Bis(phenolato)molybdenum complexes as catalyst precursors for the deoxydehydration of biomass-derived polyols

Reactivity of metal dioxo complexes

Metal dioxo chemistry and its diverse reactivity are presented with an emphasis on the mechanisms of reactivity. Work from approximately the last decade is surveyed and organized by metal. In particular, the chemistry of cis-dioxo metal complexes is discussed at length. Reactions are grouped by generic type, including addition across a metal oxo bond, oxygen atom transfer, and radical atom transfer reactions. Attention is given to advances in deoxygenation chemistry, oxidation chemistry, and reductive transformations.

First Evidence of the Double-Bond Formation by Deoxydehydration of Glycerol and 1,2-Propanediol in Ionic Liquids

Deoxydehydration (DODH) reaction of glycerol (GL) and 1,2-propanediol (1,2-PD), in ionic liquids (ILs), catalyzed by methyltrioxorhenium (MTO) and Re₂O₇, was studied in detail. To better understand the ability of ILs to improve the catalytic performance of the rhenium catalyst, several experiments, employing eight different cations and two different anions, were carried out. Among the anions, bis(trifluoromethylsulfonyl)imide (TFSI) appears to be more appropriate than PF₆ ^-, for its relatively lower volatility of the resulting IL. Regarding the choice of the most appropriate cation, the presence of a single aromatic ring seems to be a necessary requirement for a satisfying and convenient reactivity. With the aim to extend the recyclability of the catalyst, experiments involving the readdition of polyol to the terminal reaction mixture were carried out. Worthy of interest is the fact that the presence of the IL prevents the inertization process of the catalyst, allowing us to obtain the alkene also after a readdition of fresh polyol.

Perspectives for the Upgrading of Bio-Based Vicinal Diols within the Developing European Bioeconomy

The previous decade has witnessed a drastic increase of European incentives aimed at pushing forward the transition from an exclusively petro-based economy toward a strong and homogeneous bio-based economy. Since 2012, numerous programs have been developed to stimulate and promote research and innovation relying on sustainable and renewable resources. Terrestrial biomass is a virtually infinite reservoir of biomacromolecules, the biorefining of which provides platform molecules of low complexity yet with tremendous industrial potential. Among such bio-based platform molecules, polyols and, more specifically, molecules featuring vicinal diols have gained tremendous interest and have stimulated an increasing research effort from the chemistry and chemical engineering communities. This Review revolves around the most promising process conditions and technologies reported since 2012 that specifically target bio-based vicinal diols and promote their transformation into value-added molecules of wide industrial interest, such as olefins, epoxides, cyclic carbonates, and ketals.

Titania-supported molybdenum oxide combined with Au nanoparticles as hydrogen-driven deoxydehydration catalyst of diol compounds

A heterogenous catalyst for deoxydehydration (DODH) reaction was developed using less expensive Mo than Re as the active center. Combination of Mo with anatase-rich TiO2 and Au as the support...

Transition metal-catalyzed deoxydehydration: missing pieces of the puzzle

Deoxydehydration (DODH) is a transformation that converts a vicinal diol into an olefin with the help of a sacrificial reductant.

Deoxygenation reactions in organic synthesis catalyzed by dioxomolybdenum(VI) complexes

Dioxomolybdenum(VI) complexes have been applied as efficient, inexpensive and benign catalysts to deoxygenation reactions of a diverse number of compounds in the last two decades. Dioxomolybdenum complexes have demonstrated wide applicability to the deoxygenation of sulfoxides into sulfides and reduction of N-O bonds. Even the challenging nitro functional group was efficiently deoxygenated, affording amines or diverse heterocycles after reductive cyclization reactions. More recently, carbon-based substrates like epoxides, alcohols and ketones have been successfully deoxygenated. Also, dioxomolybdenum complexes accomplished deoxydehydration (DODH) reactions of biomass-derived vicinal 1,2-diols, affording valuable alkenes. The choice of the catalytic systems and reductant is decisive to achieve the desired transformation. Commonly found reducing agents involved phosphorous-based compounds, silanes, molecular hydrogen, or even glycols and other alcohols.

