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Catalytic Asymmetric Oxidation of Aryl Sulfides with a Ti/H2O/(R,R)-Diphenylethane-1,2-diol Complex: a Versatile and Highly Enantioselective Oxidation Protocol
No full textA new catalytic procedure for the asymmetric oxidation of aryl alkyl and aryl benzyl sulfides to optically active sulfoxides by hydroperoxides is described. This oxidation of sulfides is mediated by a chiral Ti complex formed in situ by reacting Ti(i-PrO)4, (R,R)-diphenylethane-1,2-diol (1), and water. The conditions of the reaction (stoichiometric composition of the catalyst, temperature, and the presence of additives and solvent) have been determined in order to reach the highest enantioselectivity and avoid the intervention of a kinetic resolution process. The oxidation protocol described herein is quite versatile as the values of chemical yields (60- 73%) and of enantioselectivity (ee 70-80%) achieved for aryl alkyl sulfides are almost independent of the nature of the aryl substituent and of the size of the alkyl group. Notably, aryl benzyl sulfides, which are poor substrates for the Ti/DET catalyzed oxidations, afforded very high ee's (92-99%) with this oxidation system
Towards a Correlation of Absolute Configuration and Chiroptical Properties of Alkyl Aryl Sulfoxides: a Coupled-Oscillator Foundation of the Empirical Mislow Rule?
No full textThe 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 σ → σ* 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 σ → σ* 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 σ → σ* transition of the S=O chromophore. In this case the Harada-Nakanishi rule also predicts a negative CD couplet for the S-configured sulf-oxides, as found experimentally
Enantiopure 1,2-Diarylethane-1,2-diols as Chiral Auxiliaries in Stereoselective Processes: Induction of Molecular and Supramolecular Chirality
No full textThis review discusses how 1,2-diarylethane-1,2-diols have been used to control stereoselective processes leading to the induction of a prevailing chirality in molecular and supramolecular structures. To this end this article is divided into three parts. (i) in the first one are described the synthesis and the stereochemical characterization of the above compounds; (ii) part two deals with the use of 1 as chiral auxiliaries in stereoselective processes (i.e. induction of a prevailing sense of chirality at a molecular level); (iii) part three discusses the use of derivatives of 1 as inducers of cholesteric mesophases in nematic solvents (i.e. induction of a prevailing sense of chirality at a supramolecular level)
Determination of the Absolute Configuration of 1-Arylethane-1,2-diols by a Nonempirical Analysis of the CD Spectra of Their 4-Biphenylboronates
No full text1-Arylethane-1,2-diols 1, reacting with 4-biphenylboronic acid 2, form the conformationally defined boronates 3 where the aryl and biphenyl chromophores assume a fixed and known relative disposition. These chromophores thus define an exciton coupled system, whose chirality (revealed by the sign of the biphenyl On band at 260 nm) allows an unambiguous assignment of the absolute configuration of the stereogenic center. This approach provides the hitherto unreported absolute configuration of diols 1c-f
Chromatographic Resolution and Elution Order of Alkyl Aryl and Aryl Benzyl Sulfoxides on Cellulose Based Chiral Stationary Phases
No full textSeveral alkyl aryl and aryl benzyl sulfoxides have been prepared in optically active form via enantioselective Ti-catalyzed oxidation of the corresponding sulfides. The absolute configuration was assigned on the basis of optical rotation while in the case of some new sulfoxides it was determined by the analysis of their circular dichroism spectra. The alkyl aryl sulfoxides have been efficiently resolved by CHIRALCEL OB chiral stationary phase (CSP) while the aryl benzyl sulfoxides were better separated on CHIRALCEL OJ CSP. In both cases the S enantiomer was always eluted first. This finding can then allow to determine the absolute configuration of alkyl aryl and aryl benzyl sulfoxides on the basis of their elution order on these CSPs
Circular Dichroism spectra and absolute configuration of some alkyl aryl sulfoxides
No full textThe absorption and circular dichroism (CD) spectra of three aryl sulfoxides, i.e. (-)-(S)-1-naphthyl methyl sulfoxide, (S)-l, (-)-(S)-1-(2-methyl)naphthyl methyl sulfoxide, (5)-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
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
No full textIn 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-CF3 substituted ligand (R,R)-1c leads to significantly lower ee and in the case of 2a a reversal of asymmetric induction is observed
Recent progress in application of spectroscopic methods for assigning absolute configuration of optically active sulfoxides
No full textIn the recent years, in addition to the more traditional methods based on X-ray diffraction and mechanistic considerations, the problem of the configurational assignment of optically active sulfoxides has been approached with spectroscopic methods. In this review the methods based on the use of NMR spectroscopy and electronic circular dichroism are described, as well as the emerging approaches based on the analysis of vibrational CD spectra, on the ab initio calculation of the optical rotation and on the cholesteric induction in nematic solvents. The advantage and limitations of each approach are discussed with a major attention to their reliability and practicalit
Determination of absolute configuration using vibrational circular dichroism spectroscopy: The chiral sulfoxide 1-(2-methylnaphthyl) methyl sulfoxide
No full textWe report the determination of the absolute configuration (AC) of the chiral sulfoxide, 1-(2-methylnaphthyl) methyl sulfoxide, 1, using vibrational circular dichroism (VCD) spectroscopy. The VCD of 1 has been measured in the mid-IR spectral region in CCl4 solution. Analysis employs the ab initio DFT/GIAO methodology. DFT calculations predict two stable conformations of 1, E and Z, Z being lower in energy than E by <1 kcal/mol. In both conformations the S-O bond is rotated from coplanarity with the naphthyl moiety by 30-40°. The predicted unpolarized absorption ("IR") spectrum of the equilibrium mixture of the two conformations permits assignment of the experimental IR spectrum in the mid-IR spectral region. The presence of both E and Z conformations is clearly evident. The VCD spectrum predicted for S-1 is in excellent agreement with the experimental spectrum of (-)-1, unambiguously defining the AC of 1 as R(+)/S(-)
Anantioselective synthesis of the fragrance tran-magnolione under asymmetric phase transfer catalysis
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