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Localized partitioning of amorphous enantioenriched aryl benzyl sulfoxides in the solid state can lead to substantial variation in enantiopurities, even for sulfoxides which do not show detectable levels of self-disproportionation of enantiomers (SDE) during chromatography on an achiral stationary phase. The importance of preparation of representative samples of enantioenriched sulfoxides for chiral HPLC to enable reproducible results is clear.

Over the past 20 years reports of sulfonyl migrations have appeared, frequently described as ‘unusual’ and ‘unexpected’. This comprehensive review compiles, for the first time, sulfonyl migrations reported over the last 20 years including formal 1,2-, 1,3-, 1,4-, 1,5-, 1,6- and 1,7-sulfonyl shifts, occurring through either radical or polar processes, either inter- or intramolecularly. Discussion of the sulfonyl migrations is structured according to reaction type, i.e. nitrogen–carbon, nitrogen–oxygen, nitrogen–nitrogen, oxygen–carbon (including anionic and non-anionic thia-Fries rearrangements), oxygen–oxygen and carbon–carbon migrations. Discussion of the underlying mechanisms for the migrations is included, with particular attention afforded to the principal techniques utilised for their elucidation, namely isotopic-labelling, crossover experiments, density functional theory calculations and electron paramagnetic resonance spectroscopy amongst others.

Highly regioselective synthetic methodology leading to densely functionalised C(3), C(4) and C(5) substituted pyrazoles 10a–q, 14a‐i and 16a–g via thermal [3+2]‐dipolar cycloaddition, of α‐diazoacetates and α‐thio‐β‐chloroacrylamides, at the sulfide, sulfoxide and sulfone levels of oxidation, is described. This method allows access to C(4)‐sulfenyl or sulfonyl pyrazoles, through migration of the sulfur substituent at the sulfide and sulfone oxidation levels, while elimination of the sulfinyl group leading to 3,5‐disubstituted pyrazoles, is observed. While the sulfide migration is readily rationalised, the carbon to carbon 1,2‐sulfonyl migration is unprecedented and mechanistically intriguing. The synthetically versatile generation of densely functionalised pyrazoles containing substituents amenable to further modification offers advantages over alternative synthetic routes. Isolation of the N‐alkylated pyrazoles 11a and 12a as by‐products from the cycloaddition through further reaction of the pyrazoles 10 with excess α‐diazoacetate, proved useful in rationalising the tautomeric behaviour evident in the NMR spectra of the pyrazoles, with the position of tautomeric equilibrium influenced by solvent and substituents.