Enantioselective cross-coupling of anhydrides with organozinc reagents: the controlled formation of carbon-carbon bonds through the nucleophilic interception of metalacycles.

The construction of carbon-carbon bonds, particularly with concomitant control of newly formed asymmetric centers, is of paramount importance for the development of synthetic routes to complex organic molecules. While cross-coupling reactions for the generation of sp(2) carbon centers are well established, similar methodology for the formation and control of sp(3)-hydridized carbon stereocenters is extremely limited. We suggest that the nucleophilic interception of metalacycles provides the means to achieve such a transformation, wherein the metal complex serves to activate electrophiles, facilitate nucleophile addition, and ultimately control stereochemistry. One means of accessing these intermediates is through the use of simple meso-carboxylic anhydrides, which upon reaction with transition metals readily generate the desired metalacycles. Interception of the metalacycle with an appropriate carbon-based nucleophile generates an enantioenriched ketoacid, the product of the asymmetric desymmetrization of achiral starting materials. Early successes with achiral nickel catalysts and organozinc reagents provided the foundations for our approach. Alkylation of both succinic and glutaric anhydrides proceeds with a wide range of organozinc nucleophiles, forming 1,4- and 1,5-ketoacids in excellent yields. This reaction manifold has been extensively examined with a detailed kinetic study and mechanistic investigations utilizing mixed zinc reagents and alkene directing groups. This work has highlighted a number of unusual phenomena, including rate-limiting reductive elimination to form an sp(3)-sp(2) carbon-carbon bond. Despite excellent results with the achiral system, to date, all efforts to render the nickel-catalyzed reaction asymmetric have been limited to modest success. Palladium and rhodium complexes, with the use of chiral P-P and P-N ligands, respectively, have been identified as competent catalysts for the enantioselective addition of organozinc reagents to anhydrides. The arylation of a series of succinic anhydrides with Ph2Zn can be achieved in greater than 95% enantioselectivity using a Pd/Josiphos catalyst. Rhodium catalysts have proven amenable for the incorporation of in situ formed organozinc reagents, nucleophiles traditionally troublesome in transition metal catalysis due to the deleterious effects of residual halide ions. Highly functionalized organozinc nucleophiles, including those containing indole and furan, participate in this chemistry to provide the corresponding 1,4- and 1,5-ketoacids in excellent yield with greater than 85% enantioselectivity. This metalacycle interception methodology is currently being expanded to the use of other systems, most notably the asymmetric [2 + 2 + 2] cycloaddition of alkenes, alkynes, and isocyanates. Ongoing studies promise the extension of existing methodology toward the development of modular, fully intermolecular three-component couplings in which both metalacycle formation and nucleophilic interception can be controlled. Ultimately, we envision the use of heterocumulenes in such methodology, providing a route to complex products utilizing CO2 as an inexpensive C1 feedstock.