Structural, vibrational, and electronic characteristics of enyne macrocycles as a function of ring strain

The cross-conjugated macrocycles 8a-e and acyclic model compound 12 have been synthesized and thoroughly analyzed by H-1 and C-13 NMR, UV, IR, and Raman spectroscopies, and by X-ray crystallography. The increasing ring strain and resultant changes in the double and triple bond orders are clearly detailed in the Raman and C-13 NMR spectra, as well as in the X-ray structures. Correlations between C-13 NMR shifts, bond angles, and Raman frequencies are essentially linear for the butadiynyl and olefinic moieties. The most highly strained system, 8a, displays interior alkylidene bond angles that are reduced to 108 degrees and alkyne angles reduced to as little as 155 degrees. The electronic absorption spectrum of 8a shows evidence of through-space homoconjugation that is manifested as a bathochromic shift of the absorption band arising from the in-plane pi-system. The absorption bands from the out-of-plane pi-electron systems of the more strained cycles 8a and 8b show slight bathochromic shifts as compared with the less strained systems 8c-e and acyclic 12, Ring strain, however, has a surprisingly small effect on the electronic absorption energies of these macrocycles.