Current Organic Chemistry, 2008, 12, 257-261 257 1385-2728/08 $55.00+.00 © 2008 Bentham Science Publishers Ltd. Sol-Gel Entrapped TPAP: An Off-the-Shelf Catalyst Set for the Clean Oxidation of Alcohols Rosaria Ciriminna a , Massimo Carraro b , Sandro Campestrini b and Mario Pagliaro a,c* a Istituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153, 90146 Palermo, Italy; b Università di Padova, Dipartimento di Scienze Chimiche and Istituto per la Tecnologia delle Membrane, CNR, via Marzolo 1, 35131 Padova, Italy; c Institute for Scientific Methodology, via U. La Malfa 153, 90146 Palermo, Italy Abstract: Whether in supercritical CO 2 or in organic solvent and using O 2 or aqueous H 2 O 2 as primary oxidants, the se- ries 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 polymer- supported analogues. This report summarizes recent findings showing the large potential of this technology for synthetic chemistry. 1. INTRODUCTION Ruthenium is an highly selective and versatile metal cata- lyst for the oxidation of alcohols [1]. In particular, the VII species tetra-n-propylammonium perruthenate (TPAP) either dissolved in solution or immobilized on polymer support is a very efficient and selective catalyst for the aerial oxidation of primary, secondary, benzyl and alkyl alcohols in non- aqueous media, including dense-phase carbon dioxide. Most functional groups such as carbon-carbon double bond, ep- oxy, indole, acetal, remain unaffected in the TPAP/O 2 con- version of alcohols which eventually are transformed into valued carbonyls [2]. However, after reaction the spent cata- lyst is isolated as a black colloidal suspension of RuO 2 which cannot be recycled. This limitates the commercial applica- tion of expensive TPAP-based catalysis only to those indus- trial processes where an extremely high degree of selectivity is needed such as in the pharmaceutical industry. In such cases, a solid heterogenized version of the catalyst is used to avoid product contamination and speed up product isolation from reactants [3]. In general, heterogenization of TPAP on commercial supports (Fig. 1) adds significant financial value [4]. For catalysis to achieve its full potential in the fine chemicals and pharmaceutical industries, where only about 10% of all productions currently use catalysis, heterogenous processes are to be developed in which reduction of waste, increased selectivity and shortened synthetic routes go along with process robustness and catalyst stability. Remarkably, in the last few years a number of selective and clean catalytic processes for alcohols oxidations have been developed that are being increasingly adopted by indus- try [5]. Among these, the series of sol-gel entrapped TPAP catalysts are versatile mediators suitable for the selective oxidation of alcohols to carbonyls with oxygen or aqueous H 2 O 2 as terminal oxidants. These materials and related oxi- *Address correspondence to this author at the Istituto per lo Studio dei Materiali Nanostrutturati, CNR, via U. La Malfa 153, 90146 Palermo, Italy; Tel +39 091 680 93 70; Fax +39 091 680 92 47; E-mail: mario.pagliaro@ismn.cnr.it Fig. (1). Supported perruthenate commercial catalysts: silica- supported (a) and polymer-supported (b) such as in FibreCat. dative processes are of urgent commercial interest to drugs, perfumery and fragrance manufacturers commercializing highly pure products (devoid of organic solvents and metals) to prevent drug contamination and alteration of olfactory test screenings. In general, the selectivity and the activity of en- trapped TPAP materials are higher than for homogenoeus perruthenate, i.e., the solid surface in these hybrid organic- inorganic catalysts participates actively in the reaction mechanism. Reactions can be carried out in conventional organic solvent with O 2 [6] or with H 2 O 2 [7] or aerobically in dense phase carbon dioxide [8] affording in all cases high yields of aldehydes or ketones with pronounced selectivity. We summarize here the achievements in each of these re- action media, and discuss how the versatility of the sol-gel entrapment process allows active site optimization affording a true off-the-shelf catalytic series suitable for application in diverse environments. 2. AEROBIC OXIDATION IN ORGANIC SOLVENT Sol-gel nanohybrid silica particles organically modified and doped with TPAP are highly efficient catalysts for the selective oxidation of alcohols to carbonyls with O 2 at low pressure (1 bar) in toluene [6]. The materials are easily pre- pared by a one-step sol-gel process and their catalytic per-