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 DegC. Primary (12C/13C) and secondary (1H/2H) kinetic isotope effects were detd. The primary KIEs were k12C/k13C = 1.041 +- 0.005 at the a position and 1.013 +- 0.006 at the b position. Secondary KIEs were kH/kD = 1.076 +- 0.005 at the a position and 1.017 +- 0.005 at the b 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: r = -0.65 for electron donors, but r = +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.
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 2002|