Base Cocatalyst Research Articles

New insights on the role of perimeter sites in oxygen and methanol activation during furfural oxidative esterification over gold supported catalysts

Au/ZrO2 catalysts (1.5 wt% Au loading) undergone to a final calcination at different temperature (423, 573, 773 and 923 K) to opportunely modulate the Au size were proven to be highly active and selective in furfural oxidative esterification to methyl-2-furoate. The catalysts were tested according to previously optimized reaction conditions, employing O2 as benign oxidant and without any base co-catalyst. The strong dependence of the catalytic activity on the Au size was previously ascribed to the presence of Au clusters able to dissociate O2 and producing atomic O that can activate methanol. However, despite the absence of clusters, also the catalysts containing larger Au nanoparticles were active, even if conversion and selectivity were remarkably lower if compared to the other samples. The nature and the role of Au active sites exposed at the surface of clusters and nanoparticles were then investigated by HR-TEM, CO chemisorption and FTIR spectroscopy of adsorbed CO, molecular oxygen, methanol and furfural to establish structure-activity relationships. Metal nanoparticles can activate oxygen and form superoxo species and atomic O, whereas experiments performed on the pre-hydrated samples revealed that Au perimeter sites at the metal-support interface play a key role in methanol activation during furfural oxidative esterification.

Heterodinuclear Zn(II), Mg(II) or Co(III) with Na(I) Catalysts for Carbon Dioxide and Cyclohexene Oxide Ring Opening Copolymerizations.

A series heterodinuclear catalysts, operating without co‐catalyst, show good performances for the ring opening copolymerization (ROCOP) of cyclohexene oxide and carbon dioxide. The complexes feature a macrocyclic ligand designed to coordinate metals such as Zn(II), Mg(II) or Co(III), in a Schiff base ‘pocket’, and Na(I) in a modified crown‐ether binding ‘pocket’. The 11 new catalysts are used to explore the influences of the metal combinations and ligand backbones over catalytic activity and selectivity. The highest performance catalyst features the Co(III)Na(I) combination, [N,N′‐bis(3,3’‐triethylene glycol salicylidene)‐1,2‐ethylenediamino cobalt(III) di(acetate)]sodium (7), and it shows both excellent activity and selectivity at 1 bar carbon dioxide pressure (TOF=1590 h−1, >99 % polymer selectivity, 1 : 10: 4000, 100 °C), as well as high activity at higher carbon dioxide pressure (TOF=4343 h−1, 20 bar, 1 : 10 : 25000). Its rate law shows a first order dependence on both catalyst and cyclohexene oxide concentrations and a zeroth order for carbon dioxide pressure, over the range 10–40 bar. These new catalysts eliminate any need for ionic or Lewis base co‐catalyst and instead exploit the coordination of earth‐abundant and inexpensive Na(I) adjacent to a second metal to deliver efficient catalysis. They highlight the potential for well‐designed ancillary ligands and inexpensive Group 1 metals to deliver high performance heterodinuclear catalysts for carbon dioxide copolymerizations and, in future, these catalysts may also show promise in other alternating copolymerization and carbon dioxide utilizations.

Maghemite as a Catalyst for Oxidation of Alcohols with Air

The possibility of using maghemite (γ-iron(III) oxide) as a catalyst for the oxidation of alcohols with atmospheric oxygen has been explored. It has been shown that maghemite is active in the oxidation of benzyl alcohols. In the case of benzyl alcohol, the oxidation leads to benzaldehyde with nearly 100% selectivity. It has been found that the addition of small amounts of basic cocatalysts can substantially increase the yield of the target products.

Bisurea and Bisthiourea H-Bonding Organocatalysts for Ring-Opening Polymerization: Cues for the Catalyst Design

A series of conformationally flexible bis(thio)urea H-bond donors plus base cocatalyst were applied to the ring-opening polymerization (ROP) of lactones. The rate of the ROP displays a strong depen...

Electron rich salen-AlCl catalysts as efficient initiators for the ring-opening polymerisation of rac-lactide

The ring-opening polymerisation of lactide is a useful means to prepare biodegradable materials with well controlled polymer architectures and bespoke material properties. While homogeneous ligand-Al-OR complexes have shown great success in this field, initiation from ligand-AlCl complexes has lagged behind. Here, we report four new salen-AlCl complexes featuring NEt2-substituents, which display high catalyst activities towards rac-lactide ring-opening polymerisation (kobs < 1.9 × 10−3 s−1) via in situ activation with a single equivalent of propylene oxide. Incorporating Lewis basic NEt2 groups into the ligand scaffold not only improves the initiation efficiency but also avoids the need for a Lewis basic co-catalyst and excess epoxide. Notably, studies of our amino-substituted catalysts reveal that the formation of a hexacoordinate aluminate species may hinder rather than enhance catalytic activity.

