Salophen-type Organocatalysts for the Cycloaddition of CO2 and Epoxides under Solvent, Halide, and Metal-Free Conditions.

8-Formyl-7-hydroxycoumarin (A) and their derived salophen-type organocatalysts L1, L2, and L3 were used for the synthesis of cyclic carbonates from carbon dioxide (CO2) and epoxides under solvent-, halide-, and metal-free conditions. According to previous optimization tests, L1 and L2 had the best catalytic activity presenting 89 and 92% conversion toward the synthesis of 3-chloropropylene carbonate (2c) using 8 bar CO2, 100 °C at 9 h. Therefore, they were used as organocatalysts to complete the catalytic screening with 11 terminal epoxides (1a-k) exhibiting the highest TOF values of 20 and 22 h-1 using 1c and 1b, respectively. Similarly, they were tested with an internal epoxide, such as cyclohexene oxide (1l) exhibiting 72% conversion, becoming the first salophen organocatalyst to obtain cis-cyclohexane carbonate (2l) in the absence of a cocatalyst. In addition, a reaction mechanism was proposed for the formation of cyclic carbonates based on experimental data and computational techniques; these contributed in establishing a probable role of CO2 pressure along the catalysis and the hydrogen bonds that favor the stabilization of the different intermediates of the reaction.

Air-Stable Cobalt(III) and Chromium(III) Complexes as Single-Component Catalysts for the Activation of Carbon Dioxide and Epoxides.

Cobalt(III) and chromium(III) salophen chloride complexes were synthesized and tested for the cycloaddition of carbon dioxide (CO2) with epoxides to obtain cyclic carbonates. The cat1, cat2, cat4, and cat5 complexes presented high catalytic activity without cocatalysts and are solvent-free at 100 °C, 8 bar, and 9 h. At these conditions, the terminal epoxides (1a-1k) were successfully converted into the corresponding cyclic carbonates with a maximum conversion of ∼99%. Moreover, cat5 was highlighted due to its capability of opening internal epoxides such as limonene oxide (1l) with a 36% conversion to limonene carbonate (2l), and from cyclohexene oxide (1m), cyclic trans-cyclohexene carbonate (2m) and poly(cyclohexene carbonate) were obtained with 15% and 85% selectivity, respectively. A study of the coupling reaction mechanism was proposed with the aid of electrospray ionization mass spectrometry (ESI-MS) analysis, confirming the single-component behavior of the complexes through their ionization due to epoxide coordination. In addition, crystallographic analysis of cat1 single crystals grown in a saturated solution of pyridine helped to demonstrate that the substitution of chloride ion by pyridine ligands to form an octahedral coordination occurs (Py-cat1), supporting the proposed mechanism. Also, a recyclability study was performed for cat5, and a total turnover number of 952 was obtained with only minor losses in catalytic activity after five cycles.

Stannylenes and Germylenes Stabilized by Tetradentate Bis(amidine) Ligands with a Rigid Naphthalene Backbone.

An unusual series of germylenes and stannylenes stabilized by new tetradentate bis(amidine) ligands RNC(R')N-linker-NC(R')NR with a rigid naphthalene backbone has been prepared by protonolysis reaction of Lappert's metallylenes [M(HMDS)2] (M = Ge or Sn). Germylenes and stannylenes were fully characterized by NMR spectroscopy and X-ray diffraction analysis. DFT calculations have been performed to clarify the structural and electronic properties associated with tetradentate bis(amidine) ligands. Stannylene L1Sn shows reactivity through oxidation, oxidative addition, and transmetalation reactions, affording the corresponding gallium and aluminum derivatives.

Approach to Circular Chemistry Preparing New Polyesters from Olive Oil.

