Neutral and Cationic Aluminum Complexes Supported by Acetamidate and Thioacetamidate Heteroscorpionate Ligands as Initiators for Ring-Opening Polymerization of Cyclic Esters

Zinc-Catalyzed Cyclization of Alkynyl Derivatives: Substrate Scope and Mechanistic Insights

Novel alkyl zinc complexes supported by acetamidate/thioacetamidate heteroscorpionate ligands have been successfully synthesized and characterized. These complexes exhibited different coordination modes depending on the electronic and steric effects of the acetamidate/thioacetamidate moiety. Their catalytic activity has been tested toward the hydroelementation reactions of alkynyl alcohol/acid substrates, affording the corresponding enol ether/unsaturated lactone products under mild reaction conditions. Kinetic studies have been performed and confirmed that reactions are first-order in [catalyst] and zero-order in [alkynyl substrate]. DFT calculations supported a reaction mechanism through the formation of the catalytically active species, an alkoxide-zinc intermediate, by a protonolysis reaction of the Zn-alkyl bond with the alcohol group of the substrate. Based on the experimental and theoretical results, a catalytic cycle has been proposed.

Improvement of catalytic activity of aluminum complexes for the ring-opening polymerization of ε-caprolactone: aluminum thioamidate and thioureidate systems

In this study, a series of Al complexes bearing amidates, thioamidates, ureidates, and thioureidates were synthesized and their catalytic activity for ε-caprolactone (CL) polymerization was evaluated. SPr-Al exhibited a higher catalytic activity than OPr-Al (3.2 times as high for CL polymerization; [CL] : [SPr-Al] : [BnOH] = 100 : 0.5 : 2; [SPr-Al] = 10 mM, conv. = 93% after 14 min at 25 °C), and USCl-Al exhibited a higher catalytic activity than UCl-Al (4.6 times as high for CL polymerization; [CL] : [USCl-Al] : [BnOH] = 100 : 0.5 : 2; [USCl-Al] = 10 mM, conv. = 90% after 15 min at 25 °C). Regardless of whether aluminum amidates or ureidates were present, thioligands improved the polymerization rate of aluminum catalysts. Density functional theory calculations revealed that the eight-membered ring [SPr-AlOMe2]2 decomposed into the four-membered ring SPr-AlOMe2. However, [OPr-AlOMe2]2 did not decompose because of its strong bridging Al-O bond. The overall activation energy required for CL polymerization was lower when using [SPr-AlOMe2]2 (18.1 kcal mol-1) as a catalyst than when using [OPr-AlOMe2]2 (23.9 kcal mol-1). This is because the TS2a transition state of SPr-AlOMe2 had a more open coordination geometry with a small N-Al-S angle (72.91°) than did TS3c of [OPr-AlOMe2]2, the crowded highest-energy transition state of [OPr-AlOMe2]2 with a larger N-Al-O angle (99.63°).

Highly Electrophilic Mononuclear Cationic Aluminium Alkoxide Complexes: Syntheses, Reactivity and Catalytic Applications

Herein, we report the synthesis of β-diketiminate-supported aluminium complexes bearing terminal alkoxide and mono-thiol functional groups: LAlOMe(Et) (2), LAlOtBu(Et) (3), and LAlSH(Et) (4), (L=[HC{C(Me)N-(2,6-iPr2 C6 H3 )}2 ]). Complexes 2 and 3 are further used as synthons to generate the fascinating cationic aluminium alkoxide complexes, [LAlOMe(μ-OMe)-Al(Et)L][EtB(C6 F5 )3 ] (5), [LAlOMe(OEt2 )][EtB(C6 F5 )3 ] (6), and [LAlOtBu(OEt2 )][EtB(C6 F5 )3 ] (8). These electrophilic cationic species are well characterized by spectroscopic and crystallographic techniques. The assessment of Lewis acidity by the Gutmann-Beckett method revealed superior Lewis acidity of the cations substituted with electron-demanding alkoxy groups in comparison to the known methyl analogue [LAlMe][B(C6 F5 )4 ]. This has been further endorsed by computational calculations to determine the NBO charges and hydride ion affinity for complexes 6 and 8. These complexes are also capable of activating triethylsilane in stoichiometric reactions. The applicability of these complexes has been realized in the hydrosilylation of ethers, carbonyls, and olefines. Additionally, the solid-state structure of a new THF stabilized aluminium halide cation [LAlCl(THF)][B(C6 F5 )4 ] (11) has also been reported.

