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Based on a modification of the DoebnerKnoevenagel reaction, a practical and highly efficient synthesis of a,b-unsaturated esters with excellent regio-and stereoselectivity was developed. The reactions are catalyzed by 4-dimethylaminopyridine in DMF at room temperature or below. Both aliphatic and aromatic aldehydes can readily be used in the process.Keywords: atom economy; Doebner-Knoevenagel reaction; malonic acid half esters; (E)-stereoselectivity; a,b-unsaturated esters The synthesis of a,b-unsaturated esters 1 from aldehydes 2 is a very common carbon-carbon bond forming reaction. Typically, the transformation is realized via the Wittig reaction [Eq. (1)] or Horner-Wadsworth À Emmons reaction [Eq. (2)] using alkoxycarbonylmethylene(triphenyl)phosphoranes 3 or trialkyl phosphonoacetates 4, respectively. [1 -3] Both reactions are reliable, high yielding, tolerate various functional groups, and are generally (E)-stereoselective. However, their main drawback, if there is one, concerns their rather modest atom economy, and as a result, considerable by-product formation. In the Wittig reaction the by-product is triphenylphosphine oxide 5, which has to be removed chromatographically and disposed. The Horner-Wadsworth À Emmons variant has the advantage of producing a water-soluble phosphate salt 6 as the by-product, which can be removed via aqueous extraction. However, the Horner-Wadsworth À Emmons reaction typically requires the use of a strong base such as sodium hydride and the phosphate salt by-product 6 is produced in stoichiometric amounts which, especially on a large scale, can challenge waste-management.An alternative yet rarely used methodology for the synthesis of a,b-unsaturated esters from aldehydes is the decarboxylative Knoevenagel-type reaction employing malonic acid half esters 7 [Eq. (3)].[4] This reaction has a significantly improved atom economy and the only by-products are water and carbon dioxide. Moreover, half-esters of malonates 7 are as inexpensive as the corresponding phosphorus-based reagents (e.g., 3 and 4) and can also be obtained from inexpensive dialkyl malonates. [5,6] The much less frequent use of this Knoevenagel (or Doebner-Knoevenagel) reaction variant can be attributed to the generally required reaction conditions, which include using an excess of the malonic half ester, catalysis with piperidine in pyridine as the solvent, and elevated temperatures. Moreover, (E)-vs. (Z)-selectivity varies and, most importantly, in the reaction with enolizable aldehydes, not a,b-but rather b,g-unsaturated esters (or their mixtures) are commonly obtained. [7] During several other projects we realized the need for a clean, by-product-free, and reliably selective methodology for the production of a,b-unsaturated esters from aldehydes that can be used on a large scale and does not require heating or strong bases. We therefore set up a study aimed at overcoming the disadvantages of the traditional Wittig, Horner-Wadsworth À Emmons, and Knoevenagel reactions. Here we report our preliminary re...

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Die Entwicklung einer Anordnung zur Untersuchung der Anlagerungswahrscheinlichkeit langsamer Elektronen an Molekeln wird beschrieben. Das Prinzip ist folgendes: In einen Gasraum mit homogenem elektrischen Zugfeld treten langsame Elektronen ein und lagern sich zum Teil an Gasmolekeln an. Die Anlagerungsorte sind statistisch verteilt. Die Laufzeit der Teilchen über eine bestimmte Driftstrecke im Zugfeld hängt vom Anlagerungsort ab. Gemessen wird die Häufigkeitsverteilung der Laufzeiten zwischen zwei zum Zugfeld senkrechten Ebenen. Als Steuerorgane dienen ebene Gitteranordnungen (Tore), die mit Rechteckspannungen moduliert werden. Das Meßergebnis liefert die Anlagerungswahrscheinlichkeit pro cm Driftweg und die Driftgeschwindigkeiten der beteiligten Trägerarten, insbesondere auch bei verwickelteren Vorgängen wie Anlagerung weiterer Molekeln an das primär gebildete Ion. Die Methode ist frei von prinzipiellen Fehlerquellen, die bei den bisher verwendeten Methoden nachzuweisen sind.Die Methode wurde auf Sauerstoff angewendet. Die gefundenen Werte werden verglichen mit Ergebnissen von Healey und Kirkpatrick sowie von Bradbury. Sie stimmen für Elektronenenergien oberhalb von 2 eV mit den von Healey und Kirkpatrick gefundenen Werten überein, für Energien unterhalb von 1,5 eV mit denen von Bradbury. Bezüglich des Mechanismus des Anlagerungsvorganges wird gefunden, daß in der Regel unmittelbar bei der Anlagerung des Elektrons ein negatives Ion der Beweglichkeit von 2,68 cm