Sterically Hindered Alkanes, II 1,1,1,3,3,3-Hexaphenylpropane

Institut für Organische Chemie der Universität Tübingen, Auf der Morgenstelle 18, D-7400 Tübingen Z. Naturforsch. 37b, 1623-1632 (1982); received October 19, 1982 X-ray, Steric Hindrance, C14-Tracer Technique, Triarylmethyl Radical, Diazomethane The structure of the title compound 3, C39H32, obtained for the first time by Schlenk and Bornhardt (1912) from triphenylmethyl (1) and diazomethane (2) has been determined by 14C-tracer technique, NMR spectroscopy and X-ray analysis. It crystallizes in the monoclinic space group C2/c with Z = 4, a = 9.943(6), b= 19.054(3), c = 14.708(2) A and ß = 101.80(2)°. The structure was solved by direct methods and refined by full-matrix least squares to R = 0.046 for all 2625 reflexions (|Fol >0) . On account of steric repulsions between the phenyl rings at both termini the central propane bond angle <p is stretched to 127.2°, whereas the bond lengths are not much affected. The X-ray results are compared with those of Blount et al. (1982), obtained on a triclinic crystal modification (Pi) of 3. A fair agreement of bond lengths and angles is observed, the slopes of the homochiral propellers, however, aro different in both modifications. The measurement of the N2evolution shows that the mechanism of formation of 3 from 1 and 2 is different in the photochemical and dark reaction. A carbene respectively a one-electron transfer mechanism is suggested. In spite of the remarkably normal CC-propane bond lengths homolytic cleavage of these bonds to give 1 is observed at 200-220 °C using the ESR technique.