ABSTRACT
The extremely robust and water-stable tetranuclear complex Ti(4)(micro-BINOLato)(6)(micro(3)-OH)(4) (1) catalyzes the direct aldol addition with high degrees of regioselectivity at the sterically more encumbered alpha-side of unsymmetrical ketones. The formation of quaternary stereocenters is described. Oxygen-containing ene components can also be used as starting material in these reactions. When used with aliphatic aldehydes, acetals 18 or acetals of aldol adducts 20 were observed. As few as 0.2 mol % loadings with this catalyst 1 were enough to complete the reactions. Mechanistical aspects of the reactions are discussed.
Subject(s)
Aldehydes/chemistry , Naphthols/chemistry , Titanium/chemistry , Acetone/chemistry , Crystallography, X-Ray , Cyclopentanes/chemical synthesis , Cyclopentanes/chemistry , Ketones/chemistry , Kinetics , Models, Molecular , Molecular Structure , Naphthols/chemical synthesis , ThermodynamicsABSTRACT
The aldol reaction is one of the most important methods for the stereoselective construction of polyketide natural products, not only for nature but also for synthetic chemistry. The tremendous development in the field of aldol additions during the last 30 years has led to more and more total syntheses of complicated natural products. This Review illustrates by means of selected syntheses of natural products the new variants of the aldol addition. This includes aldol additions with various metal enolates, as well as metal-complex-catalyzed, organocatalytic, and biocatalytic methods.
Subject(s)
Aldehydes/chemistry , Macrolides/chemical synthesis , Anti-Bacterial Agents/chemical synthesis , Boron/chemistry , Catalysis , Erythromycin/chemical synthesis , Molecular Structure , Organometallic Compounds/chemistryABSTRACT
Nonporous spherical silica nanoparticles resulting from a controlled Stöber process are covalently surface modified with redox-active molecules. Ferrocene, a ruthenium(II) complex with an N2P2Cl2 ligand set, and a sterically hindered biphenylamine are used as modifiers. The resulting materials are characterized by physical, spectroscopic, electrochemical, and chemical methods. The cyclic voltammetric behavior is studied in detail and reveals effects of charge transport by electron hopping along the surface of particles adsorbed on a Pt electrode. The ruthenium(II) complex remains catalytically active with respect to hydrogenation upon immobilization on the particles. Thus, the respective material provides a heterogenized homogeneous hydrogenation catalyst on a solid support.
Subject(s)
Nanoparticles/chemistry , Organometallic Compounds/chemistry , Ruthenium/chemistry , Silicon Dioxide/chemistry , Adsorption , Catalysis , Electrochemistry , Electrodes , Molecular Structure , Oxidation-Reduction , Particle Size , Platinum/chemistry , Porosity , Surface PropertiesABSTRACT
[reaction: see text] The extremely robust and water-stable tetranuclear complex Ti(4)(mu-BINOLato)(6)(mu(3)-OH)(4) was found to catalyze the direct aldol addition with high regioselectivities at the more steric alpha-encumbered side of unsymmetrical ketones. As few as 0.2 mol % loadings with this cluster were enough to afford complete conversions. The reaction proceeds very smoothly without a significant amount of byproducts. The formation of quaternary stereocenters is described.
