Département de chimie moléculaire

The 31P NMR chem. shift of carbene-phenylphosphinidene adducts gives a good indication of the relative $π$-accepting properties of carbenes. These adducts can be easily prepd. in two steps, by reaction of the carbene with dichlorophenylphosphine followed by redn. with Mg or KC8, and they do not have to be purified. In marked contrast with the narrow range of Tolman electronic parameter (TEP) values, the 31P NMR chem. shifts of these main-group-element adducts are considerably spread out, and are very sensitive to subtle changes in the electronic structures of carbenes. For example, noticeable differences are obsd. when imidazol-2-ylidenes are compared with their satd. homologs, but also with the corresponding annulated version. Knowing both the TEP value and the 31P NMR chem. shift of the corresponding carbene-phosphinidene adduct allows for an evaluation of the relative pure $\sigma$-donating properties of carbenes. [on SciFinder(R)]