Different donor-acceptor distance (DAD) in H-transfer and D-transfer tunneling-ready-state (TRS) leads to different TRS conformations, in terms of the steric environment and electronic structures. First, the steric effect on the 1° isotope dependence of 2° KIEs in H-transfer reactions is investigated to test the isotopically different TRS conformations. The kinetics of hydride transfer from Hantzsch ester (HEH-4,4-h,h) and HEH-4,4-d,d to 10-methylacrdinium ion (MA+) and 9,10-dimehtylacridinium ion (DMA+), the 2° CH3/CD3 KIEs at the 9-β- and 10-ε-positions of DMA+, and the 2° H/D KIE at the 9-α-position of MA+ in their reactions with the donor HEH, were determined. The β-2° KIEs at the bulkier CH3/CD3 groups would be expected to be dependent upon the 1° isotope but not the α-2° KIE. Second, the 1° isotope dependence of the Hammett correlations at the donor or acceptor would suggest the isotopically different TRS electronic structures. Here, the substituent effect on the 1° KIEs was determined for hydride transfer from 1-(substituted phenyl)-1,4-dihydropyridines (G-ANAH-4-h,h and G-ANAH-4-d,d, as donors) to 10-methylacrdinium ion (MA+). The 1° KIEs with substituents (-X) changing from electron-donating to electron-withdrawing groups,such as p-OMe, -H ,p-Cl, p-CN and m-CF3, were obtained. The observed 1o isotope effect on the 2o KIEs and the substituent effect on the 1o KIEswere explained in terms of the isotopically different conformational and electronic TRSs.

Testing the isotopically different tunneling-ready-state conformational and electronic structures in hydride transfer reactions

255th National Meeting (2018)

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