Absolute configuration of 1,5-diazepin-2-ones: A critical test case for density functional theory

The absolute configuration of a series of 4(R)-methyl-1,3,4,5-tetrahydro-2H-1,5-benzodiazepin-2-ones is assigned by comparison of experimental and calculated CD spectra using time-dependent density functional theory (TDDFT) in combination with four different functionals (B3PW91, CAM-B3LYP, M06-2X, BMK). Closest agreement with experiment is obtained with B3PW91. All compounds except those bearing an acyl group at N5 exist in two nearly isoenergetic conformations of the seven-membered ring (G ⩽ 0.5 kcal mol−1 with the local pair natural orbital coupled-electron pair approximation LPNO-CEPA/1/def2-TZVPP//B3LYP/6-311G(d,p)). Agreement between simulated and experimental CD spectra was found to critically depend on a proper description of conformer population as well as the position of the calculated longest-wavelength transitions of the individual conformers.