ORGN 398 |
| The general mechanism for electrophilic aromatic substitution involves rate-limiting addition of the electrophile to form a σ complex. In the 1950's, Brown provided evidence for this single rate-limiting step mechanism through correlated observations of intermolecular selectivity between aromatics and the intramolecular positional selectivity for an aromatic, the ‘Brown selectivity relationship.' Failures in this relationship have been observed and the origin of these failures has been controversial. We have observed a new way in which this relationship can fail involving kinetic isotope effects (KIE). In the acylation of veratrole, the experimental intermolecular and intramolecular KIEs are not consistent, providing strong evidence that the intermolecular and positional selectivities are decided in different steps. To account for this and related observations, we propose that positional selectivity in our case is determined dynamically on the downward slope of a bifurcating energy surface. Experimental and theoretical results from this study will be presented. |
|
Physical Organic Chemistry: Calculations, Mechanisms, Photochemistry, and High Energy Species
8:00 AM-11:40 AM, Tuesday, August 21, 2007 BCEC -- 254 A/B, Oral
Division of Organic Chemistry |