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

As the mechanisms of the hydride transfer reaction between 7,8-dihydro-9-methylcaffeine (CAFH) with N-methylacridinium (AcrH(+)ClO(4)(-)) and hydrogen atom transfer (HAT) reaction between 2,3-dihydrobenzo-imidazoles (BIH) with 2,2-diphenyl-1-picrylhydrazyl radical (DPPH center dot) 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 Delta G(XH/Y)(not equal) = Delta G(not equal o) (XH) + Delta G(not equal o)(Y) and thermo-kinetic parameter Delta G(not equal o). The HAT reaction mechanisms between BTH with DPPH center dot and (Bu3PhO center dot)-Bu-t were researched by Delta G(HD)(not equal o). As the Delta G(not equal o) (BTH) values in these two reactions were similar; hence, the rate determining steps of them were both HATs as the Delta G(HD)(not equal o)(Y) values of DPPH center dot and (Bu3PhO center dot)-Bu-t used for determining Delta G(not equal o) (BTH) were both in HAT reactions. The HAT reaction mechanisms between BIH with (Bu3PhO center dot)-Bu-t were also researched by thermodynamic analysis platform and kinetic isotope effect (KIE = 3.99), which confirmed that the rate determining step of BIH/(Bu3PhO center dot)-Bu-t was indeed HAT. The H-donating ability of BIH and BTH was compared by Delta G(HD)(not equal o). 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.