Mechanism of the Methyltrioxorhenium‐Catalyzed Deoxydehydration of Polyols: A New Pathway Revealed

Abstract

Polyols pathway probed: Density functional theory computations reveal that the methyltrioxorhenium-catalyzed deoxydehydration of polyols follows pathway C, which is energetically more favorable than the previously proposed pathways A and B. In addition to serving as solvent/reductant, the alcohol also acts as a shuttle to greatly facilitate various hydrogen-transfer steps.

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

“…In agreement with the computational investigation of the mechanism by Wang and co-workers, [14] we suggest that the active catalyst is XReO 3 ·L and that it can undergo reduction to XReO 2 ·L, which thus favors the right-hand side of the pathway shown in Scheme 2. To account for the fact that the rate increases if the diol concentration decreases, we suggest additionally that XReO 3 ·L can also undergo deactivation to a rhenium(VII) diolate XReO 2 (diolate) according to the following equilibrium [Eq.…”

“…In addition, the law of mass action is only obeyed in dilute solutions (not the case here), and the value of K is dependent on the polarity of the solution; therefore, a variation of the concentration of, for example, 3-octanol will not only shift the equilibrium but also change the value of K. The addition of the nonreactive nucleophile 1-decanol shifts the equilibrium shown in Equation (4) towards the active catalyst, which explains the higher observed rate in entry 10 compared to that in entry 4 in Table 1. The same is not true if 3-octanone is added, which is because of the less nucleophilic nature of ketones and, more important, the inability of ketones to stabilize the transition state for the reduction of XReO 3 ·L to XReO 2 ·L: As shown by Wang and co-workers, [14] two alcohol molecules are coordinated to the Re complex in the transition state-one is oxidized and the other, which could also be a nonreactive primary alcohol, stabilizes the transition state. Furthermore, Wang and co-workers also found that an alcohol molecule served as a H-transfer shuttle in the condensation between CH 3 ReO 3 and the diol-at low alcohol concentrations, the equilibrium in Equation (4) might, therefore, be reached more slowly.…”

“…However, they added an alkyne to capture the in situ-generated rhenium(V) center, and an investigation of the unmodified reaction would, therefore, be desirable. This pathway was also favored in a recent computational study by Wang and co-workers, [14] who additionally concluded that the reduction of rhenium(VII) to rhenium(V) by a secondary alcohol is the rate-limiting step.…”

“…The mechanism of DODH has been the subject of a number of studies, both experimental [10,11,17] and computational, [12,14,15] but the conclusions are conflicting, which in part may be because of the use of different catalysts and reductants. There is agreement on the existence of three stages in the reaction: condensation of the diol and the Re center, reduction of the Re center, and extrusion of the alkene accompanied by oxidation of the Re center, but the two main points of dispute are 1) the sequence of the condensation and reduction and 2) the identification of the rate-limiting step.…”

“…Wang and co-workers [14] suggested that the rhenium(V) species found before the condensation was CH 3 ReO(OH) 2 instead of CH 3 ReO 2 ·H 2 O (or CH 3 ReO 2 ·L), but as these two compounds cannot be discerned experimentally, this complication is omitted in the presented schemes.…”