Current Organic Chemistry - Volume 12, Issue 3, 2008

Part II of the second special issue of Current Organic Chemistry dedicated to the topic of the use of heterogeneous catalysts in organic synthesis, again, brings together experts of a few selected, important fields to highlight recent major achievements. I am pleased that by these contributions we can maintain the diversity established in the previous volume. In the opening account Mohammad Ali Zolfigol (Bu-Ali Sina University, Hamadan) and his group of experts from Iran (Farhad Shirini, Guilan University, Rasht, Peyman Salehi, Shahid Beheshti University, Tehran, and Masoumeh Abedini, Bu-Ali Sina University, Hamadan) provides an up to date survey of the use of various metal hydrogen sulfates. These recyclable catalysts can be easily handled and have been applied in a broad range of organic reactions under mild conditions. Professor Said Sebti (University of Hassan II-Mohammedia, Casablanca, Morocco) and his team of experts from Marocco (Mohamed Zahouily, University of Hassan II-Mohammedia, Mohammedia, and Hassan B. Lazrek, University of Cadi Ayyad, Marrakech), Spain (Jose Antonio Mayoral, University of Zaragoza, Zaragoza), and the United Kingdom (Duncan J. Macquarrie, University of York, Heslington) give an account of the use of phosphates as heterogeneous catalysts with the main emphasis on natural phosphates. The review provides a detailed treatment about the wide array of organic transformations in which these new catalysts have successfully been applied. The contributors of the final chapter are Habib Firouzabadi (Shiraz University, Iran) and Abas Ali Jafari (Yazd University, Iran). They review recent applications of cesium and aluminum salts of dodecatungstophosphoric acid. These efficient and eco-friendly catalysts have been applied successfully in numerous organic transformations. It is the sincere hope of both the contributing authors and myself that the readers will find the second part of this second special issue about heterogeneous catalysis in organic synthesis interesting and informative.

Sodium, magnesium, ferric, zirconium and aluminum hydrogen sulfates have been used as efficient reagents in organic chemistry. A broad range of reactions including deprotection, oxidation, C-C, C-N and C-O bond formation and cleavage took place in the presence of these reagents under mild and completely heterogeneous reaction conditions in good to high yields. In many cases the reagents were recycled and reused.

This two part review looks at the uses of phosphate based heterogeneous catalysts for a range of synthetic organic reactions. Many different phosphates are known and have been tested as catalysts in a wide range of reaction types, either as catalysts, or in combination with other species such as zinc chloride or KF. The first section deals with the wide variety of phosphates which have been utilized in organic chemistry, while the second focuses more tightly on catalysts derived from naturally occurring phosphates. In all cases, efficient reactions can be achieved and in many reactions, significant solvent effects have been noted. Of benefit is the fact that relatively benign solvents such as lower alcohols and water are often the most effective.

In this review we have paid attention to cesium and aluminum salts of dodecatungstophosphoric acid as ecofriendly and efficient heterogeneous catalysts. The structural effect and also catalytic properties of exchanging Cs+ with protons in dodecatungstophosphoric acid is discussed. Using cesium and aluminum salts of dodecatungstophosphoric acid as efficient catalysts for Friedel-Crafts reactions such as acylation and alkylation of aromatic compounds, Fries, Claisen and pinacol rearrangements, esterification, amidation, ether formation, protection of thiols, alcohols and amines, conversion of aldehydes to their acylals, ring opening of epoxides, selective protection of one hydroxy group in diols, preparation of β-hydroxythioethers and regioselective bromination of aromatic compounds are discussed.

Whether in supercritical CO2 or in organic solvent and using O2 or aqueous H2O2 as primary oxidants, the series of sol-gel entrapped TPAP hybrid silica gels is a versatile class of oxidation catalysts suitable for the highly selective conversion of alcohols to carbonyls. These materials are generally more stable and active than traditional polymersupported analogues. This report summarizes recent findings showing the large potential of this technology for synthetic chemistry.