DOI: 10.1002/adsc.200600394 Anaerobic Palladium-Catalyzed Chemoselective Oxidation of Allylic and Benzylic Alcohols with a-Bromo Sulfoxide as a Co-oxidant Nuria Rodríguez, a Mercedes Medio-Simón, a and Gregorio Asensio a, * a Departamento de Química Orgµnica, Universidad de Valencia, Avenida Vicent AndrØs EstellØs s/n, 46100 Burjassot, Valencia, Spain Fax: (+ 34)-96-354-4939; e-mail: gregorio.asensio@uv.es Received: August 3, 2006 Dedicated to Prof. M. Yus on the occasion of his 60th birthday. Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/. Abstract: A chemoselective palladium-catalyzed anaerobic oxidation of allylic and benzylic alcohols using an a-bromo sulfoxide as a co-oxidant is de- scribed for the first time. The catalyst system is simple and has a long life because of the allowance of phosphane ligands under the non-aerobic condi- tions. The advantages of the described method in- clude no overoxidation of primary alcohols to car- boxylic acids because of the mild conditions applied, the tolerance of oxygen-sensitive functionalities such as a carbon-carbon double bond, an organothio group, or a diorganoamino group and the effective preparation of a,b-unsaturated aldehydes and ke- tones, resulting from the oxidation of primary and secondary allylic alcohols, since a competitive Heck reaction with the co-oxidant does not occur. Keywords: alcohols; homogeneous catalysis; oxida- tion; palladium; sulfur Introduction The oxidation of alcohols to aldehydes and ketones is one of the most important functional group transfor- mations in organic synthesis. [1] Although many re- agents are available to accomplish this key reaction, most of them are required in stoichiometric quantities and are either toxic, hazardous or both. To circum- vent these problems, catalytic oxidation processes based on the combination of a salt of a metal such as V, Mo, Ru, Co, Cu, Ni or Pd and stoichiometric oxi- dants have been devised. [2] In this context, the devel- opment of procedures that use molecular oxygen as the sole re-oxidant has been extensively pursued for economic and ecological reasons. [3,4] However, there are some persistent problems: a) the stability of metal ligands under aerobic oxidation conditions and b) metal aggregation and precipitation, which cause cat- alyst decomposition with a considerable loss of cata- lytic activity that occurs mainly under ligand-free con- ditions. This problem is particularly important in the case of Pd. So, although Pd is perhaps one of the most active and versatile transition metals, it has re- ceived comparatively little attention in oxidation chemistry, probably because phosphanes, which are standard palladium ligands, are incompatible with aerobic oxidation conditions. Nitrogen ligands have been used with relative success, but the reaction con- ditions must be properly fixed to avoid the easy for- mation of Pd black. These key conditions are: a suffi- cient oxygen partial pressure (typically 1 atm molecu- lar oxygen) and a low molar ratio of substrate to cata- lyst (S/C: typically 20 or less). To circumvent these limitations, recent effort has been focused on the preparation of dendrimers as palladium ligands. [5] An- other important point that is associated with the diffi- culty of aerobic oxidation is the restrictions in the type of alcohol that can be oxidized. In fact, alcohols with oxygen-sensitive functionalities such as C = C, À SR and À NR 2 are not suitable substrates. An alter- native to the aerobic homogeneous transition metal oxidation of alcohols is the anaerobic version. [6] In this alternative, re-oxidation of the transition metal is generally based on a non-O-containing compound. Halogen-based co-oxidants have been used for this purpose. This approach based in the oxidation of Pd(0) by halogenated compounds has been applied not only in the oxidation of alcohols but also in other process such as the homocoupling of alkynes [7] and boronic acids [8] to give diynes and symmetrical biaryls, Adv.Synth.Catal. 2007, 349,987–991 # 2007 Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim 987 FULL PAPERS