One-pot terpolymerization of CHO, CO2 and L-lactide using chloride indium catalysts

Ring-opening copolymerization reactions of epoxides, carbon dioxide and cyclic esters to produce copolymers is a promising strategy to prepare CO₂-based polymeric materials. In this contribution, bimetallic chloride indium complexes have been developed as catalysts for the copolymerization processes of cyclohexene oxide, carbon dioxide and L-lactide under mild reaction conditions. The catalysts displayed good catalytic activity and excellent selectivity towards the preparation of poly(cyclohexene carbonate) (PCHC) at one bar CO₂ pressure in the absence of a co-catalyst. Additionally, polyester-polycarbonate copolymers poly(lactide-co-cyclohexene carbonate) (PLA-co-PCHC) were obtained via an one-pot one-step route without the use of a co-catalyst. The degree of incorporation of carbon dioxide can be easily modulated by changing the CO₂ pressure and the monomer feed, resulting in copolymers with different thermal properties.

Polylactide Perspectives in Biomedicine: From Novel Synthesis to the Application Performance

The incessant developments in the pharmaceutical and biomedical fields, particularly, customised solutions for specific diseases with targeted therapeutic treatments, require the design of multicomponent materials with multifunctional capabilities. Biodegradable polymers offer a variety of tailored physicochemical properties minimising health adverse side effects at a low price and weight, which are ideal to design matrices for hybrid materials. PLAs emerge as an ideal candidate to develop novel materials as are endowed withcombined ambivalent performance parameters. The state-of-the-art of use of PLA-based materials aimed at pharmaceutical and biomedical applications is reviewed, with an emphasis on the correlation between the synthesis and the processing conditions that define the nanostructure generated, with the final performance studies typically conducted with either therapeutic agents by in vitro and/or in vivo experiments or biomedical devices.

Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ε-Caprolactone

Trinuclear aluminum complexes bearing bipyrazoles were synthesized, and their catalytic activity for ε-caprolactone (CL) polymerization was investigated. D^Bu₂Al₃Me₅ exhibited higher catalytic activity than did the dinuclear aluminum complex L^Bu₂Al₂Me₄ (16 times as high for CL polymerization; [CL]:[D^Bu₂Al₃Me₅]:[BnOH] = 100:0.5:5, [D^Bu₂Al₃Me₅] = 10 mM, conversion 93% after 18 min at room temperature). Density functional theory calculations revealed a polymerization mechanism in which CL first approached the central Al atom and then moved to an external Al. The coordinated CL ring was opened because the repulsion of two tert-butyl groups on the ligands pushed an alkoxide initiator on an external Al to initiate CL. In these trinuclear Al catalysts, the central Al plays a role in monomer capture and then collaborates with the external Al to activate CL, accelerating polymerization.

Cationic aluminum, gallium, and indium complexes in catalysis

The introduction of cationic charge allows cationic group 13 complexes to be excellent Lewis acid catalysts. Cationic aluminum, gallium, and indium complexes in catalysis are comprehensively reviewed based on the reaction type.

Synthesis and Cytotoxic Activity Evaluation of New Cu(I) Complexes of Bis(pyrazol-1-yl) Acetate Ligands Functionalized with an NMDA Receptor Antagonist

In the present article, copper(I) complexes of bis(pyrazol-1-yl) carboxylic acid (LH), bis(3,5-dimethylpyrazol-1-yl) carboxylic acid (L²H), and bis(pyrazol-1-yl) acetates conjugated with an N-methyl-d-aspartate (NMDA) receptor antagonist (L^NMDA or L^2NMDA) and phosphane ligands (triphenylphosphine or 1,3,5-triaza-7-phosphaadamantane) were synthesized. The selection of an NMDA antagonist for the coupling with LH and L²H was suggested by the observation that NMDA receptors are expressed and play a role in different types of cancer models. All the new complexes showed a significant antitumor activity on a panel of human tumor cell lines of different histology, with cisplatin-sensitive, cisplatin-resistant, or multi-drug-resistant phenotype. Their half maximal inhibitory concentration (IC₅₀) values were in the low- and sub-micromolar range and, in general, significantly lower than that of cisplatin. Interestingly, the fact that all the complexes proved to be significantly more active than cisplatin even in three-dimensional (3D) spheroids of H157 and BxPC3 cancer cells increased the relevance of the in vitro results. Finally, morphological analysis revealed that the most representative complex 8 induced a massive swelling of the endoplasmic reticulum (ER) membrane, which is a clear sign of ER stress.

