Rapid Photoracemization of Chiral Alkyl Aryl Sulfoxides

Abstract

The photoracemization of chiral alkyl aryl sulfoxides with a photosensitizer has not been sufficiently investigated thus far. Therefore, in this study, a rapid photoracemization reaction of enantiopure alkyl aryl sulfoxides using 1 mol % 2,4,6-triphenylpyrylium tetrafluoroborate (TPT + ) was developed. Various substitution patterns were tolerated and every racemization reaction proceeded extremely fast ( k 2 = 1.77 × 10 4 –6.08 × 10 1 M –1 s –1 , t 1/2 = 0.4–114 s). Some chiral sulfoxides with easily oxidizable functional groups are not appropriate for this photoisomerization. The electrochemical potentials of the functional groups, determined via cyclic voltammetry, are useful for predicting the reactive or nonreactive groups in this photoracemization reaction. A theoretical study was conducted to clarify the sp 2 -like nature of S of the sulfoxide cation radical, which makes photoracemization easier.

Conversion of Racemic Alkyl Aryl Sulfoxides into Pure Enantiomers Using a Recycle Photoreactor: Tandem Use of Chromatography on Chiral Support and Photoracemization on Solid Support

“…Increasing light transmission to the central region of the glass tube should be key to success, and thus, light-permeable glass beads, which are almost identical in radius to C , are employed to yield a more efficient light distribution. A total of 6 g of a mixture of the glass beads and C is uniformly mixed and packed into the glass tube, and the efficiencies of the photoreactions with different ratios of glass beads and C were evaluated via the photoracemization of 2-methoxyethyl phenyl sulfoxide (+)- 1b . The injected (+)- 1b flows at 2.0 mL/min through the packed glass tube under optical irradiation (405 nm).…”

“…All resins were purchased from commercial suppliers and used as received, and the characterizations of 1a−d are described in our previous study. 12 Synthesis of 2. 13 NaOAc (1.24 g, 15.1 mmol) was added to a stirred solution of TPT + (2.00 g, 5.05 mmol) in 25 mL of CHCl 3 :H 2 O (1:1 v/v).…”

“…Various substitution patterns are tolerated, and the racemizations proceed extremely rapidly (k 2 = 1.77 × 10 4 −6.08 × 10 1 M −1 s −1 , t 1/2 = 0.4−114 s, Scheme 1). 12 The rapid photoracemizations of chiral sulfoxides should be applicable in deracemization. Although the one-pot deracemization of chiral sulfoxides using chemoenzymatic methods has been reported, 13 to the best of our knowledge, the deracemization of chiral sulfoxides utilizing an on-column rapid photoracemization reaction has never been reported.…”

“…Although B displays a somewhat low activity, A (k obs = 2.59 × 10 −2 M −1 s −1 ) and C (k obs = 1.99 × 10 −2 M −1 s −1 ) yield desirable results. Immobilized TP + exhibits a comparable catalytic activity 12 to that of soluble TPT + .…”

Photocatalytic deracemisation of cobalt(iii) complexes with fourfold stereogenicity

“…As the use of photocatalysts in diastereomerisation, 19 racemisation 20 and deracemisation reactions 21 has drawn the attention of many organic chemists, we anticipated that photocatalytic reactions allow to control the configuration of highvalent stereocentres with more than two states, such as fourfold stereogenic cobalt centres (Figure 1E). Furthermore, a photoderacemisation would grant most direct access to stereoisomerically enriched material from a racemate.…”