The historical survey of the uncatalyzed, thermal rearrangement of thiocyanates into isothiocyanates is continued, starting with a review of Otto Billeter's scientific and social career in Neuchatel. A specific look is taken at 1925 where Ludwig Claisen, just 75 years ago, had broken, in one of his last publications before he died at the very beginning of 1930, the prevailing dogma that, in rearrangements, bond breaking and making will always occur at the same atom, by unequivocal experimental proof that the thermal, uncatalyzed rearrangement of substituted allyl phenyl ethers procedes with inversion of the C-atom alignment of the migrating allylic group. In this direction, Billeter tried to prove the same for the thermal rearrangement of substituted allyl thiocyanates into the corresponding mustard oils, but failed, as he wrote in his last publication before he died in 1927. About 15 years later, Mumm and Richter were able to realize Billeter's ideas. An eye is cast on some synthetic applications of the allylic thiocyanate → isothiocyanate rearrangement and, finally, the view is extended to modern high-temperature, gas-phase rearrangements of propargyl thiocyanates and selenocyanates into the corresponding allenyl isochalkogenocyanates which are interesting starting materials for syntheses of thia- and selena-heterocycles.
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