Au-catalyzed skeletal rearrangement of O-propargylic oximes via N-O bond cleavage with the aid of a Brønsted base cocatalyst.

O-Propargylic oximes that possess an electron-withdrawing aryl group on the oxime moiety undergo Au-catalyzed skeletal rearrangements via N-O bond cleavage to afford the corresponding 2H-1,3-oxazine derivatives. Our studies show that the inclusion of a Brønsted base cocatalyst not only accelerates the reaction but also switches pathways of the skeletal rearrangement reaction, realizing divergent synthesis of heterocyclic compounds. Computational studies indicate that the elimination of propargylic proton in the cyclized vinylgold intermediate is rate-determining and both electron-withdrawing substituents at the oxime moiety and base cocatalyst facilitate the proton elimination. Moreover, the protodeauration process proceeds stepwise involving N-O bond cleavage followed by recyclization to construct the oxazine core.

H-Bonding Organocatalysts for Ring-Opening Polymerization at Elevated Temperatures

The ring-opening polymerization (ROP) kinetics of e-caprolactone and lactide with various H-bonding organocatalysts, (thio)ureas paired with an amine cocatalyst, were evaluated at temperatures up to 110 °C. In nonpolar solvent, most cocatalyst systems exhibit decomposition at high temperatures, while only two, a monourea and bis-urea H-bond donor plus base cocatalyst, are stable up to 110 °C. The onset temperature of cocatalyst decomposition must be measured under reaction conditions. In polar solvent, when the more active imidate form of the (thio)urea is favored, most cocatalyst systems become thermally stable up to 110 °C, exhibiting linear Eyring behavior, including some that were unstable in toluene. The very progress of an ROP is shown to influence the nature of the catalysts as the solution polarity changes from highly polar (at 0% conversion) to less polar at full conversion. Activation parameters are discussed, and a mechanistic explanation of the observations is proposed.

Organic Ring-Opening Polymerization Catalysts: Reactivity Control by Balancing Acidity

Organocatalysts derived from thioureas and amines exhibit high functional group tolerance and extraordinary selectivities for ring-opening relative to chain transesterification. The modest activities of the thiourea/amine catalysts prompted a detailed investigation of ureas and thiourea with organic bases for the ring-opening polymerization of lactones. An array of ureas or thioureas and organic bases were evaluated to assess the effect of the acidity of the urea (thiourea) and the basicity of the base cocatalyst on the activity for ring-opening polymerization. These studies reveal that for a given urea or thiourea stronger bases lead to faster rates. For a given base, the observed catalytic activity is highest when the acidity of the (thio)urea is closely matched with that of the B–H+. For ureas and thioureas of comparable acidity, the urea/base catalyst systems are considerably more active than the corresponding thiourea/base systems. These results are consistent with two mechanisms: one mediated by dep...

Stereoselectivity-Tailored, Metal-Free Hydrolytic Dynamic Kinetic Resolution of Morita–Baylis–Hillman Acetates Using an Engineered Lipase–Organic Base Cocatalyst

Metal-free enantiocomplementary hydrolytic dynamic kinetic resolution of Morita–Baylis–Hillman (MBH) acetates was developed using triethylamine (TEA) as a racemization catalyst and wild-type or engineered lipase B from Candida antarctica (CALB) as stereoselectivity-determining catalyst, leading to chiral MBH alcohols with tailor-made R or S configurations on an optional basis. In the TEA-WT CALB catalysis system, WT CALB displays excellent S enantioselectivity for a series of MBH acetates tested (up to 96% ee and 98% conversion). Reversal of enantioselectivity in favor of (R)-MBH alcohols (95% ee; 95% conversion) was achieved by generating a focused site-specific mutagenesis library composed of less than 20 variants. Molecular modeling explains the origin of stereoselectivity.

Triclocarban: Commercial Antibacterial and Highly Effective H-Bond Donating Catalyst for Ring-Opening Polymerization.

The antibacterial compound, triclocarban (TCC), is shown to be a highly effective H-bond donating catalyst for ring-opening polymerization (ROP) when applied with an H-bond accepting base cocatalyst. These ROPs exhibit the characteristics of "living" polymerizations. TCC is shown to possess the high activity characteristic of urea (vs thiourea) H-bond donors. The urea class of H-bond donors is shown to remain highly active in H-bonding solvents, a trait that is not displayed by the corresponding thiourea H-bond donors. Two H-bond donating ureas that are electronically similar to TCC are evaluated for their efficacy in ROP, and a mechanism of action is proposed. This "off-the-shelf" H-bond donor is among the most active and most controlled organocatalysts for the ROP of lactones.

