title: Oxygen-initiated stereoselective thermal isomerisation of a cyclobutane derivative in the solid state.
abstract: Solid-state [2+2] photochemical cycloaddition reactions have been extensively studied after the classical work of Schmidt in the 1960s. Of these, trans-1,2-bis(4'-pyridyl)ethylene (bpe) is one of the well-studied alkenes to synthesize tetrakis(4-pyridyl)cyclobutane (tpcb). However, almost all the solid-state [2+2] cycloaddition reactions of bpe yielded, almost exclusively, one of the four possible isomers, namely, the rctt-tpcb (r=regio c=cis and t=trans). Here we describe a stereoselective synthesis of the tetrahedrally disposed rtct-tpcb by the solid-state thermal isomerization of the rctt-isomer in atmospheric air. We propose that this isomerization occurs through a topochemical unimolecular mechanism by a radical chain pathway, initiated by molecular oxygen. This is supported by the nature of products formed in air and nitrogen, detection of a radical in ESR spectral studies, ESI-MS crossover experiments, VT PXRD studies along with QM, MD and docking calculations. The formation of a unique isomer by thermal isomerization may be a general phenomenon to quantitatively synthesize other useful stereoisomers from the existing isomers of cyclobutane derivatives.
authors.affiliation: Department of Chemistry, National University of Singapore, Fax: (+65) 6779-1691.
authors.forename: Anjana
authors.initials: A
authors.lastname: Chanthapally
authors.forename: Hui
authors.initials: H
authors.lastname: Yang
authors.forename: Hong Sheng
authors.initials: HS
authors.lastname: Quah
authors.forename: Richard D
authors.initials: RD
authors.lastname: Webster
authors.forename: Martin K
authors.initials: MK
authors.lastname: Schreyer
authors.forename: Ming Wah
authors.initials: MW
authors.lastname: Wong
authors.forename: Jagadese J
authors.initials: JJ
authors.lastname: Vittal
keywords: crystal engineering; cyclobutanes; cycloreversion; radical mechanism; thermal isomerization