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

Abstract

The fifteen‐membered NNNNN macrocycle Me5PACP (Me5PACP=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 Me5PACP followed by hydrogenolysis or treating with nOctSiH3 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 Me4TACD (Me4TACD=1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane). Kinetic investigations indicate that higher catalytic efficiency for the Me5PACP stabilized calcium hydride is due to easier dissociation of the dimer in solution when compared to the Me4TACD analogue.

Heterometallic Mg−Ba Hydride Clusters in Hydrogenation Catalysis

“…[16] They are also regarded intermediates in hydrogenation catalysis, a field that underwent rapid growth and development. [16][17][18][19][20][21][22][23][24][25][26][27] Their applications also include hydrogenation of challenging arenes, [26,28] Hydrogen-Isotope-Exchange, [29] CÀ F bond activation" [30][31][32] ethylene polymerization, [11] and controlled reactions with olefins to isolate first examples of Ca alkyls and a Sr ethyl complex. [11,33] However, looking back at the beginnings of Ae hydride chemistry, the research field was pioneered by a heterobimetallic system.…”

Generation of organo-alkaline earth metal complexes from non-polar unsaturated molecules and their synthetic applications

“…The syn-hydromagnesiation of the alkyne generated the vinyl magnesium intermediate, which underwent another s-bond metathesis with HB(pin) 4, to regenerate butylmagnesium hydride species with the release of the hydroborated product. 33 Solvothermal treatment of sodium hydride (42) with magnesium iodide (43) in THF was found to allow for counter ion metathesis, generating highly reactive magnesium hydride (44) [MgH 2 ] n , which could induce unprecedented antihydromagnesiation of arylalkynes such as diphenylacetylene (45) (Scheme 11A). 34 The resulting alkenylmagnesium intermediate 46 could be trapped with a series of electrophiles such as water (for 47), HB(pin) 4 (for 48), 1,2-dibromotetrachloroethane (for 49) and dimethylformamide (DMF) (for 50) to afford stereochemically well dened functionalized alkenes.…”

“…The unique hydridic reactivity of magnesium hydride (44) generated in situ from sodium hydride (42) and magnesium iodide (43) was extended further to regioselective hydromagnesiation of 1,3-enynes 56 to form an equilibrium mixture of allenylmagnesium 57 and propargylmagnesium 58 (Scheme 12). 35 Downstream functionalization of the resulting organomagnesium intermediates 57 and 58 was demonstrated by subsequent treatment with a series of alkyl and silyl halides in the presence of CuCN as a catalyst, affording polysubstituted allenes 59-62.…”

“…A more recent study revealed that the employment of 1,4,7,10,13pentamethyl-1,4,7,10,13-pentaazacyclopentadecane as the pentadentate macrocyclic ligand could enhance the catalytic activity of the calcium hydride complex toward the hydrogenation and hydrosilylation. 43 Maron discovered that b-diketiminato calcium hydride dimer 95 with no THF solvation on the calcium center undergoes hydrometallation of non-activated terminal alkenes such as 1-butene (96) in C 6 D 6 at ambient temperature to form alkylcalcium dimer 97 (Scheme 18). 44 Further treatment of 97 at 60 C liberated n-butylbenzene 99 along with the regeneration of the calcium deuteride dimer 95-d 2 .…”

“…A more recent study revealed that the employment of 1,4,7,10,13-pentamethyl–1,4,7,10,13-pentaazacyclopentadecane as the pentadentate macrocyclic ligand could enhance the catalytic activity of the calcium hydride complex toward the hydrogenation and hydrosilylation. 43 …”