Fast Equilibrium in N-(Ethoxycarbonyl)-r-2,c-7-diphenylhexahydro-1,4-diazepin-5-ones in Flattened Boat Conformations

The preferred conformations of three N-ethoxycarbonyl derivatives of r-2,c-7-diphenylhexahydro-1,4-diazepin-5-ones 9−11 have been studied using NMR spectral techniques. The N 1,N 4-bis(ethoxycarbonyl)-r-2,c-7-diphenylhexahydro-1,4-diazepin-5-ones 9 and 10 were found to prefer flattened boat conformations with fast equilibrium between two N−CO rotamers while 4-(ethoxycarbonyl)-t-3-isopropyl-r-2,c-7-diphenylhexahydro-1,4-diazepin-5-one (11) was found to prefer a chair conformation with the N−COOEt group locked in one rotameric state. Dynamic 1H NMR studies have been carried out on the N 1,N 4-bis(ethoxycarbonyl) derivatives 9 and 10, and the barriers for the N−CO rotation were found to be 49.9 and 58.0 kJ mol-1, respectively. These barriers are much lower than those observed for N-nitroso- and N-formyl-r-2,c-7-diphenylhexahydro-1,4-diazepin-5-ones (90.0 and 84.4 kJ mol-1, respectively), indicating a fast equilibrium in 9 and 10 at room temperature.