Enantioselective organic catalysis represents one of the most rapidly expanding fields of research in modern organic chemistry. The term “organic” emphasizes the advantages of performing a catalytic reaction under metal-free conditions, exploiting substoichiometric amounts of a small organic molecule which is often more stable and cheaper than its organometallic equivalent.1 Catalytic amounts of structurally complex and difficult to obtain pyridine-N-oxides have been recently used as organic Lewis bases capable of promoting the enantioselective allylation of aldehydes.2 A very short and easy entry to chiral pyridine-N-oxides was envisaged in the reaction between 2-pyridinecarboxylic acid N-oxide and commercially available, enantiomerically pure diamines. A family of N-oxides (1) was thus prepared in moderate to good yields (Scheme 1). A second family of N-oxides (2) was prepared by bis esterification of 2-(6,6’-dihydroxyphenil)-3-methyl pyridine-N-oxide with readily available enantiomerically pure carboxilic acids. 1H-NMR studies revealed that the catalysts have a strongly preferred conformation and thus stereochemically defined 3-D structures. Scheme 1 The catalytic activity of these N-oxides was tested in the reaction of benzaldehyde with allyltrichlorosilane affording the homoallylic alcohol. Among the catalysts employed, bis pyridine-N-oxides (1) afforded the desired product in up to 60% yield and up to 50% ee; mono pyridine-N-oxides (2) performed better in terms of stereoselectivity (up to 45% yield, up to 68% ee). The use of both types of catalysts 1 and 2 was extended to other aromatic aldehydes, bearing both electron-withdrawing and electron-donating substituents, affording homoallylic alcohols in up to 73% yield and 37% ee for catalysts of type 1 and up to 60% yield and 84% ee for catalysts of type 2.
|Titolo:||Pyridine- N-Oxides as New Organocatalysts for the Enantioselective Allylation of Aldehydes|
|Data di pubblicazione:||giu-2005|
|Settore Scientifico Disciplinare:||Settore CHIM/06 - Chimica Organica|
|Enti collegati al convegno:||Società Chimica Italiana. Divisione di Chimica Organica|
|Citazione:||Pyridine- N-Oxides as New Organocatalysts for the Enantioselective Allylation of Aldehydes / L. Pignataro, M. Benaglia, F. Cozzi. ((Intervento presentato al 30. convegno Summer School on Organic Synthesis “A. Corbella” tenutosi a Gargnano nel 2005.|
|Appare nelle tipologie:||14 - Intervento a convegno non pubblicato|