科研之友
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摘要
An examination of the literature on [2.2] cyclophanes reveals a loose relationship between the relative sizes of the two 'half-cyclophanes' (as measured by the parameter Delta d) and the limitations of the dominant general synthetic approaches. Direct coupling methods tend to be successful only for systems with Delta d values below 1.0 angstrom, whereas ring-contraction-based approaches are usually viable for systems with Delta d values up to 2.0 angstrom. For the very few known systems with Delta d values greater than 2.0 angstrom, aromatization-based approaches are the only ones that have been successful. The syntheses of two [2.2] cyclophanes with very large Delta d values, [2] paracyclo[2](2,7) pyrenophane (17) (Delta d =4.25 angstrom) and [2] metacyclo [2](2,7) pyrenophane (18) (Delta d 5.04 angstrom) are presented here. The syntheses hinge on a valence isomerization/dehydrogenation reaction. The crystallographically determined bend angle, theta, for 18 is 96.1 degrees. Cyclophane 18 undergoes a degenerate conformational flip, the energy barrier for which was determined to be 18.9 kcal mol(-1) by DNMR.
- 出版日期2010
