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 be en synthesized and thoroughly analyzed by H-1 and C-13 NMR, UV, IR, and Ram an spectroscopies, and by X-ray crystallography. The increasing ring strain and resultant changes in the double and triple bond orders are clearly det ailed 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 h ighly strained system, 8a, displays interior alkylidene bond angles that ar e reduced to 108 degrees and alkyne angles reduced to as little as 155 degr ees. The electronic absorption spectrum of 8a shows evidence of through-spa ce homoconjugation that is manifested as a bathochromic shift of the absorp tion band arising from the in-plane pi-system. The absorption bands from th e out-of-plane pi-electron systems of the more strained cycles 8a and 8b sh ow 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.