ChemInform Abstract: Model Studies of Palladium-Catalyzed Coupling Reactions for the “South- North” Attachment of Avermectins.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.     

Enzyme Catalysis “Reilluminated”

In a new light : The NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR; see structure, green Pchlide, yellow NADPH) is a good model to investigate catalytical processes in enzymes, as its light activation allows an immediate start of the catalyzed reaction. By irradiation with weak, short laser pulses it is possible to detect conformation changes during the reaction and thus to uncover the elementary steps of the catalytic process.

     

Direct Conversion of Nitriles into Alkene “Isonitriles”

The sequenced addition of RLi to nitriles, trapping with isopropylformate, and dehydration with phosphoryl chloride provides an efficient, direct synthesis of alkene isocyanides. The one‐pot sequence involves a series of carefully orchestrated steps: addition, formylation, tautomerization, and dehydration, with CuCN catalyzing a key equilibration of a formyl imine to an N‐formyl enamine. The resulting aromatic alkeneisocyanides, that are otherwise challenging to synthesize, engage in an unusual [4+2]‐type cycloaddition/1,3‐H shift/decyanation sequence to afford substituted naphthalenes.     

Carbocatalysis: The State of “Metal‐Free” Catalysis

The rise in global demand for crucial chemical compounds has driven immense research in the fundamental science of catalysis. Graphene and its derivatives (chemically modified graphene, CMGs) have recently emerged as a new class of heterogeneous catalyst that promises economically viable and greener routes to these compounds. Although CMGs possess unique catalytic properties, the actual active sites are often points of discussion. Current minimal understanding on the possible effects of metallic impurities on the electrocatalytic performances of these CMGs calls forth the need to raise awareness on possible metallic impurities misrepresenting the actual chemical catalytic performances of the CMGs. This Minireview highlights the latest advances in the application of CMGs as catalysts, with an emphasis on the possible effects of metallic impurities on CMG catalysis.     

Design and Enantioselective Synthesis of Cashmeran Odorants by Using “Enol Catalysis”

Novel Cashmeran odorants were designed by molecular modeling. Their short syntheses involve a novel asymmetric Brønsted acid catalyzed Michael addition of unactivated α‐substituted ketones. This key transformation was realized by utilizing a new type of enol activation catalysis and affords different cyclic ketones bearing α‐quaternary stereocenters in good to excellent yields and with high enantioselectivity. Subsequent McMurry coupling and Saegusa–Ito oxidation furnished the enantiopure target odorants, one enantiomer of which indeed possesses the typical olfactory aspects of Cashmeran.     

Computational design of S‐nitrosothiol “click” reactions

To address a long‐standing problem of finding efficient reactions for chemical labeling of protein‐based S‐nitrosothiols (RSNOs), we computationally explored hitherto unknown (3+2) cycloaddition RSNO reactions with alkynes and alkenes. Nonactivated RSNO cycloaddition reactions have high activation enthalpy (>20 kcal/mol at the CBS‐QB3 level) and compete with alternative S—N bond insertion pathway. However, the (3+2) cycloaddition reaction barriers can be dramatically lowered by coordination of a Lewis acid to the N atom of the —SNO group. To exploit this effect, we propose to use reagents with Lewis acid and a strain‐activated carbon–carbon multiple bond linked by a rigid scaffold, which can react with RSNOs with small activation enthalpies (～5 kcal/mol) and high reaction exothermicities (～40 kcal/mol). The proposed efficient RSNO cycloaddition reactions can be used for future development of practical RSNO labeling reactions. © 2013 Wiley Periodicals, Inc.     

“First‐Principles” kinetic monte carlo simulations revisited: CO oxidation over RuO2(110)

First principles‐based kinetic Monte Carlo (kMC) simulations are performed for the CO oxidation on RuO₂(110) under steady‐state reaction conditions. The simulations include a set of elementary reaction steps with activation energies taken from three different ab initio density functional theory studies. Critical comparison of the simulation results reveals that already small variations in the activation energies lead to distinctly different reaction scenarios on the surface, even to the point where the dominating elementary reaction step is substituted by another one. For a critical assessment of the chosen energy parameters, it is not sufficient to compare kMC simulations only to experimental turnover frequency (TOF) as a function of the reactant feed ratio. More appropriate benchmarks for kMC simulations are the actual distribution of reactants on the catalyst's surface during steady‐state reaction, as determined by in situ infrared spectroscopy and in situ scanning tunneling microscopy, and the temperature dependence of TOF in the from of Arrhenius plots. © 2012 Wiley Periodicals, Inc.     

Homogeneous Catalysts with a Mechanical (“Machine‐like”) Action

Molecular machines? The factors involved in and the fundamental nature of a “mechanical” action within molecular catalysis is explored in detail. Such mechanical homogeneous catalysts are rare but not unknown (see graphic). The spatial and temporal nature of their catalytic action is considered. Potential implications for mimicry of enzymes are discussed.

     

Reply to the comment on “A stationary‐wave model of enzyme catalysis”

This letter briefly outlines the grounds that led to an expression for reaction rate co‐efficients based on the incomplete gamma function of Euler. The proposed law explains the catalytic activity of enzymes as an effect of the coupling of active vibrational modes to the reaction coordinate, and predicts the observed negative curvature of the Arrhenius plots of the relevent reactions. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2011     

TMEDA in Iron‐Catalyzed Kumada Coupling: Amine Adduct versus Homoleptic “ate” Complex Formation

The reactions of iron chlorides with mesityl Grignard reagents and tetramethylethylenediamine (TMEDA) under catalytically relevant conditions tend to yield the homoleptic “ate” complex [Fe(mes)₃]^? (mes=mesityl) rather than adducts of the diamine, and it is this ate complex that accounts for the catalytic activity. Both [Fe(mes)₃]^? and the related complex [Fe(Bn)₃]^? (Bn=benzyl) react faster with representative electrophiles than the equivalent neutral [FeR₂(TMEDA)] complexes. Fe^I species are observed under catalytically relevant conditions with both benzyl and smaller aryl Grignard reagents. The X‐ray structures of [Fe(Bn)₃]^? and [Fe(Bn)₄]^? were determined; [Fe(Bn)₄]^? is the first homoleptic σ‐hydrocarbyl Fe^III complex that has been structurally characterized.     

Heterogeneous “Organoclick” Derivatization of Polysaccharides: Photochemical Thiol‐ene Click Modification of Solid Cellulose

A simple and direct method for derivatization of solid polysaccharides is presented. The novel methodology is based on the combination of organic acid‐catalyzed esterification or etherification and photochemical thiol‐ene click derivatization of a heterogeneous polysaccharide. The solid cellulose was “organoclick” modified with aryl, alkyl and polyester groups, respectively. The modification allows for a highly modular and metal free surface modification of solid polysaccharides.

     

Functionalized Cross-Linked Copolymers: A “C2-Symmetric” Solid-Phase Catalyst for Enantioselective Reactions

Similar enantiomeric excesses as for analogous monomeric sulfonamides are provided by the chiral resin 1 (connections to the polymer are shown as circles) when it is used as catalyst for the reductive alkylation of aromatic aldehydes and for the cyclopropanation of cinnamyl alcohol.