Kinetic isotope effects alpha carbon

The first report of this new type of kinetic isotope effect in a Menshutkin reaction was published by Matsson and coworkers in 198744. In this study, the alpha carbon kll/ku kinetic isotope effect was measured for the Menshutkin reaction between N,N-dimethyl-para-toluidine and labelled methyl iodide in methanol at 30 °C (equation 35). The carbon-11 labelled methyl iodide required for this study was prepared from the nC atoms produced in the cyclotron in three steps45 (equation 37). 

It is worth noting that Yamataka and coworkers47 also found large (near the theoretical maximum) alpha carbon kinetic isotope effects for the Menshutkin reactions between 3,5-disubstituted pyridines and methyl iodide (equation 38, Table 6). 

Other workers have concluded that the transition state for the Menshutkin reaction is late with more nitrogen-alpha carbon bond formation than alpha carbon-leaving group bond rupture. For instance, Harris and coworkers51 found that the secondary alpha deuterium kinetic isotope effects (Table 8) decreased when a poorer nucleophile was used in the S v 2 reactions between 3,5-disubstituted pyridines and methyl iodide in 2-nitropropane at 25 °C (equation 38, Table 8). 

TABLE 9. The carbon-12/carbon-14 and secondary alpha hydrogen-tritium kinetic isotope effects for the Sat2 reactions between Y-substituted /VA-dimethylanilines and Z-substituted benzyl X-substituted benzensulfonates in acetone at 35°C ... 

The above conclusion must be remembered when considering carbon isotope effects for elimination reactions. Certainly the alpha- and beta-carbon isotope effects for elimination from propyl trimethylammonium ion indicate a different extent of rehybridisation at the two carbon atoms in the transition state, but a more definite conclusion requires additional kinetic evidence. 

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