text: In 1957, Ronald Breslow proposed that a relatively stable nucleophilic carbene, a thiazol-2-ylidene derivative of vitamin B (thiamine), was the catalyst involved in the benzoin condensation that yields furoin from furfural.
   In this cycle, the vitamin's thiazolium ring exchanges a hydrogen atom (attached to carbon 2 of the ring) for a furfural residue. In deuterated water, the C2-proton was found to rapidly exchange for a deuteron in a statistical equilibrium.
  This exchange was proposed to proceed via intermediacy of a thiazol-2-ylidene. In 2012 the isolation of the so-called Breslow intermediate was reported.
  In 1960, Hans-Werner Wanzlick and coworkers conjectured that carbenes derived from dihydroimidazol-2-ylidene were produced by vacuum pyrolysis of the corresponding 2-trichloromethyl dihydroimidazole compounds with the loss of chloroform.
   They conjectured that the carbene existed in equilibrium with its dimer, a tetraaminoethylene derivative, the so-called Wanzlick equilibrium. This conjecture was challenged by Lemal and coworkers in 1964, who presented evidence that the dimer did not dissociate; and by Winberg in 1965. However, subsequent experiments by Denk, Herrmann and others have confirmed this equilibrium, albeit in specific circumstances.