Chiral sulfoxides are valuable in the fields of medicinal chemistry and organic synthesis. A recycle photoreactor utilizing the concept of deracemization, where a racemate is converted into a pure enantiomer, is developed and successfully applied in the syntheses of chiral alkyl aryl sulfoxides. The recycling system consists of rapid photoracemization using an immobilized photosensitizer and separation of the enantiomers via chiral high-performance liquid chromatography, and the desired pure chiral sulfoxides are obtained after 4-6 cycles. The key to the success of the system is the photoreactor site, wherein the photosensitizer 2,4,6-triphenylpyrylium is immobilized on the resin and irradiated (405 nm) to enable the rapid photoracemizations of the sulfoxides. As the green recycle photoreactor requires no chiral components, it should be a useful alternative system for application in producing chiral compounds.
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 (k2 = 1.77 × 104-6.08 × 101 M-1 s-1, t1/2 = 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 sp2-like nature of S of the sulfoxide cation radical, which makes photoracemization easier.
