Arjan W. Kleij

Salen-Complex-Mediated Formation of Cyclic Carbonates by Cycloaddition of CO2 to Epoxides

Metal complexes of salen ligands are an important class of compounds, and they have been widely studied in the past. Among their successful catalytic applications, the synthesis of cyclic carbonates by the coupling reaction of epoxides with CO(2) has received increased attention; this is mostly due to the importance of using a greenhouse gas as a feedstock for the synthesis of useful molecules. Herein the most relevant past and present research surrounding this topic is presented.

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.

Sustainable conversion of carbon dioxide: the advent of organocatalysis

The conversion of carbon dioxide (CO2), an abundant renewable carbon reagent, into chemicals of academic and industrial interest is of imminent importance to create a higher degree of sustainability in chemical processing and production. Recent progress in this field is characterised by a plethora of organic molecules able to mediate the conversion of suitable substrates in the presence of CO2 into a variety of value-added commodities with advantageous features combining cost-effectiveness, metal-free transformations and general substrate activation profiles. In this review, the latest developments in the field of CO2 catalysis are discussed with a focus on organo-mediated conversions and their increasing importance in serving as practicable alternatives for metal-based processes. Also a critical assessment of the state-of-the-art methods is presented with attention to those features that need further development to increase the usefulness of organocatalysis in the production of organic molecules of potential commercial interest.

Material Applications for Salen Frameworks

Salens are among the most widely studied ligands in chemistry. They are primarily applied in homogeneous catalysis, a use that helps to increase our knowledge of environmentally benign and cost-attractive chemical and/or pharmaceutical processes. Lately, the interest in salen chemistry has shifted to the use of these scaffolds in various other applications in which the immense versatility of the salen framework as a molecular building block can be exploited. Herein, we highlight the most recent research involving the incorporation of salen motifs in new materials and sophisticated catalysts.

A DFT Study on the Mechanism of the Cycloaddition Reaction of CO2 to Epoxides Catalyzed by Zn(Salphen) Complexes

The reaction mechanism for the Zn(salphen)/NBu4X (X = Br, I) mediated cycloaddition of CO2 to a series of epoxides, affording five-membered cyclic carbonate products has been investigated in detail by using DFT methods. The ring-opening step of the process was examined and the preference for opening at the methylene (Cβ) or methine carbon (Cα) was established. Furthermore, calculations were performed to clarify the reasons for the lethargic behavior of internal epoxides in the presence of the binary catalyst. Also, the CO2 insertion and the ring-closing steps have been explored for six differently substituted epoxides and proved to be significantly more challenging compared with the ring-opening step. The computational findings should allow the design and application of more efficient catalysts for organic carbonate formation.

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.

