Hydrogen Transfer between Anthracene Structures

Article abstract of DOI:10.1021/j100275a033

This work reports results of kinetic studies of hydrogen migration between 9,10-dihydro positions in anthracene structures.The transfer of two H atoms from 9,10-dimethyl-9,10-dihydroanthracene (AnH2) to 2-ethylanthracene (EAn) follows simple bimolecular kinetics with k/M^-1 s^-1=10^9.64+/-0.14exp<-18540+/-180/T> (250-375 deg C) At 300-350 deg C, H transfer to 9,10-dimethylanthracene led to nearly equimolar mixtures of cis- and trans-9,10-dihydroanthracene, consistent with a free radical mechanism.The rate-limiting step appears to be transfer of a single benzylic H atom from a donor molecule to an acceptor molecule, resulting in the formation of two highly stabilized free radicals.Reactions of this nature are likely to serve as major sources of free radicals in condensed-phase thermolysis reactions.Measurements of their rate constants offer a new, relatively direct means of determining bond strengths.From the above rate expression, we derive an AnH-H bond strength of 78.4+/- 1.8 kcal mol^-1.From literature data for a similar reaction (Halpern et al.) we obtain an H-Mn(CO)5 bond strength of 63 kcal mol^-1.Based on an observed lowering of the reaction rate with added anthracene, a rate constant was derived for β-H transfer from a2-ethyl-9-hydroanthryl radical to anthracene.At 350 deg C, this value was 120 M^-1 s^-1, indicating a high activation barrier (ca. 18 kcal mol^-1) for this seldom reported process.