Issue in Honor of Prof. J. Elguero and P. Molina ARKIVOC 2005 (ix) 189-199 Stereocontrolled formation of substituted imidazolidines in the reaction between N-metallated azomethine ylides and isocyanates Edurne Erkizia,a,1 Eneko Aldaba,a Yosu Vara,a Ana Arrieta,a Heinz Gornitzka,b and Fernando P. Cossío a,* a Kimika Fakultatea, Universidad del País Vasco – Euskal Herriko Unibertsitatea P. K. 1072, 20080 San Sebastián – Donostia, Spain b Laboratoire Hétérochimie Fondamentale et Appliquée, Université Paul Sabatier 118 rue de Narbonne, 31062 Toulouse Cedex 4, France E-mail: Dedicated to Professors José Elguero and Pedro Molina on the occasion of their 70th and 60th birthdays, respectively (received 14 Dec 04; accepted 18 Mar 05; published on the web 25 Mar 05) Abstract Substituted imidazolidines (and not imidazolidin-4-ones) are the unexpected cycloadducts obtained in the reaction between imines and isocyanates. The reaction is shown to take place via stepwise [3+2] cycloaddition between the N-metallated azomethine ylide formed in situ and the starting imine, followed by nucleophilic addition of the resulting imidazolidine on the sphybridized carbon atom of the isocyanate. Density-Functional Theory calculations provide a model for the mechanism of this unusual reaction and for the origins of the observed regio- and stereoselectivity. Keywords: Azomethine ylides, imines, isocyanates, imidazolidines, cycloadditions Introduction It is well-known that cumulenes, in particular ketenes and isocyanates, can participate in 1,3dipolar reactions to yield different cycloadducts (Scheme 1).2 For example, isocyanates 1 can react with azomethine imines 2 to form 1,2,4-triazolidin-3-ones 6.3 In contrast, imines 3 react with isocyanates to yield 1,3,5-triazenane-2,4-diones 7 as [2+2+2] cycloadducts.4 The reaction between nitrones 4 and isocyanates has also been described.5,6 In this case, both the C=N 5 and C=O 6 bonds of the isocyanate can participate in the [3+2] cycloaddition to yield 1,2,4oxazolidin- 5-ones 8 and 1,2,4-dioxazolidines 9, respectively. It is noteworthy that the cycloadducts 9 have been found as transient species only when chlorosulfonyl isocyanate is used ISSN 1424-6376 Page 189 ©ARKAT USA, Inc
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