Dediazoniation of Arenediazonium Ions. Part XX. Secondary deuterium isotope effects in reactions of benzenediazonium tetrafluoroborate

Secondary deuterium isotope effects were measured for the dediazoniation of [2,4,6‐2H3]‐benzenediazonium tetrafluoroborate ([2H3]‐1), and were found to be (kH/kD3)s = 1.46 in 2,2,2‐trifluoroethanol (TFE) and 1.51 in 1, 1, 1, 3, 3, 3‐hexafluoro‐2‐propanol (HFIP), respectively. These values are in agreement with the previously proposed rate determining formation of the phenyl cation (Eqn. 1), and are similar to the results obtained by Swain et al. in other solvents. The Nα‐Nβ rearrangement Eqn. 2) of β‐15N labelled 1 was studied in TFE. The percent of rearrangement after 70% dediazoniation was found to be 8.18, which corresponds to a rate constant for the rearrangement kr=4.02 · 10−6S−1. Thus the secondary deuterium isotope effect in the rearrangement in TFE (kH/kD3)r was calculated to be 1.42. The fact that the isotope effect in the rearrangement is, within experimental error, the same as that observed during dediazoniation, definitely excludes the formation of benzenespirodiazirine cation 3, which has repeatedly been discussed as an intermediate in the rearrangement. The size of the deuterium isotope effect in the rearrangement is in accordance with the rearrangement proceeding via a phenyl cation/N2 molecule pair (2).