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

The crystal and molecular structure of carbodiimides 2 (5,6,18,19-tetr adehydro-5, abenzo[d,h,m,q][1,3,10,12]tetraazacyclooctadecine) and 3 c lo-[23.3.1.1(3,7).1(11,15).1(17,21)]dotriaconta-1 11,13,15(31),17,19,2 1(30),22,23,25,27-hexadecaene) have been determined. The activation ba rriers for the racemization of carbodiimides 1 (6,7-dihydrodibenzo[d,h ][1,3]diazonine), 2, and 3 have been determined. While 1 presents a re latively high barrier (17.4 kcal mol(-1)), 2 and 3 have very low activ ation barriers (between 5 and 7 kcal mol(-1)). We tentatively conclude that open-chain and large-ring carbodiimides racemize by nitrogen inv ersion or trans-rotation while medium-size cyclic carbodiimides racemi ze by cis-rotation.