Maria Irene Donnoli

New ligands for manganese catalysed selective oxidation of sulfides to sulfoxides with hydrogen peroxide

Abstract In situ prepared manganese complexes with ligands 1 – 5 have been used in the catalytic oxidation of sulfides to sulfoxides using hydrogen peroxide at 0°C in acetone. Using ligand 4 , methyl phenyl sulfoxide is obtained in 55% yield and turnover numbers up to 250, while the formation of sulfone is almost suppressed.

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.

Enantiopure p,p′-disubstituted 1,2-diphenylethane-1,2-diols as chiral inducers in the Ti-mediated oxidation of sulfides: a case of reversal of asymmetric induction by fluorine substitution

Abstract In the asymmetric oxidation of methyl p -tolyl sulfide, ( 2a ), and benzyl phenyl sulfide ( 2b ) by TBHP, mediated by a titanium complex with enantiopure (R,R)-p,p′ -disubstituted-1,2-diphenylethane-1,2-diols, both the unsubstituted diol (R,R) - 1a and the p -OMe substituted diol (R,R) - 1b lead to sulfoxides of S configuration, with ee up to 99%. On the contrary the p -CF 3 substituted ligand (R,R) - 1c leads to significantly lower ee and in the case of 2a a reversal of asymmetric induction is observed.

Circular dichroism spectra and absolute configuration of some aryl methyl sulfoxides

The absorption and circular dichroism (CD) spectra of three aryl sulfoxides, i.e. (-)-(S)-1-naphthyl methyl sulfoxide, (S)-1, (-)-(S)-1-(2-methyl)naphthyl methyl sulfoxide, (S)-2 and (-)-(S)-9-phenanthryl methyl sulfoxide, (S)-3, have been interpreted by means of the coupled oscillator model formulated by DeVoe. Theoretical spectra have been calculated starting from input geometries provided by molecular mechanics (MMX) calculations and by employing standard spectroscopic parameters to describe the allowed transitions of the aromatic and the sulfoxide chromophores. The satisfactory agreement between the predicted and experimental spectra allows us to confirm the configurational assignment of these compounds as (-)/(S). The analysis of CD spectra, affording the right assignment of the absolute configuration (AC) of the alkyl aryl sulfoxides, then offers a practical alternative to the more complex vibrational circular dichroism spectroscopy and ab initio optical rotation calculation techniques that have been used very recently to assign the AC of (-)-2 and (-)-3.

Capillary electrophoretic resolution of chiral aryl alkyl and aryl benzyl sulphoxides using sulphated β‐cyclodextrins as chiral selectors

The enantiomeric resolution of a series of chiral aryl alkyl and aryl benzyl sulphoxides was investigated by reversed-polarity capillary electrophoresis using sulphated-β-cyclodextrins as chiral selectors. Separations were performed using fused silica capillaries with a lithium phosphate run buffer (100 mM, pH 2) containing hydroxyethylcellulose as a dynamic coating agent to virtually eliminate electroosmotic flow and sulphated-β-cyclodextrins as chiral selectors. The majority of enantiomeric pairs were resolved in the presence of heptakis-6-sulphato-β-cyclodextrin, heptakis-(2,3-diacet-yl-6-sulphato)-β-cyclodextrin, and sulphated-β-cyclodextrin (average degree of substitution = 4). In general, the electrophoretic migration order was R- before S-sulphoxide, indicating generally greater affinities between the R-enantiomers and the sulphated β-CDs.