Theoretical studies on hydrogen transfer reactions

Abstract

The present analysis reveals the following features: (i) The calculated activation barriers via both configuration mixing approach (using equations 2.1–2.4) and quantum chemical methods (using MNDO/RHF/CI and AM1/RHF/CI) for the hydrogen transfer reaction between alkanes containing primary, secondary and tertiary abstractable hydrogen, decrease in the order primary > secondary > tertiary. (ii) The rate constants of hydrogen abstraction from the primary, secondary and tertiary C–H bonds increase due to the lowering of the activation barriers on the excited state surface. (iii) While the thermal nucleophilic reactions are interpreted in terms of the DA–D?A? interaction, photochemical hydrogen transfer reactions are governed primarily by DA*–D?A? interactions up to the formation of the transition state. But both are same from the mechanistic point of view, the difference being that the latter process takes place on the excited state surface. Intramolecular ?-hydrogen abstraction reaction of aliphatic mono- and diketones involves two steps. In the first step, the molecule undergoes internal rotation to reach the favorable conformation for hydrogen abstraction from their most stable conformation. In the second step, the hydrogen abstraction takes place from the favorable conformer. The ?-hydrogen abstraction reaction in aliphatic diketones (ketones II) exhibits remarkably low rate constants relative to alkyl monoketones (ketones I). This has been interpreted as the larger radical-like reactivity (or free valence) of the carbonyl oxygen of monoketones (ketones I) than that of ?-diketones (ketones II). As the C–H bond in ?-position changes from a primary to secondary to tertiary, the relative rate constants increase in the order primary < secondary < tertiary for all the ketones studied here (i.e., ketones I–IV). The ?-hydrogen abstraction by the second keto group in ?-diketones (ketones II) is difficult, not because of any loss of radical-like reactivity of the second oxygen atom, but due to the higher rotational barrier required to attain the favorable conformation before the ?-hydrogen abstraction process can take place. The effect of the ether oxygen in a position ? to the carbonyl group in ketones III and IV leads to an overall decreased energy of activation and increased exothermicity relative to the corresponding alkyl monoketones (ketones I) and diketones (ketones II). This increased reactivity is not due to any activating influence of the ether oxygen on the carbonyl oxygen that abstracts the hydrogen, as the free valence indices on the carbonyl oxygens in both alkyl and ?-alkoxyacetones are nearly the same. The weaker C–H bonds of the alkoxy group are responsible for the increased reactivity. In ketones IV, the difference in selectivity for primary vs secondary vs tertiary hydrogen abstraction from ?-position is less compared to that observed in ?-diketones (ketones II). The intramolecular ?-hydrogen abstraction can take place very fast in methyl 2:2-dimethylcyclopropyl ketone (ketone V) from the higher vibrational level of the excited singlet state. This is due to the fact that no conformational change is required to bring hydrogen close to the oxygen atom of the carbonyl groups. The ?-hydrogen abstraction of 2-pentanone cannot take place from the vertically excited singlet state or higher vibrational level of the excited singlet state as the most stable conformers in the ground and the first excited singlet state are not favorable for hydrogen abstraction. The barrier heights for the cyclisation and disproportionation processes of the 1:4 biradicals derived from alkyl diketone (ketones II) are increased relative to those of monoketones (ketones I). The cyclisation of the 1:4 biradicals derived from ketones IV is not possible due to its unfavorable stretched conformation and have larger barrier height when compared to those of ketones III. Increased barrier heights for cyclisation process of diketone-derived biradicals relative to those of monoketones is due to the decreased free valence indices on the hydroxyl carbon atom of the diketone-derived biradicals. If the C–H bonds in the ?-position are changed from primary to secondary to tertiary, the relative heights of the barrier for cyclisation and disproportionation increase in the order primary < secondary < tertiary in both diketones and monoketones. The calculated free valence indices at the radical sites are in qualitative agreement with the above trends in reactivity.