“This document is the Accepted Manuscript version of a Published Work that appeared in final form in Org. Lett. 2016, 18, 6042-6045. copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see DOI: 10.1021/acs.orglett.6b02981. This article may be used for non-commercial purposes in accordance with the ACS guidelines published at http://pubs.acs.org/page/policy/articlesonrequest/index.html].” Palladium-Catalyzed Stereoselective Formation of Substituted Allylic Thioethers and Sulfones José Enrique Gómez,† Wusheng Guo,*,† and Arjan W. Kleij*,†,§ † Institute of Chemical Research of Catalonia (ICIQ), the Barcelona Institute of Science and Technology, Av. Països Catalans 16, 43007 – Tarragona, Spain § Catalan Institute of Research and Advanced Studies (ICREA), Pg. Lluís Companys 23, 08010 – Barcelona, Spain Supporting Information Placeholder ABSTRACT: A general method is reported for the stereoselective preparation of highly functionalized allylic thioethers. This protocol is based on a Pd-catalyzed thiolation of modular vinyl cyclic carbonate substrates and features high (Z)-selectivity, good yields, minimal waste, ample product scope and operational simplicity. A one-pot strategy was used towards the stereoselective formation of pharma-relevant allylic sulfones derived from their in situ prepared thioether precursors. Organosulfur compounds such as thioethers 1 and sulfones2 are very important building blocks in synthetic and pharmaceutical chemistry, and their derivatives were representative in around 20% of the best-selling US pharmaceuticals in 2012.3 As a subclass of sulfur-containing compounds, a number of allylic thioethers have been found to be bioactive4 and important reaction intermediates,5 and continue to attract the interest from various synthetic communities.6,7 Effective methodologies for the synthesis of allylic thioether/sulfones and related compounds include allylic substitution,8 hydrothiolation of allenes7,9 or alkynes,10 and CH bond functionalization.11 Apart from these catalytic methodologies, also stoichiometric approaches have proven to be effective in this context. 12 Despite the impressive progress noted in this area, general catalytic methodology for the stereoselective synthesis of highly substituted allylic thioethers (see structure in the blue box of Scheme 1) remains underdeveloped.13 A recent example of (Z)-selective allylic thioether and sulfone synthesis was reported by Breit et al. using hydrothiolation of allenes under Rh catalysis (Scheme 1, a),7 but this approach is limited in substitution diversity around the allylic double bond. A stereoselective construction of multi-functionalized olefin scaffolds represents a rather challenging task in synthetic chemistry.14 Our group recently developed a Pd-catalyzed stereoselective methodology for the decarboxylative functionalization of vinyl carbonates towards synthetically useful, highly substituted (Z)configured allylic amines and 1,4-but-2-ene-diols.15 Key to the high (Z) selectivity found in these transformations was the in situ generation of a six-membered palladacycle revealed by DFT calculations (Scheme 1, b).15b We envisaged that, under suitable reaction conditions, the reaction of modular vinyl carbonates and thiol nucleophiles would give access to (Z)-allylic thioethers through nucleophilic attack onto the in situ formed Pd-intermediate. Such a manifold would thus offer a practical route towards the challenging stereoselective synthesis of triand even tetra-substituted allylic thioethers and sulfones (upon oxidation) from simple and accessible precursors (Scheme 1, b). (Allylic) sulfone scaffolds are frequently observed in relevant pharmaceutical compounds16 and thus their synthesis is of significant importance.17 Scheme 1. Different Strategies for Catalytic