Effect of N, S atoms on the mechanisms of H-transfer for five-membered nitrogen-containing heterocycles

Abstract

As the mechanisms of the hydride transfer reaction between 7,8-dihydro-9-methylcaffeine (CAFH) with N-methylacridinium (AcrH⁺ClO₄^-) and hydrogen atom transfer (HAT) reaction between 2,3-dihydrobenzo-imidazoles (BIH) with 2,2-diphenyl-1-picrylhydrazyl radical (DPPH^•) were researched to both be induced by electron transfer, the reaction mechanisms of 2,3-dihydrobenzo-thiazoles (BTH) with these two substrates were studied. The thermodynamic analysis platforms were used to judge the mechanisms, and the mechanisms of these two reactions were not the same with CAFH and BIH as the E_ox (BTH) value was more positive than CAFH and BIH. A new method for inferring the reaction mechanism was proposed using the kinetic equation ΔG^≠_XH/Y = ΔG^≠o (XH) + ΔG^≠o(Y) and thermo-kinetic parameter ΔG^≠o. The HAT reaction mechanisms between BTH with DPPH^• and ^tBu₃PhO^• were researched by ΔG^≠o_HD. As the ΔG^≠o (BTH) values in these two reactions were similar; hence, the rate determining steps of them were both HATs as the ΔG^≠o_HD(Y) values of DPPH^• and ^tBu₃PhO^• used for determining ΔG^≠o (BTH) were both in HAT reactions. The HAT reaction mechanisms between BIH with ^tBu₃PhO^• were also researched by thermodynamic analysis platform and kinetic isotope effect (KIE = 3.99), which confirmed that the rate determining step of BIH/^tBu₃PhO^• was indeed HAT. The H-donating ability of BIH and BTH was compared by ΔG^≠o_HD. From BIH to BTH, the substitution of N by S not only greatly reduces the thermodynamic electron donating and H-donating capacity of the compound but also increases the H-donating ability in kinetics and HAT reaction.