Synthesis and activity of a new generation of ruthenium-based olefin metathesis catalysts coordinated with 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligands.

ion. The oxidative addition mechanism was originally proposed22i because of the lack of a strong rate dependence on polar factors and on the acidity of the medium. Later, however, the electrophilic substitution mechanism also was proposed. Recently, the oxidative addition mechanism was confirmed by investigations into the decomposition and protonolysis of alkylplatinum complexes, which are the reverse of alkane activation. There are two routes which operate in the decomposition of the dimethylplatinum(IV) complex Cs2Pt(CH3)2Cl4. The first route leads to chloride-induced reductive elimination and produces methyl chloride and methane. The second route leads to the formation of ethane. There is strong kinetic evidence that the ethane is produced by the decomposition of an ethylhydridoplatinum(IV) complex formed from the initial dimethylplatinum(IV) complex. In D2O-DCl, the ethane which is formed contains several D atoms and has practically the same multiple exchange parameter and distribution as does an ethane which has undergone platinum(II)-catalyzed H-D exchange with D2O. Moreover, ethyl chloride is formed competitively with H-D exchange in the presence of platinum(IV). From the principle of microscopic reversibility it follows that the same ethylhydridoplatinum(IV) complex is the intermediate in the reaction of ethane with platinum(II). Important results were obtained by Labinger and Bercaw62c in the investigation of the protonolysis mechanism of several alkylplatinum(II) complexes at low temperatures. These reactions are important because they could model the microscopic reverse of C-H activation by platinum(II) complexes. Alkylhydridoplatinum(IV) complexes were observed as intermediates in certain cases, such as when the complex (tmeda)Pt(CH2Ph)Cl or (tmeda)PtMe2 (tmeda ) N,N,N′,N′-tetramethylenediamine) was treated with HCl in CD2Cl2 or CD3OD, respectively. In some cases H-D exchange took place between the methyl groups on platinum and the, CD3OD prior to methane loss. On the basis of the kinetic results, a common mechanism was proposed to operate in all the reactions: (1) protonation of Pt(II) to generate an alkylhydridoplatinum(IV) intermediate, (2) dissociation of solvent or chloride to generate a cationic, fivecoordinate platinum(IV) species, (3) reductive C-H bond formation, producing a platinum(II) alkane σ-complex, and (4) loss of the alkane either through an associative or dissociative substitution pathway. These results implicate the presence of both alkane σ-complexes and alkylhydridoplatinum(IV) complexes as intermediates in the Pt(II)-induced C-H activation reactions. Thus, the first step in the alkane activation reaction is formation of a σ-complex with the alkane, which then undergoes oxidative addition to produce an alkylhydrido complex. Reversible interconversion of these intermediates, together with reversible deprotonation of the alkylhydridoplatinum(IV) complexes, leads to multiple H-D exchange

Activation of c-h bonds by metal complexes

Dioxygen activation at mononuclear nonheme iron active sites: enzymes, models, and intermediates.

Electrochemistry represents one of the most intimate ways of interacting with molecules. This review discusses advances in synthetic organic electrochemistry since 2000. Enabling methods and synthetic applications are analyzed alongside innate advantages as well as future challenges of electroorganic chemistry.

Synthetic Organic Electrochemical Methods Since 2000: On the Verge of a Renaissance

Kinetics of II-VI and III-V Colloidal Semiconductor Nanocrystal Growth: “Focusing” of Size Distributions

Asymmetric Total Synthesis of (+)-7-Deoxypancratistatin.

4-Siloxy-α-bromocrotonate: A New Reagent for (2 + 3)Annulation Leading to Oxygenated Cyclopentenes at Low Temperatures.

Biocatalysis. Fundamentals and Applications By A. S. Bommarius (Georgia Institute of Technology) and B. R. Riebel (Emory University). Wiley-VCH, Weinheim, Germany. 2004. xxiii + 611 pp. 7 × 10 1/2 in. $175.00. ISBN 3-527-30344-8.

Syntheses a partir d'acetyl-5 tetrahydro-1,2,3,4 pyridine via l'ethyl-3a perhydro furo [2,3-b] pyridinone-2

Synthesis of Eburnamonine and Dehydroaspidospermidine.

A comparison in separation efficiency between conventional chromatographic methods and newly devised time-pulsed methods (driven by either negative or positive pressure) is made. The methods are compared in terms of adsorbent amounts, solvent volumes, separation efficiency of a binary mixture, and the total time required for separation. It is found that alternating pressure or pulsed methods are superior on all counts compared to the conventional processes. This is supported by discussion of the principles as well as by the provision of detailed parameters for all experiments.

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Tomas Hudlicky

Papers

Asymmetric Total Synthesis of (+)-7-Deoxypancratistatin.

4-Siloxy-α-bromocrotonate: A New Reagent for (2 + 3)Annulation Leading to Oxygenated Cyclopentenes at Low Temperatures.

Biocatalysis. Fundamentals and Applications By A. S. Bommarius (Georgia Institute of Technology) and B. R. Riebel (Emory University). Wiley-VCH, Weinheim, Germany. 2004. xxiii + 611 pp. 7 × 10 1/2 in. $175.00. ISBN 3-527-30344-8.

Synthesis of Eburnamonine and Dehydroaspidospermidine.

Time-Pulsed Vacuum and Time-Pulsed Alternating Pressure Chromatography