Stereoselective Synthesis of Allylic Thioethers and Sulfones We started our investigations using vinyl carbonate A (Table 1) and thiophenol as a benchmark reaction. Inspired by our previous research,15 the combination of the White catalyst precursor (2.0 mol %) and bidentate phosphine L1 (DPEPhos, 5.0 mol %) was first examined at rt (Table 1, entries 1-5). Unfortunately, in different solvents no reaction was observed (DMF, THF, MeOH and CH3CN). To our delight, when Pd(dba)2 in CH3CN was used, a 16% yield of allylic thioether 1a (Z/E = 86:14) was noted (Table 1, entry 6). Increasing the temperature (Table 1, entries 8-9) gave significantly improved yield of 1a of up to 84% and higher selectivity (Z/E = 92:8). 1, entry 10). It is worth noting that no special precautions or base additives18 were required making the present protocol highly attractive from a practical point of view. Table 2. Investigated Scope in Thiol Partners to Produce Allylic Thioethers 1a-1l.a Table 1. Screening Data for the Optimization of the Reaction Conditions towards Allylic Thioether 1a.a entry R product yield (%)b Z/E 0 0 - 3 “White” L1, THF rt 0 - 4 “White” L1, CH3CN rt 0 - 5 “White” L1, MeOH rt 0 - 6 Pd(dba)2 L1, CH3CN rt 16 Pd(OAc)2 L1, CH3CN rt 0 - 8 Pd(dba)2 L1, CH3CN 50 77 91:9 83:17 4-Br-C6H4 1e 76 80:20 3-Me-C6H4 1f 95 90:10 4-MeO-C6H4 1g 83 93:7 4-COOH-C6H4 1h 98 91:9 2-pyridyl 1i 93 >99:1 4-HO-C6H4 1j 87 91:9 Adamantyl 1k 98 >99:1 12c Benzyl 1l 50 92:8 86:14 7 70 11c - rt 1d 10c rt 92:8 4-CF3-C6H4 9 L1, none 93 8c L1, DMF 1c 7 “White” 92:8 2-Me-C6H4 6 “White” 94:6 73 5c Z:Ec 90 1b 4c yield (%)b 1a 4-Me-C6H4 3 t (o C) C6H5 2 L/solv 1 entry [Pd] 1 2 9 Pd(dba)2 L1, CH3CN 70 84 92:8 10d Pd(dba)2 L1, CH3CN 70 91 94:6 11 Pd(dba)2 L2, CH3CN 70 83 94:6 12 Pd(dba)2 L3, CH3CN 70 0 - 13 Pd(dba)2 L4, CH3CN 70 0 - 14 Pd(dba)2 L5, CH3CN 70 0 - 15 Pd(dba)2 L6, CH3CN 70 0 - 16 Pd(dba)2 L7, CH3CN 70 0 - 17e Pd(dba)2 L1, CH3CN 70 73 90:10 18f Pd(dba)2 L1, CH3CN 70 92 91:9 a Reaction conditions unless otherwise stated: carbonate substrate (0.20 mmol), thiophenol (1.5 equiv), solvent (0.20 mL), catalyst (2.0 mol %), L (5.0 mol %), open to air, 12 h. bNMR yield using toluene as an internal standard. cBased on 1H NMR integration. d[Pd] = 3.0 mol %. e[Pd] = 3.0 mol %, thiophenol (1.2 equiv). f[Pd] = 3.0 mol %, thiophenol (0.20 mmol) and carbonate substrate (0.22 mmol). The catalysis was further enhanced with a higher Pd loading (3.0 mol %; 91% yield, Z/E = 94:6: Table 1, entry 10). Other phosphine ligands L2‒L7 were also tested but proved to be less productive (Table 1, entries 11-16), and a lower thiophenol amount also resulted in erosion of the yield of 1a (1.2 equiv, Table 1, entry 17). Under the optimized conditions, the use of excess of carbonate gave fairly similar results (Table 1, entry 18; 92% yield, Z/E = 91:9). Thus, the best conditions towards the formation of allylic thioether (Z)-1a were the use of Pd(dba)2 (3.0 mol %), L1 (5.0 mol %) in CH3CN at 70 oC (Table a Reaction conditions unless otherwise stated: carbonate substrate (0.20 mmol), thiol (1.5 equiv), CH3CN (0.20 mL), Pd(dba)2 (3.0 mol %), L1 (5.0 mol %), 70 oC, 12 h; bIsolated yield. cThiol (0.20 mmol), carbonate substrate (0.22 mmol). With the optimized conditions in hand, we then systematically varied the nature of both reaction partners, and first the scope in thiols was examined (Table 2). Generally, the decarboxylative thiolation approach proceeded with high stereoselectivity providing the allylic thioethers 1a-1l in good yields and Z/E ratios typically being >90:10. Both the presence of electron–withdrawing (1d, 1e, 1h and 1i) and –donating groups (1b, 1c, 1f, 1g, 1j) in the aryl thiols is tolerated, and para- meta- and ortho-substitutions (Table 2, entries 2, 3 and 6) are endorsed. Deactivated thiophenols