The transformation of cooking oils and their waste into polyesters is a challenge for circular chemistry. Herein, we have used epoxidized olive oil (EOO), obtained from cooking olive oil (COO), and various cyclic anhydrides (such as phthalic anhydride PA, maleic anhydride MA, and succinic anhydride SA) as raw materials for the preparation of new bio-based polyesters. For the synthesis of these materials, we have used the bis(guanidine) organocatalyst 1 and tetrabutylammonium iodide (Bu4NI) as cocatalyst. The optimal reaction conditions for the preparation of poly(EOO-co-PA) and poly(EOO-co-MA) were 80 °C for 5 h using toluene as solvent; however, the synthesis of poly(EOO-co-SA) required more extreme reaction conditions. Furthermore, we have exclusively succeeded in obtaining the trans isomer for MA-polyester. The obtained biopolyesters were characterized by NMR, Fourier transform infrared, thermogravimetric analysis, and scanning electron microscopy analyses. Since there are few examples of functionalized and defined compounds based on olive oil, it is innovative and challenging to transform these natural-based compounds into products with high added value.

Trapping an unusual pentacoordinate carbon atom in a neutral trialuminum complex.

A neutral trialuminum complex incorporates a pentacoordinate carbon through a methylidene bridge linking the three metal atoms. The rigid electron-deficient Al3 core stabilizes the hypercoordinate carbon atom resulting in the shortest equatorial Al-C distance reported for such an Al3-(µ3-CH2) unit.

Toward a Neutral Single-Component Amidinate Iodide Aluminum Catalyst for the CO2 Fixation into Cyclic Carbonates.

A new iodide aluminum complex ({AlI(κ4-naphbam)}, 3) supported by a tetradentate amidinate ligand derived from a naphthalene-1,8-bisamidine precursor (naphbamH, 1) was obtained in quantitative yield via reaction of the corresponding methyl aluminum complex ({AlMe(κ4-naphbam)}, 2) with 1 equiv of I2 in CH2Cl2 at room temperature. Complexes 2 and 3 were tested and found to be active as catalysts for the cyclic carbonate formation from epoxides at 80 °C and 1 bar of CO2 pressure. A first series of experiments were carried out with 1.5 mol % of the alkyl complex 2 and 1.5 mol % of tetrabutylammonium iodide (TBAI) as a cocatalyst; subsequently, the reactions were carried out with 1.5 mol % of iodide complex 3 as a single-component catalyst. Compound 3 is one of the first examples of a nonzwitterionic halide single-component aluminum catalyst producing cyclic carbonates. The full catalytic cycle with characterization of all minima and transition states was characterized by quantum chemistry calculations (QCCs) using density functional theory. QCCs on the reaction mechanism support a reaction pathway based on the exchange of the iodine contained in the catalyst by 1 equiv of epoxide, with subsequent attack of I- to the epoxide moiety producing the ring opening of the epoxide. QCCs triggered new insights for the design of more active halide catalysts in future explorations of the field.

Aluminum complexes with new non-symmetric ferrocenyl amidine ligands and their application in CO2 transformation into cyclic carbonates.

A set of alkyl aluminum complexes supported by non-symmetric ferrocenyl amidine ligands were used as catalysts for the preparation of cyclic carbonates from epoxides and carbon dioxide using Bu4NI as a co-catalyst. A modified method for the synthesis of aminoferrocene allowed us to obtain this precursor in quantitative yield. Treatment of aminoferrocene with the corresponding acetimidoyl chloride afforded the desired ferrocenyl amidine ligands L1H, (E)-N-(2,6-diisopropylphenyl)-N'-(ferrocenyl)acetimidamide, and L2H, (E)-N-(2,6-dimethylphenyl)-N'-(ferrocenyl)acetimidamide. The reaction of these ligands with 1.0 or 0.5 equiv. of AlMe3 led to the synthesis of aminoferrocene based aluminum complexes ((L1)AlMe2 (1), (L2)AlMe2 (2), (L1)2AlMe (3), and (L2)2AlMe (4)) in excellent yields, which were characterized by spectroscopic and X-ray diffraction methods. In addition, we have studied their electrochemical properties and complex 1 was found to be the most active catalyst for the formation of cyclic carbonates 6a-j from their corresponding epoxides 5a-j and CO2.

A molecular electron density theory study of the insertion of CO into frustrated Lewis pair boron-amidines: a [4 + 1] cycloaddition reaction.