Syntheses and Biological Studies of Cu(II) Complexes Bearing Bis(pyrazol-1-yl)- and Bis(triazol-1-yl)-acetato Heteroscorpionate Ligands

Copper(II) complexes of bis(pyrazol-1-yl)- and bis(triazol-1-yl)-acetate heteroscorpionate ligands have been synthesized. The copper(II) complexes [HC(COOH)(pz^Me2)₂]Cu[HC(COO)(pz^Me2)₂]·ClO₄, [HC(COOH)(pz)₂]₂Cu(ClO₄)₂ (pz^Me2 = 3,5-dimethylpyrazole; pz = pyrazole) were prepared by the reaction of Cu(ClO₄)₂·6H₂O with bis(3,5-dimethylpyrazol-1-yl)acetic acid (HC(COOH)(pz^Me2)₂) and bis(pyrazol-1-yl)acetic acid (HC(COOH)(pz)₂) ligands in ethanol solution. The copper(II) complex [HC(COOH)(tz)₂]₂Cu(ClO₄)₂·CH₃OH (tz = 1,2,4-triazole) was prepared by the reaction of Cu(ClO₄)₂·6H₂O with bis(1,2,4-triazol-1-yl)acetic acid (HC(COOH)(tz)₂) ligand in methanol solution. The synthesized Cu(II) complexes, as well as the corresponding uncoordinated ligands, were evaluated for their cytotoxic activity in monolayer and 3D spheroid cancer cell cultures with different Pt(II)-sensitivity. The results showed that [HC(COOH)(pz^Me2)₂]Cu[HC(COO)(pz^Me2)₂]·ClO₄ was active against cancer cell lines derived from solid tumors at low IC₅₀ and this effect was retained in the spheroid model. Structure and ultra-structure changes of treated cancer cells analyzed by Transmission Electron Microscopy (TEM) highlighted the induction of a cytoplasmic vacuolization, thus suggesting paraptotic-like cancer cell death triggering.

Alternating Copolymerization of Epoxides and Anhydrides Catalyzed by Aluminum Complexes

The optimization of an organoaluminum catalytic system for the copolymerization of epoxides and anhydrides is presented. For this purpose, the influence of different variables in the process, such as catalysts, cocatalyst, solvent, or substrates, has been analyzed. Kinetic studies, a proposal for the catalytic mechanism, and full characterization of the copolymers obtained are also discussed. Finally, a new copolymer, poly(limonene succinate), obtained by the optimized catalytic system is reported.

Multiple coordination modes of a new ditopic bis(pyrazolyl)methane-based ligand

A new ditopic ligand, N-(2,2-bis(pyrazolyl)ethyl)-2,2-bis(pyrazolyl)acetamide ((pz)₂CH-C(O)-NH-CH₂-CH(pz)₂, L4Pz, pz = pyrazolyl ring), comprising two bis(pyrazolyl)methane donor groups linked via an amide bridge, has been prepared from the reaction of HOOCCH(pz)₂ and H₂NCH₂CH(pz)₂. The ligand coordinates to various metallic salts (i.e. AgO₃SCF₃, PdCl₂, Re(CO)₅Br, and Fe(BF₄)₂), in either a κ²-μ-κ² or a κ³-μ-κ² fashion, depending on the coordination preferences of the metallic center. These compounds were characterized by NMR, UV-Vis and IR spectroscopy, and in solid state by single crystal X-ray diffraction. In the case of silver(i), a mono-dimensional coordination polymer was obtained, while the others were found to be discrete complexes. The synthesis and characterization of a heterobimetallic complex is also described. In solid state, all compounds are associated into supramolecular architectures via hydrogen bonding and pyrazolyl embrace interactions.

