Generation of quaternary stereocenters by asymmetric Michael reactions: enamine regiochemistry as configuration switch

Regioselective enamine formation from cyclic β‐diketones 1 is obtained by the appropriate choice of activating agent: Brønsted acid catalyzed condensation gives endocyclic enamines 3 as the thermodynamically favored products. Activation with Lewis acid BF₃ ? OEt₂ affords betaines 8 as intermediate products, which can be reacted with L ‐valine diethylamide ( 2 ) to preferentially furnish exocyclic enamines 4 as kinetic products. Derivatives with quaternary stereocenters were accessible from both isomeric enamines by using asymmetric, copper(II )‐catalyzed Michael reactions at ambient temperature. Both regioisomers afford the triketones 7 with the same constitution but bearing the opposite absolute configuration at the quaternary stereocenter. Thus, both enantiomers of the product are prepared by using the same chiral auxiliary derived from L ‐valine.     

The primitive spectrum of a semigroup of Markov operators

Positivity - For a semigroup $$\mathcal {S}$$ of Markov operators on a space of continuous functions, we use $$\mathcal {S}$$-invariant ideals to describe qualitative properties of $$\mathcal {S}$$...     

Combinatorial screening of PtTiMe ternary alloys for oxygen electroreduction

The catalytic oxygen electroreduction properties of PtTiMe (Me = Co, Cr, Cu, Fe, Mn, Mo, Ni, Pd, Ta, V, W and Zr) ternary alloys were studied using an in-house developed thin film based combinatorial high throughput method. Libraries containing discrete alloy compositions were fabricated by plasma co-sputtering and the resulting alloys were electrochemically screened by the hydrodynamic rotating disk electrode technique. Candidate catalysts were identified by comparing the activity-stability-composition relationships between the platinum titanium alloys and pure platinum standard. Among the PtTiMe alloys studied, PtTiNi, PtTiCu and PtTiV, respectively, displayed the highest catalytic oxygen electroreduction activities with a tenfold, an eightfold and a sixfold enhancement as compared to the pure platinum standard and good chemical stabilities.