Intrinsic barrier aromaticity

The reduction of aryl-substituted vinyl halides by electrochemically generated aromatic anion radicals has also been investigated in DMF (Gatti et al., 1987). Counter-diffusion behaviour at low driving forces (pp. 34, 35) does not appear as clearly as in the case of aryl halides (Fig. 11). However, analysis of the log k vs E° plot according to a quadratic activation-driving force relationship gave standard potential and intrinsic barrier values that... 

A simple diagram depicting the differences between these two complementary theories is shown in Fig. 1, which represents reactions at zero driving force. Thus, the activation energy corresponds to the intrinsic barrier. Marcus theory assumes a harmonic potential for reactants and products and, in its simplest form, assumes that the reactant and product surfaces have the same curvature (Fig. la). In his derivation of the dissociative ET theory, Saveant assumed that the reactants should be described by a Morse potential and that the products should simply be the dissociative part of this potential (Fig. Ib). Some concerns about the latter condition have been raised. " On the other hand, comparison of experimental data pertaining to alkyl halides and peroxides (Section 3) with equations (7) and (8) seems to indicate that the simple model proposed by Saveant for the nuclear factor of the ET rate constant expression satisfactorily describes concerted dissociative reductions in the condensed phase. A similar treatment was used by Wentworth and coworkers to describe dissociative electron attachment to aromatic and alkyl halides in the gas phase. ... 

The question how aromaticity in a reactant or product might affect intrinsic barriers has only recently received serious attention. Inasmuch as aromaticity is related to resonance one might expect that its development at the transition state should also lag behind proton transfer (or its loss from a reactant would be ahead of proton transfer) and hence lead to an increase in AGj, as is the case for resonance/delocalized systems. However, recent studies from our laboratory suggest the opposite behavior. 

A study on how the difference in the aromaticity between (35) and (36) may affect the intrinsic barriers to proton transfer has been reported (Scheme 18).137 The intrinsic barriers for the deprotonation of the thiophene derivative by amines and OH- have been found to be somewhat higher than for the furan analogue. This result has been attributed to a combination of steric, inductive, and n -donor effects which overshadow the aromaticity effect. 

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