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

Microbial oxidation of substituted aromatic compounds with mutant strains of Pseudomonas puridu (39D) yields the corresponding arene cis-diols in efficient quantities and >99% ee. The use of these synthons in the practical preparation of oxygenated naturalproducts is described with emphasis on the importance of symmetry considerations during synthetic planning in order to achieve enantiodivergent preparations of target compounds. The applications are highlighted by discussion of the syntheses of the following compounds from chlorobenzene : (+)- and (-)- erythrose, (+)- and (-)- ribonolactone, (+) - and (-)-trihydroxyheliotridane, (+)- and (-)-pinitol, kifunensine, lycoricidine and a model study for a short synthesis of (-)-morphine. The conclusion of the lecture places in perspective the use of halogenated aromatics in the chiral pool supply with the potential of efficiently replacing sugars. Brevity and efficiency of the synthetic sequences are emphasized.

Enzymatic Hydroxylation of Arene and Symmetry Considerations in Efficient Synthetic Design of Oxygenated Natural Products

α,β-Unsaturated esters such as (I), (III), (V), and (VII) are selectively reduced with magnesium in methanol to give the corresponding saturated esters.

Selective Reduction of α,β‐Unsaturated Esters in the Presence of Olefins.

Menthyl 2‐Bromocrotonate and Menthyl 4‐Bromocrotonate: Reagents for Chiral Vinylogous Darzen and Reformatsky Reactions.

Biocatalysis and its applications to synthesis has risen to prominence in recent years in both academic and industrial domains Several recent reviews highlight the applications of enzymes to the production of enantiomerically pure synthons for manufacturing of fine chemicals.[1-5] Various lipases are now commonly used in organic synthesis for desymmetrization of meso compounds. On the other hand, oxidoreductases have not been used so extensively in organic synthesis because there are difficulties in handling such enzymes as isolated entities and due to the necessity of recycling cofactors. For this reason, whole-cell fermentation is the method of choice in the application of oxidoreductases to organic synthesis. Both bacterial cells (E. coli recombinant organisms) and yeast are now used for such reactions as epoxidation, diol formation, and Baeyer-Villiger oxidation.

Biocatalytic Synthesis of Alkaloids and Carbohydrates: An Update

Analytically pure vinylcyclopropane was prepared from cyclopropanecarboxaldehyde by the Conia-Dauben modification of the Wittig reaction.

Tomas Hudlicky

Papers

Enzymatic Hydroxylation of Arene and Symmetry Considerations in Efficient Synthetic Design of Oxygenated Natural Products

Selective Reduction of α,β‐Unsaturated Esters in the Presence of Olefins.

Menthyl 2‐Bromocrotonate and Menthyl 4‐Bromocrotonate: Reagents for Chiral Vinylogous Darzen and Reformatsky Reactions.

Biocatalytic Synthesis of Alkaloids and Carbohydrates: An Update

Practical Synthesis of Vinylcyclopropane.