Oxygen-initiated stereoselective thermal isomerisation of a cyclobutane derivative in the solid state
Quah, Hong Sheng
Webster, Richard D.
Schreyer, Martin K.
Wong, Ming Wah
Vittal, Jagadese J.
Date of Issue2014
School of Physical and Mathematical Sciences
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.
Chemistry - a European journal
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.