Catalytic hydroboration: a study of model hydridoiridium and hydridorhodium boron complexes

The mechanism of catalytic hydroboration was studied through the use of iridium and rhodium model complexes. Oxidative addition of the B-H bond in (1,2-phenylenedioxy) borane (catecholborane) to (Me₃P)₃Ir(Cl)(H) (BO₂C₆H₄ (II) produces mer-(Me₃P}₃Ir(Cl)(H)(B0₂C₆H₄) (II), which was characterized by ¹H NMR spectroscopy and single crystal X-ray diffraction. Compound II reacted with alkynes to form vinyliridium complexes and will catalyze the hydroboration of alkynes with (1,2- phenylenedioxy)borane. The reaction of II with acetylenes was inhibited by the presence of free Lewis bases indicating that the reaction proceeds by a dissociative mechanism. Exchange of the chloride ligand in II occurred with other Lewis bases, indicating that chloride dissociation was responsible for opening up the vacant coordination site on the complex and thus providing for acetylene coordination. When the chloride ligand on II was replaced with other Lewis bases, the reactivity towards trimethylsiliylacetylene was qualitatively determined to be inversely proportional to the strength of the new ligand. The above experiments indicated that 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-alkylvinylborole esters.

The stable metallo-vinyl complex, IX, produced in the reaction of II with dimethyl acetylene dicarboxylate produced 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 observed have not yet been explained.

The rhodium complex was synthesized by oxidative addition of the B-H bond in (1,2- phenylenedioxy) borane to (Me₃P)₃RhCl producing mer-(Me₃Rh(CI)(H)(BO₂C₆H-₄) (XXIV), which was characterized by ¹H NMR spectroscopy. This complex reacted with acetylenes, but more slowly than the iridium complex, II. The resulting vinyl products were also different than those produced in the iridium case. Phosphine dissociation in XXIV was observed, indicating the possibility of a different mechanism than proposed for the iridium complex.