Giulia Fiorani

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.

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.

Substrate-Controlled Product Divergence: Conversion of CO2 into Heterocyclic Products

Substituted epoxy alcohols and amines allow substrate-controlled conversion of CO2 into a wide range of heterocyclic structures through different mechanistic manifolds. This new approach results in an unusual scope of CO2-derived products by initial activation of CO2 through either the amine or alcohol unit, thus providing nucleophiles for intramolecular epoxy ring opening under mild reaction conditions. Control experiments support the crucial role of the amine/alcohol fragment in this process with the nucleophile-assisted ring-opening step following an SN i pathway, and a 5-exo-tet cyclization, thus leading to heterocyclic scaffolds.

Highly Efficient Organocatalyzed Conversion of Oxiranes and CO2 into Organic Carbonates

A binary catalyst system based on tannic acid/NBu4X (X=Br, I) is presented as a highly efficient organocatalyst at very low catalyst loading for the coupling of carbon dioxide and functional oxiranes to afford organic carbonates in good yields. The presence of multiple polyphenol fragments within the tannic acid structure is considered to be beneficial for synergistic effects that lead to higher stabilization of the catalyst structure during catalysis. The observed turnover frequencies (TOFs) exceed 200 h(-1) and are among the highest reported to date for organocatalysts in this area of CO2 conversion. This organocatalyst system presents a useful, readily available, inexpensive, and, above all, reactive alternative for most of the metal-based catalyst systems reported to date.

Catalytic Coupling of Carbon Dioxide with Terpene Scaffolds: Access to Challenging Bio‐Based Organic Carbonates

The challenging coupling of highly substituted terpene oxides and carbon dioxide into bio-based cyclic organic carbonates catalyzed by Al(aminotriphenolate) complexes is reported. Both acyclic as well as cyclic terpene oxides were used as coupling partners, showing distinct reactivity/selectivity behavior. Whereas cyclic terpene oxides showed excellent chemoselectivity towards the organic carbonate product, acyclic substrates exhibited poorer selectivities owing to concomitant epoxide rearrangement reactions and the formation of undesired oligo/polyether side products. Considering the challenging nature of these coupling reactions, the isolated yields of the targeted bio-carbonates are reasonable and in most cases in the range 50-60 %. The first crystal structures of tri-substituted terpene based cyclic carbonates are reported and their stereoconnectivity suggests that their formation proceeds through a double inversion pathway.

Preparation of CO2/Diene Copolymers: Advancing Carbon Dioxide Based Materials†

Despite its physical and chemical inertness, carbon dioxide (CO2) continues to be an attractive and alternative carbon synthon as it is abundant, renewable, readily available, and inexpensive. The inertness of CO2 poses a huge challenge given the energy input that is required for its transformation and/or functionalization. A successful example of CO2 reutilization is the development of atom-economic catalytic processes that are based on high-energy reactants (such as epoxides and oxetanes) leading to new functional molecules, such as organic carbonates, polycarbonates, and polyether– polycarbonate-based polymers. Over the last ten years, various efficient catalysts that are active towards CO2/epoxide couplings have been developed for the stereocontrolled preparation of functional cyclic carbonates as well as stereoregular functional polymers. Furthermore, some of these promising catalytic systems are currently employed in commercially feasible industrial processes that exploit CO2 fixation using ethylene and propylene oxide as reaction partners. These processes furnish poly(ethylene carbonate), poly(propylene carbonate), and polyethercarbonate–polyol mixtures with a tailored narrow molecular-weight distribution, which are of further (potential) use in polyurethane synthesis. The applicability of this type of polymerization reaction is still limited to the synthesis of polycarbonates and polyethercarbonates and has not been extended to another ambitious and challenging (commercial) target, the preparation of polyesters by direct copolymerization of CO2 with ethylene or dienes, thus far. This copolymerization reaction is particularly appealing as it represents a link between different renewable resources, such as CO2, and inexpensive, widely available petroleum-derived alkenes, thus allowing a potential evolution towards more sustainable materials. The main obstacles that prevent a successful copolymerization of these monomers include 1) a high energy barrier associated with the alternating copolymerization between ethylene/polyene and CO2, which requires excess ethylene insertion to ensure endergonic CO2 insertion, and 2) a kinetic barrier that arises from the high activation energy for CO2 insertion into the growing polymeric chain relative to polyethylene or polypropylene chain growth. Nozaki and co-workers have now reported a reproducible and highly customizable procedure for the preparation of CO2/diene copolymers. [8b,c] Key to this success was the innovative use of an alternative polymerization strategy that circumvents the thermodynamic and kinetic barriers associated with direct CO2/butadiene copolymerization. In particular, they have employed a known metastable d-lactone, 3ethylidene-6-vinyltetrahydro-2H-pyran-2-one (1), which can be easily obtained by telomerization of CO2 and butadiene in the presence of a palladium/phosphine ligand catalytic system (Scheme 1). Lactone 1 has been extensively studied by the groups of Behr and others over the past 30 years as a promising functional organic intermediate and versatile synthetic building block. The optimized preparation of 1 both on laboratory and pilot-plant scales, which minimizes the formation of undesired telomerization side products, should thus be regarded as a milestone in this area.

