Impact of the Methylene Bridge Substitution in Chelating NHC-Phosphine Mn(I) Catalyst for Ketone Hydrogenation.

Systematic modification of the chelating NHC-phosphine ligand (NHC = N-heterocyclic carbene) in highly efficient ketone hydrogenation Mn(I) catalyst fac-[(PhPCHNHC)Mn(CO)Br] has been performed and the catalytic activity of the resulting complexes was evaluated using acetophenone as a benchmark substrate. While the variation of phosphine and NHC moieties led to inferior results than for a parent system, the incorporation of a phenyl substituent into the ligand methylene bridge improved catalytic performance by ca. 3 times providing maximal TON values in the range of 15000-20000.

Hydrosilylation Reactions Catalyzed by Rhenium.

Hydrosilylation is an important process, not only in the silicon industry to produce silicon polymers, but also in fine chemistry. In this review, the development of rhenium-based catalysts for the hydrosilylation of unsaturated bonds in carbonyl-, cyano-, nitro-, carboxylic acid derivatives and alkenes is summarized. Mechanisms of rhenium-catalyzed hydrosilylation are discussed.

Manganese and rhenium-catalyzed selective reduction of esters to aldehydes with hydrosilanes.

The selective reduction of esters to aldehydes, via the formation of stable alkyl silyl acetals, was, for the first time, achieved with both manganese, -Mn(CO)- and rhenium -Re(CO)- catalysts in the presence of triethylsilane as reductant. These two methods provide a direct access to a large variety of aliphatic and aromatic alkyl silyl acetals (30 examples) and to the corresponding aldehydes (13 examples) upon hydrolysis. The reactions proceeded in excellent yields and high selectivity at room temperature under photo-irradiation conditions (LED, 365 nm, 40 W, 9 h).

Rhenium-Catalyzed Reduction of Carboxylic Acids with Hydrosilanes.

Re(CO) efficiently catalyzes the direct reduction of various carboxylic acids under mild conditions (rt, irradiation 350 or 395 nm). While aliphatic carboxylic acids were readily converted to the corresponding disilylacetals with low catalyst loading (0.5 mol %) in the presence of EtSiH (2.

Phosphine-NHC Manganese Hydrogenation Catalyst Exhibiting a Non-Classical Metal-Ligand Cooperative H Activation Mode.

Deprotonation of the Mn NHC-phosphine complex fac-[MnBr(CO) (κ P,C-Ph PCH NHC)] (2) under a H atmosphere readily gives the hydride fac-[MnH(CO) (κ P,C-Ph PCH NHC)] (3) via the intermediacy of the highly reactive 18-e NHC-phosphinomethanide complex fac-[Mn(CO) (κ P,C,C-Ph PCHNHC)] (6 a). DFT calculations revealed that the preferred reaction mechanism involves the unsaturated 16-e mangana-substituted phosphonium ylide complex fac-[Mn(CO) (κ P,C-Ph P=CHNHC)] (6 b) as key intermediate able to activate H via a non-classical mode of metal-ligand cooperation implying a formal λ -P-λ -P phosphorus valence change. Complex 2 is shown to be one of the most efficient pre-catalysts for ketone hydrogenation in the Mn series reported to date (TON up to 6200).

Base-Free Generation of Organic Electron Donors from Air-Stable Precursors.

Organic electron donors (OEDs) are powerful reducing agents recognized for their potential in the reduction of challenging substrates and in original applications. Nonetheless, their low stability in atmospheric oxygen or over time complicates their manipulation and storage. To overcome these constraints and enhance OED practicality, new air- and moisture-stable aminopyridinium carboxylate and carbonate precursors were synthesized and thermally activated to generate the potent electron donor in situ.