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.

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.

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.

Zinc versus Magnesium: Orthogonal Catalyst Reactivity in Selective Polymerizations of Epoxides, Bio-derived Anhydrides and Carbon Dioxide

Developing selective polymerizations from complex monomer mixtures is an important challenge. Here, dinuclear catalysts allow selective polymerization from mixtures of sterically hindered tricyclic anhydrides, carbon dioxide and epoxides to yield well-controlled copoly(ester-carbonates). Surprisingly, two very similar homogeneous catalysts differing only in the central metal, zinc versus magnesium, show very high but diametrically opposite monomer selectivity. The selectivity is attributed to different polymerization kinetics and to steric factors associated with the anhydrides.

A Multiphase Protocol for Selective Hydrogenation and Reductive Amination of Levulinic Acid with Integrated Catalyst Recovery

At 60-150 °C and 15-35 bar H₂ , two model reactions of levulinic acid (LA), hydrogenation and reductive amination with cyclohexylamine, were explored in a multiphase system composed of an aqueous solution of reactants, a hydrocarbon, and commercial 5 % Ru/C as a heterogeneous catalyst. By tuning the relative volume of the immiscible water/hydrocarbon phases and the concentration of the aqueous solution, a quantitative conversion of LA was achieved with formation of γ-valerolactone or N-(cyclohexylmethyl)pyrrolidone in >95 and 88 % selectivity, respectively. Additionally, the catalyst could be segregated in the hydrocarbon phase and recycled in an effective semi-continuous protocol. Under such conditions, formic acid additive affected the reactivity of LA through a competitive adsorption on the catalyst surface. This effect was crucial to improve selectivity for the reductive amination process. The comparison of 5 % Ru/C with a series of carbon supports demonstrated that the segregation phenomenon in the hydrocarbon phase, never previously reported, was pH-dependent and effective for samples displaying a moderate surface acidity.

Advancements and Complexities in the Conversion of Lignocellulose Into Chemicals and Materials

This Perspective describes the challenges and objectives associated to the development of new chemical technologies for the conversion of lignocellulose (non-food or waste) into chemicals and materials; it also provides an outlook on the sources, potential products, and issues to be addressed.

Novel Regioselective Synthesis of 1,3,4,5-Tetrasubstituted Pyrazoles and Biochemical Valuation on F1FO-ATPase and Mitochondrial Permeability Transition Pore Formation

An efficient, eco-compatible, and very cheap method for the construction of fully substituted pyrazoles (Pzs) via eliminative nitrilimine-alkene 1,3-dipolar cycloaddition (ENAC) reaction was developed in excellent yield and high regioselectivity. Enaminones and nitrilimines generated in situ were selected as dipolarophiles and dipoles, respectively. A deep screening of the employed base, solvent, and temperature was carried out to optimize reaction conditions. Recycling tests of ionic liquid were performed, furnishing efficient performance until six cycles. Finally, a plausible mechanism of cycloaddition was proposed. Then, the effect of three different structures of Pzs was evaluated on the F1FO-ATPase activity and mitochondrial permeability transition pore (mPTP) opening. The Pz derivatives' titration curves of 6a, 6h, and 6o on the F1FO-ATPase showed a reduced activity of 86%, 35%, and 31%, respectively. Enzyme inhibition analysis depicted an uncompetitive mechanism with the typical formation of the tertiary complex enzyme-substrate-inhibitor (ESI). The dissociation constant of the ESI complex (Ki') in the presence of the 6a had a lower order of magnitude than other Pzs. The pyrazole core might set the specific mechanism of inhibition with the F1FO-ATPase, whereas specific functional groups of Pzs might modulate the binding affinity. The mPTP opening decreased in Pz-treated mitochondria and the Pzs' inhibitory effect on the mPTP was concentration-dependent with 6a and 6o. Indeed, the mPTP was more efficiently blocked with 0.1 mM 6a than with 1 mM 6a. On the contrary, 1 mM 6o had stronger desensitization of mPTP formation than 0.1 mM 6o. The F1FO-ATPase is a target of Pzs blocking mPTP formation.

Highly Efficient Organocatalyzed Conversion of Oxiranes and CO2 into Organic Carbonates

Invited for the cover of this issue is the group of Arjan Kleij at the Institute of Chemical Research of Catalonia (ICIQ). The image shows how naturally occurring compounds such as tannic acid can be used to devise highly efficient CO2 conversion catalysis. The Full Paper itself is available at 10.1002/cssc.201500710.

[OSSO]-Type Chromium(III) Complexes for the Reaction of CO2 with Epoxides

In this work, four new mononuclear Cr(III) complexes (2-5) bearing bis-thioether-diphenolate, [OSSO]-type ligands, were synthesized and characterized. These complexes in combination with bis(triphenylphosphine)iminium chloride (PPNCl) promoted the coupling of CO2 with epoxides. Depending on the type of substrate and the conditions, the reaction results in the selective formation of either polycarbonate or cyclic carbonate. For example, the reactions in the presence of complex 2 led to the exclusive formation of poly(cyclohexene carbonate, PCHC) from cyclohexene oxide (CHO) (TOF up to 39 h-1 , at T=45-100 °C, time=24 h, pCO2 =20 bar, epoxide/2 (mol/mol)=1000, and PPNCl/2=0.5-2.0 mol %). Under the same conditions and PPNCl/2=0.5-5.0 mol %, the reactions of CO2 with styrene oxide (SO), epichlorohydrin (ECH), 1,2 epoxydodecane (EDD), and allyl glycidyl ether (AGE) have shown selective conversion to the corresponding cyclic carbonates (TOF up to 41 h-1 ).