A new synthetic method for the preparation of protonated-NHCs and related compounds

Protonated versions of N-heterocyclic carbenes (NHC,H+) are classically prepared by closing the ring through the introduction of the CH+ fragment. Here we report a totally different synthetic approach, which can be viewed as the addition of a 1,3-diazaallyl anion to a compound featuring two leaving groups (hereafter named “di-electrophile”). Using 1,3- and 1,4-dibromides, six- and seven-membered NHC,H+s have been prepared in good yields. Similarly, with 1,3,2-dioxathiolane-2,2-dioxide as a di-electrophile, imidazolidinium salts were obtained. To illustrate its broad scope of application, this synthetic route has been expanded to the preparation of protonated cyclic amino alkyl carbenes (CAACs) and amino thio carbenes, using 1-aza-allyl and 1,3-azathio-allyl anions, respectively.

Graphical abstractThe protonated form of NHCs and a variety of related heterocyclic salts, of various ring sizes, are readily prepared by addition of (di)hetero-allyl anions to compounds featuring two leaving groups.Figure optionsDownload full-size imageDownload as PowerPoint slide