Isomers of Diformyl[2.2]paracyclophane; X-Ray Structure Determinations of the Pseudo-ortho Isomer and Two Polymorphs of the Pseudo-meta Isomer

Abstract

Pseudo-ortho- or 4,16-diformyl[2.2]paracyclophane (1) and two polymorphs of pseudo-meta- or 4,13-diformyl[2.2]paracyclophane (2) all display the usual features of [2.2]paracyclophane strain (lengthened C-C bonds and widened C-C(sp3)-C angles in the bridges, narrower sp2 ring angles at the bridgehead atoms, and flattened boat conformations of the rings). All bulk samples were racemates. Polymorph 2a crystallizes in space group P21/n with one molecule in a general position, whereas 2b crystallizes in space group C2 with two independent molecules, each with crystallographic twofold symmetry. All three molecules of 2 are different rotamers with respect to the formyl groups; in 2a one is endo and one exo to the neighbouring bridge, whereas in 2b both formyls are exo in one molecule and endo in the other. In all compounds, the packing patterns are preponderantly associated with C-H・ ・ ・O contacts. In 1 the molecules are connected to form tubes parallel to the short a axis. 2a consists of two interconnected layer structures. One is parallel to (100) and involves chains of molecules parallel to [01̄1]; the other is parallel to (001) and involves chains of molecules parallel to the b axis. 2b consists of two hexagonal layers, one for each independent molecule, parallel to (001). One layer contains bifurcated (C-H・ ・ ・ )2O systems, whereas the single H・ ・ ・O interactions in the other are long and markedly bent.