title: New Models for the Study of the Racemization Mechanism of Carbodiimides. Synthesis and Structure (X-ray Crystallography and (1)H NMR) of Cyclic Carbodiimides.
abstract: The crystal and molecular structure of carbodiimides 2 (5,6,18,19-tetradehydro-5,12,13,18,25,26-hexahydrotetrabenzo[d,h,m,q][1,3,10,12]tetraazacyclooctadecine) and 3 (8,10,22,24-tetraazapentacyclo[23.3.1.1(3,7).1(11,15).1(17,21)]dotriaconta-1(29),3,5,7(32),8,9,11,13,15(31),17,19,21(30),22,23,25,27-hexadecaene) have been determined. The activation barriers for the racemization of carbodiimides 1 (6,7-dihydrodibenzo[d,h][1,3]diazonine), 2, and 3 have been determined. While 1 presents a relatively high barrier (17.4 kcal mol(-)(1)), 2 and 3 have very low activation barriers (between 5 and 7 kcal mol(-)(1)). We tentatively conclude that open-chain and large-ring carbodiimides racemize by nitrogen inversion or trans-rotation while medium-size cyclic carbodiimides racemize by cis-rotation.
authors.affiliation: Instituto de Química Médica, CSIC, Juan de la Cierva 3, E-28006 Madrid, Spain.
authors.forename: Pedro
authors.initials: P
authors.lastname: Molina
authors.forename: Mateo
authors.initials: M
authors.lastname: Alajarín
authors.forename: Pilar
authors.initials: P
authors.lastname: Sánchez-Andrada
authors.forename: Juan Server
authors.initials: JS
authors.lastname: Carrió
authors.forename: Martín
authors.initials: M
authors.lastname: Martínez-Ripoll
authors.forename: J. Edgar
authors.initials: JE
authors.lastname: Anderson
authors.forename: María Luisa
authors.initials: ML
authors.lastname: Jimeno
authors.forename: José
authors.initials: J
authors.lastname: Elguero