Direct Deoxydehydration of Cyclic trans-Diol Substrates: An Experimental and Computational Study of the Reaction Mechanism of Vanadium(V)-based Catalysis*

The deoxydehydration of carbohydrates represents a key target to leverage renewable biomass resources chemically. Using a vanadium(V)-based catalyst, it was possible to directly deoxydehydrate cyclic trans-diol substrates. Accompanying mechanistic characterisation of this process by density functional calculations pointed to an energetically tractable route for deoxydehydration of cyclic trans-diol substrates involving stepwise cleavage of the diol C-O bonds via the triplet state; experimentally, this was supported by light dependence of the reaction. Calculations also indicated that cyclic cis-diols and a linear diol substrate could additionally proceed by a concerted singlet DODH mechanism. This work potentially opens a new and cost-effective way to efficiently convert carbohydrates of trans-diol stereochemistry into alkenes.

N-Donor Ligand Supported “ReO2+”: A Pre-Catalyst for the Deoxydehydration of Diols and Polyols

A selected number of tetradentate N2Py2 ligand-supported ReO2+ complexes and a monodentate pyridine-supported ReO2+ complex have been investigated as catalysts for the deoxydehydration (DODH) of diols and polyols. In situ 1H NMR experiments showed that these N-donor ligand-supported ReO2+ complexes are only the pre-catalyst of the DODH reaction. Treatment of (N2Py2) ReO2+ with an excess amount of water generates an active species for DODH catalysis; use of the Re-product of this reaction shows a much shorter induction period compared to the pristine complex. No ligand is coordinated to the “water-treated” complex indicating that the real catalyst is formed after ligand dissociation. IR analysis suggested this catalyst to be a rhenium-oxide/hydroxide oligomer. The monodentate pyridine ligand is much easier to dissociate from the metal center than a tetradentate N2Py2 ligand, which makes the Py4ReO2+-initiated DODH reaction more efficient. For the Py4ReO2+-initiated DODH of diols and biomass-based polyols, both PPh3 and 3-pentanol could be used as a reductant. Excellent olefin yields are achieved.

Oxo-Rhenium-Catalyzed Radical Addition of Benzylic Alcohols to Olefins

Although carbon radicals generated from a variety of alcohol derivatives have proven valuable in coupling and addition reactions, the direct use of alcohols as synthetically useful radical sources is less known. In this report, benzylic alcohols are shown to be effective radical precursors for addition reactions to alkenes when treated with triphenylphosphine or piperidine with the catalyst ReIO₂(PPh₃)₂ (I).

Influence of the pendant arm in deoxydehydration catalyzed by dioxomolybdenum complexes supported by amine bisphenolate ligands

Molybdenum complexes devoid of a strongly coordinating pendant arm result in enhanced catalytic activity.

Deoxydehydration of polyols catalyzed by a molybdenum dioxo-complex supported by a dianionic ONO pincer ligand

Deoxydehydration (DODH) is the net reduction of diols and polyols to alkenes or dienes and water. Molybdenum cis-dioxo bis-phenolate ONO complexes were synthesized and have been shown to be active for DODH. Catalysts were screened for activity at 150-190 °C, and appreciable yields of up to 59% were obtained. PPh₃, Na₂SO₃, Zn, C, 3-octanol and 2-propanol were screened as reductants. Additionally, the reactivities of a variety of diols were screened. With (R,R)-(+)-hydrobenzoin as substrate, DODH occurs via a mechanism where reduction of the Mo catalyst is a result of diol oxidation to form two equivalents of aldehyde. These reactions result in complete conversion and near quantitative yields of trans-stilbene and benzaldehyde.

Deoxydehydration of vicinal diols by homogeneous catalysts: a mechanistic overview

Deoxydehydration (DODH) is an important reaction for the upconversion of biomass-derived polyols to commodity chemicals such as alkenes and dienes. DODH can be performed by a variety of early metal-oxo catalysts incorporating Re, Mo and V. The varying reduction methods used in the DODH catalytic cycle impact the product distribution, reaction mechanism and the overall yield of the reaction. This review surveys the reduction methods commonly used in homogeneous DODH catalyst systems and their impacts on yield and reaction conditions.

Deprotonation of a formato ligand by a cis-coordinated carbyne ligand within a bis(phenolate) tungsten complex

Deprotonation of a formato ligand by a cis-coordinated propylidyne ligand in a tungsten(vi) complex [(OSSO)W([triple bond, length as m-dash]CEt)(OCHO)] (3) that contains a tetradentate bis(phenolato) ligand (OSSO = {1,4-dithiabutanediyl-2,2'-bis(4,6-di-tert-butyl-phenolato)}) gave the dioxo complex [(OSSO)WO2] (4) along with CO, ethylene and propylene as major products of decomposition.