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Recent Advances and Applications of Reductive Desulfurization In&Nbsp Tetrahedron 70 (2014) 8983e9027 Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet Tetrahedron report number 1054 Recent advances and applications of reductive desulfurization in organic synthesis Jana Rentner, Marko Kljajic, Lisa Offner, Rolf Breinbauer * Institute of Organic Chemistry, NAWI Graz, Graz University of Technology, Stremayrgasse 9, A-8010 Graz, Austria article info Article history: Received 28 March 2014 Available online 5 July 2014 Keywords: Desulfurization Radical reaction Raney-nickel Synthon Thiol Total synthesis Contents 1. Introduction . ............................................... 8983 2. Mechanistic considerations . ............................................... 8984 3. Thiophenes as masked carbon synthons . .......................................... 8986 4. Saturated S-heterocycles as masked carbon-synthons . .................................. 8995 5. Linchpin strategy for the assembly of building blocks . .................................. 8997 6. S-heterocycles for deoxygenation of carbonyl groups . .................................. 8999 7. S-containing functional groups for tuning reactivity . .................................. 9003 8. S-heterocycles for improved selectivity and molecular recognition . ................... 9006 9. Acyclic S-compounds for improved selectivity and molecular recognition . ................... 9009 10. Other applications in organic synthesis . ...........................................9015 11. Reductive desulfurization in peptide chemistry . ...................................9021 12. Conclusions . ............................................... 9024 Acknowledgements . ......................9024 13. Abbreviations . ............................................... 9024 References and notes . ......................9025 Biographical sketch . ......................9027 1. Introduction tungsten, or nickel sulfide catalysts, the true potential of this re- action in the lab scale total synthesis of natural products, bi- While the reductive desulfurization of thiols, thioethers, and S- ologically active compounds, or new materials has not been containing heterocycles is performed on a multi-million ton scale in exploited yet. down-stream oil processing in the production of gasoline, kerosene In this report we want to give a summary about the opportu- and Diesel fuel using heterogeneous molybdenum, cobalt, nities offered by reductive desulfurization as a synthetic tool in organic synthesis and highlight its applications as carbon-synthons or for tuning the reactivity and selectivity of reactions. There have * Corresponding author. E-mail address: [email protected] (R. Breinbauer). been comprehensive reviews about the desulfurization of thio http://dx.doi.org/10.1016/j.tet.2014.06.104 0040-4020/Ó 2014 Elsevier Ltd. All rights reserved. 8984 J. Rentner et al. / Tetrahedron 70 (2014) 8983e9027 compounds with Raney-nickel by van Tamelen1 and Hauptmann2 the 1940’s establishing Raney-Ni as a reagent for reductive de- from 1962 and a book chapter by Gol’dfarb3 from 1986. This re- sulfurization. Soon the desulfurization of thioketals (e.g., 5)16 and view article builds on these earlier review articles to describe the thiophenes (e.g., 6)17,18 was realized by other groups (Scheme 2). current state of this subject and complements review articles about reductive desulfonylation,4,5 which will not be covered in this re- port, as this article focuses on non-oxidized sulfur species, such as 2. Mechanistic considerations thiols, thioethers, and thiophenes. This review also will not discuss the metal-catalyzed C(sp2)-SR cleavage as it has found numerous As listed in Scheme 2 Raney-Ni has historically been the first applications for the removal of RS-substituents as a strategic reagent, which enabled the desulfurization of thiophenes and thi- transformation in heterocyclic chemistry and can be reliably ac- oethers and is until today the reagent of choice for these trans- e complished by a variety of methods.6 10 We have structured the formations. Activated Raney-Ni can be categorized into seven material according to the fields of application, using the following different generations W-1 up to W-7, all types differing in reaction e categories: 1) thiophenes and saturated S-heterocycles as carbon time for Raney-Ni synthesis and workup conditions.19 23 Murray fragments, 2) assembly of organic frameworks using the linchpin Raney reported the first Raney-Ni catalyst in 1927 without immo- strategy, 3) deoxygenation of carbonyl groups via thioketalization, bilization of the metal on a surface.24 W-2 to W-7 type Raney-nickel 4) increasing the reactivity of reactions by tuning the electronic catalysts are advancements of this first