Topochemically controlled photodimerisation of crystalline methyl 6-isobutenyl-2-methyl-4-oxocyclohex-2-enecarboxylate : a chemical and X-ray crystallographic study

Abstract

The chemistry of the cyclobutano-dimer (3), formed when methyl 6-isobutenyl-2-methyl-4-oxocyclohex-2-ene-carboxylate (1) is irradiated with u.v. light in the crystalline state, is studied. Particular attention is given to the Baeyer–Villiger reactions of derivatives of (3) and the ensuing lactone isomerisations. This leads to a chemical proof of the antiparallel-cis,anti,cis-orientation of the dimer, the 1,2-substituents being trans-equatorial.

The topology of the system is examined in detail by X-ray crystallographic studies of the cyclohexenone (1) end its cyclobutano-dimer (3). Although the centre-to-centre distance of the nearest-neighbour double bonds falls within the usual range (3.86 Å), the geometry is apparently not ideal. Neighbouring monomer molecules move ca. 2.3 Å within the ring plane, as well as ca. 2.2 Å towards each other perpendicular to the ring plane, in order to take up the final dimer geometry.