Synthesis of dibenzyl carbonate: towards a sustainable catalytic approach

At 90 °C, in the presence of CsF/α-Al2O3 or [P8,8,8,1][H3COCO2] as catalysts, a straightforward protocol was set up for the synthesis of dibenzyl carbonate (DBnC) via the transesterification of dimethyl carbonate (DMC) with an excess of benzyl alcohol. The two catalysts were used in amounts as low as 1% mol (with respect to the limiting reagent DMC). Best results were achieved with CsF/α-Al2O3 that allowed a simpler and reproducible isolation of DBnC in yields up to 70%. Moreover, both the catalyst and the excess BnOH were recovered and could be recycled. The evaluation of mass index (MI) and cost index for the investigated procedure confirmed the economic sustainability and the choice of a rational mass flow throughout the reaction: the method was in the top 7 among 21 protocols selected as the best available options for the synthesis of DBnC.

Luminescent dansyl-based ionic liquids from amino acids and methylcarbonate onium salt precursors: synthesis and photobehaviour

Five new luminescent ionic liquids (LILs) derived from tryptophan (Trp), phenylalanine (Phe) and the dipeptide Gly-Gly functionalized with a dansyl chromophore moiety, were synthesized by an original protocol involving both green reagents/solvents such as non-toxic dimethyl carbonate (DMC: MeOCO2Me) and 2-propanol, and reaction conditions. In particular, DMC was used for: (i) the synthesis of methyltrioctyl methylcarbonate onium salts [Q1mmn][MeOCO2] (Q = N, m = 1, n = 8; Q = P, n = m = 4, 8) by P- and N-methylation of trioctylphosphine and trioctylamine, respectively, and (ii) acid-catalyzed esterifications of Trp and Phe to produce the corresponding methyl esters (Trp-OMe and Phe-OMe). Both reactions proceeded with >90% isolated yields and a mass index (esterifications) as low as 4.5. 2-propanol was used as the solvent for N-dansylation reactions where Trp-OMe and Gly-Glyethyl ester hydrochloride (Gly-Gly-OEt) were coupled to dansyl chloride (DNS-Cl) as a luminescent precursor. A final anion metathesis step between methylcarbonate onium salts and N-dansyl amino acid derivatives gave desired LILs of general formula [Q1mmn][DNS-X] (X = Trp, Phe, and Gly-Gly) in quantitative yields and with by-products minimization. Upon excitation (λex = 340 nm) in MeCN, all LILs exhibited green luminescence with emission quantum yields in the range of 33–41% and monoexponential emission lifetimes of 12.6 ± 0.5 ns. Moreover, each compound showed a remarkable hypsochromic shift in the peak emission wavelength when dissolved in solvents of decreasing polarity (from water to MeCN, toluene and CH2Cl2, respectively). A photostability test by a 350 nm continuous excitation on thin films of LILs proved that, after 10 min, the GlyGly derivative fully retained its PL intensity, while this (intensity) decreased from 10 to 25% for other LILs.

Chapter 13:Iron Complex-based Catalysts

This chapter illustrates some of the immense advancements made using iron catalysis in synthetic chemistry to develop more efficient, environmentally benign and cheap catalytic strategies towards organic compounds with high added value. Some key areas of interest have been identified including C–H activation, crosscoupling reactions between various carbon-sourced nucleophiles and electrophiles and biomimetic oxidation reactions. A compilation of key achievements is made in these transformations that have been shown to be important to both academic and industrial practitioners of catalytic chemistry with a strong focus on the most recent developments in the aforementioned areas. Some general conclusions are drawn at the end of this chapter and areas of further research are identified to stimulate the research communities to further amplify and optimise the use of green, iron-based chemical transformations.

Cover Picture: Highly Efficient Organocatalyzed Conversion of Oxiranes and CO2 into Organic Carbonates (ChemSusChem 19/2015)

The Cover picture shows the chemical structure of tannic acid (a naturally occurring polyphenol) being used as a catalyst component of a highly effective, binary organocatalyst for the formation of organic carbonates from epoxides and CO2 . This organocatalyst proved to be highly active at low catalyst concentrations and resulted in initial catalyst turnover frequencies of >200 h(-1) , thus being rather competitive compared with most of the reported metal-based systems. Using this type of polyphenols as effective catalyst components (representing cheap, renewable, and accessible scaffolds) holds great promise for devising new and improved catalytic processes for CO2 conversion. Thereby, the overall sustainability can be increased and practical, metal-free catalysts could be identified. More details can be found in the Full Paper by A. W. Kleij et al. on page 3248 in Issue 19, 2015 (DOI: 10.1002/cssc.201500710).