DEPROTONATION OF AMINIUM CATIONS - A CIDNP STUDY OF RELATIVE GROUP REACTIVITIES

We have studied the photoreactions of several tertiary aliphatic amines R2NR' with the sensitizer 9,10-anthraquinone in acetonitrile-d3 by means of pseudo-steady state measurements of chemically induced dynamic nuclear polarization (CIDNP). These reactions involve aminium cations as intermediates, which are then deprotonated either within the cage (by the radical anion of the sensitizer) or outside the cage (by surplus amine). The deprotonation pathway taken can be determined from the CIDNP intensity patterns in the reaction products. With methyl, ethyl and isopropyl substituents, we have exclusively observed deprotonation outside the cage, whereas deprotonation within the cage dominates in the case of allyl substituents. This different behaviour can be explained by an increase of the rate of in-cage proton transfer relative to the cage life with increasing driving force DELTAG(dep)0 of this reaction. DELTAG(dep)0 was estimated from electrochemical data and the results of AM1 calculations. The relative rates of the deprotonation outside the cage were found to be 1 for methyl, 0.4 for ethyl, and less than 0.005 for isopropyl. The higher selectivity in these experiments in comparison with results reported in the literature for the sensitizer stilbene is due to the lower reactivity: with stilbene, in-cage deprotonation occurs for all the amines investigated, owing to the much more negative reduction potential of this sensitizer, and DELTAG(dep)0 for these reactions is about four times the free enthalpy for the deprotonation of the aminium cations by the amines, which takes place in the 9,10-anthraquinone-sensitized photoreactions.