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Copper-Catalyzed (Transfer) Hydrogenation Reactions and Coupling Reactions by Intercepting Reactive Inter- Mediates Thereof

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Copper-Catalyzed (Transfer) Hydrogenation Reactions and Coupling Reactions by Intercepting Reactive Inter- Mediates Thereof SYNTHESIS0039-78811437-210X © Georg Thieme Verlag Stuttgart · New York 2020, 52, 2483–2496 short review 2483 en Syn thesis L. T. Brechmann, J. F. Teichert Short Review Catch It If You Can: Copper-Catalyzed (Transfer) Hydrogenation Reactions and Coupling Reactions by Intercepting Reactive Inter- mediates Thereof Lea T. Brechmann Johannes F. Teichert* 0000-0003-1043-8092 Institut für Chemie, Technische Universität Berlin, Straße des 17. Juni 115, 10623 Berlin, Germany [email protected] Received: 21.05.2020 1 Introduction: H2-Mediated C–C Bond- Accepted after revision: 18.06.2020 Published online: 13.07.2020 Forming Reactions DOI: 10.1055/s-0040-1707185; Art ID: ss-2020-z0281-sr The interception of reactive intermediates of catalytic Abstract The key reactive intermediate of copper(I)-catalyzed alkyne transformations is a typical approach to unravel new reac- semihydrogenations is a vinylcopper(I) complex. This intermediate can tivity in method development for synthetic chemistry. Es- be exploited as a starting point for a variety of trapping reactions. In this manner, an alkyne semihydrogenation can be turned into a pecially for the construction of C–C bonds, the use of or- dihydrogen-mediated coupling reaction. Therefore, the development of ganometallic reagents has proven to be extremely effec- copper-catalyzed (transfer) hydrogenation reactions is closely inter- tive.1,2 However, the use of a stoichiometric organometallic twined with the corresponding reductive trapping reactions. This short review highlights and conceptualizes the results in this area so far, with H2-mediated carbon–carbon and carbon–heteroatom bond-forming reactions emerging under both a transfer hydrogenation setting as well as with the direct use of H2. In all cases, highly selective catalysts are required that give rise to atom-economic multicomponent coupling re- actions with rapidly rising molecular complexity. The coupling reactions are put into perspective by presenting the corresponding (transfer) hy- drogenation processes first. 1 Introduction: H2-Mediated C–C Bond-Forming Reactions 2 Accessing Copper(I) Hydride Complexes as Key Reagents for Cou- pling Reactions; Requirements for Successful Trapping Reactions 3 Homogeneous Copper-Catalyzed Transfer Hydrogenations 4 Trapping of Reactive Intermediates of Alkyne Transfer Semi- This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. Johannes Teichert (left) is currently a junior professor at Technische hydrogenation Reactions: First Steps Towards Hydrogenative Universität Berlin. He studied at Philipps-Universität Marburg (diploma Alkyne Functionalizations thesis with Prof. Gerhard Hilt) and at Université Paul Sabatier in Tou- 5 Copper(I)-Catalyzed Alkyne Semihydrogenations louse, France. In 2006, he joined the group of Prof. Ben L. Feringa at 6 Copper(I)-Catalyzed H2-Mediated Alkyne Functionalizations; Rijksuniversiteit Groningen, the Netherlands for his Ph.D. in the field of Trapping of Reactive Intermediates from Catalytic Hydrogena- asymmetric catalysis. After a postdoctoral stay with Prof. Jeffrey Bode at tions ETH Zurich, he started his independent research group in Berlin. Johannes’ research interests include method development in organic 6.1 A Detour: Copper(I)-Catalyzed Allylic Reductions, Catalytic Gen- synthesis, transition-metal catalysis and especially reductive transfor- eration of Hydride Nucleophiles from H2 mations employing H2. 6.2 Trapping with Allylic Electrophiles: A Copper(I)-Catalyzed Hydro- allylation Reaction of Alkynes Lea Brechmann (right) is currently working in the group of Johannes Teichert. She studied at Philipps-Universität Marburg and at the Univer- 6.3 Trapping with Aryl Iodides sity of Santiago de Compostela before joining the group of Gerhard Hilt 7 Conclusion for her M.Sc. degree. Since 2017 she has been studying for her Ph.D. focusing on the development of new transition-metal-catalyzed syn- Key words cross-coupling, dihydrogen, copper, catalysis, alkynes thetic methods by activating dihydrogen as the hydride source. © 2020. Thieme. All rights reserved. Synthesis 2020, 52, 2483–2496 Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany 2484 Syn thesis L. T. Brechmann, J. F. Teichert Short Review reagent in, e.g., a cross-coupling reaction, inevitably leads to Exemplary of these is a rhodium-catalyzed enantioselective the formation of a stoichiometric amount of metal-based reductive coupling reaction of enyne 3 with -ketoester 4 waste. The possible avoidance of such waste by means of a (Scheme 1, c).14 Key to this transformation is the activation catalytic process based on dihydrogen (H2) serves as a of dihydrogen by a rhodium complex consisting of bis(1,5- prime inspiration for developing new reactions. This