Catalytic asymmetric C-H insertion reactions of vinyl carbocations

From the preparation of pharmaceuticals to enzymatic construction of natural products, carbocations are central to molecular synthesis. Although these reactive intermediates are engaged in stereoselective processes in nature, exerting enantiocontrol over carbocations with synthetic catalysts remains challenging. Many resonance-stabilized tricoordinated carbocations, such as iminium and oxocarbenium ions, have been applied in catalytic enantioselective reactions. However, their dicoordinated counterparts (aryl and vinyl carbocations) have not, despite their emerging utility in chemical synthesis. We report the discovery of a highly enantioselective vinyl carbocation carbon-hydrogen (C-H) insertion reaction enabled by imidodiphosphorimidate organocatalysts. Active site confinement featured in this catalyst class not only enables effective enantiocontrol but also expands the scope of vinyl cation C-H insertion chemistry, which broadens the utility of this transition metal-free C(sp³)-H functionalization platform.

Electrochemical Fluorination of Vinyl Boronates through Donor-Stabilized Vinyl Carbocation Intermediates

The electrochemical generation of vinyl carbocations from alkenyl boronic esters and boronates is reported. Using easy-to-handle nucleophilic fluoride reagents, these intermediates are trapped to form fully substituted vinyl fluorides. Mechanistic studies support the formation of dicoordinated carbocations through sequential single-electron oxidation events. Notably, this electrochemical fluorination features fast reaction times and Lewis acid-free conditions. This transformation provides a complementary method to access vinyl fluorides with simple fluoride salts such as TBAF.

C–H Functionalization Reactions of Phenyl and Vinyl Carbocations Paired with Weakly Coordinating Anions

Carbocations have played a central role in the chemical sciences for over a century. In a synthetic setting, most methods utilize stabilized tricoordinate carbocations, while there are far fewer examples of reactions featuring nonstabilized dicoordinate cations. Here, we provide an overview of recent developments in the generation of high-energy carbocations mediated by weakly coordinating anions and the C–H insertion reactions of such carbocations. Moreover, we discuss mechanistic studies of these catalytic C–H insertion reactions aimed at furthering our understanding of the reactive nature of these rarely invoked cationic intermediates.

1 Introduction

2 Background: Phenyl Carbocations

3 Silylium/Carborane-Catalyzed C–H Insertion Reactions of Phenyl Carbocations

4 Silane-Fueled, Weakly Coordinating Anion-Catalyzed, Reductive C–H Insertion Reactions of Vinyl Carbocations

5 C–H Insertion Reactivity of Vinyl Carbocations under Basic Conditions

6 Conclusion and Outlook

Urea-Catalyzed Vinyl Carbocation Formation Enables Mild Functionalization of Unactivated C-H Bonds

Herein we report the 3,5-bistrifluoromethylphenyl urea-catalyzed functionalization of unactivated C-H bonds. In this system, the urea catalyst mediates the formation of high-energy vinyl carbocations that undergo facile C-H insertion and Friedel-Crafts reactions. We introduce a new paradigm for these privileged scaffolds where the combination of hydrogen-bonding motifs and strong bases affords highly active Lewis acid catalysts capable of ionizing strong C-O bonds. Despite the highly Lewis-acidic nature of these catalysts that enables triflate abstraction from sp² carbons, these newly found reaction conditions allow for the formation of heterocycles and tolerate highly Lewis-basic heteroaromatic substrates. This strategy showcases the potential utility of dicoordinated vinyl carbocations in organic synthesis.

Vinyl Carbocations Generated under Basic Conditions and Their Intramolecular C-H Insertion Reactions

Here we report the surprising discovery that high-energy vinyl carbocations can be generated under strongly basic conditions, and that they engage in intramolecular sp³ C-H insertion reactions through the catalysis of weakly coordinating anion salts. This approach relies on the unconventional combination of lithium hexamethyldisilazide base and the commercially available catalyst, triphenylmethylium tetrakis(pentafluorophenyl)borate. These reagents form a catalytically active lithium species that enables the application of vinyl cation C-H insertion reactions to heteroatom-containing substrates.