Rotamers and isomers in the fulgide series. Part 3. Structures of the bis(4-methoxy-3-methylbenzylidene)succinic anhydrides

Abstract

The conformation of bisarylsuccinic anhydrides is a function of steric interaction between the aryl rings and the reactivity of the substituents that they carry. To explore the second factor a methyl group has been introduced for the present study. For the first time a stable E,Z-isomer has now been isolated as a result. The structures of the E,E- and Z,Z-isomers have been determined crystallographically. Crystals of the E,E-isomer have a= 7.835(1), b= 8.966(1), c= 25.725(3)Å, space group P2₁2₁2₁, Z= 4; and for Z,Z a= 25.506(5), b= 7.896(1), c= 20.058(3)Å, β= 102.75(1)°, space group C2/c, Z= 8. All efforts to obtain sufficiently large crystals of the E,Z-product have failed, but it has been characterised in terms of NMR evidence. The dehydrogenation product from ring closure of the E,Z-isomer has been characterised from its NMR spectrum and confirmed crystallographically, a= 13.132(4), b= 8.239(1), c= 17.242(7)Å, β= 100.84(3)°, space group P2₁/c. Atomic coordinates from this structure are used to optimize the E,Z-structure by molecular modelling.