Homogeneous and Nanoparticle Gold-Catalyzed Hydrothiocyanation

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Cite This: Org. Lett. XXXX, XXX, XXX−XXX

Homogeneous and Nanoparticle Gold-Catalyzed Hydrothiocyanation of Haloalkynes Xiaojun Zeng,† Bocheng Chen,‡ Zhichao Lu,† Gerald B. Hammond,*,† and Bo Xu*,‡ †

Department of Chemistry, University of Louisville, Louisville, Kentucky 40292, United States College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shanghai, China

‡

Org. Lett. Downloaded from pubs.acs.org by IDAHO STATE UNIV on 04/09/19. For personal use only.

S Supporting Information *

ABSTRACT: The first homogeneous and heterogeneous nanoparticle goldcatalyzed addition of sulfur nucleophiles to alkynes was developed. More specifically, gold-catalyzed hydrothiocyanation of haloalkynes gave good yields and good stereoselectivity of vinyl thiocyanates. Furthermore, a sulfurbased gold catalyst (PPh3AuSCN) has shown a unique reactivity in goldcatalyzed reactions such as the cyclization of N-propargylic amides.

O

Although homogeneous or nanoparticle gold-catalyzed carbon-heteroatom bond formation, via nucleophilic addition to π-activated alkynes, is over three decades old,21 the strong affinity between the gold and sulfur has hindered progress in gold-catalyzed C−S bond formation.21a In fact, organosulfur compounds are extensively used to derivative nanoparticle gold surfaces22 for therapeutic, diagnostics and spectroscopic applications.21f,23 The homogeneous gold-catalyzed the addition of organosulfur compounds to form a Csp3-S bond has been reported by several groups,24 but there has been only one example (Scheme 1a) that reported a homogeneous gold

rganosulfur compounds are ubiquitous and of great importance in material and pharmaceutical chemistry.1 Organosulfur compounds are also synthetically useful building blocks. Among organosulfur compounds,2 thiocyanates are found in antiparasitic compounds,3 psammaplin B,4 and fasicularin.5 The CN group in thiocyanates could be considered a protecting group and an excellent leaving group. Thiocyanates have been used extensively as thiol precursors.6 Thiocyanates could be transformed into many other organosulfur compounds such as thioethers,7 disulfides,8 phosphonothioates,9 and trifluoromethyl sulfides.10 Furthermore, thiocyanates can be regarded as less-toxic cyanide sources, thus providing a complementary method for constructing the C−CN (sp2 and sp3) bond.11 Lastly, the dual role of thiocyanates (sulfur electrophile and nitrile nucleophile) offers a platform for the construction of diverse heterocyclic compounds.12 A wide-ranging set of alkyl,13 aromatic,14 alkenyl,15 and alkynyl16 thiocyanate derivatives have been prepared via simple substitutions or photocatalytic or transition-metal catalyzed methodologies. In general, the use of transition metal catalysis in thiocyanate synthesis is challenging because most transition metal catalysts could be easily poisoned by sulfur-containing compounds.17 As a result, few methods have recently been reported using transition metal catalysis to prepare thiocyanates. Examples include the synthesis of aryl thiocyanates via copper-catalyzed cross-coupling,18 silver-catalyzed syntheses of vinyl thiocyanates,19 silver-catalyzed alkynylthiocyanates,16 and palladium-catalyzed the thiocyanation of 2-aminofurans.20 We envisioned a new tactic for transition metal-catalyzed incorporation of thiocyanates to organic compounds (namely, the homogeneous and nanoparticle gold-catalyzed thiocyanate addition to haloalkynes to produce Z-vinyl thiocyanates, exclusively), which we are pleased to report now. © XXXX American Chemical Society

Scheme 1. Gold-Catalyzed Addition of Sulfur Nucleophiles to Alkynes

Received: February 26, 2019

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DOI: 10.1021/acs.orglett.9b00728 Org. Lett. XXXX, XXX, XXX−XXX

Letter

Organic Letters mediated organosulfur addition to alkynes to afford Csp2-S containing compounds.25 Moreover, this transformation required stoichiometric amounts of gold and showed poor regioselectivity. Along this line, Lee and co-workers26 have reported a gold(I)-catalyzed addition of thiols and thioacids to cyclopropenes (Scheme 1b), but highly reactive 3,3-disubstituted cyclopropenes were needed. To the best of our knowledge, there are no reports on both homogeneous and heterogeneous nanoparticle gold-catalyzed addition of sulfur nucleophiles to alkynes. We decided to use haloalkynes as starting material, not only because they are versatile intermediates27 but also because of their synthetic potential, as we demonstrated in the sulfonic acid28 and hydrochloride29 addition to haloalkynes that furnished multifunctional alkenes. In the beginning, we evaluated solvents that could favor the conversion of bromoalkyne 1a to vinyl thiocyanate 2a in the presence of KSCN under homogeneous gold PPh3AuCl catalysis (Table 1,