Ambient Fixation of Carbon Dioxide using a ZnIIsalphen Catalyst

The use of carbon dioxide (CO2) as a C-1 renewable feedstock in organic synthesis is currently receiving considerable interest. Despite its relative inertness (it is both kinetically and thermodynamically highly stable), CO2 offers a number of advantages as it is a cheap, non-toxic, and virtually inexhaustible chemical reagent. The biggest challenge in (catalytically) converting CO2 into useful organic matter [1d, 2] is the use of low energy-intensive reaction conditions to maximize the overall process sustainability. The quest for the development of such green processes remains open, even though recent contributions have shown that CO2 conversion at either low temperature and/or ambient pressure is feasible. With cyclic/poly-carbonate formation being one of the most successful applications within the context of CO2 fixation, North and co-workers demonstrated that cyclic carbonate synthesis through cycloaddition of CO2 to epoxides catalyzed by bimetallic Al -salen complexes can be performed under ambient pressure and temperature. Conversely, polycarbonate synthesis may also be achieved under mild conditions (pCO2 = 0.1 MPa and/or 25 8C) while maintaining good conversion rates. We recently reported on Zn-centered salphen [salphen = N,N’-phenylene1,2-bis-salicylidene] complexes active in cyclic carbonate synthesis that showed reasonable activity at 45 8C and 1 MPa of CO2 pressure, with dichloromethane as the solvent. Herein we communicate that cyclic carbonate synthesis can be effectively achieved under extremely mild conditions (25 8C, pCO2 = 0.2 MPa) and with a green reaction medium. This represents one of the very few catalytic processes that can exclusively produce cyclic carbonate structures under ambient conditions. We previously reported that the Zn ion in salphen complexes shows increased Lewis-acid behavior as a result of the constrained geometry imposed by the ligand scaffold. To assess the relative reactivity of these Zn(salphen)s and their utility as effective catalysts in cyclic carbonate formation (Table 1), we first examined a series of M(salphen) catalysts including metal ions that have proven to be useful in cyclic carbonate synthesis. 9] In this series the salphen scaffold was identical allowing a direct comparison between the activities of the metal complexes. As expected, the complexes based on Pd, Cu, and Ni (1 a–c) proved to be inactive under the applied conditions. The Mn complex (1 d, 2 %), the Al complexes (1 e–f, 23–36 %) and Co complex (1 g, 45 %) only gave very low to moderate yields of the desired product. However, the Cr complex (1 h, 82 %) and the Zn complex 2 a (80 %) proved to be the most active systems within this series. Unlike the synthesis of the Cr complex, the preparation of Zn(salphen) 2 a does not require for oxygenand/or moisture-free conditions. Furthermore, the use of Zn should not give rise to any toxicity issues related to a change of the metal oxidation state. The facile synthesis of 2 a (one step, nearly quantitative yield and economical) and its advantageous catalytic properties make it a preferred catalyst for cyclic carbonate synthesis as compared to catalyst structures 1 a–1 h. It should be noted that the Al catalyst 1 e proved to be rather insoluble in DCM; therefore we compared the more soluble Al complex 1 f with Zn complex 2 b to evaluate the relative activity. Although 1 f is initially soluble under these catalytic conditions, after the reaction a precipitate was noted, which may have affected turnover. Next, we investigated how the changing of the salphen scaffold (using Zn as the active metal centre) influences the cycloaddition reaction using epoxyhexane as a benchmark subTable 1. Conversion of epoxyhexane and CO2 into the corresponding cyclic carbonate by complexes 1 a–h and 2 a–b.

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.

Effective chirogenesis in a bis(metallosalphen) complex through host-guest binding with carboxylic acids.

Transfer of chiral information through supramolecular interactions (chirogenesis) has been observed in many natural systems including DNA and proteins, and is nowadays widely used in the development of smart artificial and biomimetic materials. The induction of chirality in bis(metalloporphyrins) for example, has been successfully applied in assigning the absolute configuration of amines, diamines and aminoamides, aminoalcohols and epoxyalcohols, and diols by using a circular dichroism (CD) protocol. Effective chirality transfer with carboxylic acids, however, has proven to be highly difficult, and has only been achieved by using potassium carboxylate salts followed by tedious extractions, or by the addition of a huge excess of substrate to a metal-free host. The low efficiency of chiral induction with these previous methods is mainly due to the relatively weak host–guest interactions with carboxylic acid groups. To overcome this problem, we have designed a bis[Zn(salphen)] complex 1 (salphen = N,N’-phenylenebis(salicylimine)), which, similar to 2,2’-biphenol units, exists in dynamic equilibrium between two chiral conformations (S and R enantiomers; see Scheme 1). We reasoned that the energy barrier of rotation increases upon binding of a ditopic ligand to the Lewis acidic Zn centers. 12] Herein, we demonstrate that 1 binds very strongly with acetic acid and that axial chirality can be effectively induced by exchange for chiral a-substituted carboxylic acids, with the practical advantage that substrate derivatization or use of excessive substrate is not required. Compound 1 was prepared in a single step by reaction of a bis(salicylaldehyde) molecule with two equivalents of a monoimine precursor and Zn(OAc)2 in the presence of pyridine. Subsequent precipitation from MeOH afforded the product in excellent yield (73 %) and purity (see the Supporting Information). Characterization by NMR spectroscopy indicated the presence of one equivalent of acetic acid, which had formed as a by-product in the synthesis. Slow evaporation of a solution of the product in toluene/ MeCN 1:1 resulted in single crystals suitable for X-ray analysis (Figure 1). The solid-state structure revealed that the two Zn centers of 1 are bridged by AcOH (through the oxygen atoms of the carboxylic acid unit) to give a complex with 1:1 stoichiometry (1 AcOH). As anticipated, both the S and the R conformer were present in a 1:1 ratio in the unit cell; each conformer has the same dihedral angle (44.98) and Zn–O(acetate) distance (2.01 ). This distance is identical to that previously found in a related acetate-bridged complex. Since the exact position of the acidic proton of AcOH could not be resolved by X-ray diffraction, the proton was placed in Scheme 1. Conformational isomerism of 1. The tBu groups are omitted for clarity in the line drawings of the conformers. P denotes righthandedness and M left-handedness.