Dioxomolybdenum(VI) Complexes with Acylpyrazolonate Ligands: Synthesis, Structures, and Catalytic Properties

Abstract

The complexes [Mo(O)2(QR)2] [R = cyclohexyl (1), ethylcyclopentyl (2), hexyl (3), and neopentyl (4)] have been obtained in good yields by treatment of [Mo(O)2(acac)2] with 2 equivalents of acylpyrazolone compounds HQR [HQR = 3‐methyl‐1‐phenyl‐4‐alkylcarbonyl‐5‐pyrazolone; R = cyclohexyl (HQCy), ethylcyclopentyl (HQEtCp), hexyl (HQHe), neopentyl (HQnPe)]. They were isolated as yellow crystalline solids and characterized spectroscopically [IR, 1H and 13C(1H) NMR] and structurally (X‐ray for 2 and 3). The deoxygenation of selected epoxide substrates to alkenes by employing compounds 1 and 3 as catalysts and PPh3 as the oxygen acceptor showed good activities in toluene. The use of the ionic liquid [C4mim]PF6 as solvent gave lower yields, but the resulting catalytic system could be conveniently recycled. The [Mo(O)2(QR)2] derivatives 1 and 3 were also found to be moderately active catalysts for the deoxydehydration of vicinal diols.

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

“…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. [17] From an economic point of view, the scarcity and high cost of Re [57] makes it a challenge to use a Re-based catalyst for large amounts of biomass, but the direct proof for catalyst deactivation by the substrate shown here for Re could, nonetheless, be an equally important issue for the other transition metals (Mo [39,58] and V [59] ) that have been used as DODH catalysts.…”

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

“…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.…”

Expanding the Scope of Biomass‐Derived Chemicals through Tandem Reactions Based on Oxorhenium‐Catalyzed Deoxydehydration

“…[1] Although preliminary studies on ruthenium, [2] vanadium, [3] molybdenum, [4] and rhenium carbonyl catalysts (Re 2 (CO) 10 or BrRe(CO) 5 ) [5] have been reported, most precedents of DODH employ high-valent oxorhenium catalysts in conjunction with various reductants, such as phosphines, [6] H 2 , [7] and NaSO 3 . However, the conversion of biomass faces the fundamental challenge that saccharides and their polyol derivatives, the most basic platform chemicals accessible from cellulose, are too oxygen-rich to be compatible with the current petroleumbased infrastructure.…”

“…Likewise, d-chiro-inositol, muco-inositol, and allo-inositol all gave high benzene yields (> 45 %), without forming cyclohexa-2,5diene-1,4-diol and cyclohexa-3,5-diene-1,2-diol. Trisubstituted alkene 6 gave more dehydration product [16] than DODH product, possibly because the electron-donating alkyl group stabilizes the allylic carbocation intermediate (entry 4). [14] To examine this hypothesis and gain insights into the origin of the observed exquisite selectivity in polyol DODH reactions, we tested a series of 2-ene-1,4-diol and 2,4-diene-1,6-diol substrates and indeed observed the unprecedented 1,4-DODH and 1,6-DODH reactions (Table 1).…”