Current Catalysis - Volume 6, Issue 1, 2017

Background: The hetero-Diels-Alder (HDA) reaction is an important method for the construction of heterocyclic six-membered rings. The HDA reaction involving the carbonyl functionality as a dienophile or as a heterodiene makes dihydropyran ring accessible, which is present in many biologically active compounds. However, reactivity of the carbonyl group as a dienophile or as heterodiene is much less and for successful completion of the reaction, usually a catalyst is required. In view of this, there are always attempts afoot to devise more effective and specific catalysts for this purpose. Methods: The catalysts used for these reactions are generally of two types, namely the Lewis acid (LA) catalysts and organocatalysts. A large number of LA catalysts used in these reactions belong to the main group metals. However, some of these catalysts are the derivatives of the hard transition metals and a few lanthanide elements with a variety of ligands. Besides, in recent years a variety of organocatalysts based on different principles, such as enamine formation and hydrogen-bonding have been developed. Results: The use of a catalyst results in lowering of the activation energy barrier thus facilitating occurrence of the HDA reaction. Depending on the nature of the diene and the catalyst, the reaction may occur through a concerted or a stepwise mechanism. There are a few reports about the theoretical investigation of the mechanism. The catalysts have been used both homogeneously and heterogeneously grafted on polymers. The latter technique makes it possible to recycle the catalyst several times without losing its activity. The use of chiral catalysts makes it possible to obtain the cycloadducts in high enantiomeric excess (ee). In some cases, it has been possible to increase enantioselectivities by tuning the coordination sphere of the metallic ion. The use of organocatalysts has also been successful. The present article overviews the use of both types of the catalysts for the HDA reactions of the carbonyl compounds during the last 15 years. Conclusion: It is possible to accomplish HDA reactions with the carbonyl functionality successfully by the use of an appropriate catalyst, Lewis acid based catalyst or organocatalyst. By the use of chiral catalysts, asymmetric HDA reactions can be accomplished affording the products with high enantio- or diastereoselectivities

Background: Development of straightforward and atom/step-economic approaches for new C-C bond formation from readily available starting materials is one of the fundamental goals in the area of chemical synthesis. Among these strategies, Pd-catalyzed allylic alkylation represents one of the powerful synthetic tools and has been widely utilized in organic synthesis. The direct use of allylic alcohol itself instead of its derivatives, such as carbonates, amines, acetates, and halides, as π-allyl fragment source for Pd-catalyzed allylic alkylation presents higher atom-/step-economy and is much more practical and attractive. During the last decade, this topic of recognized importance has become an emerging area, and significant progresses have been achieved. On the other hand, the scope of carbon-nucleophiles was further enlarged that gives various useful allylic compounds through the powerful strategy. This review summarized recent advances in such research area. Methods: We searched a number of research literature from bibliographic databases and combined with our previous work on this topic. Then recent advances in such research area are summarized. Results: Despite the great efforts that have been undertaken in the last decade to improve and extend the nucleophilic allylic substitution reaction catalyzed by palladium, there are still some challenges that need to be addressed. Palladium-catalyzed activation of allylic alcohols without any additional activators represents one of these challenges and research studies are still in progress in this direction. Conclusion: New strategy for palladium-catalyzed nucleophilic allylic alkylation represents important contribution, and this review summarized recent advances in such research area.

Background: To alleviate the crisis from depletion in non-renewable fossil reserves, catalytic conversion of the most abundant and renewable lignocellulosic biomass to biofuels and value-added chemicals has received much attention. In this respect, γ-valerolactone (GVL) derived from carbohydrates has gained considerable attention, as it can be further converted to biofuels and value-added chemicals via different catalytic processes such as hydrogenation, ring-opening, decarboxylation, isomerization and oligomerization. This work aims to review the most relevant research in the catalytic upgrading of GVL into fuel additives, biofuels and various valuable chemicals. Focus is also placed on the corresponding reaction pathways and catalytic mechanisms. Methods: We searched peer-reviewed research literatures in Web of Science using focused review questions. In this review, focus was primarily put on the catalytic upgrading of GVL into biofuels and valuable chemicals, such as (i) 2-methyltetrahydrofuran and isooctane, (ii) valeric biofuels and 5-nonanone, (iii) liquid alkenes fuels, #136;iv#137;1,4-pentanediol and aromatic hydrocarbons, and#136;v#137;128;polymers. Meanwhile, the properties of GVL and the recent advances in the production of GVL from biomass and its derivatives were also described in the introduction section of this review. Results: Ninety-two papers were included in the review. A part of papers (i.e., 16) briefly outlined the properties of GVL and its wide range of potential applications. Twenty-seven papers summarized recent advances in the catalytic production of GVL from biomass and its derivatives. The majority of papers (49) described recent advances in catalytic upgrading of GVL into bio-fuels and valuable chemicals. Among them, ten papers depicted the upgrading of GVL to 2-methyltetrahydrofuran and isooctane, sixteen papers illustrated the conversion of GVL to valeric biofuels and 5-nonanone, seven papers described the transformation of GVL into liquid alkenes fuels, seven papers reviewed the catalytic production of 1,4-pentanediol and aromatic hydrocarbons from GVL, and the upgrading of GVL to polymers were described in nine papers. This review not only depicted the upgrading pathways of GVL but also focused on the corresponding catalytic mechanisms. Conclusion: The findings of this review confirm the importance of upgrading of GVL to bio-fuels and valuable chemicals. Meanwhile, the review suggests that designing efficient and multifunctional catalysts is crucial for various routes to upgrade GVL.

