Towards a correlation of absolute configuration and chiroptical properties of alkyl aryl sulfoxides: a coupled-oscillator foundation of the empirical Mislow 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 bathochromic effect exerted by the nitro group in the para position of the phenyl sulfoxides indicates that the sulfur atom acts as an electron donor moiety towards the phenyl ring. Such behavior requires a significant 2p(C)-3sp3(S) overlap, and therefore the phenyl (and p-substituted phenyl) sulfoxides 1-12, as well as the 2-naphthyl sulfoxides 15 and 16, must assume a conformation which permits such orbital overlap. The steric effect of the peri hydrogen in 1-naphthyl-substituted compounds 13 and 14 does not allow a conformation of this type, and in these compounds the above-mentioned 2p(C) and 3sp3(S) orbitals are positioned in almost orthogonal planes. This conformational difference is clearly shown by the absorption spectra: compounds 1-12, 15, and 16 show the lowest energy sigma --> sigma* transition of the sulfoxide chromophore at approximately 250 nm, indicating the existence of a conjugated S=O chromophore. In contrast, the corresponding absorption in 13 and 14 occurs at about 200 nm, indicating the presence of an isolated S=O chromophore. The CD spectra of 13 and 14 show a negative, couplet-like feature between 250 and 200 nm. This spectral feature can be interpreted in terms of exciton coupling between the allowed sigma --> sigma* transition of the isolated S=O chromophore at 200 nm and the 1B transition of the naphthalene chromophore. In fact, the Harada-Nakanishi rule predicts a negative CD couplet for an S-configured sulfoxide in the conformation found by UV analysis, as 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 1-12, taking into account the coupling of the benzene chromophore 1La transition with the sigma --> sigma* transition of the S=O chromophore. In this case the Harada-Nakanishi rule also predicts a negative CD couplet for the S-configured sulfoxides, as found experimentally.
Rosini
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Dipartimento di Chimica, Università della Basilicata, Potenza, Italy. rosini@unibas.it
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