ENHANCEMENT OF THROUGH-SPACE AND THROUGH-BOND PI-ORBITAL INTERACTIONS - SYNTHESES AND PROPERTIES OF PERMETHYLATED AND PERSPIROCYCLOPROPANATED CYCLOTETRADECA-1,3,6,9,12-PENTAYNE

The permethylated and perspirocyclopropanated cyclotetradeca-1,3,6,9,12-pentaynes 1 and 2 have been synthesized and completely characterized by H-1 NMR, C-13 NMR, UV, IR, and mass spectroscopy, as well as by X-ray crystal structure analysis. In the permethylated pentayne, compression of the internal C-C-C bond angles at the saturated carbon atoms flanking the diyne (103.8 degrees) enhances the through-space pi-orbital interactions and causes a bathochromic shift in the long wavelength UV absorption maximum (lambda(max) 266 nm) relative to that of reference compounds (lambda(max) 255-259 nm). In the perspirocyclopropanated pentayne, wider internal C-C-C bond angles at the corresponding carbon atoms (109.2 degrees) reduce the through-space pi-orbital interactions, but the through-bond pi-orbital interactions are enhanced by spirocyclopropanation and cause a shift in the long wavelength UV absorption maximum to even longer wavelength (lambda(max) 273 nm).