Oxygen-carbon bond dissociation enthalpies of benzyl phenyl ethers and anisoles. An example of temperature dependent substituent effects

For some time it has been assumed that the direction and magnitude of the e ffects of Y-substituents on the Z-X bond dissociation enthalpies (BDE's) in compounds of the general formula 4-YC(6)H(4)Z-X could be correlated with t he polarity of the Z-X bond undergoing homolysis. Recently we have shown by DFT calculations on 4-YC6H4CH2-X (X = H, F, Cl, Br) that the effects of Y on CH2-X BDE's are small and roughly equal for each X, despite large change s in C-X bond polarity. We then proposed that when Y have significant effec ts on Z-X BDE's it is due to their stabilization or destabilization of the radical. This proposal has been examined by studying 4-YC6H4O-X BDE's for X = H, CH3, and CH2C6H5 both by theory and experiment. The magnitudes of the effects of Y on O-X BDE's were quantified by Hammett type plots of Delta B DE's vs sigma+ (Y). Calculations reveal that changes in O-X BDE's induced b y changing Y are large and essentially identical (rho (+) = 6.7-6.9 kcal mo l(-1)) for these three classes of compounds. The calculated pf values are c lose to those obtained experimentally for X = H at ca. 300 K and for X = CH 2C6H5 at ca. 550 K. However, early literature reports of the effects of Y o n O-X BDE's for X = CH3 with measurements made at ca. 1000 K gave rho (+) a pproximate to 3 kcal mol(-1). We have confirmed some of these earlier, high -temperature O-CH3 BDE's and propose that at 1000 K, conjugating groups suc h as -OCH3 are essentially free rotors, and no longer lie mainly in the pla ne of the aromatic ring. As a consequence, the 298 K DFT-calculated Delta B DE for 4-OCH3-anisole of -6.1 kcal mol(-1) decreases to -3.8 kcal mol(-1) f or free rotation, in agreement with the ca. 1000 K experimental value. In c ontrast, high-temperature O-CH3 Delta BDE's for three anisoles with strongl y hindered substituent rotation are essentially identical to those that wou ld be observed at ambient temperatures. We conclude that substituent effect s measured at elevated temperatures may differ substantially from those app ropriate for 298 K.