Syntheses and biological studies of nitroimidazole conjugated heteroscorpionate ligands and related Cu(I) and Cu(II) complexes

Copper(I) and copper(II) complexes of 5-nitroimidazole conjugated heteroscorpionate ligands have been synthesized. In particular, the new 2,2-bis(pyrazol-1-yl)-N-(2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl)acetamide ligand (L^HMN) was synthesized by direct coupling of preformed side chain acid with 5-nitroimidazole and its coordination chemistry was investigated towards Cu(I) and Cu(II) acceptors and compared with that of the related 2,2-bis(3,5-dimethyl-1-H-pyrazol-1-yl)-N-(2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethyl)acetamide ligand (L^MeMN). The copper(II) complexes {[(L^MeMN)₂Cu]Cl₂} and {[(L^HMN)₂Cu]Cl₂} were prepared by the reaction of CuCl₂·2H₂O with L^HMN or L^MeMN ligands in methanol solution. The water soluble copper(I) complexes {[(L^MeMN)Cu(PTA)₂]}(PF₆) and {[(L^HMN)Cu(PTA)₂]}(PF₆) were prepared by the reaction of Cu(CH₃CN)₄PF₆ and 1,3,5-triaza-7-phosphaadamantane (PTA) with L^HMN or L^MeMN ligands in acetonitrile solution. The new Cu(I) and Cu(II) complexes as well as the corresponding uncoordinated ligands were evaluated for their cytotoxic activity against 2D monolayer cultures of multiple human cancer cell lines and 3D-cultured HCT-15 colon cancer spheroids. Morphological analysis by Transmission Electron Microscopy (TEM) revealed the induction of a massive cytoplasmic vacuolization consistent with a paraptotic-like cancer cell death.

Versatile organoaluminium catalysts based on heteroscorpionate ligands for the preparation of polyesters

A series of alkyl aluminium complexes based on heteroscorpionate ligands were designed as catalysts for the ring-opening polymerisation of cyclic esters and ring-opening copolymerisation of epoxides and anhydrides. Treatment of AlX3 (X = Me, Et) with ligands bpzbeH [bpzbe = 1,1-bis(3,5-dimethylpyrazol-1-yl)-3,3-dimethyl-2-butoxide], bpzteH [bpzte = 2,2-bis(3,5-dimethylpyrazol-1-yl)-1-para-tolylethoxide], and (R,R)-bpzmmH [(R,R)-bpzmm = (1R)-1-{(1R)-6,6-dimethyl-bicyclo[3.1.1]-2-hepten-2-yl}-2,2-bis(3,5-dimethylpyrazol-1-yl)ethoxide] for 2 hours at 0 °C afforded the mononuclear dialkyl aluminium complexes [AlMe2{κ2-bpzbe}] (1), [AlEt2{κ2-bpzbe}] (2), [AlMe2{κ2-(R,R)-bpzmm}] (3) and [AlEt2{κ2-(R,R)-bpzmm}] (4), and the dinuclear dialkyl complexes [AlMe2{κ2-bpzte}]2 (5) and [AlEt2{κ2-bpzte}]2 (6). The molecular structures of the new complexes were determined by spectroscopic methods and confirmed by X-ray crystallography. The alkyl-containing aluminium complexes can act as highly efficient single-component initiators for the ring-opening polymerisation of ε-caprolactone and l-lactide and for the ring-opening copolymerisation of cyclohexene oxide and phthalic anhydride to give a range of biodegradable polyesters.

Synthesis and characterization of neutral and cationic aluminum complexes supported by a furfuryl-containing aminophenolate ligand for ring-opening polymerization of ε-caprolactone

Mononuclear aluminum complexes supported by novel furfuryl-containing aminophenolate ligand are reported along with CL polymerization.

Impact of aryloxy initiators on the living and immortal polymerization of lactide

This report describes two different methodologies for the synthesis of aryl end-functionalized poly(lactide)s (PLAs) catalyzed by indium complexes.

