Catalytic Deoxydehydration of Diols to Olefins by using a Bulky Cyclopentadiene‐based Trioxorhenium Catalyst

Abstract

A bulky cyclopentadienyl (Cp)-based trioxorhenium compound was developed for the catalytic deoxydehydration of vicinal diols to olefins. The 1,2,4-tri(tert-butyl)cyclopentadienyl trioxorhenium (2) catalyst was synthesised in a two-step synthesis procedure. Dirhenium decacarbonyl was converted into 1,2,4-tri(tert-butyl)cyclopentadienyl tricarbonyl rhenium, followed by a biphasic oxidation with H2 O2 . These two new three-legged compounds with a 'piano-stool' configuration were fully characterised, including their single crystal X-ray structures. Deoxydehydration reaction conditions were optimised by using 2 mol % loading of 2 for the conversion of 1,2-octanediol into 1-octene. Different phosphine-based and other, more conventional, reductants were tested in combination with 2. Under optimised conditions, a variety of vicinal diols (aromatic and aliphatic, internal and terminal) were converted into olefins in good to excellent yields, and with minimal olefin isomerisation. A high turnover number of 1400 per Re was achieved for the deoxydehydration of 1,2-octanediol. Furthermore, the biomass-derived polyols (glycerol and erythritol) were converted into their corresponding olefinic products by 2 as the catalyst.

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

“…There are two simple solutions to the problem that the DODH of the diol is impeded by the diol itself: 1) A more potent reductant (e.g., hydrogen) able to reduce rhenium(VII) diolates can be employed or 2) a catalyst with sterically demanding ligands, [18] which prevent the formation of rhenium-(VII) diolates, can be used and thus render the catalyst reducible by a secondary alcohol. A third more challenging solution would be to add small, equimolar amounts of the diol and the reductant gradually, which would ensure that the diol concentration is kept so low that the reaction is constantly in the "acceleration" regime; it is simultaneously necessary to keep the diol concentration sufficiently high to prevent an irreversible deactivation of the catalyst, presumably by reduction to catalytically inactive Re nanoparticles.…”

“…To further corroborate the two assignments, a similar preequilibration using 18 O-labeled [44] CH 3 ReO 3 was performed as described above. The two bands of CH 3 O is 1:8).…”

Hydrodeoxygenation of Vicinal OH Groups over Heterogeneous Rhenium Catalyst Promoted by Palladium and Ceria Support

“…[16,21,20,27] Because the substrate, main product, and active element of our system are similar to those of CH 3 ReO 3 -catalyzed deoxydehydration, the key steps can be similar in both systems. The proposed mechanism for simultaneous hydrodeoxygenation over ReO x -Pd/CeO 2 catalyst is shown in Scheme 2.…”

“…3) These systems typically use other reductants than H 2 and lower yields are obtained if H 2 is used as the reductant. [16,20] Simultaneous hydrodeoxygenation of straightchain vicinal diols over heterogeneous Re catalysts has been reported; however, the yields of alkenes ( % 50 %) were lower than those obtained with homogeneous catalysts ( % 95 %), and the TON and TOF per Re atom were very low (< 20 and < 5 h À1 , respectively; Table S1, entries [27][28][29][30]. [27,28] In addition, there are no reports on the reuse of heterogeneous catalysts for deoxydehydration without decrease in activity up to now.…”

“…On the other hand, Re, V, and Mo homogeneous catalysts, especially Re, have been reported to be active in deoxydehydration (didehydroxylation) of vicinal OH groups to give alkenes. [15][16][17][18][19][20][21][22][23][24][25] In combination with the hydrogenation of the produced alkene, deoxydehydration transforms two vicinal OH groups to H atoms, and the reaction can be regarded as simultaneous hydrodeoxygenation (Scheme 1). [26] However, there are several problems in deoxydehydration systems in the literature (Table S1): 1) It is difficult to separate these homogeneous catalysts from the reaction mixture.…”