also showed sufficient reactivity (cf., synthesis of 1d and 1h). Variation of the thiol partners allows to include synthetically useful aryl-halides (1e), benzoic acid (1h), pyridyl (1i) and phenoxy (1j) groups. Apart from aryl thiols, alkyl thiols also proved to be feasible substrates leading to their respective allylic thioethers in high stereoselectivty (1k and 1l, Z/E > 92:8). In some cases, the catalytic procedure was optimized by using a slight excess of vinyl carbonate substrate. The (Z) configuration of the major isomer in all cases was supported by 1H NOESY (Supporting Information, SI), and for 1a also by X-ray analysis (see SI). Subsequently, the vinyl carbonate reaction partner was varied (Table 3) giving access to a wider range of highly functionalized allylic thioethers 2a2l in moderate to excellent yields (up to 97%) with high stereocontrol in most cases (Z/E ratios of at least >80:20). Both aryl and alkyl substituents in the carbonate reagent (Table 3, “R”) were tolerated. Different functionalities such as benzoic esters, aryl nitrile and furyl groups (2e, 2f and 2i) can be readily introduced further amplifying the product diversity. Upon using more sterically demanding vinyl carbonates that incorporate naphthyl- or cyclohexyl substituents, the catalytic procedure was less productive (2g and 2j; 51% and 48% yield, respectively). In the case of product 2l we observed the formation of a branched allylic thioether which affected the yield of the linear derivative (see SI for details). The observed lower Z/E ratios observed for 2f and 2k (Table 3) may be the result of the oxa-palladacycle (see Scheme 1b) being in equilibrium with a non-cyclic intermediate at 70 ºC. The carbonate Rsubstituent in the acyclic species can affect the olefin geometry prior to attack of the thiol nucleophile through possible  alkene isomerization.19 A higher tendency to form such an acyclic Pdintermediate causes loss in stereocontrol. (Z/E = 94:6).20 This one-pot strategy was then utilized to prepare different, highly functionalized sulfones (Table 5, entries 2-8). Gratifyingly the oxidation step did not interfere with the presence of other (functional) groups under these conditions including benzyl (4d), pyridyl (4e), methylester (4f), halide (4g) and in all cases primary alcohol and internal alkene groups. The same level of stereoselectivity was found in the sulfone synthesis, and generally good to excellent yields for 4a‒4h were obtained. X-ray analysis of 4a unambiguously confirmed the (Z)configuration of this tri-substituted allylic sulfone (see SI). Table 4. Preparation of Elusive Tetra-Substituted Thioethers 3a-3da Table 3. Investigated Scope in Vinyl Carbonate Partners to Produce Allylic Thioethers 2a-2la entry R1 1 2 product yield (%)b Z/E Ph 3a 48 >99:1 Ph 3b 62 94:6 R2 R3 p-Me-C6H4 Ph Ph Me entry R product yield (%)b Z/E 1 4-Me-C6H4 2a 70 94:6 3 Ph Me 4-MeO-C6H4 3c 65 98:2 2 4-F-C6H4 2b 84 83:17 4 Ph Me 2-Me-C6H4 3d 37 90:10 3 4-Br-C6H4 2c 88 87:13 4 4-Ph-C6H4 2d 97 94:6 5 4-CO2Me-C6H4 2e 80 84:16 6 4-CN-C6H4 2f 91 67:33 7 2-naphthyl 2g 51 89:11 8 3-Cl-C6H4 2h 71 80:20 9 2-furyl 2i 99 Cy 2j 48 C10H21 2k 87 70:30 12 Me 2l 40d Table 5. One-Pot Synthesis of Highly Substituted Sulfonesa 84:16 11c Reaction conditions unless otherwise stated: carbonate substrate (0.20 mmol), thiophenol (1.5 equiv), CH3CN (0.20 mL), Pd(dba)2 (3.0 mol %), L1 (5.0 mol %), 70 oC, 12 h. bIsolated yield. >99:1 10c a 80:20 a Reaction conditions unless otherwise stated: carbonate substrate (0.20 mmol), thiophenol (1.5 equiv), CH3CN (0.20 mL), Pd(dba)2 (3.0 mol %), L1 (5.0 mol %), 70 oC, 12 h. bIsolated yield. cThiophenol (0.20 mmol), carbonate substrate (0.22 mmol). dBranched product (30%) also formed. R1 1 Ph 2 In order to further challenge the newly developed catalytic