S. Eisler et al., Structural, vibrational, and electronic characteristics of enyne macrocycles as a function of ring strain, J AM CHEM S, 122(29), 2000, pp. 6917-6928
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.