Synthesis and Stereochemical Characterization of Optically Active 1,2-Diarylethane-1,2-diols: Useful Chiral Controllers in the Ti-mediated Enantioselective Sulfoxidation MARIA IRENE DONNOLI, PATRIZIA SCAFATO, STEFANO SUPERCHI, AND CARLO ROSINI* Dipartimento di Chimica, Universita ` della Basilicata, Potenza, Italy Dedicated to Professor Koji Nakanishi on the occasion of his 75th birthday ABSTRACT The series of phenylsubstituted 1,2-diphenylethane-1,2-diols 2a–h was prepared in high chemical (70–80%) and optical yields (∼90%) by Sharpless syn- dihydroxylation of the corresponding (E)-1,2-diarylethenes, in turn obtained by Mc- Murry or Wittig reactions. The enantiomeric excesses of the samples were determined by HPLC analysis using Chiralcel OD chiral stationary phase (CSP). This CSP was able to resolve all the diols, except for 2g, with  values ranging between 1.10–1.64. In all cases the (R,R) antipode was eluted first. (R,R) absolute configuration was assigned to the dextrorotatory (CHCl 3 ) diols 2a–h by analyzing the CD spectra of their 2,2-dimethyl- 1,3-dioxolanes 3a–h. In fact, the CD spectra of all these dioxolanes present a positive couplet (210–180 nm range) which can be nonempirically related to an (R,R) absolute configuration of the two stereocenters. Chirality 13:258–265, 2001. © 2001 Wiley-Liss, Inc. KEY WORDS: absolute configuration; asymmetric dihydroxylation; circular dichroism; 1,2-diarylethane-1,2-diols We have been recently interested 1 in the preparation of optically active alkyl aryl sulfoxides by asymmetric cataly- sis 2 and described a versatile procedure for the catalytic asymmetric oxidation of sulfides which affords sulfoxides in good chemical yields and enantiomeric excesses. In our protocol the catalytic precursor is formed in situ by react- ing suitable amounts of Ti(i-PrO) 4 ,H 2 O, and enantiopure 1,2-diphenylethane-1,2-diol. Since many different 1,2- diarylethane-1,2-diols are, in principle, available by means of catalytic asymmetric dihydroxylation 3 of (E)-1,2- diarylethenes, it should be possible to study the effects of both the steric and electronic nature of the aryl moieties on the chemical and stereochemical outcomes of the sulfoxi- dation reaction. In order to obtain useful information on the mechanism of the sulfoxidation we decided to prepare and test the enantiopure diols 2a–h. The diols 2a–d, p- substituted with either electron-withdrawing or electron- donating groups, were chosen to test the effect of the elec- tronic features of the aromatic moiety. In the ortho- substituted diols 2e,f, both electronic and steric effects could influence the mechanism and then the outcome of the reaction. Finally, the fluoro-substituted diols 2g,h were chosen, together with 2a,c,e, in order to better define and, possibly, understand the unexpected stereochemical out- come observed with mono CF 3 -substituted diols. The latter diols in fact provided an inversion of asymmetric induction in the sulfoxidation reaction with respect to simple 1,2- diphenylethane-1,2-diol. 1c In this article we describe the preparation and the com- plete stereochemical characterization (nonempirical as- signment of absolute configuration by CD spectroscopy, determination of enantiomeric purity) of the diols 2a–h. It is interesting to note that a systematic study aimed at com- pletely characterizing these compounds by using CD spec- troscopy has not been reported so far. Compounds 2b, 4 2f, 5 and 2h 6 have been previously described in optically active form but CD data have been reported only for 2f. Its absolute configuration was, however, assigned empirically simply by comparing its CD spectrum with that of the analogous 4-methoxy-substituted diol. The results obtained employing compounds 2a–h as chiral controllers in the Ti-mediated asymmetric oxidation of sulfides to sulfoxides will be discussed in a forthcoming article. MATERIALS AND METHODS General HPLC analyses were performed at room temperature with a JASCO PU-1580 pump equipped with a Varian 2550 UV detector and Daicel Chiralcel OD [cellulose tris(3,5- dimethylphenylcarbamate)] and Chiralcel OJ [cellulose tris(p-methylbenzoate)] chiral stationary phases. Melting points were determined with a Kofler hot-stage apparatus Contract grant sponsors: Universita ` della Basilicata (Potenza), and MURST (Roma). *Correspondence to: Prof. Carlo Rosini, Dipartimento di Chimica, Univer- sita ` della Basilicata, via Nazario Sauro, 85, 85100 Potenza, Italy. E-mail: rosini@unibas.it Received for publication 30 October 2000; Accepted 22 December 2000 CHIRALITY 13:258–265 (2001) © 2001 Wiley-Liss, Inc.