protocol for stereoselective allylic thioether formation, the synthesis of elusive tetra-substituted derivatives was attempted and the results are listed in Table 4. To our delight, the installation of both alkyl- and aryl-substituents (R2 = Ph, Me) at the position of the allylic scaffold is feasible while maintaining excellent stereoselectiviy (Table 4, entries 1 and 2: 3a and 3b, Z/E > 94:6). However, the preparation of functionalized allylic thioethers, derived from internal olefin substrates, failed under these conditions probably because of steric reasons (see SI for details). Other combinations of R1 and R2 were then also probed (cf., synthesis of 3c and 3d) and gave the targeted allylic thioethers in high to excellent stereoselectivity. We then set out to develop a one-pot strategy towards the stereoselective synthesis of highly substituted allylic sulfones by combining the Pd-catalyzed allylic thioether formation and in situ oxidation. Various conditions were tested using vinyl carbonate A and thiophenol as a model reaction (see SI for more details). We were pleased to find that a combination of (NH4)6Mo7O214H2O and H2O2 (after initial and in situ formation of allylic thioether 1a) gave 90% isolated yield of sulfone product 4a (Table 5, entry 1) with excellent stereocontrol entry Ph 3 Ph R2 product yield (%)b Z/E Ph 4a 90 94:6 4-MeO-C6H4 4b 83 93:7 3-Me-C6H4 4c 89 90:10 4 Ph Benzyl 4d 48 91:9 5c Ph 2-pyridyl 4e 91 87:13 6 4-CO2Me-C6H4 Ph 4f 76 84:16 7 4-F-C6H4 Ph 4g 77 93:7 8 4-Me-C6H4 Ph 4h 72 93:7 a Reaction conditions: (i) carbonate substrate (0.20 mmol), thiol (1.5 equiv), CH3CN (0.20 mL), Pd(dba)2 (3.0 mol %), L1 (5.0 mol %), 70 oC, 12 h; (ii) (NH4)6Mo7O214H2O, H2O2 (4 equiv, 30 wt % in H2O), MeOH (0.20 mL), rt, 1 h; X-ray structure measured for sulfone (Z)-4a, see SI. bIsolated yield. c Thiol (0.20 mmol), carbonate substrate (0.22 mmol). In summary, we herein report a general catalytic method for the (Z)-selective preparation of a diverse series of highly functionalized and substituted allylic thioethers and sulfones. This methodology is based on a Pd-catalyzed decarboxylative functionalization of readily available and modular vinyl cyclic carbonates, and features minimal waste release, wide scope and operationally simplicity. Based on our previous mechanistic investigations,15b the (Z)-selectivity in the present catalytic protocol is ascribed to a nucleophilic attack of the thiol reagent onto an in situ generated (Z)-configured six-membered palladacycle that guides the stereoselective course of the developed process for the (Z) allylic thioethers.  ASSOCIATED CONTENT Supporting Information Experimental details and copies of relevant NMR and IR spectra for all new products, X-ray data in cif format. This material is available free of charge via the Internet at http://pubs.acs.org.  AUTHOR INFORMATION Corresponding Author * wguo@iciq.es * akleij@iciq.es Notes The authors declare no competing financial interest.  ACKNOWLEDGMENT We thank ICIQ, ICREA, and the Spanish Ministerio de Economía y Competitividad (MINECO) through projects CTQ-2014–60419R, and the Severo Ochoa Excellence Accreditation 2014–2018 through project SEV-2013–0319. Eduardo C. Escudero-Adán and Dr. Eddy Martin (ICIQ)† are acknowledged for the X-ray analysis of compounds 1a and 4a. J.E.G. acknowledges MINECO and ICIQ for a Severo Ochoa/FPI pre-doctoral fellowship. W.G thanks the Cellex foundation for a postdoctoral fellowship.  REFERENCES (1) For general reviews: (a) Mellah, M.; Voituriez, A.; Schulz, E. Chem. Rev. 2007, 107, 5133. (b) Kondo, T.; Mitsudo, T. Chem. Rev. 2000, 100, 3205. (c) Beletskaya, I. P.; Ananikov, V. P. Chem. Rev. 2011, 111, 1596. (d) Castarlenas, R.; Giuseppe, A. D.; Pérez-Torrente, J. 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