Efficient reduction of cyclic and linear organic carbonates catalyzed by a readily available earth alkaline catalyst has been achieved. The described homogenous reaction based on a ligand-free magnesium catalyst provides an indirect route for the conversion of CO 2 into valuable alcohols. The reaction proceeds with high yields under mild reaction conditions, with low catalyst loading and short reaction times, and shows a broad applicability toward various linear and cyclic carbonates. Additionally, it can be applied for the depolymerization of polycarbonates.
The addition of a
B–H bond to an unsaturated bond (polarized
or unpolarized) is a powerful and atom-economic tool for the synthesis
of organoboranes. In recent years, s-block organometallics have appeared
as alternative catalysts to transition-metal complexes, which traditionally
catalyze the hydroboration of unsaturated bonds. Because of the recent
and rapid development in the field of hydroboration of unsaturated
bonds catalyzed by alkali (Li, Na, K) and alkaline earth (Mg, Ca,
Sr, Ba) metals, we provide a detailed and updated comprehensive review
that covers the synthesis, reactivity, and application of s-block
metal catalysts in the hydroboration of polarized as well as unsaturated
carbon–carbon bonds. Moreover, we describe the main reaction
mechanisms, providing valuable insight into the reactivity of the
s-block metal catalysts. Finally, we compare these s-block metal complexes
with other redox-neutral catalytic systems based on p-block metals
including aluminum complexes and f-block metal complexes of lanthanides
and early actinides. In this review, we aim to provide a comprehensive,
authoritative, and critical assessment of the state of the art within
this highly interesting research area.
The zinc acetate promoted asymmetric hydrosilylation of various ketones and imines under solvent‐free conditions was examined by using an unprecedented low catalyst loading. Exposure of ketones to 0.05 mol % Zn‐based chiral diamine complex in the presence of triethoxysilane afforded enantioenriched alcohols in excellent yields (up to 98 %) and enantioselectivities (up to 97 % ee). This methodology also allowed for the chemoselective 1,2‐reduction of α,β‐unsaturated ketones and imines.
A new reduction of carbamates to N-methyl amines is presented. The magnesium-catalyzed reduction reaction allows the conversion of cyclic and linear carbamates, including N-Boc protected amines, into the corresponding N-methyl amines and amino alcohols which are of significant interest due to their presence in many biologically active molecules. Furthermore, the reduction can be extended to the formation of N-trideuteromethyl labeled amines.
A regio-and stereoselective magnesium catalyzed hydrostannylation of internal and terminal alkynes has been developed. Excellent yields and selectivities are obtained for a wide range of terminal and internal symmetrical and unsymmetrical alkynes by using this alkaline earth metal catalyst as effective alternative to transition metal catalysts.
Zinc acetate complexes with ac hiral diphenylethylenediamine (DPEDA)-derived ligand have been proved to be efficient catalysts for the enantioselective hydrosilylation of aryl ketones.R eplacing pyrophoric dialkylzinc with the readily available zinc salt simplifies the procedures and provides excellent conversions (up to > 99%) and enantioselectivities(eesupt o9 7%).Keywords: asymmetric synthesis;c hiral alcohols; hydrosilylation;reduction;zinc Asymmetric hydrosilylation of prochiral ketones followed by hydrolysis of the resulting silylether is avaluable tool for the synthesis of enantiopure secondary alcohols,w hich are important building blocks in pharmaceutical, agrochemical, fragrance and flavoring chemistry. Reductiono fk etones,i ncluding asymmetrich ydrosilylation, was commonly carriedo ut using platinum-group metals such as Ru, Rh or Ir.  Recently,e nvironmentally benign and less expensive catalysts have also been exhaustively exploredl eading to broad acceptance of earth-abundant metals such as zinc  and iron  in asymmetric synthesis. Particularly stimulating progress has been observed recently in the asymmetric hydrosilylation of ketones promoted by ironc omplexes.S ince Nishiyama  and Beller  reported the first examples of Fe(OAc) 2 -catalyzed asymmetric hydrosilylation with (EtO) 2 MeSiH and (EtO) 3 SiH, manyi ron-based catalysts have been studied for the asymmetric hydrosilylation of ketones. However, ironc omplexes are often sensitive to the reactionc onditions due to the easy oxidation of iron(II)a nd also require relatively elevatedt emperatures. Only recently, an example of am ore reactive and enantioselective iron complex with the boxmi ligand has been presented by Gade.  In contrast to iron, zinc has only one oxidation state anda sarelatively more stable elements hould deserve certain attention. Surprisingly,s uccessfula pplications of Zn catalysts for asymmetric hydrosilylation are mostly limited to diethylzinc. Theprotocolinvolving application of Et 2 Zn with chiral ligands for the enantioselective reductiono fc arbonyl compounds with PMHS was developed by Mimouni nt he late 1990s. Theb est results for the asymmetric hydrosilylation of acetophenonew ere achieved when diamine ligands were used (eesu pt o8 8%). Zinc complexes with various chiral diamine scaffolds were furtheri nvestigated by Walsh,  Carpentier  andM ikami  providing chiral alcohols with good enantioselectivities (up to 85% ee). Va riationso fc hiral diaminocyclohexane (DACH)-based ligands have been also demonstrated  including an interesting applicationo f am acrocyclic oligoamine for asymmetric reduction with eesu pt o8 9%.  In contrast to well-developed applicationo fdiethylzinc with various ligands,h ydrosililation promoted by inorganic Zn salts is stilli ni ts infancy.I n2 009, Nishiyama developed more sustainable catalysts for asymmetrich ydrosilylation by replacing the highly reactive and hazardous dialkylzinc with zinca cetate. Ligandsb earing thiophene ri...
A highly enantioselective method for the synthesis of β‐hydroxy esters via reductive aldol reaction of acrylates with aryl and heteroaromatic ketones is described. In situ generated catalyst composed of zinc acetate and chiral diamine afforded enantioenriched tertiary alcohols in high yields and with excellent enantioselectivity (up to 91% ee). This is also the first successful application of the zinc hydride reagent in stereoselective reductive aldol reactions of ketones.
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