Background: The preparation of hierarchical Y is of practical importance for industrial applications. Here, we utilized NaY or NH4Y as parent material to investigate its influence on catalytic performance of hierarchical Y prepared by post-treatment method. It was found that the hierarchical Y obtained by treating NaY exhibited superior catalytic performance than that by NH4Y in the benzylation of aromatics with benzyl chloride and the acid-washing step after alkali-leaching could not help improve the catalytic performance. Methods: The techniques of XRD, N2 sorption, SEM, NH3-TPD, and XPS were used to characterize the textural properties of catalysts. The benzylation reactions of aromatics with benzyl chloride were used as probe reactions to evaluate the catalytic performance of samples. Results: NaY and NH4Y were used as parent materials to prepare hierarchical Y by performing a variety of acid and base treatments. The type of parent Y had a big influence on catalytic performance of the obtained hierarchical Y. The hierarchical Y prepared by using NaY exhibited better catalytic performance than that by using NH4Y in the benzylations. The catalyst MY-Na prepared without acid wash exhibited superior catalytic performance than MY-Na with acid wash and parent Y in the benzylation of aromatics with benzyl chloride. In the benzylation of benzene, BC conversion over MY-Na is nearly twice as that over HY, which could be mainly attributed to the improvement of mass transfer ability. In the case of p-xylene or mesitylene with relatively large molecular size, BC conversion over MY-Na is three times more than that over HY. The inaccessibility of acid sites in micropores should be partly responsible for the low catalytic activity of HY. Conclusion: A catalyst with good combination of accessible acid sites, mesopore volume and external surface area would give superior catalytic activity in the benzylation. Further, hierarchical Y could be reused and almost maintain the initial catalytic activity. It has been demonstrated that hierarchical Y has potential applications in Friedel-Crafts alkylations, especially of large molecules.

Background: TiO2 is one of the most promising photocatalysts in photoelectrocatalytic (PEC) water splitting due to its high chemical stability, photocorrosion resistance, nontoxicity and low cost. One dimensional TiO2 nanowire arrays are expected to provide a large surface area and a direct electrical pathway for rapid photogenerated electron transport. However, the photocatalytic performances of TiO2 nanowire arrays are largely limited by the surface charge recombination because of the large amount of surface defects. Methods: In this work, novel TiO2/Al2O3 core-shell nanowire arrays with conformal Al2O3 surface passivation layer were investigated for PEC water splitting. Rutile TiO2 nanowire arrays were successfully grown on carbon cloth by hydrothermal method, and atomic layer deposition (ALD) was used to deposit uniform Al2O3 layer on TiO2 nanowires. X-ray diffraction, transmission electron microscopy, scanning electron microscopy, photoelectrochemical measurements, PEC water splitting performance of TiO2/Al2O3 core-shell nanowire arrays were systematically investigated in this work. Results: Compared with that of pure TiO2 nanowire arrays, the photocurrent response of the TiO2/Al2O3 core-shell nanowire arrays is enhanced about 1.7 times under simulated solar light irradiation. And the PEC hydrogen production (bias potential=0.2 V) under simulated solar light irradiation presents 5.6 times enhancement on the TiO2 nanowire arrays after Al2O3 passivation coating. Conclusion: The improved performance of PEC hydrogen production of TiO2 is attributed to the reduction of surface recombination caused by the chemical passivation and field-effect passivation effect of Al2O3 passivation layer. The results provide a potential way to develop efficient photoelectrodes in PEC water splitting system.

Background: The simply prepared and characterized alkaline starch (AS) by a new co-grinding protocol represents a superior catalytic activity in two- or three-component synthesis of hydroquinazolinones from either 2-aminobenzonitrile, 2-aminobenzamide, or isatoic anhydride. All AS-catalyzed reactions occur in higher yield and faster than the earlier methodologies. These performances are due to the several basic sites and special polymeric structure of this green bioorganic base catalyst. Methods: The alkaline starch (AS) was prepared by a new method based on co-grinding of starch with solid NaOH at room temperature. AS has been characterized by SEM, FT-IR spectroscopy, XRF, and base capacity. The efficacy of AS catalyst has been examined in the multi-strategic synthesis of hydroquinazolinones in water or under solvent free conditions. Results: While AS consists of elements of C, H, O, and Na, it exhibits high base capacity, thermal stability, reusability, and high catalytic activity in more than 40 different sets of experiments for effective synthesis of hydroquinazolinones. Conclusion: The first co-grinding preparation and application of alkaline starch as a biocompatible, cost effective, and reusable base catalyst has been developed. AS is a superior catalyst for highly efficient synthesis of hydroquinazolinones from either 2-aminobenzonitrile, 2-aminobenzamide, or isatoic anhydride in water or under solvent-free conditions.