Regioselective Hydrosilylation of Olefins Catalyzed by a Molecular Calcium Hydride Cation

Abstract

Chemo‐ and regioselectivity are often difficult to control during olefin hydrosilylation catalyzed by d‐ and f‐block metal complexes. The cationic hydride of calcium [CaH]+ stabilized by an NNNN macrocycle was found to catalyze the regioselective hydrosilylation of aliphatic olefins to give anti‐Markovnikov products, while aryl‐substituted olefins were hydrosilyated with Markovnikov regioselectivity. Ethylene was efficiently hydrosilylated by primary and secondary hydrosilanes to give di‐ and monoethylated silanes. Aliphatic hydrosilanes were preferred over other commonly employed hydrosilanes: Arylsilanes such as PhSiH3 underwent scrambling reactions promoted by the nucleophilic hydride, while alkoxy‐ and siloxy‐substituted hydrosilanes gave isolable alkoxy and siloxy calcium derivatives.

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

“…The Ca⋅⋅⋅Ca distance is 3.7178(10) Å in 3 b and longer than that in the Me 4 TACD analogue (3.6306(11) Å). The Ca‐H distances of 2.24(2) and 2.27(2) Å in 3 b are in the same range as for C (2.22(3) to 2.34(3) Å) [7c] . A folded coordination pattern in Me 4 TACD stabilized complexes has been observed only in the solid state for [(Me 4 TACD) 2 Mg 2 (μ‐O 2 CH) 2 ][B(C 6 H 3 ‐3,5‐Me 2 ) 4 ] [9] but is more common for larger macrocyclic ligands [10] .…”

“…Complex 3 b decomposes at 50 °C within 6 h. In the 1 H NMR spectrum of 3 b , the hydride resonance appears at δ =4.63 ppm which is in the expected region for molecular calcium hydrides [3a, 7b, c] . The resonance is shifted downfield when compared to the hydride resonance at δ =4.46 ppm in C and slightly upfield shifted compared to [(Me 4 TACD) 2 Ca 2 (μ‐H) 3 ][B(C 6 H 3 ‐3,5‐Me 2 ) 4 ] at δ =4.71 ppm [7b, c] . A solution of 3 b in [D 8 ]THF reacted with D 2 (1 bar) to give 3 b ‐ d 2 and HD within 3 h. This reaction is faster than that observed for C (8 h).…”

“…Compared to transition‐metal and lanthanide‐based systems, alkaline earth metal catalysts for hydrofunctionalization are still underdeveloped and understanding structure–activity/selectivity relationships in these systems appears crucial for their improvement. Previously we have introduced the NNNN macrocycle Me 4 TACD (Me 4 TACD=1,4,7,10‐tetramethyl‐1,4,7,10‐tetraazacyclododecane) derived from the twelve‐membered cyclen to support [CaH] + as dinuclear hydride cations [(Me 4 TACD) 2 Ca 2 (μ‐H) 2 (thf)][BAr 4 ] 2 ( C , Ar=C 6 H 3 ‐3,5‐Me 2 ) and shown their reactivity toward inactivated olefins [7b, c] . Evidently, this macrocycle is capable of kinetically stabilizing highly electrophilic fragments such as [CaH] + , although the mononuclear species [(Me 4 TACD)CaH(thf) x ] + remains elusive.…”

Molecular Zinc Hydride Cations [ZnH]+: Synthesis, Structure, and CO2 Hydrosilylation Catalysis

“…[21] Therefore we applied tertiary alkylsubstituted silanes such as n BuMe 2 SiH or Et 3 SiH which are more likely to be inert toward this exchange. [22] TheC O 2 hydrosilylation using these hydrosilanes was benchmarked by using 2mol %of2a or 2b in THF at 70 8 8C (Table 1).…”

Molekulare Zinkhydridkationen [ZnH]+: Synthese, Struktur und CO2‐Hydrosilylierungskatalyse

“…[21] Daher haben wir alkylsubstituierte Hydrosilane wie n BuMe 2 SiH oder Et 3 SiH verwendet, welche eher gegen einen solchen Austausch inert sind. [22] Die Hydrosilylierung von CO 2 unter Verwendung dieser Hydrosilane wurde durch 2mol % 2a oder 2b in THF bei 70 8 8Cg etestet (Tabelle 1).…”