The search for exceptions in the highly enantioselective titanium catalysed oxidation of aryl benzyl sulfides

Circular Dichroism and TDDFT Investigation of Chiral Fluorinated Aryl Benzyl Sulfoxides

“…Some of the CD spectra of compounds 1b-k have been reported previously. [15,27,29] They were not discussed in detail but only employed to assign the absolute configuration on the basis of Mislow's rule. [22,23] We will see below, however, that at least in the case of sulfoxides 1b, 1d, and 1e, the employment of Mislow's rule is questionable.…”

“…The complete characterization of all compounds has been reported previously. [15,[27][28][29] CD spectra were recorded with a Jasco J-710 spectropolarimeter with the following measurement parameters: scan speed, 50 nm/min; band-width, 1 nm; response, 1 sec; 4-8 accumulations. Sample concentrations and cell path lengths were 3.2-3.6 mm for sulfoxides 1a,b,e,f,i,j,k (with 0.01 cm cell), 36.6 mm for 1c, 60.6 mm for 1d, 42.4 mm for 1g, and 48.2 mm for 1h (all with 1 cm cell).…”

“…[28] Further work in search of sulfides that could potentially give problems in the enantioselective oxidation revealed that the behavior of pentafluorobenzyl pentafluorophenyl sulfide is almost unique, although other substituents at the aromatic rings, such as iodine and hydroxymethyl group, can also lower the reaction yield and/or enantioselectivity. [29] To explore further the role played by fluorine substitution and to complete our survey, we report herein a CD investigation of ten fluorinated aryl benzyl sulfoxides (1b-k, Scheme 1), including TDDFT calculations of their CD spectra. Once again, although most of the considered substrates behaved in a manner that was similar to standard aryl benzyl sulfoxides, the perfluorination of the phenyl and/or benzyl ring is responsible for profound differences in the nature of the excited states, which is not fully appreciated in experimental spectra but clearly emerges from TDDFT calculations.…”

Stereochemical analysis of β‐keto sulfoxides by circular dichroism

“…Racemic sulfoxides, used to set up the HPLC chiral separations, were synthesized by standard mCPBA oxidation in methylene chloride. Chiral sulfoxides were prepared according to the previously reported procedure, by substituting carbon tetrachloride with the less toxic n ‐hexane as a solvent . The following procedure is representative.…”

“…During the years, some of us developed an enantioselective protocol for the oxidation of prochiral sulfides, based upon hydroperoxides in the presence of catalytic amounts of a complex between titanium and ( S,S )‐hydrobenzoin . This procedure usually works at room temperature, produces low amounts of side products, and is particularly effective in the synthesis of enantiopure aryl benzyl sulfoxides.…”