J. Henrique Teles

Zinc Silicates: Very Efficient Heterogeneous Catalysts for the Addition of Primary Alcohols to Alkynes and Allenes

There is an astonishing parallel between the mechanism generally accepted for the addition of water to CO2 catalyzed by the enzyme carbonic anhydrase and the mechanism calculated for the addition of methanol to allene catalyzed by the naturally occurring zinc silicate hemimorphite. The latter reaction was investigated in detail following the observation that hemimorphite as well as an amorphous zinc silicate prepared in situ are excellent heterogeneous catalysts for the addition of primary alcohols to alkynes and allenes [Eq. (1)].

A stable carbene as π-acceptor electrochemical reduction to the radical anion

Abstract The electrochemical reduction of the carbene 1 reversibly yielded the corresponding radical anion 2 , which was characterized by ESR measurements. Ab initio calculations provided a better understanding of its structure and stability.

Zinksilicate: hochwirksame heterogene Katalysatoren für die Addition primärer Alkohole an Alkine und Allene

Eine erstaunliche Parallele existiert zwischen dem allgemein anerkannten Mechanismus fur die vom Enzym Carboanhydrase katalysierte Addition von Wasser an CO2 und dem Mechanismus, der fur die Addition von Methanol an Allen, katalysiert durch das naturlich auftretende Zinksilicat Hemimorphit, berechnet wurde. Die letztgenannte Reaktion wurde genau untersucht, da sowohl Hemimorphit als auch ein in situ hergestelltes, amorphes Zinksilicat ausgezeichnete heterogene Katalysatoren fur die Addition primarer Alkohole an Alkine und Allene sind [Gl. (1)].

The Role of Conjugative Interaction in Stable Carbenes of the 1,2,4-Triazol-5-ylidene Type and their Energy of Dimerization. An Ab Initio Study

Abstract The role of conjugative interaction in stable carbenes of the 1,2,4-triazol-5-ylidene type was studied at different levels of ab initio theory employing model compounds. In the case of parent 4,5-dihydro-1 H-1,2,4-triazol-5-ylidene this type of interaction was found to be of similar importance as in 1,2,4-triazole and pyridine. Five stationary points were located on the hypersurface of the dimer (C2N3H3)2 . ΔE for all considered dimerization reactions was found to be negative, and the change of energy associated with the formation of the most stable isomer is about -9.5 kcal/mol. In spite of a high value for the singlet-triplet gap of about 90 kcal/mol for the carbene, the heavy atom skeleton of the most stable dimer is close to planarity. This finding is in perfect accordance with the predictions of other authors regarding the geometry of dimers from singlet carbenes stabilized by π-conjugation.