DOI: 10.1002/cctc.201100239 Microwave-Assisted Meyer–Schuster Rearrangement of Propargylic Alcohols Catalyzed by the Oxovanadate Complex [V(O)Cl(OEt) 2 ] Antonio AntiÇolo, [a] Fernando Carrillo-Hermosilla,* [a] Victorio Cadierno, [b] Joaquín García- lvarez,* [b] and Antonio Otero [a] Introduction a,b-Unsaturated carbonyl compounds are useful building blocks in organic synthesis, advanced intermediates in the manufacture of aromas and fragrances, and key structural units in a large number of biologically active natural prod- ucts. [1] Such species are usually obtained by using aldol- or Knoevenagel-type condensation reactions and by using Wittig and Horner–Wadsworth–Emmons olefination processes. [1, 2] However, in addition to their low atom economy, [3] these classi- cal methods generally require strongly basic media and, there- fore, functional group compatibility and selectivity issues can be problematic. An alternative method of synthesis is the iso- merization of propargylic alcohols through a formal 1,3-shift of the hydroxyl moiety, which is known as the Meyer–Schuster rearrangement (Scheme 1). [4] However, despite the accessibility of the starting materials, which can be easily generated by simple addition of metallat- ed alkynes to aldehydes or ketones, and its complete atom economy, the Meyer–Schuster rearrangement has remained largely forgotten. This is attributable to the low selectivities often shown by the traditional protocols, which are based on the use of strong Brønsted acids under harsh reaction condi- tions. [5] In particular, starting from substrates able to undergo a competitive Rupe-type rearrangement (Scheme 2), [6] non-regio- selective transformations are usually observed. With the gradual emergence of new synthetic approaches based on the use of metal catalysts, more selective and effi- cient under milder reaction conditions, [7, 8] the Meyer–Schuster rearrangement has begun to play a more prominent role within the toolbox of synthetic organic chemists. In particular, remarkable results have been recently reported by several groups in tandem processes that involved the combination of this isomerization reaction with condensation, [9] cycloconden- sation, [10] asymmetric hydrosilylation, [11] Nazarov-type electro- cyclization, [12] Michael-type addition, [13] oxirane ring-opening, [14] and allylic alkylation [15] reactions, all of them allowing rapid access to elaborated structures from readily available propar- gylic alcohols. [16] Clearly, the inclusion of this textbook transfor- mation in future research programs is highly dependent on the availability of simple and effective catalytic systems, an area of research open for many opportunities of new discoveries. Metal oxides and oxo complexes are among the most popu- lar catalysts employed for the isomerization of alkynols be- cause they show a remarkable selectivity toward the Meyer– Schuster as opposed to the Rupe reaction. [7] This selectivity stems from the mechanism of action of these catalysts, which A general and efficient procedure for the selective Meyer– Schuster isomerization of both, terminal and internal alkynols, has been developed by using catalytic amounts of the readily accessible oxovanadium(V) complex [V(O)Cl(OEt) 2 ]. Reactions proceeded smoothly in toluene at 80 8C under microwave irra- diation to provide the corresponding a,b-unsaturated carbonyl compounds in excellent yields and short times without the assistance of any additive. Scheme 1. The Meyer–Schuster rearrangement of propargylic alcohols. Scheme 2. The Rupe rearrangement of propargylic alcohols. [a] Prof. Dr. A. AntiÇolo, Dr. F. Carrillo-Hermosilla, Prof. Dr. A. Otero Departamento de Química Inorgµnica, Orgµnica y Bioquímica Facultad de Ciencias Químicas, Campus de Ciudad Real Universidad de Castilla-La Mancha Campus Universitario, 13071 Ciudad Real (Spain) Fax: (+ 34) 926-295-318 E-mail : Fernando.Carrillo@uclm.es [b] Dr. V. Cadierno, Dr. J. García-lvarez Departamento de Química Orgµnica e Inorgµnica IUQOEM (Unidad asociada al CSIC) Universidad de Oviedo Juliµn Claveria 8, 33006 Oviedo (Spain) Fax: (+ 34) 985-103-446 E-mail : garciajoaquin@uniovi.es Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cctc.201100239. ChemCatChem 2012, 4, 123 – 128  2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 123