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Carbonyl Addition (carbonyl + addition)
Selected AbstractsChemInform Abstract: Hydroxyalkyl Thiazolines, a New Class of Highly Efficient Ligands for Carbonyl Additions.CHEMINFORM, Issue 17 2009Michael Bauer Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] C-Glycosidations of a 2-Ketohexosyl Bromide with Electrophilic, Radical, and Nucleophilic Anomeric CarbonsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 16 2003Frieder W. Lichtenthaler Abstract The susceptibility of acylated 2-ketohexosyl halides to C-homologation is demonstrated with the easily accessible tri- O -benzoyl-,- D - arabino -hexos-ulosyl bromide 1 as the model compound. C-Glycosidation with an electrophilic anomeric carbon requires prior carbonyl protection, to avoid carbonyl addition by the C-nucleophile, for example, as the cyanohydrin. Silver triflate-promoted reaction with the silylenol ether of acetophenone then efficiently yields the ,-phenacyl product. With thermal (AIBN) or photochemical induction, 1 smoothly generates an anomeric radical , comparatively electrophilic, due to its capto-dative substitution , which exclusively traps hydrogen in the presence of tributyltin and electron-deficient alkenes. With allyltributylstannanes, however, it reacts with high stereoselectivity to afford ,- C -allyl glycosiduloses. The ,-bromoketone functionality in ulosyl bromide 1 is susceptible to Reformatsky conditions: treatment with zinc-copper couple readily generates the 1,2-enolate, a most simple anomeric nucleophile, which effectively adds to aldehydes to give ,- C -hydroxyalkyl glycosiduloses or ,- C -disaccharides (with sugar aldehydes) with a high degree of double stereoselection. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Mechanistic Investigation of Chiral Phosphoric Acid Catalyzed Asymmetric Baeyer,Villiger Reaction of 3-Substituted Cyclobutanones with H2O2 as the OxidantCHEMISTRY - A EUROPEAN JOURNAL, Issue 10 2010Senmiao Xu Dr. Abstract The mechanism of the chiral phosphoric acid catalyzed Baeyer,Villiger (B,V) reaction of cyclobutanones with hydrogen peroxide was investigated by using a combination of experimental and theoretical methods. Of the two pathways that have been proposed for the present reaction, the pathway involving a peroxyphosphate intermediate is not viable. The reaction progress kinetic analysis indicates that the reaction is partially inhibited by the ,-lactone product. Initial rate measurements suggest that the reaction follows Michaelis,Menten-type kinetics consistent with a bifunctional mechanism in which the catalyst is actively involved in both carbonyl addition and the subsequent rearrangement steps through hydrogen-bonding interactions with the reactants or the intermediate. High-level quantum chemical calculations strongly support a two-step concerted mechanism in which the phosphoric acid activates the reactants or the intermediate in a synergistic manner through partial proton transfer. The catalyst simultaneously acts as a general acid, by increasing the electrophilicity of the carbonyl carbon, increases the nucleophilicity of hydrogen peroxide as a Lewis base in the addition step, and facilitates the dissociation of the OH group from the Criegee intermediate in the rearrangement step. The overall reaction is highly exothermic, and the rearrangement of the Criegee intermediate is the rate-determining step. The observed reactivity of this catalytic B,V reaction also results, in part, from the ring strain in cyclobutanones. The sense of chiral induction is rationalized by the analysis of the relative energies of the competing diastereomeric transition states, in which the steric repulsion between the 3-substituent of the cyclobutanone and the 3- and 3,-substituents of the catalyst, as well as the entropy and solvent effects, are found to be critically important. [source] Vinyl-, Propargyl-, and Allenylsilicon Reagents in Asymmetric Synthesis: A Relatively Untapped Resource of Environmentally Benign ReagentsCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2009Marcus Abstract Green carbanion surrogates: Organotrialkylsilanes occupy a niche in the array of ,green' carbon nucleophiles open to chemists that few other reagents can fill. Despite being known for over 30 years, it is only recently that their true worth in stereoselective carbonyl addition and related processes has started to emerge, primarily due to their low reactivity. It is our hope that this minireview will make the true worth of these reagents more widely known, setting the stage for expanded usage of these versatile yet benign reagents in organic synthesis. An up-to-date in-depth review of the current virtues and limitations in the realm of carbonyl addition reactions with allenyl-, propargyl-, and vinylsilicon reagents, encompassing numerous practical as well as pedagogical principles is presented. Comparisons of chemo-, regio-, and stereoselectivity and reactivity are drawn. Synthetic applications and challenges associated with each class of organosilane are discussed threading together the prospects of these green carbanion surrogates. [source] | ||||||||||