P,O-Phosphinophenolate zinc(II) species: synthesis, structure and use in the ring-opening polymerization (ROP) of lactide, ε-caprolactone and trimethylene carbonate

The P,O-type phosphinophenol proligands (1·H, 2-PPh2-4-Me-6-Me-C6H2OH; 2·H, 2-PPh2-4-Me-6-(t)Bu-C6H2OH) readily react with one equiv. of ZnEt2 to afford in high yields the corresponding Zn(II)-ethyl dimers of the type [(κ(2)-P,O)Zn-Et]2 (3 and 4) with two μ-OPh bridging oxygens connecting the two Zn(II) centers, as determined by X-ray diffraction (XRD) studies in the case of 3. Based on diffusion-ordered NMR spectroscopy (DOSY), both species 3 and 4 retain their dimeric structures in solution. The alcoholysis reaction of Zn(II) alkyls 3 and 4 with BnOH led to the high yield formation of the corresponding Zn(II) benzyloxide species [(κ(2)-P,O)Zn-OBn]2 (5 and 6), isolated in a pure form as colorless solids. The centrosymmetric and dimeric nature of Zn(II) alkoxides 5 and 6 in solution was deduced from DOSY NMR experiments and multinuclear NMR data. Though the heteroleptic species 5 is stable in solution, its analogue 6 is instable in CH2Cl2 solution at room temperature to slowly decompose to the corresponding homoleptic species 8via the transient formation of (κ(2)-P,O)2Zn2(μ-OBn)(μ–κ(1):κ(1)-P,O) (6′). Crystallization of compound 6 led to crystals of 6′, as established by XRD analysis. The reaction of ZnEt2 with two equiv. of 1·H and 2·H allowed access to the corresponding homoleptic species of the type [Zn(P,O)2] (7 and 8). All gathered data are consistent with compound 7 being a dinuclear species in the solid state and in solution. Data for species 8, which bears a sterically demanding P,O-ligand, are consistent with a mononuclear species in solution. The Zn(II) alkoxide species 5 and the [Zn(P,O)2]-type compounds 7 and 8 were evaluated as initiators of the ring-opening polymerization (ROP) of lactide (LA), ε-caprolactone (ε-CL) and trimethylene carbonate (TMC). Species 5 is a well-behaved ROP initiator for the homo-, co- and ter-polymerization of all three monomers with the production of narrow disperse materials under living and immortal conditions. Though species 7 and 8 are ROP inactive on their own, they readily polymerize LA in the presence of a nucleophile such as BnOH to produce narrow disperse PLA, presumably via an activated-monomer ROP mechanism.

Synthesis and catalytic application of magnesium complexes bearing pendant indolyl ligands

Three novel indole-based ligand precursors [HIndPh(R), R = methoxy, HIndPh(OMe) (); thiomethoxy, HIndPh(SMe) (); and N,N'-dimethylamino, HIndPh(NMe2) ()] have been synthesized via Sonogashira and cyclization reactions with moderate to high yield. Reactions of these ligand precursors with 0.6 equivalent of Mg(n)Bu2 in THF afforded the magnesium bis-indolyl complexes , respectively. All the ligand precursors and related magnesium complexes have been characterized by NMR spectroscopy and elemental analyses. The molecular structures are reported for compounds and . Under optimized conditions, compound demonstrates efficient catalytic activities towards the ring opening polymerization of l-lactide and ε-caprolactone in the presence of BnOH.

The ring-opening polymerization of ε-caprolactone and l-lactide using aluminum complexes bearing benzothiazole ligands as catalysts

A series of aluminum complexes bearing benzothiazole ligands was synthesized and the ring-opening polymerization of ε-caprolactone (CL) and l-lactide (LA) using these aluminum complexes as catalysts was studied.

Different mechanisms at different temperatures for the ring-opening polymerization of lactide catalyzed by binuclear magnesium and zinc alkoxides

Two binuclear magnesium and zinc alkoxides supported by a bis-salalen type dinucleating heptadentate Schiff base ligand were synthesized and fully characterized. The two complexes are efficient initiators for the ring-opening polymerization (ROP) of L-lactide, affording polymers with narrow polydispersities and desirable molecular weights. Interestingly, the mechanisms for the ROP of lactide are different at different temperatures. At a high temperature of 130 °C, a coordination-insertion mechanism is reasonable for the bulk melt polymerization of lactide. At a low temperature, the alkoxide cannot initiate the ROP reaction; however, upon the addition of external benzyl alcohol into the system, the ROP of lactide can smoothly proceed via an "activated monomer" mechanism. In addition, these complexes display slight stereo-selectivity for the ring-opening polymerization of rac-lactide, affording partially isotactic polylactide in toluene with a Pm value of 0.59.

