CHARACTERIZATION OF THE ELECTROLYTICALLY INDUCED ISOMERIZATION OF 1-PHENYL-1-HEXYNE .1. CYCLIC VOLTAMMETRY AND DIGITAL-SIMULATION

In dimethylformamide containing tetra-n-butylammonium perchlorate, cyclic voltammograms for 1-phenyl-1-hexyne at a concentration less than 1 mM exhibit a single, irreversible reduction wave at glassy carbon. When the concentration of 1-phenyl-1-hexyne is higher, the cyclic voltammograms are more complicated. On the second and succeeding negative-going scans, a new wave is observed with a peak potential 200 mV more positive than that for the alkyne; and, when the concentration of residual water and the rate of potential scan are low, a pre-peak is seen prior to the wave for the alkyne on the first negative-going scan. These phenomena are due to electrolytically induced isomerization of 1-phenyl-1-hexyne to 1-phenyl-1,2-hexadiene, the alllene being easier to reduce than the alkyne. Digital simulation of the cyclic voltammograms for 1-phenyl-1-hexyne, plus simulation of the behavior of electroactive products (1-phenyl-1-hexene and 1-phenyl-1,2-hexadiene) formed upon reduction of the alkyne, has served to elucidate the mechanism for the alkyne-to-allene rearrangement. Two important mechanistic steps are required to model the isomerization correctly: (i) the rate-determining step in the rearrangement is deprotonation of 1-phenyl-1-hexyne by electrogenerated bases and (ii) protonation of the conjugate base of the alkyne by the parent alkyne is a rapid process. © 1990.