The insertion of CO into hydrogenated boron-amidine 1 yielding five-membered diazaborolone (5DAB) 3 has been studied within the molecular electron density theory (MEDT) at the DFT ωB97X-D/6-311G(d,p) level. This is a domino process comprised of two consecutive reactions: (i) the dehydrogenation of 1 yielding the frustrated Lewis pair (FLP) boron-amidine 4, which quickly equilibrates with four-membered diazaborolone (4DAB) 2; and (ii) the addition of CO into FLP 4, yielding the final 5DAB 3. Analysis of the Gibbs free energies indicates that the extrusion of H2 demands a high ΔG≠ of 28.6 kcal·mol-1, being endergonic by 6.7 kcal·mol-1. The subsequent addition of CO into FLP 4 presents a low ΔG≠ of 15.0 kcal·mol-1; formation of 5DAB 3 being exergonic by -5.7 kcal·mol-1 from hydrogenated boron-amidine 1. An analysis of the bonding changes along the insertion of CO in a smaller FLP model indicates that this reaction can be considered a [4 + 1] cycloaddition reaction taking place via a five-membered pseudocyclic transition state associated with a two-stage one-step mechanism. Analysis of the conceptual DFT reactivity indices suggests that the initial attack of CO on FLP 4 is an acid/base process in which the carbenoid carbonyl character allows CO to participate as a Lewis base, rather than a nucleophilic/electrophilic interaction. The results arising from the analysis of the Parr functions, however, coincide with this behaviour.

New imidoyl-indazole platinum (II) complexes as potential anticancer agents: Synthesis, evaluation of cytotoxicity, cell death and experimental-theoretical DNA interaction studies.

Four new neutral N,N imidoyl-indazole ligands (L1, L3, L6, L7) and six new Pt(II)-based complexes (C1-5 and C7) were synthesized and characterized by spectroscopic and spectrometric techniques. Additionally, compounds L6, L7, C3, C5 and C7 were analyzed using X-ray diffraction. An evaluation of cytotoxicity and cell death in vitro for both ligands and complexes was performed by colorimetric assay and flow cytometry, in four cancer cell lines and VERO cells as the control, respectively. Cytotoxicity and selectivity demonstrated by each compound were dependent on the cancer cell line assayed. IC50 values of complexes C1-5 and C7 were lower than those exhibited for the reference drug cisplatin, and selectivity of these complexes was in general terms greater than cisplatin on three cancer cell lines studied. In HL60 cells, complexes C1 and C5 exhibited the lowest values of IC50 and were almost five times more selective than cisplatin. Flow cytometry results suggest that each complex predominantly induced necrosis, and its variant necroptosis, instead of apoptosis in all cancer cell lines studied. DNA binding assays, using agarose gel electrophoresis and UV-visible spectrophotometry studies, displayed a strong interaction only between C4 and DNA. In fact, theoretical calculations showed that C4-DNA binding complex was the most thermodynamic favorable interaction among the complexes in study. Overall, induction of cell death by dependent and independent-DNA-metal compound interactions were possible using imidoyl-indazole Pt(II) complexes as anticancer agents.

The Role of Co-Activation and Ligand Functionalization in Neutral Methallyl Nickel(II) Catalysts for Ethylene Oligomerization and Polymerization.

A detailed quantum chemical study that analyzed the mechanism of ethylene oligomerization and polymerization by means of a family of four neutral methallyl NiII catalysts is presented. The role of the boron co-activators, BF3 and B(C6 F5 )3 , and the position of ligand functionalization (ortho or para position of the N-arylcyano moiety of the catalysts) were investigated to explain the chain length of the obtained polymers. The chain initialization proceeded with higher activation barriers for the ortho-functionalized complexes (≈19 kcal mol-1 ) than the para-substituted isomers (17-18 kcal mol-1 ). Two main pathways were revealed for the chain propagation: The first pathway was favored when using the B(C6 F5 )3 co-activated catalyst, and it produced long-chain polymers. A second pathway led to the ß-hydrogen complexes, which resulted in chain oligomerization; this pathway was preferred when the BF3 co-activated catalysts were used. Otherwise, the termination of longer chains occurred via a stable hydride intermediate, which was formed with an energy barrier of about 14 kcal mol-1 for the B(C6 F5 )3 co-activated catalysts. Significant new insights were made into the reaction mechanism, whereby neutral methallyl NiII catalysts act in oligomerization and polymerization processes. Specifically, the role of co-activation and ligand functionalization, which are key information for the further design of related catalysts, were revealed.

