the indole core. Utility of this method is highlighted in a modular formal synthesis of an S1P₁ agonist precursor developed by Merck.

Recent developments in the use of aza-Heck cyclizations for the synthesis of chiral N-heterocycles

Aza-Heck cyclizations are an emerging method for the construction of chiral N-heterocyclic systems. In these processes, an activated N-O bond replaces the C-X bond (X = halide, OTf) used in conventional Heck reactions, with the associated aza-Pd(ii)-intermediate engaging pendant alkenes in a Heck-like manner. This perspective article commences with an historical overview of the area, which stems from Narasaka's seminal studies using oxime esters as the initiating motif. The scope and mechanism of associated chiral N-heterocyclic methodologies are then outlined, including cascade processes that enable diverse alkene 1,2-carboaminations. The recent emergence of new N-O donors and the realization of highly enantioselective aza-Heck cyclizations are then discussed. Collectively, these studies suggest that the aza-Heck approach can underpin a broad family of redox-neutral and enantioselective C-N bond forming processes.

Enantioselective Narasaka-Heck cyclizations: synthesis of tetrasubstituted nitrogen-bearing stereocenters

The first examples of highly enantioselective Narasaka-Heck cyclizations are described. A SPINOL-derived P,N-ligand system enables Pd-catalyzed 5-exo cyclization of a range of oxime esters with sterically diverse trisubstituted alkenes to generate dihydropyrroles containing tetrasubstituted nitrogen-bearing stereocenters in 56 to 86% yield and 90 : 10 to 95 : 5 e.r. These processes are rare examples of reactions that proceed via enantioselective migratory insertion of alkenes into Pd-N bonds, and the first where trisubstituted alkenes are used to generate tetrasubstituted stereocenters with high enantioselectivity.

Palladium-Catalyzed Enantioselective Intermolecular Coupling of Phenols and Allylic Alcohols

An enantioselective intermolecular coupling of oxygen nucleophiles and allylic alcohols to give β-aryloxycarbonyl compounds is disclosed using a chiral pyridine oxazoline-ligated palladium catalyst under mild conditions. As opposed to the formation of traditional Wacker-type products, enantioselective migratory insertion is followed by β-hydride elimination toward the adjacent alcohol. Deuterium labeling experiments suggest a syn-migratory insertion of the alkene into the Pd-O bond. A broad scope of phenols, various allylic alcohols, and an alkyl hydroperoxide are viable coupling partners in this process.

Dichotomous mechanistic behavior in Narasaka-Heck cyclizations: electron rich Pd-catalysts generate iminyl radicals

Pd-catalyzed cyclizations of oxime esters with pendant alkenes are subject to an unusual ligand controlled mechanistic divergence. Pd-systems modified with electron deficient phosphines (e.g. P(3,5-(CF3)2C6H3)3) promote efficient aza-Heck cyclization, wherein C-N bond formation occurs via alkene imino-palladation. Conversely, electron rich ligands, such as P(t-Bu)3, cause deviation to a SET pathway and, in these cases, C-N bond formation occurs via cyclization of an iminyl radical. A series of mechanistic experiments differentiate the two pathways and the scope of the hybrid organometallic radical cyclization is outlined. This study represents a rare example in Pd-catalysis where selection between dichotomous mechanistic manifolds is facilitated solely by choice of phosphine ligand.

Copper catalyzed Heck-like cyclizations of oxime esters

Copper catalyzed Heck-like cyclizations of oxime esters are described. This new protocol is an effective alternative to Pd-based systems and highlights the value of replacing precious metal catalysts with cheaper and more sustainable variants.