Kinetic Isotope Effects in Cycloreversion of Rhenium (V) Diolates

Abstract

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 degrees C. Primary ((12)C/(13)C) and secondary ((1)H/(2)H) kinetic isotope effects were determined. The primary KIEs were k(12C)/k(13C) = 1.041 +/- 0.005 at the alpha position and 1.013 +/- 0.006 at the beta position. Secondary KIEs were k(H)/k(D) = 1.076 +/- 0.005 at the alpha position and 1.017 +/- 0.005 at the beta position. Computational modeling (B3LYP/LACVP+) located a transition state for concerted cycloreversion of styrene from TpRe(O)(OCH(2)CHPh) 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: rho = -0.65 for electron donors, but rho = +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.

Rhenium‐Catalyzed Deoxydehydration of Diols and Polyols

“…[47] The apparent contrast to previous results was explained by a Hammett study that revealed that two different mechanisms were in competition, thus indicating a highly flexible transition state structure. [47] The apparent contrast to previous results was explained by a Hammett study that revealed that two different mechanisms were in competition, thus indicating a highly flexible transition state structure.…”

In Situ Spectroscopic Investigation of the Rhenium‐Catalyzed Deoxydehydration of Vicinal Diols

“…The conclusion was supported by an observed inability of the rhenium(V) diolate to extrude stilbene, but this observation is contradictory to every other paper on the subject, [24][25][26][27][28][29][30] and more work is needed to confirm it or provide an alternative explanation.…”

Diradical versus Concerted Mechanisms for the Dihydroxylation of Protoanemonin by OsO4 and OsO4−NH3 − The Effect of the Base in the Reaction

“…Comparable results have been obtained in studies of the cycloreversion of rhenium() diolates (an analogous metal oxide system), which reflect a probability that highly asyn-chronous concerted and diradical mechanisms are involved. [37] …”