12) give slightly better results. To our surprise, ligandless gold chloride gave an excellent yield of 2a (Table 1, entry 13). We found noteworthy to observe that NaAuCl·H2O produced a complex mixture (Table 1, entry 14). Next, we studied the effect of the thiocyanate sources in the transformation (Table 1, entries 15−19). The alkali metal counterions produced similar results (Table 1, entries 15−17). In contrast, CuSCN completely inhibited the reaction (Table 1, entry 18). When we substituted the copper(I) with ammonium, the yield of 2a increased to 94% (Table 1, entry 19). We extended our investigation with gold nanoparticles (Table 1, entries 20−23). Various thiocyanates sources were employed. The product was detected in good yield (Table 1, entry 23) when NH4SCN was used. When the loading of NH4SCN was increased to 4 equiv, the yield of 2a was dramatically improved to 92%. With optimized conditions in hand, we proceeded to evaluate the scope of the homogeneous gold-catalyzed antihydrothiocyanation reaction (Figure 1). Aryl bromoalkyne

Table 1. Optimization of the Hydrothiocyanation Reaction.a

entry

[Au]

MSCN

solvent

yieldb

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

PPh3AuCl PPh3AuCl PPh3AuCl PPh3AuCl PPh3AuCl XPhosAuCl BrettphosAuCl t BuXPhosAuC S PhosAuCl DppfAuCl iPrAuCl (RO)3PAuClc AuCl·SMe2 NaAuCl·H2O AuCl·SMe2 AuCl·SMe2 AuCl·SMe2 AuCl·SMe2 AuCl·SMe2 Ti2O/Aud Ti2O/Aud Ti2O/Aud Ti2O/Aud Ti2O/Aud

KSCN KSCN KSCN KSCN KSCN KSCN KSCN KSCN KSCN KSCN KSCN KSCN KSCN KSCN KSCN NaSCN LiSCN CuSCN NH4SCN KSCN NaSCN LiSCN NH4SCN NH4SCNe

DMF HOAC TFA dioxane DMSO HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc HOAc

17 70 complex trace trace 52 48 40 67 66 68 73 84 complex 91 92 90 trace 94 67 67 65 74 92

Figure 1. Scope of homogeneous gold-catalyzed thiocyanation of haloalkynes. Reaction conditions: 1 (0.2 mmol), NH4SCN (0.4 mmol, 2.0 equiv), and AuCl·SMe2 (5 mol %) in HOAc (0.8 mL) under 70 °C 8 h. All yields are isolated yields. Superscript b: a 1 mmol scale, HOAc (4 mL) was used, 70 °C, and 12 h. Superscript c: NH4SCN (4.0 equiv) was used.

substrates bearing a variety of functional groups (1a−1m) were investigated. We found that aromatic alkynes substituted with chlorine in the ortho, meta, or para positions (2b−d) afforded the desired products in similar yields. When 1 mmol of 1a was subjected to the standard conditions, the desirable product was obtained in 79% yield. Methyl (2e), fluorine (2f) and electron withdrawing groups such as ketone (2g), ester (2h), nitrile (2i), and sulfonate (2j) were well-tolerated, yielding the corresponding products in 73−85% yield. It should be noted that the acid-sensitive substrate bearing an -OAc group (2k) was only obtained in a relatively low yield. Substrates containing heteroaromatic moieties such as thiophene (2l), and the aromatic diyne (2m) were also compatible with the reaction conditions. In general, aliphatic haloalkynes exhibited higher reactivity than the aromatic haloalkynes. Cyclohexene (2n), disubstituted alkene (2q), benzyl ester (2p), phthalimide (2o), and a cholesterol derivative furnished the corresponding vinyl thiocyanate products in good to excellent yields. Additionally, a carboxylic

a

Conditions: 1a (0.2 mmol), MSCN (0.4 mmol, 2.0 equiv), gold catalyst [Au] (5 mol %) in HOAc (0.8 mL) under 70 °C for 8 h. bGC yields. cR: 2,4-di-tBu-C6H3-. d2 mol % was used. eNH4SCN (4.0 equiv) was used.

entries 1−5). To our delight, we found that the 2a could be produced in 70% yield in a protic solvent (HOAc). Generally, homogeneous gold catalysis requires reliable ligands30 and suitable counter-ions.31 When we examined the ligand effects (Table 1, entries 6−13) we found that sterically demanding ligands gave a low yield of product (Table 1, entries 6−8), but a bidentate ligand (Table 1, entry 10), an NHC carbene ligand (Table 1, entry 11), or an electron-rich ligand (Table 1, entry B

DOI: 10.1021/acs.orglett.9b00728 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters acid (2r) was tolerated, providing a synthetic handle for further synthetic transformations. To further underscore the wide range of the reaction scope, we used chloroalkyne substrates (2u−2x). Again, excellent yields and exclusive anti-selectivity were obtained. Various functional groups such as tert-butyl (2v), and trifluoromethoxy (2w), were compatible with the reaction conditions. It should be noted that some functional groups such as −OH and −Cl were esterified or thiocyanated (2s, 2x). Similarly, we investigated the scope of the nanoparticle gold -catalyzed hydrothiocyanation of haloalkynes (Figure 2).

