title: Origin of the synchronicity in bond formation in polar Diels-Alder reactions: an ELF analysis of the reaction between cyclopentadiene and tetracyanoethylene.
abstract: The origin of the synchronicity in C-C bond formation in polar Diels-Alder (P-DA) reactions involving symmetrically substituted electrophilic ethylenes has been studied by an ELF analysis of the electron reorganization along the P-DA reaction of cyclopentadiene (Cp)  with tetracyanoethylene (TCE)  at the B3LYP/6-31G* level. The present study makes it possible to establish that the synchronicity in C-C bond formation in P-DA reactions is controlled by the symmetric distribution of the electron-density excess reached in the electrophile through the charge transfer process, which can be anticipated by an analysis of the spin electron-density at the corresponding radical anion. The ELF comparative analysis of bonding along the DA reactions of Cp  with ethylene and with TCE  asserts that these DA reactions, which have a symmetric electron reorganization, do not have a cyclic electron reorganization as the pericyclic mechanism states. Due to the very limited number of cases of symmetrically substituted ethylenes, we can conclude that the synchronous mechanism is an exception of DA reactions.
authors.affiliation: Universidad de Valencia, Departamento de Química Orgánica, Burjassot, Valencia, Spain. domingo@utopia.uv.es
authors.forename: Luis R
authors.initials: LR
authors.lastname: Domingo
authors.forename: Patricia
authors.initials: P
authors.lastname: Pérez
authors.forename: Jose A
authors.initials: JA
authors.lastname: Sáez