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Surprises from a classic: The Breslow intermediate of triazolylidene-catalyzed benzoin condensations was characterized for the first time by NMR spectroscopy in its keto form (K, energetic minimum). The hitherto unknown spiro-dioxolane S, generated from the carbene catalyst and two equivalents of aldehyde, is the resting state of the catalytic system.
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Berkessel et al. (2010) studied this question.
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