Scheme 2. Stoichiometric Reactions of L-Au-SCN Complex

the insolubility of PPh3AuSCN, we added 0.2 mL of DCE); the reaction produced 2a in 45% yield (Scheme 3, III). When Scheme 3. PPh3AuSCN-Catalyzed Reactions

Figure 2. Scope of the nanoparticle gold-catalyzed thiocyanation of haloalkynes. Reaction conditions: 1 (0.2 mmol), NH4SCN (0.4 mmol, 2 equiv), and TiO2/Au (2 mol %) in HOAc (0.8 mL) under 70 °C and 8 h. All yields are isolated yields. Superscript b: d4-DOAc was used. Superscript c: d4-DOAc was used. Superscript d: NH4SCN (4.0 equiv) was used.

we added 2.0 equiv of PPh3AuSCN, the yield increased to 75% (Scheme 2c). These experiments revealed that the bond between the gold and thiocyanate bond could be broken in the presence of acetic acid. Recently, we have reported a widely applicable carbon-based counter-ion-based cationic gold catalyst.31 Based on our success on gold-catalyzed hydrothiocyanation reactions, it appears that a sulfur-based counter-ion-based gold catalyst may have promising reactivity in other gold-catalyzed reactions. To assess the applicability of PPh3AuSCN, we carried out some classic cationic gold-catalyzed reactions (Scheme 3). First, we investigated the intermolecular addition of N-hydroxyl benzotriazole 5 to octyne 6 (Scheme 3a). Unfortunately, only 24% of product 7 was obtained. However, when we conducted a gold-catalyzed intermolecular oxidation reaction (Scheme 3b), the desired product, oxazole 9, was isolated in 70% yield. Next, we found that PPh3AuSCN performed well in the hydration of propargyl acetate (Scheme 3c). Finally, we investigated the cyclization of N-propargylic amides. Usually, the gold(I)-catalyzed 5-exo-dig cyclization of propargyl amide 12 yields an alkylidene oxazoline product.33 To our surprise, oxazole 13 (Scheme 3d) was isolated in the presence of PPh3AuSCN catalyst, which is consistent with the action of a gold(III) catalyst.34 In contrast to homogeneous gold, we were able to recycle the commercially available TiO2/Au (eq 1). Indeed, we implemented five straight runs of the hydrothiocyanation under the optimized conditions without significant loss of catalytic activity. The nanoparticle gold could be simply recycled by simple filtration after each run.

Initially, we conducted a deuterium-labeling experiment that revealed that acetic acid was the major proton source. Different functionalities on the aromatic ring were examined, such as the electron donating methyl group (3a) the electron deficient ester (3b), bromo (3c), and unprotected alkyne (3d). All of these substrates afforded products in moderate to good yields. We probed the reactions of a range of aliphatic bromoalkynes under the same conditions; electron-neutral substituents such as n-hexyl (3e) and cyclohexane (3i), the electron-withdrawing nitrile (3f) benzoyl (3g), and diyne (3h) performed quite well. Even the estrone-tethered substrate (3j) afforded the corresponding halo vinyl thiocyanate in moderate yield and selectivity. Lastly, we found that both aliphatic and aromatic chloroalkynes could be readily converted to vinyl thiocyanates products. Gold(I) thiocyanate complexes have been considered as chemotherapeutic agents and have been used clinically.32 However, the reactivity of gold thiocyanate has not been studied in any detail. To gain some insight on the mechanism of the reaction just reported, we synthesized the hypothetical intermediate PPh3AuSCN via a two-phase substitution reaction32b (Scheme 2a). Exposure of PPh3AuSCN to the standard reaction conditions (replacing the AuCl·SMe2) afforded the product 2a in 90% yield (Scheme 2b). This result indicated that L-Au-SCN might be generated in situ and then participated in the catalytic cycle. In support of this hypothesis, we used stoichiometric amounts of PPh3AuSCN (in the absence of NH4SCN) in the optimized reaction (due to C

DOI: 10.1021/acs.orglett.9b00728 Org. Lett. XXXX, XXX, XXX−XXX

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Organic Letters

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In summary, we have developed a homogeneous gold- and nanoparticle gold-catalyzed protocol for the hydrothiocyanation of haloalkynes. Furthermore, the PPh3AuSCN catalyst has shown a unique reactivity. Further investigation on the use of L-Au-SCN in new reactions is under way in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00728. Additional details on the experimental methods and NMR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Gerald B. Hammond: 0000-0002-9814-5536 Bo Xu: 0000-0001-8702-1872 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Science Foundation (grant no. CHE-1401700) for financial support. B.X. acknowledges the financial support of the National Science Foundation of China (grant no. NSFC-21472018).

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DOI: 10.1021/acs.orglett.9b00728 Org. Lett. XXXX, XXX, XXX−XXX