STEREOCHEMICAL CONSEQUENCES OF A PARITY RESTRICTION ON DYNAMIC GEARING IN TRIS(9-TRIPTYCYL)GERMANIUM CHLORIDE AND TRIS(9-TRIPTYCYL)CYCLOPROPENIUM PERCHLORATE

Crystal and molecular structures of tris(9-triptycyl)germanium chloride (1) and tris(9-triptycyl)cyclopropenium (2) perchlorate were determined. Crystals of 1 are trigonal, space group R3c (No. 161), with a = b = 13.090 (3) Å, c = 43.58 (1) Å, Z = 6. Crystals of 2 perchlorate are hexagonal, space group P63/m (No. 176), with a = b = 15.145 (5) Å, c-13.617 (4) Å, Z = 2. Both 1 and 2 are located on crystallographic 3-fold rotation axes, with site symmetries C3for 1 and C3hfor 2. The same symmetries are maintained in solution at room temperature on the NMR time scale. The enantiomers of 1 were separated by HPLC on a column of (-)-poly(triphenylmethyl methacrylate). The racemization barrier for 1 is 19.0 kcal mol-1and the topomerization barrier of 2 is similar in magnitude, as judged by the 22.2 and 23.4 kcal mol-1required for the interconversion of the achiral and DL diastereomers of bis(9-triptycyl)(2,3-dimethyl-9-triptycyl)cyclopropenium ion (3). Considering that barriers of less than 5 kcal mol-1need to be overcome to achieve correlated disrotation of 9-triptycyl groups in bis(9-triptycyl)methane and related compounds, these observations show that uncorrelated as well as correlated rotation is mechanically disallowed in a closed cyclic array consisting of an odd number of securely meshed gears, thus providing compelling evidence for the operation of a mechanical (parity) selection rule at the molecular level. © 1990, American Chemical Society. All rights reserved.