Eight-membered heterocycles containing two phosphorus atoms. X-Ray diffraction and nuclear magnetic resonance studies of 2,6-dimethyl-1,3-dioxa-2,6-diphosphacyclo-octane 2,6-disulphides and 2,6-diselenides

Abstract

trans- and cis-2,6-dimethyl-1,3-dioxa-2,6-diphosphacyclo-octane 2,6-disulphides (1) and (2), and trans- and cis-2,6-diselenides (3) and (4), have been investigated by means of ¹H (2D,J-resolved), ¹³C, and ³¹P n.m.r. spectroscopy and X-ray diffraction. In the solid state the cis-isomers adopt a crown conformation and the trans-isomers a boat-chair conformation. The orientations of P–X (X = S or Se) and P–Me bonds are quasi-axial and quasi-equatorial, respectively. Room-temperature n.m.r. data can be interpreted on the basis of the X-ray data. A fast enantiomerization between two boat-chair conformations of opposite chirality characterizes the trans-isomers, whereas the cis-isomers exist mainly in a crown conformation similar to that observed in the crystal.