摘要: Since the unambiguous isolation of the first free N-heterocyclic carbene (NHC) IAd in 1991 by Arduengo et al.[1], these compounds have received a huge amount of attention from across the chemical community [2]. As stable examples of coordinatively unsaturated, electronically deficient carbene compounds, much of the earlier interest resulted from their status as academic curiosities. However, as studies were conducted on their properties and reactivity, the full potential of NHCs in many different areas of chemistry was revealed. As strong σ-donors to metal centers, NHCs are nowadays widely used as ancillary ligands in organometallic chemistry including in industrially important catalytic transformations [3], rivaling phosphines, and cyclopentadienyls in this role. Many NHC–metal adducts are also attracting attention in materials science and as potential metallopharmaceuticals [4]. The strong binding properties and stabilizing features of NHCs have also led to many applications not involving metals. For example, boron–NHC adducts have been widely studied in a number of different contexts [5], whereas some classes of π-accepting NHCs have been shown to activate small molecules such as ammonia [6]. It is the reactivity of NHCs as organocatalysts, however, that forms the basis of this book [7]. First observed by Ukai et al. in a thiazolium salt-mediated benzoin condensation in 1943 [8], NHCs have proved efficient catalysts for umpolung reactions of aldehyde substrates, reacting via so-called Breslow intermediates, which can be considered acyl anion equivalents [9]. Alongside these processes, alternative transformations involving a wide …