Further Pseudopericyclic Reactions:  An ab Initio Study of the Conformations and Reactions of 5-Oxo-2,4-pentadienal and Related Molecules

Abstract

It has been proposed that all pseudopericyclic reactions are allowed, regardless of the number of electrons involved. 5-Oxo-2,4-pentadienal (1) is a vinylog of formylketene, and thus its reactions provide a test of this proposal. Ab initio (MP4(full, SDQ)/D95**//MP2/6-31G* + ZPE) calculations were carried out on all the conformations and several reactions of 1, as well as related systems. One of the eight possible conformations of 1 (zZz1) does not exist, but closes without a barrier, via a pseudopericyclic pathway to pyran-2-one (3). IR spectra of 1 in Ar matrices are consistent with three conformations (zZe1, eZz1, and eZe1). Calculated rotational barriers from zZe1 and eZz1 to 3 are consistent with observed kinetics for the of decay of 1. The structures of s-Z- and s-E-3-hydroxy-1,2-propadien-1-one (18 and 19) and the vinylogous cumulene 16 are predicted to have strongly bent geometries. The barrier to internal proton transfer from 16 is only 0.8 kcal/mol, while the barrier from 18 is 23.7 kcal/mol. Barrier heights for these and other pseudopericyclic reactions are shown to correlate with (1) the nucleophilicity and electrophilicity of the reacting centers, (2) the exothermicity of the reaction, and (3) deviations from the ideal geometries for the reaction. Significantly, the barrier heights do not correlate with the number of electrons involved in the reaction.

Fluorine effect on pericyclic and pseudopericyclic processes: Evidences and ab initio theory

“…Electrocyclic ring opening (ERO) of pyrone has been found to be pseudeopericyclic by Birney and coworkers 27 and later Rodríguez and coworkers 28 have confirmed this by using NBO analysis. We [29][30] have studied the thermolysis of 2-pyrone and 6-halo-2-pyrone and have found that fluorine substitution substantially altered the potential energy surface for ERO, sigmatropic rearrangement and electrocyclic ring closing (ERC) of pyrones.…”

Computational Insights on the Lone Pair Induced Barrier Modulation in the Thermal Rearrangement of 6-Halo-2-Pyrones

“…18 Geometry optimizations have been carried out at MP2 level with 6-31g(d) basis-set because (i) MP2 gives fairly a good geometry and energy parameters of molecules in the ground state, 19 (ii) electron correlation is important in cases where intramolecular proton shifts are considered. 2,13 The minima have all real frequencies while TSs (Transition states) have a single imaginary frequency and the animation of this imaginary frequency has confirmed the transition states. Free energies have been calculated from thermochemical parameters obtained from frequency calculations.…”

“…This ketene aldehyde intermediate, a vinylog of well studied α-oxoketenes 3−12 resembles the α-oxoketenes in having a pseudopericyclic ring closure and sigmatropic hydrogen shift characterized by a planar or near planar transition state. 2 Later on Reva et al 13 have explored the whole pathway of rearrangement of pyrathione to thiapyrone. Recently, Alajarin and coworkers 14 have studied the migratory aptitude of halogens and other electron donor groups during the conversion of ketenimines into quinoline4 (3H)-ones.…”

Evidence for Nonstatistical Dynamics in the Wolff Rearrangement of a Carbene

“…In hindsight, one can perhaps rationalize the unexpected facility of the rearrangement by identifying it as a pseudopericyclic 17 rather than a truly pericyclic reaction. The hypothetical electron transfers in the pseudopericyclic process are shown schematically in Figure 4.…”