Amine Additives Greatly Expand the Scope of Asymmetric Hydrosilylation of Imines

Formal Anti-Markovnikov Addition of Water to Olefins by Titanocene-Catalyzed Epoxide Hydrosilylation: From Stoichiometric to Sustainable Catalytic Reactions

Here, the evolution of the titanocene-catalyzed hydrosilylation of epoxides that yields the corresponding anti-Markovnikov alcohols is summarized. The study focuses on aspects of sustainability, efficient catalyst activation, and stereoselectivity. The latest variant of the reaction employs polymethylhydrosiloxane (PMHS), a waste product of the Müller-Rochow process as terminal reductant, features an efficient catalyst activation with benzylMgBr and the use of the bench stable Cp₂TiCl₂ as precatalyst. The combination of olefin epoxidation and epoxide hydrosilylation provides a uniquely efficient approach to the formal anti-Markovnikov addition of H₂O to olefins.

Tackling N-Alkyl Imines with 3d Metal Catalysis: Highly Enantioselective Iron-Catalyzed Synthesis of α-Chiral Amines

A readily activated iron alkyl precatalyst effectively catalyzes the highly enantioselective hydroboration of N‐alkyl imines. Employing a chiral bis(oxazolinylmethylidene)isoindoline pincer ligand, the asymmetric reduction of various acyclic N‐alkyl imines provided the corresponding α‐chiral amines in excellent yields and with up to >99 % ee. The applicability of this base metal catalytic system was further demonstrated with the synthesis of the pharmaceuticals Fendiline and Tecalcet.

Titanium catalysis for the synthesis of fine chemicals – development and trends

Atlas as a Titan(ium) is holding the earth-abundant chemistry world. Titanium is the second most abundant transition metal, is a key player in important industrial processes (e.g. polyethylene) and shows much promise for diverse applications in the future.

Early Main Group Metal Catalysts for Imine Hydrosilylation

The efficient catalytic reduction of imines with phenylsilane is achieved by using the potassium, calcium and strontium based catalysts [(DMAT)K (THF)]_∞ , (DMAT)₂ Ca⋅(THF)₂ and (DMAT)₂ Sr⋅(THF)₂ (DMAT=2-dimethylamino-α-trimethylsilylbenzyl). Eight different aldimines and the ketimine Ph₂ C=NPh could be successfully reduced by PhSiH₃ at temperatures between 25-60 °C with catalyst loadings down to 2.5 mol %. Also, simple amides like KN(SiMe₃ )₂ or Ae[N(SiMe₃ )₂ ]₂ (Ae=Ca, Sr, Ba) catalyze this reaction. Activities increase with metal size. For most substrates the activity increases along the row K<Ca<Sr<Ba. Fastest conversion was found for imines with alkyl substituents at N and aryl rings at C, for example, PhC(H)=NtBu, while tBuC(H)=NtBu or PhC(H)=NPh react much slower. Reasonable functional group tolerance is observed. The proposed metal hydride mechanism is supported by stoichiometric reactions using a catalyst model system, isolation of intermediates and DFT calculations.

Click chemistry enables quantitative chiroptical sensing of chiral compounds in protic media and complex mixtures

Click reactions have become powerful synthetic tools with unique applications in the health and materials sciences. Despite the progress with optical sensors that exploit the principles of dynamic covalent chemistry, metal coordination or supramolecular assemblies, quantitative analysis of complex mixtures remains challenging. Herein, we report the use of a readily available coumarin conjugate acceptor for chiroptical click chirality sensing of the absolute configuration, concentration and enantiomeric excess of several compound classes. This method has several attractive features, including wide scope, fast substrate fixation without by-product formation or complicate equilibria often encountered in reversible substrate binding, excellent solvent compatibility, and tolerance of air and water. The ruggedness and practicality of this approach are demonstrated by comprehensive analysis of nonracemic monoamine samples and crude asymmetric imine hydrogenation mixtures without work-up. Click chemosensing addresses increasingly important time efficiency, cost, labor and chemical sustainability aspects and streamlines asymmetric reaction development at the mg scale.

Palladium-Catalyzed Reductive Coupling Reaction of Terminal Alkynes with Aryl Iodides Utilizing Hafnocene Difluoride as a Hafnium Hydride Precursor Leading to trans-Alkenes

Herein, we describe a reductive cross-coupling of alkynes and aryl iodides by using a novel catalytic system composed of a catalytic amount of palladium dichloride and a promoter precursor, hafnocene difluoride (Cp₂ HfF₂ , Cp=cyclopentadienyl anion), in the presence of a mild reducing reagent, a hydrosilane, leading to a one-pot preparation of trans-alkenes. In this process, a series of coupling reactions efficiently proceeds through the following three steps: (i) an initial formation of hafnocene hydride from hafnocene difluoride and the hydrosilane, (ii) a subsequent hydrohafnation toward alkynes, and (iii) a final transmetalation of the alkenyl hafnium species to a palladium complex. This reductive coupling could be chemoselectively applied to the preparation of trans-alkenes with various functional groups, such as an alkyl group, a halogen, an ester, a nitro group, a heterocycle, a boronic ester, and an internal alkyne.

Imidazolium-based ionic liquid-catalyzed hydrosilylation of imines and reductive amination of aldehydes using hydrosilane as the reductant

Efficient imidazolium-based ionic liquid-catalyzed hydrosilylation of imines and reductive amination using a catalytic amount of [BMIm][FeCl₄].

Amine synthesis via transition metal homogeneous catalysed hydrosilylation

This review summarizes the preparation of amines involving homogeneous transition metal catalysed hydrosilylation including reductions of imines, amides, nitro and nitriles, reductive aminations and N-methylation of amines with CO₂.

Iron-catalyzed transfer hydrogenation of imines assisted by an iron-based Lewis acid

An iron-catalyzed transfer hydrogenation of N-aryl and N-alkyl imines using isopropanol as the hydrogen donor is reported for the first time.

Highly diastereo- and enantioselective CuH-catalyzed synthesis of 2,3-disubstituted indolines

A diastereo- and enantioselective CuH-catalyzed method for the preparation of highly functionalized indolines is reported. The mild reaction conditions and high degree of functional group compatibility as demonstrated with substrates bearing heterocycles, olefins, and substituted aromatic groups, renders this technique highly valuable for the synthesis of a variety of cis-2,3-disubstituted indolines in high yield and enantioeselectivity.

Triclorosilane-mediated stereoselective synthesis of β-amino esters and their conversion to highly enantiomerically enriched β-lactams

A highly stereoselective trichlorosilane-mediated reduction of N-benzyl enamines was developed; the combination of a low cost, easy to make metal-free catalyst and an inexpensive chiral auxiliary allowed to perform the reaction on substrates with different structural features often with total control of the stereoselectivity. By easy deprotection through hydrogenolysis followed by conversion of β-aminoester to 2-azetidinones, the synthesis of enantiomerically pure β-lactams (>98% e.e.) was successfully accomplished.

Enantioselective reduction of ketones and imines catalyzed by (CN-box)Re(V)-oxo complexes

The development and application of chiral, non-racemic Re(V)-oxo complexes to the enantioselective reduction of prochiral ketones is described. In addition to the enantioselective reduction of prochiral ketones, we report the application of these complexes to 1) a tandem Meyer-Schuster rearrangement/reduction to access enantioenriched allylic alcohols and 2) the enantioselective reduction of imines.