Calcium Hydride Cation Dimer Catalyzed Hydrogenation of Unactivated 1-Alkenes and H2 Isotope Exchange: Competitive Ca-H-Ca Bridges and Terminal Ca-H Bonds.

ABSTRACT

Recently, it was shown that the double Ca-H-Ca bridged calcium hydride cation dimer complex [LCaH2 CaL]2+ (macrocyclic ligand L=NNNN-tetradentate Me4 TACD) exhibited remarkable activity in catalyzing the hydrogenation of unactivated 1-alkenes as well as the H2 isotope exchange under mild conditions, tentatively via the terminal Ca-H bond of cation monomer LCaH+ . In this DFT mechanistic work, a novel substrate-dependent catalytic mechanism is disclosed involving cooperative Ca-H-Ca bridges for H2 isotope exchange, competitive Ca-H-Ca bridges and terminal Ca-H bonds for anti-Markovnikov addition of unactivated 1-alkenes, and terminal Ca-H bonds for Markovnikov addition of conjugation-activated styrene. THF-coordination plays a key role in favoring the anti-Markovnikov addition while strong cation-π interactions direct the Markovnikov addition to terminal Ca-H bonds.