Bis- and Tris-Urea H-Bond Donors for Ring-Opening Polymerization: Unprecedented Activity and Control from an Organocatalyst.

A new class of H-bond donating ureas was developed for the ring-opening polymerization (ROP) of lactone monomers, and they exhibit dramatic rate acceleration versus previous H-bond mediated polymerization catalysts. The most active of these new catalysts, a tris-urea H-bond donor, is among the most active organocatalysts known for ROP, yet it retains the high selectivity of H-bond mediated organocatalysts. The urea cocatalyst, along with an H-bond accepting base, exhibits the characteristics of a "living" ROP, is highly active, in one case, accelerating a reaction from days to minutes, and remains active at low catalyst loadings. The rate acceleration exhibited by this H-bond donor occurs for all base cocatalysts examined. A mechanism of action is proposed, and the new catalysts are shown to accelerate small molecule transesterifications versus currently known monothiourea catalysts. It is no longer necessary to choose between a highly active or highly selective organocatalyst for ROP.

Development of a syn-Selective Mannich Reaction of Aldehydes with Propargylic Imines by Dual Catalysis: Asymmetric Synthesis of Functionalized Propargylic Amines.

Direct coupling of enolizable aldehydes with C-alkynyl imines is realized affording the corresponding propargylic Mannich adducts of syn configuration, thus complementing previous methods that gave access to the anti-isomers. The combination of proline and a urea Brønsted base cocatalyst is key for the reactions to proceed under very mild conditions (3-10 mol % catalyst loading, dichloromethane as solvent, -20 °C, 1.2 molar equivalents of aldehyde) and with virtually total stereocontrol (syn/anti ratio up to 99:1; ee up to 99 %). Some possibilities of further chemical elaboration of adducts are also briefly illustrated.

Inner- and Outer-Sphere Roles of Ruthenium Phosphido Complexes in the Hydrophosphination of Alkenes

An inner-sphere synthetic cycle for the hydrophosphination of alkenes is proposed, based on observed [2 + 2] cycloaddition of a wide range of alkenes at a coordinatively unsaturated Ru═PR2 complex. Key intermediates in the cycle were prepared, and their reactions with various organic acid/base pairs were examined to identify both new ruthenium precursors and base cocatalysts that allow turnover of the proposed cycle. Two new cationic ruthenium indenyl phosphine complexes were isolated and structurally characterized. Although preliminary screening studies show the moderate activity of these and related neutral phosphido complexes for catalytic hydrophosphination of acrylonitrile by both HPPh2 and HPCy2, and comparable activity for the hydrophosphination of tert-butyl acrylate by HPPh2, no activity was observed for the analogous hydrophosphination of 1-hexene. This is attributed to strong binding of the substrate phosphine to the unsaturated, planar Ru═PR2 fragment generated in situ, which inhibits the inne...

Connecting Neutral and Cationic Pathways in Nickel-Catalyzed Insertion of Benzaldehyde into a C–H Bond of Acetonitrile

Nickel catalysts supported by diethylamine- or aza-crown ether-containing aminophosphinite (NCOP) pincer ligands catalyze the insertion of benzaldehyde into a C–H bond of acetonitrile. The catalytic activity of neutral (NCOP)Ni(OtBu) and cationic [(NCOP)Ni(NCCH3)]+ are starkly different. The neutral tert-butoxide precatalysts are active without any added base and give good yields of product after 24 h, while the cationic precatalysts require a base cocatalyst and still operate much more slowly (120 h in typical runs). A series of in situ spectroscopic studies identified several intermediates, including a nickel cyanoalkoxide complex that was observed in all of the reactions regardless of the choice of precatalyst. Reaction monitoring also revealed that the neutral tert-butoxide precatalysts decompose to form the cationic acetonitrile complex during catalysis; this deactivation involves alkoxide abstraction and can be hastened by the addition of lithium salts. While the deactivated cationic species is inac...

Ketone Hydrogenation with Iridium Complexes with “non N–H” Ligands: The Key Role of the Strong Base

Ferrocenyl phosphine thioether ligands (PS), not containing deprotonatable functions, efficiently support the iridium catalyzed ketone hydrogenation in combination with a strong base cocatalyst. Use of an internal base ([Ir(OMe)(COD)]2 in place of [IrCl(COD)]2) is not sufficient to ensure activity and a strong base is still necessary, suggesting that the active catalyst is an anionic hydride complex. Computational investigations that include solvent effects demonstrate the thermodynamically accessible generation of the tetrahydrido complex [IrH4(PS)]− and suggest an operating cycle via a [Na+(MeOH)3···Ir–H4(PS)] contact ion pair with an energy span of 18.2 kcal/mol. The cycle involves an outer sphere stepwise H–/H+ transfer, the proton originating from H2 after coordination and heterolytic activation. The base plays the dual role of generating the anionic complex and providing the Lewis acid cocatalyst for ketone activation. The best cycle for the neutral system, on the other hand, requires an energy span...

