A Density Functional Theory Study Clarifying the Reactions of Conjugated Ketenes with Formaldimine. A Plethora of Pericyclic and Pseudopericyclic Pathways

Abstract

The reactions of vinylketene (1a), imidoylketene (1b), and formylketene (1c) with formaldimine (2) were studied at the B3LYP/6-31G* level. For the cycloadditions of these conjugated ketenes with 2, several possible pathways to both [4 + 2] and [2 + 2] products were examined. The lowest energy [2 + 2] pathways are, in most cases, calculated to be stepwise, forming the products via rate-determining conrotatory electrocyclization of zwitterionic intermediates. However, concerted transition structures analogous to the ketene plus ethene [2 + 2] cycloaddition reaction were also located; the existence of multiple transition states offers a resolution to a long-standing controversy regarding the mechanism of ketene plus imine cycloadditions. Both stepwise and concerted [4 + 2] pathways were calculated for 2b and for 2c; both these pathways are pseudopericyclic. The inherently low barriers associated with pseudopericyclic transition states provide an explanation of the experimental preference for [4 + 2] cycloadditions of alpha-oxoketenes and predict [4 + 2] cycloadditions should also be favored for imidoylketenes. For a vinylketene constrained to a Z-geometry, the concerted [4 + 2] cycloaddition is also predicted to be the lowest energy pathway. An explanation is offered for the unusual thermal equilibration from a six-membered ring (3d) to a four-membered ring (4d) observed by Sato et al. Transition structures for facile pseudopericyclic 1,3- and 1,5-hydrogen shifts in the zwitterions were also calculated.

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

“…The role interchange means a "disconnection" in the cyclic array of overlapping orbitals because the atomic orbitals switching functions are mutually orthogonal'. After two decades, it was extensively studied by Birney and coworkers [2][3][4][5][6] and they proposed that the pseudopericyclic reactions would have the following characteristics: Very low activation energies, nearly-planar transition states and symmetry allowedness. Later many methods based on the magnetic properties such as NICS, 7,8 ACID 9-12 have been developed to characterize pseudeopericyclic reactions.…”

Chemistry of Acyl(imidoyl)ketenes: IX. Synthesis and Thermolysis of 3-Aroyl-8-chloro-1,2-dihydro-4H-pyrrolo[2,1-c][1,4]benzoxazine-1,2,4-triones

“…The evolution of chemistry of acyl(imidoyl)ketenes is hampered by insufficient development of convenient methods for generation thereof from available initial compounds (unlike the chemistry of monosubstituted analogs, acyl ketenes and imidoyl ketenes) [2][3][4][5]. One of the simplest and the most available procedure for generating acyl(imidoyl)ketenes is thermal decarbonylation of 4-acyl-2,3(1H)-purrolediones.…”

Cycloaddition and Electrocyclic Reactions of Vinylketenes, Allenylketenes, and Alkynylketenes

“…Thus, although the concerted pathway is consistent with the observed data in many cases, a stepwise process provides an equally convincing explanation of the results. Computational studies at the B3LYP/6-31G* level of the reaction of vinylketene with formaldimine find a concerted transition structure for formation of lactam 8, 33 but other studies at this level find only a stepwise process for this reaction, with the formation of a zwitterionic intermediate (Scheme 12). 34 Both conrotatory and disrotatory transition structures of similar energy are found for the cyclization of the zwitterionic intermediate to lactam 8, and both are considered possible for this reaction.…”