The Zinc Mediated Condensation of Amino Acid Esters with Imines to b-Lactams

An experimentally attractive stereoselectie 'one pot' synthesis of beta-lactams is described. This route is based on the zinc mediated condensation of an alpha-amino acid ester with an imine via a zinc ester enolate. Making use of proper substituents in both the amino acid ester and the imine the stereochemistry of the C-C bond forming step can be completely controlled

Periphery-Functionalized Organometallic Dendrimers for Homogeneous Catalysis

The use of carbosilane based dendrimers as molecular scaffolds for the attachment of organomettallic Ni 'pincer' complexes that are active as a catalyst in the Kharasch addition reaction is described. It is shown that increasing steric crowding at the dendrimer periphery results in decreased catalyst activity, most likely as a consequence of an intramolecular redox process. Furthermore, preliminary investigations have shown that the application of this catalyst system in a continous process using a membrane reactor is possible

Organotin(IV) compounds 2-(Me2NCHR)C6H4(SnR'R'Br) with a fixed s-cis arrangement of the chiral carbon and tin centers

{2}-2] have been synthesized and characterized in which R is H, Me, Et, i-Pr or t-Bu. The crystal structure of one of these compounds (R = t-Bu, R' = Me and R'' = Ph) has been determined. The tin center has a distored trigonal-bipyramidal coordination geometry. The organic ligands occupy the equatorial sites, while the more electronegative Br and N ligands are in the axial positions. As a requirement of the space group symmetry the unit cell contains 8 enantiomeric pairs of diastereoisomers (R){C}(S){S}{n} and (S){C}(R){S}{n}. The given configuration at the chiral benzylic carbon atom is combined with a configuration at the chiral tin atom in such a way that the sterically most favorable diastereoisomer is formed.The influence of the steric requirements of the ortho-substituent Z = HRNMe{2} in 2-ZC{6}H{4} (SnRR'Br) on the configuration of the Sn center has been studied. These compounds contain a fixed s-cis Z-C-C-Sn orientation. }1{H and }1{}1{}9{Sn NMR spectroscopic studies show that in solution at low temperatures (-25}o{C) all these compounds are pentacoordinate as a result of intramolecular coordination. At higher temperatures a fluxional process becomes operative involving Sn-N bond dissociation/association. However, even at +110}o{C the Sn center is configurationally stable on the NMR time scale. For the compounds in which the Sn atom is also a chiral center (R' = Me and R'' = Ph) in solution there is an equilibrium between two diastereoisomers, the ratio being dependent on the bulk of the benzylic substituent R. For R = t-Bu the equilibrium lies completely to the side of the sterically most favorable one