A Theoretical Investigation of the Ni(II)-Catalyzed Hydrovinylation of Styrene

We report a detailed and full computational investigation on the hydrovinylation reaction of styrene with the Ni(II)-phospholane catalytic system, which was originally presumed to proceed through a cationic mechanism involving a nickel hydride intermediate. The following general features emerge from this study on a specific catalyst complex that was found to give quantitative yield and moderate selectivity: (a) the activation barrier for the initiation (18.8 kcal/mol) is higher than that for the reaction due to a low-lying square-planar pentenyl chelate intermediate originating from a Ni(II)-allyl catalyst precursor. Consequently there is an induction period for the catalysis; (b) the exit of product from the catalyst is via a β-H-transfer step instead of the usual β-H elimination pathway, which has a very high activation energy due to a trans effect of the phospholane ligand; (c) the turnover-limiting and enantio- determining transition state is also the β-H-transfer; (d) because of the absence of a hydride intermediate, the unwanted isomerization of the product is prevented; (e) since the enantio-discrimination is decided at the H-transfer stage itself, the configuration of the product in a catalytic cycle influences the enantioselectivity in the subsequent cycle; (f) the trans effect of the sole strong ligand in the d8 square-planar Ni(II), the stability of the η3-benzyl intermediate, and the availability of three coordination sites enable regioselective hydrovinylation over the possible oligomerization/polymerization of the olefin substrates and linear hydrovinylation. This work has also confirmed the previously recognized role of the hemilabile group at various stages in the mechanism

Paraconic Acids: The Natural Products from Lichen Symbiont

Paraconic acids, belonging to the class of g-butyrolactone natural products, display a broad range of biological activities such as antibiotic and antitumor properties. Consequently a great number of synthetic strategies have been devised for them, ranging from diastereoselective and chiral pool approaches to the application of asymmetric catalysis. This review gives a critical account on the different methods developed that lead to paraconic acids of great structural variety

Anomeric Spiro-Annulated Glycopyranosides: An Overview of Synthetic Methodologies and Biological Applications

International audienceOrganic chemistry developed a series of synthetic strategies toward spiro-annulated carbohydrates as potential pharmaceutical drugs or developed new organic synthetic methodologies. The present chapter gives a general overview of the spiro-annulation of carbohydrates at the anomeric position. The main synthetic strategies can be summarized in five paths. Intramolecular cyclizations can be performed through two short tethers with their reactive ends generating th