Catalytic behaviour in the ring-opening polymerisation of organoaluminiums supported by bulky heteroscorpionate ligands

A series of alkyl organoaluminium complexes based on bulky heteroscorpionate ligands were designed as catalysts for the ring-opening polymerisation of cyclic esters. Thus, the treatment of AlX3 (X = Me, Et) with bulky acetamide or thioacetamide heteroscorpionate ligands nbptamH (1) [nbptamH = N-naphthyl-2,2-bis(3,5-dimethylpyrazol-1-yl)thioacetamide], fbpamH (2) [fbpamH = N-fluorenyl-2,2-bis(3,5-dimethylpyrazol-1-yl)acetamide], ptbptamH (3) [ptbptamH = N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide], ntbptamH (4) [ntbptamH = N-naphthyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)thioacetamide], ptbpamH (5) [ptbpamH = N-phenyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)acetamide] and (S)-mtbpamH (6) [(S)-mtbpamH = (S)-(−)-N-α-methylbenzyl-2,2-bis(3,5-di-tert-butylpyrazol-1-yl)acetamide] for 1 hour at 0 °C afforded the dialkyl aluminium complexes [AlX2{κ(2)-nbptam}] (X = Me 7, Et 8), [AlX2{κ(2)-fbpam}] (X = Me 9, Et 10), [AlX2{κ(2)-ptbptam}] (X = Me 11, Et 12), [AlX2{κ(2)-ntbptam}] (X = Me 13, Et 14), [AlX2{κ2(-)ptbpam}] (X = Me 15, Et 16) and [AlX2{κ(2)-(S)-mtbpam}] (X = Me 17, Et 18). The structures of the complexes were determined by spectroscopic methods and the X-ray crystal structure of 14 was also established. The alkyl-containing aluminium complexes 7–18 can act as efficient single-component initiators for the ring-opening polymerisation of ε-caprolactone and rac-lactide. The polymerisations are living, as evidenced by the narrow polydispersities of the isolated polymers and the linear nature of the number average molecular weight versus conversion plot. Finally, a comparative study of ring-opening polymerisation for new bulky heteroscorpionate aluminium initiators and the less congested aluminium analogues is reported.

Optimizing ring-opening polymerization of ε-caprolactone by using aluminum complexes bearing amide as catalysts and their application in synthesizing poly-ε-caprolactone with special initiators and other polycycloesters

A series of aluminum complexes bearing amidate ligands, including acylamide, sulfonamide, and aryl carbamate, was synthesized.

Synthesis and structural characterization of amido heteroscorpionate rare-earth metal complexes and hydroamination of aminoalkenes

New amide heteroscorpionate rare-earth complexes were developed and used as efficient catalysts for the intramolecular hydroamination of aminoalkenes.

Synthesis of new heteroscorpionate iridium(i) and iridium(iii) complexes

The reactivity of different heteroscorpionate ligands based on bis(pyrazol-1-yl)methane, with different iridium-(i) and -(iii) precursors is reported.

Well-controlled ring-opening polymerization of cyclic esters initiated by dialkylaluminum β-diketiminates

A series of aluminum alkyl complexes nacnacRAlMe2 (2a-2h) bearing aliphatic N-substituted β-diketiminate ligands (nacnacRH = N,N'-dialkyl-2-amino-4-iminopent-2-enes), were prepared from the reaction of trialkyl aluminum and the corresponding β-diketimines. All these aluminum complexes were characterized by NMR, elemental analysis and HRMS spectroscopy. The molecular structures of complexes 2a, 2c and 2h were confirmed by single-crystal X-ray diffraction. These aluminum alkyl complexes show notable activity towards the ring-opening polymerization of ε-caprolactone in the absence and presence of alcohol, and the resulting polymers have narrow molecular weight distributions.

Synthesis of cyclic carbonates catalysed by aluminium heteroscorpionate complexes

A trimetallic aluminium complex catalyses the synthesis of cyclic carbonates from epoxides and CO₂ at room temperature and pressure.