Dynamic gearing and residual stereoisomerism in labeled bis-(9-triptycyl)methane and related molecules. Synthesis and stereochemistry of bis (2,3-dimethyl-9-triptycyl)methane, bis (2,3-dimethyl-9-triptycyl)carbynol, and bis (1,4-dimethyl-9-triptycyl)methane

The substitution patterns of bis(9-triptycyl)methane (1) and -carbinol (2), two molecular bevel gear systems, have been classified according to the number of residual stereoisomers remaining under the full operation of the gearing mode and in the presence or absence of gear-slippage constraints. Substitution patterns have been identified in which enantiomeric conformations are interconverted by gearing through exclusively chiral pathways. Bis(2,3-dimethyl-9-triptycyl)methane (3) and -carbinol (4) have each been separated into residual meso and dl isomers, and the isomers have been unambiguously identified by NMR spectroscopy, meso-3 (3a) and dl-3 (3b) are interconverted at elevated temperatures by a gear-slippage mechanism; the diastereomerization barrier (ΔG‡) is 34 kcal mol−1 at 145-165 °C. The racemization barrier of 3b, which also involves gear slippage, is found to be <22 kcal mol−1. These findings are in accord with an empirical force field (EFF) prediction of 30 kcal mol−1 for the gear-slippage barrier in 1. The methyl groups in the 1- and 1′-positions of meso-bis(1,4-dimethyl-9-triptycyl)methane (5a) restrict cogwheeling of the 9-triptycyl groups, and a gearing barrier is therefore measurable by dynamic NMR. The measured ΔG‡ of 14 kcal mol'1 is comparable to a barrier of 17 kcal mol−1 calculated (EFF) for the conformational interconversion by the gearing mechanism. The gear-slippage barrier, as measured by conversion of 5a to the dl isomer (5b), is 39 kcal mol−1 at 215 °C. The present work describes the first measurement of stereoisomerization barriers resulting from restricted gearing and gear slippage.