Integration of aerobic oxidation and intramolecular asymmetric aza-Friedel-Crafts reactions with a chiral bifunctional heterogeneous catalyst

A new class of chiral bifunctional heterogeneous materials composed of Au/Pd nanoparticles and chiral phosphoric acids as active orthogonal catalysts was prepared by utilizing a facile pseudo-suspension co-polymerization method. It was found that this heterogeneous catalyst was capable of facilitating the sequential aerobic oxidation-asymmetric intramolecular aza-Friedel-Crafts reaction between benzyl alcohols and N-aminoethylpyrroles. Moreover, the designed chiral heterogeneous catalyst could be recovered and reused several times without significant loss of activity or enantioselectivity.

Amberlyst-15 catalysed oxidative esterification of aldehydes using a H2O2 trapped oxidant as a terminal oxidant

A simple and efficient method has been developed for the selective oxidative esterification of aldehydes using commercially available Amberlyst-15 as a catalyst.

Cross-dehydrogenative coupling for the intermolecular C-O bond formation

The present review summarizes primary publications on the cross-dehydrogenative C-O coupling, with special emphasis on the studies published after 2000. The starting compound, which donates a carbon atom for the formation of a new C-O bond, is called the CH-reagent or the C-reagent, and the compound, an oxygen atom of which is involved in the new bond, is called the OH-reagent or the O-reagent. Alcohols and carboxylic acids are most commonly used as O-reagents; hydroxylamine derivatives, hydroperoxides, and sulfonic acids are employed less often. The cross-dehydrogenative C-O coupling reactions are carried out using different C-reagents, such as compounds containing directing functional groups (amide, heteroaromatic, oxime, and so on) and compounds with activated C-H bonds (aldehydes, alcohols, ketones, ethers, amines, amides, compounds containing the benzyl, allyl, or propargyl moiety). An analysis of the published data showed that the principles at the basis of a particular cross-dehydrogenative C-O coupling reaction are dictated mainly by the nature of the C-reagent. Hence, in the present review the data are classified according to the structures of C-reagents, and, in the second place, according to the type of oxidative systems. Besides the typical cross-dehydrogenative coupling reactions of CH- and OH-reagents, closely related C-H activation processes involving intermolecular C-O bond formation are discussed: acyloxylation reactions with ArI(O2CR)2 reagents and generation of O-reagents in situ from C-reagents (methylarenes, aldehydes, etc.).

Palladium-catalysed oxidative cross-esterification between two alcohols

Oxidative cross-esterification between two alcohols was achieved by using PdCl₂(PPh₃)₂ as the sole catalyst and benzyl chloride as the oxidant.

Tandem oxidative processes catalyzed by polymer-incarcerated multimetallic nanoclusters with molecular oxygen

Heterogeneous catalysis and one-pot tandem reactions are key for efficient and practical organic syntheses and for green and sustainable chemistry. Heterogeneous catalysts can be recovered and reused. These catalysts can be applied to efficient systems, such as continuous-flow systems. Tandem reactions often proceed via highly reactive but unstable intermediates. Tandem reactions do not require workup or much purification of the intermediate. This Account summarizes recent developments that we have made in the field of multifunctional heterogeneous metal nanocluster catalysts for use in tandem reactions based on aerobic oxidation reactions as key processes. We constructed our heterogeneous metal nanoclusters via two important procedures--microencapsulation and cross-linking--using polystyrene-based copolymers with cross-linking moieties. These frameworks can efficiently stabilize small metal nanoclusters to maintain high catalytic activity without aggregation and leaching of nanoclusters. Aggregation and leaching are prevented by weak but multiple interactions between metal nanocluster surfaces and benzene rings in the copolymer as well as by the physical envelopment of cross-linked polymer backbones. Small nanoclusters, including multimetallic alloy clusters (nanoalloys), can be "imprisoned" into these cross-linked polymer composites. The term we use for these processes is polymer incarceration. Direct oxidative esterifications were achieved with polymer-incarcerated (PI) Au nanocluster catalysts. Amides were synthesized from alcohols and amines under aerobic oxidative conditions with PI bimetallic nanocluster catalysts composed of Au and Fe-group metals that formed separated nanoclusters rather than alloys. Oxidative lactam formation from amino alcohols was also achieved. On the other hand, imines could be prepared selectively from alcohols and amines with PI Au-Pd bimetallic nanoclusters. We also achieved the integration of the aerobic oxidation of allylic alcohols and the following Michael reaction catalyzed by trimetallic PI catalysts containing Au-Pd alloy nanoclusters and tetraalkoxyborates as cross-linkers. All of these heterogeneous catalysts could be recovered by simple operations and reused without significant loss of activity or any leaching of metals. We have demonstrated that the polymer incarceration method enables the simultaneous immobilization of several metals, with which we can achieve one-pot tandem oxidative processes using molecular oxygen as an oxidant within the multifunctional heterogeneous catalysts. Suitable choices of metals and bimetallic structures are crucial for the reactivity and the selection of reaction pathways.

