Capturing 2,2-Diamino Enols: A “Missing Link” in Umpolung Reaction of Aldehydes by N-Heterocyclic Carbenes

Abstract

The 56-years old “Breslow intermediate” has played a most important role in the umpolung reaction of aldehydes triggered by carbenes such as N-heterocyclic carbenes. Chemists have paid enormous efforts to capture the Breslow intermediate (or its analogues); however, it still remains a challenge. In this review, recent progresses in this field are briefly summarized.