Hydrogen Atom Transfer from 1,n-Alkanediamines to the Cumyloxyl Radical. Modulating C-H Deactivation Through Acid-Base Interactions and Solvent Effects

A time-resolved kinetic study on the effect of trifluoroacetic acid (TFA) on the hydrogen atom transfer (HAT) reactions from 1,n-alkanediamines (R2N(CH2)(n),NR2, R = H, CH3; n = 1-4), piperazine, and 1,4-dimethylpiperazine to the cumyloxyl radical (CumO(center dot)), has been carried out in MeCN and DMSO. Very strong deactivation of the alpha-C-H bonds has been observed following nitrogen protonation and the results obtained have been explained in terms of substrate basicity, of the distance between the two basic centers and of the solvent hydrogen bond acceptor ability. At [substrate] <= 1/2 [TFA] the substrates exist in the doubly protonated form HR2N+(CH2)(n)N+R2H, and no reaction with CumO(center dot) is observed. At 1/2 [TFA] < [substrate] <= [TFA], HAT occurs from the C-H bonds that are alpha to the nonprotonated nitrogen in R2N(CH2)(n)N+R2H. At [substrate] > [TFA], HAT occurs from the alpha-C-H bonds of R2N(CH2)(n)NR2, and the mesured k(H) values are very close to those obtained in the absence of TFA. Comparison between MeCN and DMSO clearly shows that in the monoprotonated diamines R2N(CH2)(n)N+R2H remote C-H deactivation can be modulated through solvent hydrogen bonding.