The benzoin condensation: Charge tagging of the catalyst allows for tracking by mass spectrometry

• We have synthesized a novel charge-tagged N-heterocyclic carbene (thiazolylidene).
• Sulfonate-tagged thiazolylidene was used to catalyze the umpolung benzoin condensation.
• Sulfonate-tagged catalyst was also used to fish out reaction intermediates for MS detection.
• Results support Breslow, not dimer mechanism for benzoin condensation.

A novel thiazolium with a sulfonate charge tag was synthesized to test the feasibility of tracking the progress of a thiazolylidene-catalyzed benzoin condensation reaction using electrospray ionization-mass spectrometry (ESI–MS). Intermediates in the benzoin condensation were “fished” out of a reaction mixture and detected using MS. Tandem MS and calculations were used to support structural assignments. The results are consistent with the Breslow mechanism. These data show the viability of synthesizing negatively charged compounds that will both catalyze and track reactions involving N-heterocyclic carbene organocatalysis, which are becoming increasingly prevalent in organic synthesis.

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