Synthesis of 4(3H)quinazolinimines with selective cytotoxic effect on human acute promyelocytic leukemia cells.

We synthesized a new family of six 4(3H)quinazolinimines based on the reaction between (E)-N-(2-cyanophenyl)benzimidoyl chloride and substituted anilines reaching the formation of their corresponding C2, N3-substituted quinazoliniminium chlorides. This method provides novel, direct and flexible access to diverse substituted 4(3H)quinazolinimines. New compounds obtained following the proposed synthesis were fully characterized and, including the thirteen 4(3H)quinazolinimines synthesized by this method and previously reported by us, were used to study its cytotoxic effect on neoplastic cell lines. The mechanism involved in cell toxicity was also studied. Results showed that these compounds were highly cytotoxic, in particular on Human Promyelocytic Leukemia cells (HL60) and Chronic Myelogenous Leukemia cells (K562) when compared with conventional antineoplastic drugs such as etoposide and cisplatin. The mechanism associated to cytotoxic effect was mainly apoptosis, which not was decreased by antioxidant addition, thereby suggesting that the compounds exert apoptotic death through a mechanism unrelated with oxidative stress.

Dialkylboron guanidinates: syntheses, structures and carbodiimide de-insertion reactions.

The synthesis of novel dialkylboron guanidinates is reported: the symmetrical compounds, (Me2N)C(NR)2BR'2 [R = iPr, R' = Nrb (1); R = Cy, R' = Nrb (2); R = iPr, R' = Cy (3); R = R' = Cy (4); R = 2,6-iPr2-C6H3; R' = Cy (5); Nrb = exo-2-norbornyl] and the asymmetrically coordinated {iPr(H)N}C(NiPr)(NAr)BCy2 [Ar = Ph (6), 4-Me-C6H4 (7), 4-tBu-C6H4 (8)] were prepared by the salt metathesis method from the appropriate lithium guanidinates and chloroboranes. Moreover, the bis(dicyclohexylboron)guanidinate(-2) {iPr(Cy2B)N}C(NiPr){N(4-tBu-C6H4)}BCy2 (9) was also prepared from the corresponding dilithium guanidinate Li2[{N(4-tBu-C6H4)}C(NiPr)2] and ClBCy2. The structures of compounds 1, 3, 6 and 9 were confirmed by X-ray diffraction and all displayed a chelate coordination of the guanidinate ligand to the BR'2 fragment, the latter displaying an additional BCy2 attached to the exocyclic N atom. Solutions of compounds 1-4 reached an equilibrium with the aminoboranes Me2NBR'2 [R' = Nrb (10), Cy (11)] and the corresponding carbodiimides, which was slow at 25 °C. The thermodynamic parameters for these equilibria are also reported. The activation parameters for the equilibrium for compound 1 have been calculated after a kinetic study. Compounds 5-8, with one or two N-aryl fragments bound to a B centre, are more robust and need higher temperatures (80 °C) and prolonged times to give similar carbodiimide de-insertion reactions.

The performance of methallyl nickel complexes and boron adducts in the catalytic activation of ethylene: a conceptual DFT perspective.

In this work, global and local descriptors of chemical reactivity and selectivity are used to explain the differences in reactivities toward ethylene of methallyl nickel complexes and their B(C6F5)3 and BF3 adducts. DFT calculations were used to explain why nickel complexes alone are inactive in ethylene polymerization while their boron adducts can activate it. It is shown that chemical potential, hardness, electrophilicity and molecular electrostatic potential surfaces describe fairly well the reactivity and selectivity of these organometallic systems toward ethylene. Experimental data indicates that addition of a borane molecule to nickel complexes changes dramatically their reactivity-behavior that is confirmed computationally. Our results show that bare complexes are unable to activate ethylene-a Lewis base-because they also behave as Lewis bases. The addition of the co-catalyst-a Lewis acid-turns the adducts into Lewis acids, making them active towards ethylene.

