Valeri Leich

Enantioselective hydrosilylation with chiral frustrated Lewis pairs.

The development of transition-metal-catalyzed asymmetric hydrogenation could be stated as the cradle of modern enantioselective catalysis. Since the early asymmetric hydrogenation example from Knowles and Sabacky in 1968, the method has rapidly advanced over the years into an important tool in academia and chemical industry. In general, for these transformations the development of effective transition-metal complexes having chiral ligands was a basic prerequisite. However, since the pioneering work of Stephan and coworkers in 2006, the field of homogenous hydrogenation has been extended to the possibility of metal-free hydrogenation based on the utilization of frustrated Lewis pairs (FLPs) for hydrogen activation. Combinations of the strong Lewis acid tris(perfluorophenyl)borane (B(C6F5)3) with a variety of sterically encumbered Lewis bases—phosphines, nitrogen bases, and carbon-derived bases—can be used to activate hydrogen at ambient conditions. The concept was subsequently broadened from variations of the Lewis base to modifications of the Lewis acid structure, which resulted in intramolecular FLPs and borane derivatives with increased activity and stability. Furthermore, these chemical peculiarities rapidly found application in catalytic hydrogenation reactions. Some of the FLPs were found to serve as catalysts for the hydrogenation of imines, nitriles, and functionalized alkenes. In the absence of bulky Lewis bases also imine substrates could adopt the function of the FLP partner, and B(C6F5)3 was discovered to be sufficient as the catalyst for their hydrogenation. Additionally, recent mechanistic investigations and preparative experiments corroborated the assumption that for asymmetric transformations, the element of chirality has to be favorably incorporated into the Lewis acid structure. In early experiments employing a-pinene-derived chiral borane, asymmetric reduction of imines was achieved, albeit with low enantioselectivity (13% ee). With these initial findings the synthesis of effective chiral Lewis acids for application in asymmetric hydrogenation reactions was envisioned. On the basis of this concept, the first example of the highly enantioselective hydrogenation of imines with chiral FLPs is demonstrated herein. The initial example with a-pinene-derived chiral borane confirmed the effectiveness of this catalyst structure. However, the stability of the Lewis acid emerged as a major drawback. For the further investigations a chiral borane derived from camphor was considered to be a more suitable structural motif. Reaction of (1R)-(+)-camphor (1) with phenylmagnesium bromide (2) resulted in the tertiary alcohol 3 (Scheme 1). Subsequent dehydration with thionyl chloride/

Hydrosilylation catalysis by an earth alkaline metal silyl: synthesis, characterization, and reactivity of bis(triphenylsilyl)calcium

Bis(triphenylsilyl)calcium [Ca(SiPh3)2(thf)] obtained in high yield as a crystalline ether adduct catalyzes the hydrosilylation of activated C-C double bonds efficiently and regioselectively.

Formation of a Cationic Calcium Hydride Cluster with a "Naked" Triphenylsilyl Anion by Hydrogenolysis of Bis(triphenylsilyl)calcium.

Protonolysis of bis(triphenylsilyl)calcium [Ca(SiPh3)2(THF)4] (1; THF = tetrahydrofuran) with the NNNN-type macrocyclic amido triamine (Me3TACD)H (TACD = 1,4,7-triazacyclododecane) gave the heteroleptic calcium complex [Ca(Me3TACD)SiPh3] (2) in quantitative yield. Hydrogenolysis of 2 gave the cationic tricalcium dihydride cluster [Ca3H2(Me3TACD)3](+)(SiPh3)(-)·2THF (4a) in high yield with concomitant formation of HSiPh3. In the crystal, 4a consists of a cluster cation and a free triphenylsilyl anion. (1)H NMR spectroscopy and deuterium labeling experiments confirmed the selective cleavage of dihydrogen by the highly polar Ca-Si bond in 1.

Molecular Calcium Hydride: Dicalcium Trihydride Cation Stabilized by a Neutral NNNN-Type Macrocyclic Ligand.

Hydrogenolysis of bis(triphenylsilyl)calcium containing the neutral NNNN-type macrocyclic amine ligand Me4TACD [Ca(Me4TACD)(SiPh3)2] (2), gave the cationic dinuclear calcium hydride [Ca2H3(Me4TACD)2](SiPh3) (3), characterized by NMR spectroscopy, single-crystal X-ray analysis, and DFT calculations. Compound 3 reacted with deuterium to give the deuteride [D3]-3.

Hypervalent Hydrosilicates Connected to Light Alkali Metal Amides: Synthesis, Structure, and Hydrosilylation Catalysis

When light alkali metal amides [M(Me3 TACD)]n (M=Li, Na, K; (Me3 TACD)H=1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane) were treated with H2 SiPh2 in THF, [M{(H2 SiPh2 )Me3 TACD}(thf)x ] containing a pendant hypervalent dihydridosilicate were formed and characterized by elemental analysis, NMR/IR spectroscopy, and single-crystal X-ray crystallography. The lithium complex catalyzed the hydrosilylation of styrene derivatives under mild conditions with anti-Markovnikov regioselectivity.

CCDC 1565211: Experimental Crystal Structure Determination

Related Article: Lara E. Lemmerz, Valeri Leich, Daniel Martin, Thomas P. Spaniol, Jun Okuda|2017|Inorg.Chem.|||doi:10.1021/acs.inorgchem.7b02233