Christopher J. Whiteoak

A Powerful Aluminum Catalyst for the Synthesis of Highly Functional Organic Carbonates

An aluminum complex based on an amino triphenolate ligand scaffold shows unprecedented high activity (initial TOFs up to 36,000 h(-1)), broad substrate scope, and functional group tolerance in the formation of highly functional organic carbonates prepared from epoxides and CO(2). The developed catalytic protocol is further characterized by low catalyst loadings and relative mild reaction conditions using a cheap, abundant, and nontoxic metal.

Merging Sustainability with Organocatalysis in the Formation of Organic Carbonates by Using CO2as a Feedstock

The use of phenolic compounds as organocatalysts is discussed in the context of the atom-efficient cycloaddition of carbon dioxide to epoxides, forming useful cyclic organic carbonate products. The presence and cooperative nature of adjacent phenolic groups in the catalyst structure results in significantly enhanced catalytic efficiencies, allowing these CO(2) fixation reactions to operate efficiently under virtually ambient conditions. The cooperative effect has also been studied by computational methods. Furthermore, when the cycloaddition reactions are carried out on a larger scale and under solvent-free conditions, further enhancements in activity are observed, combined with the advantageous requirement of reduced loadings of the binary organocatalyst system. The reported system is among one of the mildest and most effective metal-free catalysts for this conversion and contributes to a much more sustainable development of organic carbonate production; this feature has not been the main focus of previous contributions in this area.

Highly active aluminium catalysts for the formation of organic carbonates from CO2 and oxiranes

Al(III) complexes of amino-tris(phenolate) ligand scaffolds have been prepared to attain highly Lewis acidic catalysts. Combination of the aforementioned systems with ammonium halides provides highly active catalysts for the synthesis of organic carbonates through addition of carbon dioxide to oxiranes with initial turnover frequencies among the highest reported to date within the context of cyclic carbonate formation. Density functional theory (DFT) studies combined with kinetic data provides a rational for the relative high activity found for these Al(III) complexes, and the data are consistent with a monometallic mechanism. The activity and versatility of these Al(III) complexes has also been evaluated against some state-of-the-art catalysts and the combined results compare favorably in terms of catalyst construction, stability, activity, and applicability.

Carbon Dioxide as a Protecting Group: Highly Efficient and Selective Catalytic Access to Cyclic cis-Diol Scaffolds†

The efficient and highly selective formation of a wide range of (hetero)cyclic cis-diol scaffolds using aminotriphenolate-based metal catalysts is reported. The key intermediates are cyclic carbonates, which are obtained in high yield and with high levels of diastereo- and chemoselectivity from the parent oxirane precursors and carbon dioxide. Deprotection of the carbonate structures affords synthetically useful cis-diol scaffolds with different ring sizes that incorporate various functional groups. This atom-efficient method allows the simple construction of diol synthons using inexpensive and accessible precursors and green metal catalysts and showcases the use of CO2 as a temporary protecting group.

Conversion of oxiranes and CO2 to organic cyclic carbonates using a recyclable, bifunctional polystyrene-supported organocatalyst

The development of a heterogeneous one-component bifunctional catalyst system able to catalyse the conversion of carbon dioxide and oxiranes to organic cyclic carbonates at low temperature (45 °C) is reported. The bifunctional system can be easily recycled and reactivated when required. When compared with other heterogeneous organocatalysts for the same transformation, the reported catalyst is active at much milder temperatures, thus emphasising the optimal sustainability profile of the new catalyst system.

High activity and switchable selectivity in the synthesis of cyclic and polymeric cyclohexene carbonates with iron amino triphenolate catalysts

Iron(III) amino triphenolate complexes were studied as catalysts for the reaction of carbon dioxide (CO2) with cyclohexene oxide, which can lead to the formation of cyclic carbonate and/or polycarbonate products. Both types of compound are relevant, but for their practical application it is crucial to be able to control the selectivity of the reaction. By working under solvent-free, green conditions with CO2 in the supercritical state and by tailoring the nature and the relative amount of the co-catalyst (Bu4NX or PPNX, where X is a halide) used in combination with the iron(III) complex, we have been able to enhance the catalytic efficiency and achieve a selective and high-yield synthesis of either the cyclic or the polymeric product. The studied reaction is relevant in the context of green chemistry as it provides an atom-efficient route for the conversion of CO2, which is an inexpensive, widely available, renewable and non-toxic feedstock, into valuable products.

