Computational Replication of the Abnormal Secondary Kinetic Isotope Effects in a Hydride Transfer Reaction in Solution with a Motion Assisted H-Tunneling Model

Citations Over Time

Abstract

We recently reported abnormal secondary deuterium kinetic isotope effects (2° KIEs) for hydride transfer reactions from alcohols to carbocations in acetonitrile (Chem. Comm. 2012, 48, 11337). Experimental 2° KIE values were found to be inflated on the 9-C position in the xanthylium cation but deflated on the β-C position in 2-propanol with respect to the values predicted by the semi-classical transition-state theory. No primary (1°) isotope effect on 2° KIEs was observed. Herein, the KIEs were replicated by the Marcus-like H-tunneling model that requires a longer donor-acceptor distance (DAD) in a lighter isotope transfer process. The 2° KIEs for a range of potential tunneling-ready-states (TRSs) of different DADs were calculated and fitted to the experiments to find the TRS structure. The observed no effect of 1° isotope on 2° KIEs is explained in terms of the less sterically hindered TRS structure so that the change in DAD due to the change in 1° isotope does not significantly affect the reorganization of the 2° isotope and hence the 2° KIE. The effect of 1° isotope on 2° KIEs may be expected to be more pronounced and thus observable in reactions occurring in restrictive environments such as the crowded and relatively rigid active site of enzymes.

Related Papers

• → Isotope Effects on Hydride Transfer Reactions from Transition Metal Hydrides to Trityl Cation. An Inverse Isotope Effect for a Hydride Transfer(1999)75 cited
• → Use of Fluorine Kinetic Isotope Effects in the Study of Steric Effects in Nucleophilic Aromatic Substitution Reactions(1998)14 cited
• → Effect of steric hindrance on the rates and kinetic isotope effects of the reactions of 1-nitro-1-(4-nitrophenyl)alkanes with TBD and MTBD bases in THF(1998)23 cited
• → Kinetic hydrogen isotope effects in intermolecular hydride transfer from arylalkanes to 9-arylfluoren-9-yl cations(1981)7 cited
• → Secondary α-hydrogen kinetic isotope effects in nucleophilic additions to ferrocenyl-stabilised carbocations(1979)1 cited