Structure of N-nitroso-r-2,c-7-diphenylhexahydro-1,4-diazepin-5-one

t-3-Isopropyl-r-2,c-7-diphenylhexahydro-1,4-diazepin-5-one, C20H24N2O

“…The nitroso derivative (III) of the analogous diazepinone (Priya, Shamala, Viswamitra, Senthilkumar & Jeyaraman, 1992) has been reported to adopt the boat conformation, while the title compound (II)…”

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

“…On lowering the temperature down to −55 °C, doubling of the signals for benzylic protons at C2 and C7 as well as those for CH 2 and CH 3 protons of carbethoxy group at N1 was observed. The doubling of signals at low temperature indicates that the carbethoxy group at N1 position may adopt a coplanar orientation with respect to the C2−N1−C7 plane of the molecule with weaker N−CO conjugation than in N−NO of the N -nitroso derivatives. − …”

“…The relative influences of the allylic strain (A 1,3 -strain), torsional strain, resonance energy (due to the delocalization of the lone pair of electrons on nitrogen into the carbonyl group), and 1,3-diaxial strain, etc., over the conformational preferences of N -nitroso- r- 2, c -6-diphenylpiperidines 1a − j , 1a,d N -nitroso-3-azabicyclo[3.3.1]nonanes 2a − c and 3a , b , 1d,2a,b N -nitroso- and N -formyl- r -2, c -7-diphenylhexahydro-1,4-diazepin-5-ones 4a − j , 3a-c and N 1 , N 4 -dinitroso- r -2, c -7-diphenylhexahydrodiazepines 5a − c , 3c,d have been studied in our laboratory.…”

“…All these nitroso derivatives were found to exist in an equilibrium between syn and anti rotamers at room temperature with coplanar orientation of the nitroso group. − The delocalization of the nitrogen lone pair into the double bond of the NN=O system results in restricted rotation about the N−N bond (Figure ) leading to a gain in resonance energy 5 (60−95 kJ mol -1 ) while the allylic strain , between the coplanar nitroso group and the equatorial substituents at the α-carbon tends to destabilize the conformation. In order to relieve the allylic strain the molecule may flip over to the alternate chair, , twist-chair 1a, conformations or flattened boat conformations with pseudo-axial phenyl groups, 3a-c etc., retaining the planar C2(C6)NN=O function.

1 N−N and N−C rotational equilibria.

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“…These derivatives prefer flattened boat conformations with fast equilibrium between two N−CO rotamers (Figure ) with energy barriers of 50−58 kJ mol -1 . In contrast, the energy barriers for N−NO rotation in N -nitroso- r -2, c -6-diphenylpiperidines 1a−j , 1a,d N -nitroso- r -2, c -4-diphenyl-3-azabicyclo[3.3.1]nonanes, (e.g., 2a−c , 1d , N -nitroso- r -2, c -7-diphenylhexahydro-1,4-diazepin-5-ones 4a−d , 3a,c and N−CO rotation in N -formyl- r -2, c -7-diphenylhexahydro-1,4-diazepin-5-ones 4f−j 3c are about 70−90 kJ mol -1 . …”