New iron pyridylamino-bis(phenolate) catalyst for converting CO2 into cyclic carbonates and cross-linked polycarbonates.

The atom-efficient reaction of CO2 with a variety of epoxides has been efficiently achieved employing iron pyridylamino-bis(phenolate) complexes as bifunctional catalysts. The addition of a Lewis base co-catalyst allowed significant reduction in the amount of iron complex needed to achieve high epoxide conversions. The possibility of controlling the selectivity of the reaction towards either cyclic carbonate or polycarbonate was evaluated. An efficient switch in selectivity could be achieved when cyclic epoxides such as cyclohexene oxide and the seldom explored 1,2-epoxy-4-vinylcyclohexane were used as substrates. The obtained poly(vinylcyclohexene carbonate) presents pending vinyl groups, which allowed post-synthetic cross-linking by reaction with 1,3-propanedithiol. The cross-linked polycarbonate displayed a substantial increase in the glass transition temperature and chemical resistance, thus opening new opportunities for the application of these green polymers.

Supported gold nanoparticle-catalyzed hydration of alkynes under basic conditions.

TiO2-supported nanosize gold particles catalyze the hydration of alkynes using morpholine as a basic cocatalyst. Unlike most homogeneous cationic gold catalysts, the TiO2-Au/morpholine system is weakly basic and is compatible with acid-sensitive functional groups (e.g., silyl ethers, ketals) or with a strongly coordinating group such as pyridine. What's more, this gold catalyst can be recycled by simple filtration and works well in flow reactors.

Cooperative Hydrogen-Bond Pairing in Organocatalytic Ring-Opening Polymerization.

Thiourea (TU)/amine base cocatalysts are commonly employed for well-controlled, highly active “living” organocatalytic ring-opening polymerizations (ROPs) of cyclic esters and carbonates. In this work, several of the most active cocatalyst pairs are shown by 1H NMR binding studies to be highly associated in solution, dominating all other known noncovalent catalyst/reagent interactions during ROP. One strongly binding catalyst pair behaves kinetically as a unimolecular catalyst species. The high selectivity and activity exhibited by these ROP organocatalysts are attributed to the strong binding between the two cocatalysts, and the predictive utility of these binding parameters is applied for the discovery of a new, highly active cocatalyst pair.

Synthesis of substituted isoquinolines via iminoalkyne cyclization using Ag(i) exchanged K10-montmorillonite clay as a reusable catalyst

Monosubstituted isoquinolines are synthesized in good to excellent yields by the Ag(I)-exchanged K10-montmorillonite clay catalyzed ring closure of iminoalkynes. This silver catalyzed ring closure is highly effective in cyclizing aryl- and alkyl-substituted iminoalkynes at 100 °C and accommodates a variety of iminoalkynes, thus affording a convenient route to the synthesis of isoquinolines. The other salient features of this procedure are its reusability, environmentally benign nature, high yield, operational simplicity with fewer steps, easy separation, and minimization of metallic wastes. The reaction proceeds smoothly in moderate yields and tolerates various functional groups. The solid catalyst can be readily recovered and reused. Notably, no additional base or other co-catalysts are needed. A plausible mechanism is proposed in which isoquinolines are formed via simultaneous bifunctional acid-base catalysis by Ag(I) clay.

ChemInform Abstract: Enhanced Rate and Selectivity by Carboxylate Salt as a Basic Cocatalyst in Chiral N‐Heterocyclic Carbene‐Catalyzed Asymmetric Acylation of Secondary Alcohols.

The rate and enantioselectivity of chiral NHC-catalyzed asymmetric acylation of alcohols with an adjacent H-bond donor functionality are remarkably enhanced in the presence of a carboxylate cocatalyst. The degree of the enhancement is correlated with the basicity of the carboxylate. With a cocatalyst and a newly developed electron-deficient chiral NHC, kinetic resolution and desymmetrization of cyclic diols and amino alcohols were achieved with extremely high selectivity (up to s = 218 and 99% ee, respectively) at a low catalyst loading (0.5 mol %). This asymmetric acylation is characterized by a unique preference for alcohols over amines, which are not converted into amides under the reaction conditions.