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

The fifteen-membered NNNNN macrocycle Me 5 PACP (Me 5 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 5 PACP followed by hydrogenolysis or treating with n OctSiH 3 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 4 TACD (Me 4 TACD = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane). Kinetic investigations indicate that higher catalytic efficiency for the Me 5 PACP stabilized calcium hydride is due to easier dissociation of the dimer in solution when compared to the Me 4 TACD analogue.