The Role of Anions in Guanidinium-Catalyzed Chiral Cation Ion Pair Catalysis.

Abstract

ConspectusCatalysts drive asymmetric transformations by orchestrating a network of covalent and noncovalent interactions that precisely regulate the reactivity and stereoselectivity. Ion pair catalysis, developed based on the inherent strength and long-range nature of ionic interactions, has demonstrated high catalytic efficiency and broad applicability. While chiral cationic catalysts have long been central to this field, the critical roles of their counteranions have historically been overlooked. Over the past 15 years, we have developed a class of N-sp²-hybridized guanidinium chiral cation ion pair catalysts, which have been widely applied in enantioselective reactions. In this Account, we present new insights into these catalysts, revealing how the roles of anions, acting as substrates, reagents, and cocatalysts, can be strategically leveraged to achieve remarkable enantioselectivity across a wide range of organic transformations.When acting as substrates, anions, such as sulfinates, thiocarboxylates, and azides, are rendered reactive through intimate ion pairing with the chiral guanidinium moiety. This strategy facilitates desymmetrization processes, exemplified by the conversion of sulfinates to enantioenriched sulfinate esters and the remote desymmetrization of cis-dibromocyclohexanone via sequential S_N2 and acyl transfer steps. Mechanistically, halogenophilic S_N2X pathways (e.g., thiocarboxylate substitutions at sterically hindered tertiary carbons) bypass traditional steric limitations, while dynamic kinetic resolution of racemic bromides via azide substitution highlights the interplay between ion exchange and interfacial dynamics.Anions generated in situ from stoichiometric reagents give rise to highly reactive intermediates such as enolates, sulfenates, and hypervalent silicates, which form ion pairs with chiral cations, enabling enantioselective transformations. For instance, enolates displace tertiary bromides via an S_N2X mechanism (frontside attack), circumventing steric hindrance. Sulfur alkylation of sulfenamides yields chiral sulfilimines, while fluoride-activated acylsilanes undergo Brook-like rearrangements through penta-coordinate silicates. Silicon hydrides activated by fluoride form hydridosilicates, enabling enantioselective conjugate reductions of chromones and coumarins. The versatility of ion pairing is further illustrated by α-cyano carbanions in Pd-catalyzed decarboxylative allylic alkylations and is extended to a cooperative catalytic system, where DMAP-generated nucleophiles enable enantioselective phospha-Michael additions via dynamic cation-exchange activation.The utilization of inorganic anions as cocatalysts further expands the scope of chiral cation ion pair catalysis. Peroxytungstate anions synergize with chiral cations to enable the epoxidation of allylic amines, while peroxomolybdate facilitates the N-oxidation of tertiary amines with high enantioselectivity. Beyond inorganic systems, organic anions also serve as effective cocatalysts. Notably, preformed pentanidium pyridinyl-sulfonamide ion pairs have been shown to catalyze the enantioselective Steglich rearrangement with high efficiency. This strategy extends to bisguanidinium sulfonated-phosphine/Pd ion pair catalysis, achieving stereocontrol in allylic amination of Morita-Baylis-Hillman substrates.By systematically delineating the diverse roles of anions in guanidinium ion pair catalysis, this Account highlights new mechanistic insights and synthetic applications, paving the way for further advancements in asymmetric catalysis through precise control of ionic interactions.