Role of Benzylic Deprotonation in Nickel-Catalyzed Benzylic Dehydrogenation

Comprehensive Mechanistic Analysis of Palladium- and Nickel-Catalyzed α,β-Dehydrogenation of Carbonyls via Organozinc Intermediates

Introducing degrees of unsaturation into small molecules is a central transformation in organic synthesis. A strategically useful category of this reaction type is the conversion of alkanes into alkenes for substrates with an adjacent electron-withdrawing group. An efficient strategy for this conversion has been deprotonation to form a stabilized organozinc intermediate that can be subjected to α,β-dehydrogenation through palladium or nickel catalysis. This general reactivity blueprint presents a window to uncover and understand the reactivity of Pd- and Ni-enolates. Within this context, it was determined that β-hydride elimination is slow and proceeds via concerted syn-elimination. One interesting finding is that β-hydride elimination can be preferred to a greater extent than C-C bond formation for Ni, more so than with Pd, which defies the generally assumed trends that β-hydride elimination is more facile with Pd than Ni. The discussion of these findings is informed by KIE experiments, DFT calculations, stoichiometric reactions, and rate studies. Additionally, this report details an in-depth analysis of a methodological manifold for practical dehydrogenation and should enable its application to challenges in organic synthesis.

Aromatic nitrogen scanning by ipso-selective nitrene internalization

Nitrogen scanning in aryl fragments is a valuable aspect of the drug discovery process, but current strategies require time-intensive, parallel, bottom-up synthesis of each pyridyl isomer because of a lack of direct carbon-to-nitrogen (C-to-N) replacement reactions. We report a site-directable aryl C-to-N replacement reaction allowing unified access to various pyridine isomers through a nitrene-internalization process. In a two-step, one-pot procedure, aryl azides are first photochemically converted to 3H-azepines, which then undergo an oxidatively triggered C2-selective cheletropic carbon extrusion through a spirocyclic azanorcaradiene intermediate to afford the pyridine products. Because the ipso carbon of the aryl nitrene is excised from the molecule, the reaction proceeds regioselectively without perturbation of the remainder of the substrate. Applications are demonstrated in the abbreviated synthesis of a pyridyl derivative of estrone, as well as in a prototypical nitrogen scan.

Cluster Preface: Modern Nickel-Catalyzed Reactions

Ruben Martin is a professor at the Institute of Chemical Research of Catalonia (ICIQ), Tarragona, Spain. He received his Ph.D. in 2003 from the University of Barcelona under the guidance of Prof. Antoni Riera. In 2004, he moved to the Max-Planck Institut für Kohlenforschung as a Humboldt postdoctoral fellow with Prof. Alois Fürstner. In 2005, he undertook further postdoctoral studies at MIT with Prof. Stephen L. Buchwald as a MEC-Fulbright fellow. In 2008, he began his independent career as an assistant professor at the ICIQ (Tarragona). In 2013, he was promoted to associate professor and shortly after to ICREA Research Professor. Ruben Martin has focused his career on designing synthetically useful Ni-catalyzed methodologies for streamlining the preparation of added-value chemicals from simple precursors without losing sight of mechanistic considerations, when appropriate.

Gary A. Molander is a professor at the University of Pennsylvania, Philadelphia, United States. He completed his undergraduate studies in chemistry at Iowa State University under the tutelage of Prof. Richard C. Larock. He earned his Ph.D. at Purdue University under the direction of Prof. Herbert Brown and undertook postdoctoral training with Prof. Barry Trost at the University of Wisconsin, Madison. He began his academic career at the University of Colorado, Boulder, moving to the University of Pennsylvania in 1999, where he is currently Professor of Chemistry. His research interests have focused on the utilization of organolanthanides, Pd-catalyzed cross-coupling reactions with trifluoro­borate salts, and the merger of photoredox catalysis and Ni catalysis for tackling a priori uphill transformations under visible-light irradiation for accessing valuable scaffolds in both academic and pharmaceutical laboratories.

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