Deciphering the Molecular Mechanism of Intramolecular Reactions from the Perspective of Bonding Evolution Theory

Abstract

The molecular mechanisms of three intramolecular rearrangements (I, the rearrangement of allyloxycycloheptatriene to yield tricyclic ketones; II, the cycloaddition of a nitrone-alkene to render two tricyclic isoxazolidines; and III, the decomposition of N-carbamoyl-L-proline in tetrahydro-1H-pyrrolo[1,2-c]imidazole-1,3(2H)-dione plus water, or tetrahydro-1H,3H-pyrrolo[1,2-c]oxazole-1,3-dione plus ammonia) have been studied by means of the bonding evolution theory (BET). The thermal rearrangement I is composed by a sigmatropic rearrangement coupled to an intramolecular Diels–Alder reaction. The sigmatropic reaction comprises four steps: (1) rupture of an O-C single chemical bond, (2) transformation of a C-O single to double bond, (3) creation of pseudo-radical centers on carbon atoms coupled with a double C-C bond evolving to single and the other C-C double bond migration, and (4) formation of the new C-C single bond. For the Diels–Alder reaction, the process can be described as an initial formation of up to four monosynaptic V(C) basins in two successive steps, coupled with the loss of the double bond character of the three initial double bonds, followed by the consecutive formation of two new C-C bonds, with the new double C-C bond formation sensed in between the formation of the first and the second C-C bonds. For reaction II, the bond forming process is described by the depopulation of N-C and C-C double bonds with the creation of a V(N) and two V(C) monosynaptic basins, followed by an O-C and C-C bond-forming processes via the creation of V(O,C) and V(C,C) disynaptic basins. Finally, for the thermal decomposition III, the reaction mechanism for the water elimination takes place in four events which can be summarized as follows: (1) the depopulation of V(N) with the formation of C-N, (2) the rupture of the C-O bond with transfer of its population to V(O), (3) the restoration of an N nitrogen lone pair via H-N bond cleavage, and (4) the formation of O-H illustrating the water molecule release. For the case of deamination, the events (1) and (2) correspond to the breaking and forming process of H-O and H-N bonds, respectively, while last events deal with the C-O bond formation and the elimination of the NH3 molecule.