Octet-defying molecules: Understanding the electronic properties of carbenes and isolation of a stable nitrene.

摘要

For a long time, carbenes were considered as intermediates so reactive that their isolation was deemed impossible. From the seminal works of Curtius and Staudinger to their isolation as stable molecules, carbenes led to tremendous advances in numerous fields of chemistry, from synthetic applications such as organocatalysis and transition metal chemistry, to advances in the fields of medicine and materials. In addition, a paradigm shift recently emerged with the use of stable carbenes for the activation of small molecules and the stabilization of highly reactive species, an infringement on a domain that remained exclusive to transition metal complexes. The wide scope of application of carbenes is intrinsically related to the extremely varied yet uncommon electron configuration and the understanding of the latter is of critical importance. The first part of this manuscript describes the development of a simple and inexpensive experimental method to probe the electronic properties of carbenes. The 31P NMR spectrometry of carbene-phosphinidene adducts allows the assessment of the pi-accepting abilities of carbenes, and by correlation with other experimental probes, the deconvolution of sigma-donating and pi-accepting properties. With this knowledge in mind, we will attempt to extend the scope of known stable carbenes through the development of new carbene families. Finally, we will apply our understanding of carbene stabilization mechanisms to the arduous quest for their nitrogen analogs, the so-far elusive nitrenes. Almost three decades after the isolation of the first stable carbene, a phosphinocarbene, we will report the synthesis of the first stable phosphinonitrene. This last part is a perfect example of the use of carbenes and their derivatives to stabilize highly reactive species, illustrating the previously mentioned paradigm shift.