Kinetic Isotope Effects in Cycloreversion of Rhenium (V) Diolates

Cycloreversion of 4-methoxystyrene from the corresponding Tp‘Re(O)(diolato) complex (Tp‘ = hydrido-tris-(3,5-dimethylpyrazolyl)borate) was measured competitively for various isotopomers at 103 °C. Primary (¹²C/¹³C) and secondary (¹H/²H) kinetic isotope effects were determined. The primary KIEs were k_12C/k_13C = 1.041 ± 0.005 at the α position and 1.013 ± 0.006 at the β position. Secondary KIEs were k_H/k_D = 1.076 ± 0.005 at the α position and 1.017 ± 0.005 at the β position. Computational modeling (B3LYP/LACVP*+) located a transition state for concerted cycloreversion of styrene from TpRe(O)(OCH₂CHPh) exhibiting dramatically different C−O bond lengths. A Hammett study on cycloreversions of substituted styrenes from a series of Tp‘Re(O)(diolato) showed dichotomous behavior for electron donors and electron-withdrawing groups as substituents:  ρ = −0.65 for electron donors, but ρ = +1.13 for electron-withdrawing groups. The data are considered in light of various mechanistic proposals. While the extrusion of 4-methoxystyrene is concluded to be a highly asynchronous concerted reaction, the Hammett study reflects a likelihood that multiple reaction mechanisms are involved.