Synthesis of new asymmetric substituted boron amidines - reactions with CO and transfer hydrogenations of phenylacetylene.

The syntheses of the new asymmetric substituted boron amidines [N'-(2,6-diisopropylphenyl)-N-(pentafluorophenyl)acetimidamide]bis(pentafluorophenyl)borate () and [N'-(2,6-diisopropylphenyl)-N-(4-cyanophenyl)acetimidamide]bis(pentafluorophenyl)borate () were achieved by reaction of one equivalent of HB(C6F5)2 and the respective amidines and . These adducts, bearing electron withdrawing groups, showed thermally induced H2 elimination forming the four-membered cyclic diazaborate derivatives and . These new species were characterized by spectroscopic methods. X-ray diffraction studies have been carried out on , and . To prevent undesired reactions at the nitrile group, one equivalent of B(C6F5)3 was added to yielding the -B(C6F5)3 nitrile adduct . Compound underwent thermally induced dehydrogenation to give the four-membered cyclic diazaborate derivative . CO was inserted into the ring systems of and forming the five-membered diazaborolone derivatives and . Phenylacetylene reacted stoichiometrically with the asymmetric substituted boron amidines , and to give styrene by double H transfer.

Synthesis of 4(3H)quinazolinimines by reaction of (E)-N-(aryl)-acetimidoyl or -benzimidoyl chloride with amines.

An efficient one-step method to access 4(3H)quinazolinimines by reaction of phenylchloroimines with 2-aminobenzonitrile is described. The reaction of (E)-N-(2-cyanophenyl)benzimidoyl chloride with substituted anilines that yields a number of their corresponding C2, N3-substituted quinazoliniminium chlorides or neutral products is also reported. These methods provide direct and flexible access to diverse substituted iminoquinazolines substituted at the C2, N3-positions. All the new compounds were fully characterized and six examples are given with their single-crystal X-ray structure.

A computational and conceptual DFT study on the mechanism of hydrogen activation by novel frustrated Lewis pairs.

A computational and conceptual density functional theory (DFT) study on the mechanism of molecular hydrogen activation by a set of three frustrated Lewis pairs (FLPs) was performed at the ωB97X-D/6-311G(d,p) level of theory. A reduced model and other two prototypes derived from experimental data, based on the donor nitrogen and acceptor boron atoms, were used. Analysis based on the energy results, geometries and the global electron density transfer at the TSs made it possible to obtain some interesting conclusions: (i) despite the well-known very low reactivity of molecular hydrogen, the catalytic effectiveness of the three FLPs produces reactions with almost unappreciable activation energies; (ii) the reactions, being exothermic, follow a one-step mechanism via polarised TSs; (iii) there are neither substituent effects on the kinetics nor on the thermodynamics of these reactions; (iv) the activation of molecular hydrogen seems to be attained when the N-B distance in the FLP derivatives is around 2.74 Å; and (v) the proposed FLP model is consistent with the behaviour of the experimental prototypes. Finally, the ability of the three FLPs as efficient catalysts was evaluated studying the hydrogenation of acetylene to yield ethylene.

Synthesis of [(π-cyano-P-nacnac)Cp] and [(π-cyano-nacnac)Cp]-zirconium complexes, and their remote activation for ethylene polymerization.

In this contribution the synthesis and characterization of two distinct, yet similar, zirconium complexes, [(π-P-nacnac-CN)Cp] and [(π-nacnac-CN)Cp] zirconium dichloride were discussed. In addition to the complexes, Lewis acid adducts [B(C(6)F(5))(3)] of the complexes were isolated and characterized as well. It was found that while structurally similar, the complexes behave distinctly upon adduct formation and in their ethylene polymerizations.

Highly dispersed clay-polyolefin nanocomposites free of compatibilizers, via the in situ polymerization of alpha-olefins by clay-supported catalysts.

In situ polymerization by certain transition metal catalysts supported on and activated by acid-treated montmorillonite produces well-dispersed clay-polyolefin nanocomposites, without requiring either organic surfactants to be present in the clay phase or modification of the polyolefin structure.