| dc.description.abstract | Catalytic synthesis of borylated organic molecules has been of great interest given the synthetic utility of borylated material in cross-coupling reactions. However, a glaring absence from the synthetic chemist’s repertoire was the catalytic synthesis of borylated alkynes. In 2013, the Ozerov group disclosed pincer iridium complexes capable of performing the dehydrogenative borylation of terminal alkynes (DHBTA). In the following years, second generation catalysts were disclosed using the 2,2’-bis(dialkylphosphino)diphenylamide (PNP) ligand.
Here we continue the development of PNP ligated iridium DHBTA catalysts and disclose a full mechanistic investigation coupling experimental results with computational investigation. Also disclosed is the first example of a readily available, air-stable iridium DHBTA precatalyst. We have discovered that the DHBTA reaction is moderately tolerant to air and moisture with sufficient pinacolborane present. The ease of handling this new precatalyst will hopefully inspire widespread use of catalytic methods for the construction of alkynylboronates.
Inspired by our success with (POCOP)Rh(Ar)(SAr’) and (PNP)Rh(Ar)(SAr’) in C–S bond formation reactions, (POCOP)Co(Ar)(SAr’) and (PNP)Co(Ar)(SAr’) were synthesized and their efficacy in C–S bond formation was investigated. The (POCOP)Co and (PNP)Co platforms were determined to be incapable of supporting catalysis. The former was prone to reductive elimination involving the pincer aryl and the latter was plagued by a swift comproportionation reaction after concerted two electron C–S reductive elimination. The mechanism of thiolate scrambling and C–S coupling in the (PNP)Co system was investigated in detail. A series of aryl halides were added to the (PNP)Co system to capture cobalt after reductive elimination hopefully eliciting oxidative addition to the unsaturated (PNP)Co fragment; however, halogen atom abstraction predominated. The halogen atom abstraction and scrambling of ligands between cobalt centers was investigated.
Esterified phenols are attractive substrates for Suzuki-style coupling reactions given the ubiquity of phenols and the stability of the corresponding ester. To this end, the (PNP)Co system was applied to the C–O cleavage of esters. Although the active CoI/III comproportionation reaction prevents catalysis, this is the first documented example of cobalt mediated aryl ester C–O bond activation.
Lastly, the synthesis, characterization, and structural characterization of various cobalt silylene complexes are disclosed. | en |
