Synthesis and reaction chemistry of boryliridium hydride complexes formed by oxidative addition of catecholborane to iridium(I): Lessons for metal-catalyzed hydroboration

The mechanism of catalytic hydroboration was studied through the use of an iridium model complex. Oxidative addition of the B–H bond in (1,2-phenylenedioxy)borane (catecholborane) to (Me3P)3Ir(COE)(Cl) (COE = cyclooctene) produces mer-(Me3P)3Ir(Cl)(H)(BO2C6H4) (1). Compound 1 reacted with alkynes to form vinyliridium complexes and will catalyze the hydroboration of alkynes with (1,2-phenylenedioxy)borane. The mechanism of catalytic hydroboration of acetylenes with catecholborane involves: oxidative addition of the B–H bond to the iridium center, followed by chloride dissociation and acetylene coordination, migratory-insertion into the Ir–H bond to form the metallo-vinyl complex, and finally reductive elimination to produce trans-alkylvinylboronate esters. A stable metallo-vinyl complex was produced in the reaction of 1 with dimethylacetylene dicarboxylate and displayed two isomers in solution, one of which showed fluxional behavior. Single crystal X-ray diffraction elucidated a single solid-state structure, but the structures of the isomers in solution and the fluxional properties were studied with NMR spectroscopy and DFT calculations.