title: The role of methoxy group in the Nazarov cyclization of 1,5-bis-(2-methoxyphenyl)-1,4-pentadien-3-one in the gas phase and condensed phase.
abstract: ESI-protonated 1,5-bis-(2-methoxyphenyl)-1,4-pentadien-3-one (1) undergoes a gas-phase Nazarov cyclization and dissociates via expulsions of ketene and anisole. The dissociations of the [M + D](+) ions are accompanied by limited HD scrambling that supports the proposed cyclization. Solution cyclization of 1 was effected to yield the cyclic ketone, 2,3-bis-(2-methoxyphenyl)-cyclopent-2-ene-1-one, (2) on a time scale that is significantly shorter than the time for cyclization of dibenzalacetone. The dissociation characteristics of the ESI-generated [M + H](+) ion of the synthetic cyclic ketone closely resemble those of 1, suggesting that gas-phase and solution cyclization products are the same. Additional mechanistic studies by density functional theory (DFT) methods of the gas-phase reaction reveals that the initial cyclization is followed by two sequential 1,2-aryl migrations that account for the observed structure of the cyclic product in the gas phase and solution. Furthermore, the DFT calculations show that the methoxy group serves as a catalyst for the proton migrations necessary for both cyclization and fragmentation after aryl migration. An isomer formed by moving the 2-methoxy to the 4-position requires relatively higher collision energy for the elimination of anisole, as is consistent with DFT calculations. Replacement of the 2-methoxy group with an OH shows that the cyclization followed by aryl migration and elimination of phenol occurs from the [M + H](+) ion at low energy similar to that for 1.
authors.affiliation: Department of Chemistry, Sacred Heart College, Thevara, Cochin, Kerala, India.
authors.forename: June
authors.initials: J
authors.lastname: Cyriac
authors.forename: Justin
authors.initials: J
authors.lastname: Paulose
authors.forename: Mathai
authors.initials: M
authors.lastname: George
authors.forename: Marupaka
authors.initials: M
authors.lastname: Ramesh
authors.forename: Ragampeta
authors.initials: R
authors.lastname: Srinivas
authors.forename: Daryl
authors.initials: D
authors.lastname: Giblin
authors.forename: Michael L
authors.initials: ML
authors.lastname: Gross
mesh_terms: D003500:Cyclization; D005740:Gases / Q000737:chemistry*; D007477:Ions; D007659:Ketones / Q000737:chemistry*; D008958:Models, Molecular; D053719:Tandem Mass Spectrometry
chemical_list: D005740:Gases; D007477:Ions; D007659:Ketones