DOI: 10.1002/chem.201301787 A Survey of Sulfide Ligands for Allylic C À H Oxidations of Terminal Olefins Chi “Chip” Le, Kamala Kunchithapatham, William H. Henderson, Christopher T. Check, and James P. Stambuli* [a] The allylic C ÀH oxidation of terminal olefins with palladi- um catalysts is a useful method to functionalize small mole- cules. [1] The recent use of catalytic palladium is an advance- ment over earlier allylic oxidations that employed selenium or mercury. [2] Although great strides have been made to reduce the quantity of metal used in the initial catalytic re- actions, there are still many unmet challenges that lessen the utility of this chemistry: high catalyst loading, regio-, diaster- eo- and enantioselectivity, low reaction yields, and function- al group tolerance. Rappoport and co-workers originally reported the use of stoichiometric PdACHTUNGTRENNUNG(OAc) 2 with terminal alkenes in acetic acid to yield the vinylic acetate as the major product, or the allyl- ic acetate as the major product when DMSO was employed as a co-solvent. [3] The use of DMSO demonstrated the abili- ty of weakly coordinating ancillary ligands to control the product formation and regioselectivity of the allylic acetate (Scheme 1). Following Rappoports work, several groups have combined various ligands with palladium salts to gen- erate catalysts that promote allylic oxidation reactions. Many different classes of ligands have been employed in- cluding: sulfides, [4] sulfoxides, [5] pincer ligands, [6] and aromat- ic amines [7] (Scheme 2). Despite the great advances made since the first report of palladium-promoted allylic oxidation reactions, an ideal catalyst for this transformation is still un- discovered. Although sulfoxides are well known ligands for transiton metals, [8] sulfides are less prevalent. [9] Our group reported the first examples of allylic oxidation reactions of terminal olefins using only 5 mol % of a catalyst consisting of Pd- ACHTUNGTRENNUNG(OAc) 2 and ligand 4. [4] In that same paper, we reported the first palladium-catalyzed allylic oxidation reactions of termi- nal alkenes with oxygen as the super stoichiometric oxidant. The success of our unusual palladium catalyst prompted us to survey a library of sulfide ligands in order to improve re- action yields and selectivities, and better understand what properties of the sulfide ligand allow an active and selective catalyst. This information could then be utilized to rationally design the next generation palladium–sulfide catalysts for al- lylic oxidation reactions. The sulfide ligands screens were conducted using 1-dode- cene (7) as the test substrate. Our novel mixed sulfide ether ligand, 4, yielded 52 % of the linear allylic acetate under our previously optimized reaction conditions : 5 mol % Pd- ACHTUNGTRENNUNG(OAc) 2 , 5 mol % ligand, and 2 equivalents of BQ (benzoqui- none) in AcOH with dodecene (Table 1, entry 1). To probe the importance of the oxygen atom in 4, the ether was re- placed with a methylene linker. (Table 1, entries 2–4) show that the oxygen atom in ligand 4 is not required for activity. Furthermore, the identity of the alkyl group did not drasti- cally change the outcome of the reaction until the trifluoro- methylethyl group (14) was added to the ligand scaffold (7 % yield, entry 5). The decreased conversion when com- bining ligand 14 with PdACHTUNGTRENNUNG(OAc) 2 is likely caused by the de- creased coordination of the less basic ligand to the palladi- um center. The decreased coordination of the external sul- fide ligand would cause the palladium acetate and other generated palladium salts within the reaction, to precipitate from solution at a greater rate than in the presence of a better coordinating ligand. Methylphenyl sulfide provided [a] C. Le, K. Kunchithapatham, W.H. Henderson, C. T. Check, Prof. J.P. Stambuli Department of Chemistry and Biochemistry The Ohio State University Evans Laboratory, 100 W. 18th Avenue, Columbus, OH 43210 (USA) Fax: (+ 1) 614-292-1685 E-mail: stambuli@chemistry.ohio-state.edu Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201301787. Scheme 1. The effect of DMSO on allylic oxidations of terminal olefins reported by Rappoport. Scheme 2. Ligands previously employed in the allylic oxidation of termi- nal olefins. Chem. Eur. J. 2013, 19, 11153 – 11157  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 11153 COMMUNICATION