Proton transfers in aromatic systems: How aromatic is the transition state?

Abstract

The question as to what extent aromaticity in a reactant or product is expressed in the transition state of a reaction has only recently received serious attention. Inasmuch as aromaticity is related to resonance, one might expect that, in a reaction that leads to aromatic products, its development at the transition state should lag behind bond changes as is invariably the case for the development of resonance in reactions that lead to delocalized products. However, recent experimental and computational studies on proton transfers from carbon acids suggest the opposite behavior, i.e., the development of aromaticity at the transition state is more advanced than the proton transfer. The evidence for this claim is based on the determination of intrinsic barriers that show a decrease with increasing aromaticity. According to the Principle of Nonperfect Synchronization (PNS), this decrease in the intrinsic barrier implies a disproportionately large amount of aromatic stabilization of the transition state. Additional evidence for the high degree of transition state aromaticity comes from the calculation of aromaticity indices such as HOMA, NICS, and the Bird Index. Possible reasons why the degree to which aromaticity and resonance are expressed at the transition state is different are discussed.

Effect of Transition State Aromaticity and Antiaromaticity on Intrinsic Barriers of Proton Transfers in Aromatic and Antiaromatic Heterocyclic Systems; An ab Initio Study

“…This early development of aromaticity leads to a lowering of the intrinsic barrier as required by the PNS for the early development of a product stabilizing factor, and is in keeping with Nature’s principle of always choosing the lowest energy path. As discussed in detail elsewhere, ,, the transition state aromaticity in our reactions should not be confused with the aromaticity of the transition state in pericyclic reactions such as [4 + 2] cycloadditions and others . In these latter reactions aromaticity is mainly a characteristic of the transition state while the reactants and products are not aromatic or less so than the transition state and hence the low barrier is not a PNS effect.…”

“…Our results for the 1H + -BH / 1-BH and 1H + -AlH / 1-AlH systems also reenforce previous tentative conclusions that in proton transfers that lead to an antiaromatic product the development of antiaromaticity lags behind transfers. , The result is again a lowering of the intrinsic barrier, as required by the PNS for the late development of a product de stabilizing factor and again consistent with the notion of choosing the lowest energy path. Note that this barrier-lowering effect is the opposite of the barrier enhancing effects in [2 + 2] cycloadditions and related reactions that have antiaromatic transition states with barriers so high as to render the reactions to become “forbidden.”…”

“…Such constraints do not apply in the case of aromaticity; on the contrary, only relatively minor progress in the creation of the appropriate orbitals or their optimal alignment seems to be required for aromatic stabilization to become effective. For more elaborate discussions on this fundamental difference between how aromaticity and resonance/delocalization affect intrinsic barriers, refs , , and should be consulted.…”

Kinetic Study on Michael-Type Reactions of β-Nitrostyrenes with Cyclic Secondary Amines in Acetonitrile: Transition-State Structures and Reaction Mechanism Deduced from Negative Enthalpy of Activation and Analyses of LFERs

“…Michael-type reactions of olefins activated by strong electron-withdrawing groups (e.g., 1 – 4 ) are an important type of reaction not only for synthetic chemists but also for mechanistic investigators. Accordingly, numerous synthetic and mechanistic studies have been carried out. − The Michael-type reactions of activated olefins with anionic nucleophiles have generally been reported to proceed through a stepwise mechanism in which nucleophilic attack is the rate-determining step (RDS). ,, The reactions of activated olefins with amine nucleophiles have also been reported to proceed through a zwitterionic intermediate (T ± ) with imbalanced transition states (TSs) in aqueous solution, in which the delocalization of the negative charge into the activating group lags behind the C–N bond formation (i.e., the principle of nonperfect synchronization). , …”