NEW MODELS FOR THE STUDY OF THE RACEMIZATION MECHANISM OF CARBODIIMIDES - SYNTHESIS AND STRUCTURE (X-RAY CRYSTALLOGRAPHY AND H-1 NMR) OF CYCLIC CARBODIIMIDES

摘要

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]tetraazacyclooctadec ine) and 3 (8,10,22,24-tetraazapentacyclo-[23.3.1.1(3,7).1(11,15).1(17,21)]dotriacont a-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. [References: 28]