ap- cyclooctadiene)rhodium(I) trifluoromethanesulfonate and proach can be driven from the vantage point of the desire R-Xylyl-WALPHOS (6) followed by the main C–C bond- for atom economy/efficiency,3,4 and can also serve as a forming reaction. This and related methods13 are impressive fruitful inspiration for method development in synthetic in themselves, as the resulting overall transformations are chemistry. elegant and display high chemoselectivity (none of the Dihydrogen can be viewed as an easily accessible and, in alkenes formed in Scheme 1, c suffer any additional over- principle, a sustainably producible and abundant resource, hydrogenation), next to the obvious atom economy. Indeed, and although its use in catalytic hydrogenations is by far H2-mediated processes such as the one highlighted had 5–7 the prime use of H2 in synthesis, the aforementioned fea- been put forward in the literature as being highly sought- tures render it highly desirable as a key reagent for catalytic after.15 The transformations highlighted in Scheme 1 re- C–C (and C–heteroatom) bond-forming reactions. The op- volve around the use of carbonyl and/or carboxyl acceptors. portunity to replace common organometallic reagents with Progressing from these functional groups to other coupling catalytic processes based on H2 was recognized a long time partners would substantially broaden this elegant ap- ago. Probably the most famous H2-mediated C–C bond- proach. forming reactions are the Fischer–Tropsch process (Scheme Departing from noble metal catalysts (mostly based on 1, a)8,9 and alkene hydroformylation (the oxo process, rhodium, iridium and ruthenium) could offer such an op- 10–12 Scheme 1, b). Emanating from this, catalytic H2-mediat- portunity. In particular, base metals, which have only re- ed C–C bond-forming reactions beyond C1 building blocks cently regained popularity in catalytic hydrogenations and bear great potential in synthetic chemistry. In particular, H2 activation processes, might lead the way to such a new rhodium-catalyzed coupling reactions of alkenes/alkynes approach.5,16 and carbonyl derivatives have been intensively studied.13 a) Fischer–Tropsch chemistry [Co] or [Fe] n CO + excess H2 CnH2n+2 + CnH2n + n H2O mixture of products depending on the molar ratio of H2 and CO b) catalytic hydroformylation/oxo process H O [Rh] or [Co] H H R + CO + 2 R 1 2 c) modern H2-mediated C–C bond formation This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. 2 mol% Rh(COD)2OTf 2 mol% (R)-Xylyl-WALPHOS (6) O 1 mol% Ph3CCO2H R1 O R1 2 1 bar H2 + R OR3 OR3 1,2-dichloroethane, 60 °C 2 O H R OH 3 4 5 1 2 R = aryl, alkyl R = aryl, alkyl PAr2 R3 = Me, Et PAr2 Fe Me H Ar = 3,5-xylyl R-Xylyl-WALPHOS 6 Scheme 1 Overview of catalytic, H2-mediated C–C bond-forming processes. Only the most common metals are displayed for the processes in (a) and (b) © 2020. Thieme. All rights reserved. Synthesis 2020, 52, 2483–2496 2485 Syn thesis L. T. Brechmann, J. F. Teichert Short Review In recent years, many studies employing copper as a cat- σ-bond metathesis alytic metal for reductive coupling reactions have been put forward (see Section 3), and first endeavors into copper- – HOR1 H2 L Cu OR1 catalyzed H2-mediated bond-forming reactions have ap- n 8 peared.17–19 One of the key advantages of copper catalysis is HH the fact that it opens up the possibility for widening the re- BDE H–H: 104 kcal mol–1 action space beyond carbonyl/carboxyl derivatives as cou- 1 LnCu–OR LnCu–H pling partners, which have been prevalent thus far (see 7 11 above). This review summarizes the developments based 1 on copper catalysts for reductive coupling reactions of LnCu OR alkynes under hydrogenative or transfer hydrogenative H–E H E – EOR1 9 10 20 conditions and aims at discussing the various approaches. hydride donor BDE Si–H: 92 kcal mol–1 E = SiR3, BR2, SnR3 2 Accessing Copper(I) Hydride Complexes as Scheme 2 Similar yet different: Formation of copper(I) hydrides from Key Reagents for Coupling Reactions; Re- H2 vs hydroelement reagents via -bond metathesis quirements for Successful Trapping Reactions structural motif for the formation of copper(I) hydrides: In Fundamental for the development of H2-mediated the presence of a hydrosilane as the hydride source, cop- bond-forming processes is the efficient in situ generation of per(I) hydride 11 is known to be formed via -bond me- copper(I) hydrides, which has to be compatible with all tathesis of the hydrosilane and copper(I) complexes 7 bear- other envisaged downstream reactions. Copper(I) hydrides ing a Cu–O bond, liberating silyl ether 10 as a by-product (E 25,26 are widely used and well-studied reagents in organic syn- = Si) (Scheme 2, bottom). The activation of H2 leading to thesis, probably best known for their prototypical reactivi- copper(I) hydrides 11 resembles that of the hydrosilane ty in the conjugate reduction (i.e., 1,4-reductions) of ,- pathway (Scheme 2, top). Based on only circumstantial evi- 27 unsaturated carbonyl or carboxyl compounds, or 1,2-reduc- dence, activation of H2 had been suggested in the 1950s.
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