catalyst and show in general properties of reagents, 5) increasing the selectivity of reactions by increased reactivity mainly to varying functional groups. In parallel, conformational restriction, and 6) as functional handles in peptide its stability decreases. These catalyst generations must be synthe- chemistry (Scheme 1). Naturally, certain applications would fit into sized shortly before use and cannot be stored over a longer period. more than one category, so we arbitrarily classified such application Another very important fact is their extremely high inflammability into a single category to avoid overlap. increasing with the catalyst generation. All Raney-nickel catalysts The first examples of desulfurization of thioethers (e.g., 1),11 S- incorporate emerging hydrogen during its synthesis from nickel- containing amino acids (e.g., 2e3),11,12 or biotin methyl ester ealuminium alloy under basic conditions. Due to the highly pyro- e (4)13 15 have been reported by Mozingo et al. from Merck & Co in phoric properties of all Raney-nickel catalysts they are only stored Scheme 1. Key transformation using reductive desulfurization. J. Rentner et al. / Tetrahedron 70 (2014) 8983e9027 8985 Scheme 2. Historically first examples of reductive desulfurizations. and handled as suspension. Stating the exact amount of catalyst for thiophene25 and thioether26 on Raney-Ni and thiophene on a reaction is therefore impossible. Ni(111),27 X-ray absorption spectroscopy of dibenzothiophene on Because of its complex nature as a skeletal catalyst resulting Ni/ZnO28 and more recently of computational DFT calculations of from alkaline leaching of a NiAl-alloy the investigation of the re- thiophene adsorption and dissociation on various Ni crystal faces.29 action mechanism is complicated by various Ni crystal faces and the From these studies a consensual mechanistic proposal can be de- surface decoration with adsorbed hydrogen or oxygen species. lineated (Scheme 3). Adsorption of thiophene on the Ni surface Therefore, our current knowledge about the mechanism of de- results in the geometric distortion of the adsorbed molecule and sulfurization on Ni surfaces is based on studies on model systems, the loss of aromaticity of the thiophene ring. Donation from the such as X-ray photoelectron spectroscopy studies (XPS) under ul- sulfur lone pair to the Ni-atoms is paralleled by back donation from trahigh vacuum (UHV) conditions of the chemisorption of Ni d-orbitals to the p-electrons of the carbon framework.29 Scheme 3. Plausible mechanism for desulfurization of thiophenes on Ni surfaces. 8986 J. Rentner et al. / Tetrahedron 70 (2014) 8983e9027 Depending on the crystal face several mechanistic pathways are transformations for thioether cleavage enabled by lithium/ethyl- discussed for the following CeS dissociation step, which very likely amine or LiDBB. involves a metallocycle intermediate. The CeS dissociation step has a surprisingly low activation barrier. Under UHV conditions CeS 3. Thiophenes as masked carbon synthons cleavage occurs on Raney-Ni for thiophene25 and dipropylsulfide26 at around 170 K. The hydrogenation of the olefins of the formed Building on the first historic precedence described above in olefin species is believed to have a higher activation barrier. While Scheme 2 Gol’dfarb et al. have extensively studied in a series of it is well known that saturated S-heterocycles and thioethers are publications in the 1960’s (mostly written in Russian) the use of more reactive than thiophene in the desulfurization reaction with thiophene as a C -building block. In Scheme 5 an elegant synthetic Raney-Ni and industrial HDS catalysts, such as Mo and Co, current 4 sequence is depicted, in which thiophenes are transformed to ali- evidence suggests that the S from thiophene is removed directly phatic amino acids. For example, thiophene 13 can be converted without prior hydrogenation as this process would be energetically into a-aminoenanthic acid 14 in 53% yield. 5-Acetamidothiophene- unfavourable.30 2-carboxylic acid (15) was desulfurized into d-acetamidovaleric While the spectroscopic proof of a postulated metallathiacycle acid (16) in formidable 96% yield, whereas bicyclic derivative 17 on a Ni surface is still awaited, there are several homogeneous was transformed into ε-ethyl-ε-caprolactam (18) in 82% yield metal complexes known, which make these species very plausi- (Scheme 5).34 ble.31 Jones et al. have for example, shown that [(dippe)NiH] (I) 2 Gol’dfarb’s strategy was applied by Mandolini et al. in an early readily reacts with thiophene to form the nickelthiacycle II, which synthesis of (rac)-muscone (21)(Scheme 6).35 The macrocycle was has been characterized by X-ray crystallography (Scheme 4).32 formed via an intramolecular FriedeleCrafts acylation
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