A Powerful Chiral Counterion Strategy for Asymmetric Transition Metal Catalysis

Abstract

Traditionally, transition metal-catalyzed enantioselective transformations rely on chiral ligands tightly bound to the metal to induce asymmetric product distributions. Here we report high enantioselectivities conferred by a chiral counterion in a metal-catalyzed reaction. Two different transformations catalyzed by cationic gold(I) complexes generated products in 90 to 99% enantiomeric excess with the use of chiral binaphthol-derived phosphate anions. Furthermore, we show that the chiral counterion can be combined additively with chiral ligands to enable an asymmetric transformation that cannot be achieved by either method alone. This concept of relaying chiral information via an ion pair should be applicable to a vast number of metal-mediated processes.

Deconvolution of the Mechanism of Homogeneous Gold-Catalyzed Reactions

“…The asymmetric intramolecular hydroalkoxylation and hydroamination of allenes reported by Toste 12 remains one of few examples of a highly enantioselective transition metal catalyzed reaction in which a counteranion acts as the sole source of chirality (Scheme 1).…”

“…with lower enantioselectivity, than with dppm(AuCl)2/L1-Ag (18 h at room temperature), 12 it might be possible to rule out dissociation of the phosphate as the rate determining step although a full kinetic study will be needed to verify this. On the other hand, attempts to observe any intermediates of a catalytic cycloisomerization of 1 (25 mol%, R 1 , R 2 = (CH 2 ) 5 , R 3 = Me, XH = OH) using EXAFS were unsuccessful due to the very fast reaction (<1 minute at the concentration required for EXAFS).…”

A combination of directing groups and chiral anion phase-transfer catalysis for enantioselective fluorination of alkenes

“…More recently the anionic conjugate bases of chiral phosphoric acids (i.e., chiral phosphate anions) have received attention as counterions for positively charged electrophilic intermediates, wherein ion pairing with the phosphate anion provides a suitable chiral environment for subsequent enantioselective transformations of the electrophilic species (11)(12)(13). The generality of the concept of ion pair catalysis has allowed the application of these highly tunable scaffolds to a diverse range of transformations in which carbonyl or imine activation is clearly inapplicable (15)(16)(17). As part of both aforementioned modes of reactivity, hydrogen bonds between chiral phosphoric acids or phosphate anions and reacting functionality are often invoked as crucial interactions for achieving high levels of enantioselectivity (11)(12)(13)(14).…”

A porphyrin complex of Gold(I): (Phosphine)gold(I) azides as cation precursors

“…Solvate adducts of gold(I) cations have been isolated, and these can serve as silver-free catalysts (25,(35)(36)(37)(38). Acidolysis of organogold(I) species can afford gold(I) cations (39), but coordination of the conjugate base cannot necessarily be discounted (40). Simple and ready access to (phosphine)gold(I) cations remains desirable.…”