UC San Diego
Taming Carbene Copper(I)-Hydride Complexes
- Author(s): Romero, Erik Anthony
- Advisor(s): Bertrand, Guy
- et al.
Although initially reported in 1844, copper(I)-hydrides were not widely studied until the isolation of “Stryker’s reagent” in 1971. Following this development, researchers reported a myriad of chemical transformations employing other transient copper-hydride complexes as highly stereo-, regio-, and enantioselective catalysts. Each of these processes, however, rely on the ability of Cu-H bonds to insert into substrate π-bonds. By focusing on reactions involving this mechanistic step, chemists have limited the potential applications of copper-hydrides to reactions within this niche.
The first part of this manuscript discusses the ability of CAAC copper complexes to affect dehydrogenative borylation reactions of terminal acetylenes under mild conditions. Moreover, during this study, we uncovered convincing evidence that points to the surprising involvement of a copper-hydride complex in the key dihydrogen extrusion step of the mechanistic cycle. We further show that more basic anions on copper switch the selectivity of the catalyst to favor formation of traditional anti-Markovnikov hydroboration products. Lastly, the active catalysts for both reactions were shown to tolerate a wide range of synthetically useful functional groups.
The second part of this manuscript describes our attempts to utilize extremely sterically encumbering NHC ligands to stabilize and characterize the first monomeric, monoligated copper(I)-hydride complex. Although this species exists as a dimer in the solid state, we confirmed the presence of an equilibrium in solution through variable temperature 1H NMR experiments and extracted the kinetic and thermodynamic parameters for this dimerization process. We then applied our synthetic strategy to the isolation and crystallographic characterization of the first neutral monoligated silver(I)-hydride. Preliminary experiments indicate that its reactivity mirrors that of copper-hydrides as opposed to gold-hydrides. Refocusing on copper, we employed borane stabilized copper-hydride complexes as benchtop stable surrogates for their highly reactive LCu-H counterparts in the hydrogenation of CO2 with H2. Excitingly, we found that this process is enhanced by an in situ generated amino-borane co-catalyst, thereby affording a rare example of tandem catalysis involving both a transition-metal complex and a classical Lewis pair.
The final chapter discusses the uncatalyzed dehydrogenative coupling of alcohols, amines and thiols with pinacolborane and 9-borabicyclononane, a result which contrasts with literature precedent.