X-ray Crystal Structures and ab Initio Calculations on the Photochemically Formed Dewar Isomers of the 4(3H)-Pyrimidinone Derivatives

Abstract

The structures of the 5-oxo-2,6-diazabicyclo[2.2.0]hex-2-enes (imine Dewar pyrimidinones) 2a-h have been studied by X-ray diffraction analysis and ab initio calculations at the HF/6-31G(d,p) and MP2/6-31G(d,p) levels of theory. The crystal structures of 1-alkyl-3-tert-butyl-6-methyl imine Dewar pyrimidinones 2a-c have been determined at low temperature. The X-ray diffraction studies revealed that both the 2-azetidinone and dihydroazete rings are almost planar, and their eight bond angles are nearly 90 degrees (81-100 degrees ). The C1-N2(=C3) bond distance in the dihydroazete ring is longer due to the bond angle strain by ca. 0.06 Å than that of a typical straight-chain imine molecule. Full geometry optimizations on the imine Dewar isomers (2a and 2d-h) show deviations between the HF and MP2 geometries. The MP2 structure of the 3-tert-butyl-1,6-dimethyl imine Dewar pyrimidinone 2a is compared with those of the X-ray diffraction. The results of the calculations are found to be in good agreement with the X-ray diffraction data. The full geometry optimizations are necessary with the inclusion of electron correlation for the highly strained molecules. The ab initio calculations of the 2-azetidinone 9, 3,4-dihydroazete 11, and N-ethylidenemethylamine 12 have been carried out at the HF and MP2/6-31G(d,p) levels of theory to compare with the structures, orbital hybridization, bond orders, and charge distributions of the Dewar pyrimidinones 2. The theoretical results reveal that the MP2 C1-N2 bond distance of the Dewar pyrimidinone 2a is consistent with the abnormally elongated X-ray C1-N2 bond distance. The electronegative N2 and N6 atoms in the Dewar pyrimidinones 2 give the great positive charge on the C1 atom. The smaller bond angles (ca. 90 degrees ) in the Dewars 2 than typical sp(2) and sp(3) bond angles (ca. 120 and 109 degrees ) increase p character in the endocyclic bonds and decrease p character in the exocyclic bonds.

Quantum chemical study of several monocyclic complex β‐lactam C‐3, C‐4, and N‐derivatives, and β‐ring model molecules

“…2‐Azetidinone, by contrast, is almost planar. It is the simplest four‐membered lactam that has been extensively studied by x‐ray 20, 21, by the joined (ED + MW) 22, and by ab initio methods 6, 20, 23–25. Its hydrolysis and the influence of the solvent have been analyzed theoretically 26.…”

Molecular Modeling of Anti-Bredt Compounds

“…In 10 and 13 this is impossible, because the bridgehead bond is not part of a six-member ring so both species have a diradical character (Table 3). Similar arguments apply to [11,18] and [14,20] pairs, where 11 and 14 are the more stable isomers. This can be rationalized by suggesting that while six-member rings efficiently relieve bridgehead strain in all four isomers, 18 and 20 retain highly strained fourmember rings.…”

“…14 CdC bond length in norbornene is 1.336 Å, while imine CdN bond lengths are in the range of 1.286-1.292 Å, as determined by single-crystal X-ray diffraction. 14 The species whose CC/ CN bond lengths were much longer than these values were investigated for possible diradical character. This was achieved by UB3LYP/6-311G** method.…”

“…The diradical character permits lengthening of a normally short CC/CN double bond and thus a reduction in SE. To investigate this possibility, we compared the calculated CC/CN bond lengths for 3 − 8 (Table ) with experimental CC/CN bond lengths in norbornene and strained bicyclic imines . CC bond length in norbornene is 1.336 Å, while imine CN bond lengths are in the range of 1.286−1.292 Å, as determined by single-crystal X-ray diffraction .…”

“…To investigate this possibility, we compared the calculated CC/CN bond lengths for 3 − 8 (Table ) with experimental CC/CN bond lengths in norbornene and strained bicyclic imines . CC bond length in norbornene is 1.336 Å, while imine CN bond lengths are in the range of 1.286−1.292 Å, as determined by single-crystal X-ray diffraction . The species whose CC/CN bond lengths were much longer than these values were investigated for possible diradical character.…”