Synthesis and Conformational Analysis of 2,11-Disila[3.3]metacyclophanes

Silyl-tethered [3.3]metacyclophanes were prepared and subjected to conformational analysis. The results show that these compounds exist in unprecedented anti-rich metacyclophane forms. In the case of 2,2,11,11-tetrasubstituted 2,11-disila[3.3]metacyclophanes, anti conformers are lower in energy than their syn counterparts. Evidence for this assignment comes from the results of 1H NMR spectroscopic and X-ray crystallographic analyses, along with molecular orbital calculations. However, the 2,2,11,11-tetrahydro derivative displays the opposite behavior, which means that the syn conformer is thermodynamically most stable. The syn/anti ratios of these compounds in solution are governed by temperature and solvent polarity in a manner that can be explained by considering the mixed contributions of ΔΔG and dipole moments.