Nickel-Catalyzed Stereoselective Coupling Reactions of Benzylic and Alkyl Alcohol Derivatives.

Abstract

ConspectusNickel-catalyzed reactions of alkyl alcohol derivatives leverage the high prevalence of hydroxyl groups in natural products, medicinal agents, and synthetic intermediates to provide access to C(sp3)-rich frameworks. This Account describes our laboratory's development of stereospecific and stereoconvergent C-C bond forming reactions employing C(sp3)-O and C(sp3)-N electrophiles. In the context of development of new transformations, we also define fundamental characteristics of the nickel catalysts.Part I details the nickel-catalyzed cross-coupling reactions developed by our group which hinges on stereospecific formation of stable π-benzyl intermediates. Acyclic and cyclic ethers, esters, carbamates, lactones, and sulfonamides undergo Kumada-, Suzuki-, and Negishi-type coupling reactions to produce enantioenriched products with high fidelity of stereochemical information. We describe extension to include ring-opening reactions of saturated heterocycles to afford acyclic 1,3-fragments in high diastereomeric ratios. We also describe our advances in stereospecific nickel-catalyzed cross-electrophile coupling reactions. Tethered C-O and C-X electrophiles proved fruitful for construction of a variety of carbocyclic frameworks. We report an intramolecular cross-electrophile coupling of benzylic pivalates with aryl bromides for the synthesis of indanes and tetralins. We found that 4-halotetrahydropyrans and 4-halopiperidines readily undergo stereospecific ring contraction to afford substituted cyclopropanes. Mechanistic investigations are consistent with closed-shell intermediates, a Ni(0)/Ni(II) cycle, and an intramolecular SN2-type reaction of a key organonickel intermediate to form the cyclopropane. Building toward more complex cascade reactions, we have demonstrated that 2-alkynyl piperidines incorporate MeMgI in a dicarbofunctionalization of the alkyne to afford highly substituted vinyl cyclopropanes.In Part II we present our development of stereoconvergent reactions of alkyl alcohol derivatives. In order to expand the utility of the intramolecular XEC reaction, we sought to employ unactivated alkyl electrophiles. Specifically, alkyl dimesylates engage in intramolecular XEC reactions to form alkyl cyclopropanes. In contrast to our previous work, these reactions proceed through open-shell intermediates and favor stereoconvergent formation of the trans-cyclopropane. Enantioselective aldol reactions can be employed in syntheses of 1,3-diols which furnish enantioenriched cyclopropanes in high ee. Experimental and computational evidence reveals that MeMgI mediates formation of alkyl iodides in situ. The coupling reaction initiates with halogen atom abstraction at the secondary alkyl iodide. The alkyl Ni(II) complex then proceeds through a stereospecific SN2-type ring closure to form cyclopropane. In an effort to increase functional group compatibility in the synthesis of cyclopropanes from alkyl dimesylates we developed a zinc-mediated reaction of 1,3-dimesylates prepared from medicinal analogues. In challenging nickel-catalyzed intramolecular cross-electrophile coupling we were also able to show that vicinal carbocycles can be prepared under similar conditions, affording vicinal cyclopentyl-cyclopropyl motifs in high yield.In Part III we discuss our recent findings on the role of ligand identity in catalyst selectivity for stereospecific vs stereoablative mechanisms for oxidative addition. We demonstrate multivariable control of mechanism, where the choice of substrate and ligand work together to promote open- or closed-shell intermediates. In divergent reactions of 4-halotetrahydropyrans we observe distinct ligand preference for reactions at the C(sp3)-O center or the C(sp3)-Cl center. These findings are the source of continued investigations in our laboratory.