Calcium Hydride Catalysts for Olefin Hydrofunctionalization: Ring-Size Effect of Macrocyclic Ligands on Activity.

The fifteen-membered NNNNN macrocycle Me PACP (Me PACP=1,4,7,10,13-pentamethyl-1,4,7,10,13-pentaazacyclopentadecane) stabilized the [CaH] fragment as a dimer with a distorted pentagonal bipyramidal coordination geometry at calcium. The hydride complex was prepared by protonolysis of calcium dibenzyl with the conjugate acid of Me PACP followed by hydrogenolysis or treating with OctSiH of the intermediate calcium benzyl cation. The calcium hydride catalyzed the hydrogenation and hydrosilylation of unactivated olefins faster than the analogous calcium complex stabilized by the twelve-membered NNNN macrocycle Me TACD (Me TACD=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). Kinetic investigations indicate that higher catalytic efficiency for the Me PACP stabilized calcium hydride is due to easier dissociation of the dimer in solution when compared to the Me TACD analogue.