CHARGE MAPPING IN CARBANIONS - WEAK CHARGE DEMAND OF THE CYANO GROUP AS ASSESSED FROM A C-13 NMR-STUDY OF CARBANIONS OF ALPHA-ACTIVATED ACETONITRILES AND PHENYLACETONITRILES - BREAKDOWN OF A MYTH

The C-13 NMR values of the carbanions XCH-CN 1--12- and their phenyl homologs PhC-(X)CN 13--24- were obtained in DMSO. By using previously proposed pi charge-C-13 shift relationships (eqs 3 and 4), it is possible to compute the C-13 NMR shift of the carbanionic carbon. In these relationships, the charge-dependent contribution of the C-13 NMR shift is described by the charge demands (c(X)) of the different X groups. These quantities have been previously defined as the fraction of pi charge transferred (delocalized) from an adjacent carbanionic center to group X. It is shown that it is possible to correlate with a high degree of precision (slope 0.96, r = 0.993) computed and experimental shifts not only for the XCH-CN and the PhC-(X)CN series (10 and 12 points, respectively), but also for 13 other -CHXY carbanions 25--36-. The only system that does not conform to the predictions of eq 3 is the ylid 11-, obtained by deprotonation of cyanomethylenetrimethylammonium iodide. The C-13 NMR shifts of the phenylthio-substituted carbanions 12- and 26- are reproduced using a zero charge demand for the phenylthio group. This result experimentally validates theoretical indications that the stabilization provided by the PhS substituent to a alpha carbanionic center is essentially nonmesomeric. Equations 1-4 are therefore confirmed as being highly predictive and of broad scope. It is definitely concluded that charge demands (c(X)) efficiently rank various organic functionalities X for their mesomeric electron-withdrawing power. The modest charge demand of the cyano group c(CN) in alpha-cyano carbanions is fully confirmed. In Particular, c(CN) is smaller than the charge demand of the 2- and 4-pyridyl groups c2py and c4py. This leads to a double-bond character of the bond between the carbanionic carbon and the carbon atom of the heterocycle in the anion. Because these results do not allow the description of RCH-CN alpha-cyano carbanions as RCH=C=N- ketene imine nitranions, discussion is offered as to how this can be reconciled with the considerable increase in the acidity of cyanocarbon acids.