Towards a correlation of absolute configuration and chiroptical propertiesof alkyl aryl sulfoxides: A coupled-oscillator foundation of the empiricalMislow rule?

The absorption and circular dichroism (CD) data for a series of alkyl aryl sulfoxides 1-16 of known S configuration have been analyzed. The strong bat hochromic effect exerted by the nitro group in the para position of the phe nyl sulfoxides indicates that the sulfur atom acts as an electron donor moi ety towards the phenyl ring. Such behavior requires a significant 2p(C) - 3 sp(3)(S) overlap, and therefore the phenyl land p-substituted phenyl) sulfo xides 1-12, as well as the 2naphthyl sulfoxides 15 and 16, must assume a co nformation which permits such orbital overlap. The steric effect of the per i hydrogen in 1-naphthyl-substituted compounds 13 and 14 does not allow a c onformation of this type, and in these compounds the above-mentioned 2p(C) and 3sp3(S) orbitals are positioned in almost orthogonal planes. This confo rmational difference is clearly shown by the absorption spectra: compounds 1-12, 15, and 16 show the lowest energy sigma -> o*: transition of the sulf oxide chromophore at approximately 250 nm, indicating the existence of a co njugated S=O chromophore. In contrast, the corresponding absorption in 13 a nd 14 occurs at about 200 nm, indicating the presence of an isolated S=O ch romophore. The CD spectra of 13 and 14 show a negative, couplet-like featur e between 250 and 200 nm. This spectral feature can be interpreted in terms of exciton coupling between the allowed sigma->sigma* transition of the is olated S=O chromophore at 200nm and the LB transition of the naphthalene ch romophore. In fact, the Harada-Nakanishi rule predicts a negative CD couple t for an S-configured sulfoxide in the conformation found by UV analysis, a s found experimentally. The CD spectrum of 13 is quantitatively reproduced by DeVoe coupled-oscillator calculations, strongly implying that a coupled- oscillator mechanism is operative in determining the optical activity of 13 and 14. This approach has also tentatively been extended to the conjugated sulfoxides 112, taking into account: the coupling of the benzene chromopho re L-1(a) transition with the sigma->sigma*:" transition of the S=O chromop hore. In this case the Harada-Nakanishi rule also predicts a negative CD co uplet for the S-configured sulfoxides, as found experimentally.