A highly active Zn(salphen) catalyst for production of organic carbonates in a green CO2 medium

Zn(salphen), in combination with Bu4NI, was studied as a binary catalyst system for CO2-fixation in the context of organic carbonate formation. The catalytic potential of this binary catalyst system was considerably improved by working in a solvent-free, CO2-rich environment, thereby increasing the overall contact between the reagents and catalyst. Under these green conditions, excellent conversion and selectivity towards the cyclic carbonate product were obtained with epoxides that are generally less prone to undergo cycloaddition with carbon dioxide. The effect of the reaction conditions and the type of co-catalyst employed together with Zn(salphen) were systematically investigated and optimised.

Reactivity control in iron(III) amino triphenolate complexes: comparison of monomeric and dimeric complexes.

Iron(III) amino triphenolate complexes with different substituents in the ortho-position of the phenolate moiety (R = H, Me, tBu, or Ph) have been synthesized by the reaction of iron(III) chloride and the sodium salt (Na(3)L(R)) of the requisite ligand. The complexes have been shown to be of either monomeric ([FeL(R)(THF)]) or dimeric ([FeL(R)](2)) nature by a combination of X-ray diffraction, (1)H NMR, solution magnetic susceptibility, and cyclic voltammetry studies. These analytical studies have shown that the monomeric and dimeric [FeL(R)] complexes behave distinctively, and that the dimer stability is a function of the ortho-positioned groups. Both the dimeric as well as monomeric complexes were tested as catalysts for the catalytic cycloaddition of carbon dioxide to oxiranes, and the data show that the monomeric complexes are able to mediate this conversion with significantly higher activities than the dimeric complexes. This difference in reactivity is controlled by the substitution pattern on the ligand L(R), and is in line with the catalytic requisite of binding of the epoxide substrate by the iron(III) center.

Vanadium Catalyzed Synthesis of Cyclic Organic Carbonates

Vanadium complexes bearing easily synthesized, differently functionalized salen and salphen ligands were prepared and tested for their ability to catalyze the cycloaddition of carbon dioxide to epoxides resulting in cyclic organic carbonates. The reactivity of the prepared catalysts dramatically increases when a coordinating hydroxyl group is present as a substituent in the organic epoxide. The commercially available [VO(acac)2] complex was used as reference compound, and, in this case, we found that V10O26⋅(NBu4)4 was formed during the catalytic reactions. This compound, characterized by X‐ray diffraction analysis, is likely the active catalyst, and it results in significantly better yields of cyclic carbonates compared to those obtained with Schiff base containing vanadyl complexes. The high activity of the mixed polyoxo vanadyl‐vanadate complex marks it as a powerful catalyst within the context of CO2 fixation chemistry.

New iron pyridylamino-bis(phenolate) catalyst for converting CO2 into cyclic carbonates and cross-linked polycarbonates

The atom-efficient reaction of CO2 with a variety of epoxides has been efficiently achieved employing iron pyridylamino-bis(phenolate) complexes as bifunctional catalysts. The addition of a Lewis base co-catalyst allowed significant reduction in the amount of iron complex needed to achieve high epoxide conversions. The possibility of controlling the selectivity of the reaction towards either cyclic carbonate or polycarbonate was evaluated. An efficient switch in selectivity could be achieved when cyclic epoxides such as cyclohexene oxide and the seldom explored 1,2-epoxy-4-vinylcyclohexane were used as substrates. The obtained poly(vinylcyclohexene carbonate) presents pending vinyl groups, which allowed post-synthetic cross-linking by reaction with 1,3-propanedithiol. The cross-linked polycarbonate displayed a substantial increase in the glass transition temperature and chemical resistance, thus opening new opportunities for the application of these green polymers.