title: Facile 1,3- and 1,5-Chlorine Migration.
abstract: High-level ab initio molecular orbital calculations, using the G2(MP2,SVP) theory (and semiempirical methods) have been used to examine several 1,3- and 1,5-chlorine migrations. It is found that the interaction of chlorine lone pair electrons with a low-lying LUMO accelerates the Cl shift dramatically (lone pair-LUMO-mediated pericyclic reaction). The activation barriers for the 1,3-migration in chloro oxo ketene 1 (Cl(C=O)CH=C=O) and the 1,5-migration in (2-(chlorocarbonyl)vinyl)ketene 2 (Cl(C=O)CH=CHCH=C=O) are only 53 and 61 kJ mol(-)(1), respectively, compared to the 216 and 173 kJ mol(-)(1) barriers for the corresponding unassisted 1,3- and 1,5-sigmatropic shifts of Cl in 3-chloro-1-propene and 5-chloro-1,3-pentadiene. The transition structures for 1 and 2 reveal that migration of the chlorine atoms takes place in the molecular planes. The 1,5-chlorine shift in 6-chlorocyclohexa-2,4-dienone (3) has a significantly higher barrier due to a lack of appropriate orbital interaction. The related 1,3-shift in the (chlorocarbonyl)imine-alpha-chloro isocyanate system is also dramatically accelerated compared with conventional pericyclic 1,3-Cl migration.
authors.affiliation: Department of Chemistry, The University of Queensland, Brisbane, Queensland 4072, Australia.
authors.forename: Rainer
authors.initials: R
authors.lastname: Koch
authors.forename: Ming Wah
authors.initials: MW
authors.lastname: Wong
authors.forename: Curt
authors.initials: C
authors.lastname: Wentrup