Predicting gold-mediated catalytic oxidative-coupling reactions from single crystal studies

Though metallic gold is chemically inert under ambient conditions, its surface is extremely reactive and selective for many key oxidative chemical transformations when activated by atomic oxygen. A molecular-level understanding of the mechanism of these processes could allow researchers to design "green" catalytic processes mediated by gold-based materials. This Account focuses on the mechanistic framework for oxidative-coupling reactions established by fundamental studies on oxygen-activated Au(111) and the application of these principles to steady-state catalytic conditions. We also discuss the importance of the paradigms discovered both for predicting new oxidative-coupling reactions and for understanding existing literature. The mechanistic framework for the oxidative coupling of alcohols on gold surfaces predicts that new oxidative-coupling reactions should occur between amines and aldehydes and amines and alcohols as well as through alcohol carbonylation. Adsorbed atomic oxygen on the gold surface facilitates the activation of the substrates, and nucleophilic attack and β-H elimination are the two fundamental reactions that propagate the versatile chemistry that ensues. In the self-coupling of primary alcohols, adsorbed atomic oxygen first activates the O-H bond in the hydroxyl group at ∼150 K, which forms the corresponding adsorbed alkoxy groups. The rate-limiting step of the self-coupling reaction is the β-H elimination reaction of alkoxy groups to form the corresponding aldehydes and occurs with an activation barrier of approximately 12 kcal/mol. The remaining alkoxy groups nucleophilically attack the electron-deficient aldehyde carbonyl carbon to yield the adsorbed "hemiacetal". This intermediate undergoes facile β-H elimination to produce the final coupling products, esters with twice the number of carbon atoms as the starting alcohols. This mechanistic insight suggests that cross-coupling occurs between alcohols and aldehydes, based on the logic that the nucleophilic reaction should be independent of the origin of the aldehydes, whether formed in situ or introduced externally. As a further example, adsorbed amides, formed from deprotonation of amines by atomic oxygen, can also attack aldehydes nucleophilically to yield the corresponding amides. Our mechanistic framework can also explain more elaborate gold-mediated chemistry, such as a unique carbonylation reaction via two subsequent nucleophilic attacks. These model studies on well-defined Au(111) at low pressure predict steady-state catalytic behavior on nanoporous gold under practical conditions. The fundamental principles of this research can also explain many other oxygen-assisted gold-mediated reactions observed under ambient conditions.

Direct oxidative esterification of alcohols

Recent progress in the direct esterification of alcohols from readily available materials under oxidative conditions is discussed in this perspective.

Discovery of a metalloenzyme-like cooperative catalytic system of metal nanoclusters and catechol derivatives for the aerobic oxidation of amines

We have discovered a new class of cooperative catalytic system, consisting of heterogeneous polymer-immobilized bimetallic Pt/Ir alloyed nanoclusters (NCs) and 4-tert-butylcatechol, for the aerobic oxidation of amines to imines under ambient conditions. After optimization, the desired imines were obtained in good to excellent yields with broad substrate scope. The reaction rate was determined to be first-order with respect to the substrate and catechol and zero-order for the alloyed Pt/Ir NC catalyst. Control studies revealed that both the heterogeneous NC catalyst and 4-tert-butylcatechol are essential and act cooperatively to facilitate the aerobic oxidation under mild conditions.

Potential of Gold Nanoparticles for Oxidation in Fine Chemical Synthesis

In recent years supported gold nanoparticles have emerged as efficient catalysts with considerable synthetic potential for liquid-phase oxidation reactions based on molecular oxygen as oxidant. Here we critically review the most attractive applications related to the selective oxidation of functional groups containing O, N, or Si heteroatoms. The reactions include the oxidation of alcohols, aldehydes, and organosilanes; the diverse transformations of amines; benzylic oxidations; and some one-pot multistep reactions starting with alcohol or amine oxidation. In complex liquid-phase transformations relying on bifunctional catalysis, appropriate choice of the support is frequently more important than the size of the gold particles. In some oxidation reactions gold nanoparticles outperform the traditional platinum-group metal catalysts, but the latest results indicate the superiority of bimetallic particles containing gold and platinum, palladium, or rhodium. The environmentally benign nature of the transformations is discussed.

Oxidative transformation of N-substituted 2-aminophenols to 2-substituted benzoxazoles catalyzed by polymer-incarcerated and carbon-stabilized platinum nanoclusters

The preparation of 2-substituted benzoxazoles was achieved through a sequential aerobic oxidation – enolization – oxidative cyclization reaction of N-substituted 2-aminophenols catalyzed by platinum nanoclusters supported on a polymer / carbon black composite material.