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Generation of Gold(I) Carbenes by Retro‐Buchner Reaction

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Generation of Gold(I) Carbenes by Retro‐Buchner Reaction DOI: 10.1002/cctc.201801201 Reviews 1 Very Important Paper 2 3 Generation of Gold(I) Carbenes by Retro-Buchner Reaction: 4 5 From Cyclopropanes to Natural Products Synthesis 6 [a, b] [a, b] [a, b] 7 Mauro Mato, Cristina Garcı´a-Morales, and Antonio M. Echavarren* 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 ChemCatChem 2019, 11,53–72 53 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley VCH Donnerstag, 03.01.2019 1901 / 120891 [S. 53/72] 1 Reviews 1 2 The gold(I)-catalyzed retro-Buchner (decarbenation, retrocyclo- cycloadditions with different partners, giving rise to a wide 3 propanation) reaction of 7-substituted cycloheptatrienes has range of relevant structures. This review covers the develop- 4 emerged as a safe and powerful alternative for the generation ments made on the generation of gold(I) carbenes via 5 of gold(I) carbenes. These carbenes can cyclopropanate alkenes, decarbenation reactions, and the applications of these reactions 6 take part in intramolecular reactions, or engage in higher in organic synthesis. 7 8 9 1. Introduction spectroscopically and structurally characterized.[8,9] These recent 10 findings have provided new insight into the electronic structure 11 Over the past two decades, homogeneous gold(I) catalysis has of the AuÀC bond in these key gold(I) intermediates. Ultimately, 12 arisen as one of the most powerful tools for the construction of experimental and theoretical work has shown that the nature 13 molecular complexity. Since its dawn in 1998, when a of gold(I) carbenes is better accounted for as a continuum that 14 groundbreaking report on the gold(I)-catalyzed hydration of ranges from metal-stabilized singlet carbenes to metal-coordi- 15 alkynes was published,[1] gold catalysis became a hot topic in nated carbocations.[6c,7] 16 organic chemistry. Due to its p-Lewis acidity, cationic gold(I) Gold(I) carbenes can be generated through different 17 complexes have been extensively used to selectively activate p methods in catalytic transformations (Scheme 2). Among these 18 CÀC bonds, especially alkynes, triggering chemical transforma- 19 tions that give rise to complex structures that are difficult to 20 access by other means, in an atom-economic fashion.[2] The 21 total synthesis of biologically relevant products stands out 22 among the applications that have been found for these new 23 methodologies based on homogeneous gold catalysis.[3] 24 Many gold(I)-catalyzed transformations have been proposed 25 to proceed via highly electrophilic cationic gold(I) carbenes as 26 intermediates (Scheme 1).[2,4,5] Despite the importance of these 27 28 29 30 31 32 33 Scheme 1. Carbocation and carbene resonance forms used to describe gold (I) carbenes. L=phosphines, phosphites or NHC carbene ligands among 34 others. 35 36 37 intermediates, their structure is somewhat controversial and 38 has been interpreted by different authors as gold(I)-stabilized 39 carbocations or gold(I) carbenes (Scheme 1).[6,7] The extension 40 of the p-backbonding interaction, which is at the core of the 41 discussion, influences the relative contributions of the carbene Scheme 2. General methods for the catalytic generation of gold(I) carbenes. 42 and the carbocation resonance forms, and therefore, the 43 reactivity of these electrophilic species. These different inter- 44 pretations can be understood because it was not until recently strategies, the cycloisomerization of 1,n-enynes has attracted 45 that examples of well-defined cationic gold(I) carbenes were special attention to the synthetic community, and has been 46 extensively explored in many different contexts.[2f,h,j,10] 47 Other examples include the cycloaddition of diene-allenes 48 [a] M. Mato, C. Garca-Morales, Prof. Dr. A. M. Echavarren to form gold vinylidene intermediates,[11] the 1,2-acyloxy Institute of Chemical Research of Catalonia (ICIQ) [12] 49 Barcelona Institute of Science and Technology (BIST) migration of propargylic carboxylates or ethers, and the 50 Av. Pasos Catalans 16 nucleophilic addition of N-oxides or sulfoxides to electrophilic 51 43007 Tarragona (Spain) (h2-alkyne)gold(I) complexes.[13,14] In addition to reactions involv- E-mail: [email protected] 52 [b] M. Mato, C. Garca-Morales, Prof. Dr. A. M. Echavarren ing alkynes, transformations based on gold(I)-catalyzed decom- 53 Departament de Qumica Analtica i Qumica Orgnica position of diazocompounds,[5,15] and opening of cyclopro- 54 Universitat Rovira i Virgili penes[16] have also been proposed to form these highly C/Marcel·li Domingo s/n 55 43007 Tarragona (Spain) electrophilic species. 56 This manuscript is part of the Anniversary Issue in celebration of 10 years of Driven by the high interest of the scientific community on 57 ChemCatchem. the plethora of transformations that are proposed to take place ChemCatChem 2019, 11,53–72 www.chemcatchem.org 54 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Wiley VCH Donnerstag, 03.01.2019 1901 / 120891 [S. 54/72] 1 Reviews 1 through metal carbene intermediates, great efforts have been cover the different transformations that were discovered over 2 made to generate them in an efficient manner. In spite of this the past 10 years through the generation of gold(I) carbenes 3 fact, most strategies present limitations, such as lack of general- via retro-cyclopropanation, especially using the retro-Buchner 4 ity (very specific substrates are employed, such as 1,n-enynes), reaction of 7-substituted cycloheptatrienes. 5 the requirement of potentially undesired functional groups that 6 end up in the reaction products, or the preparation and 7 handling of potentially dangerous materials, such as diazo 2. Historical Perspective 8 compounds. 9 In 2008, Chen and coworkers observed for the first time the 2.1. The Buchner Ring Expansion 10 formation and reactivity of a gold carbene via retro-cyclo- 11 propanation in gas phase, using electrospray ionization tandem The Buchner ring expansion is the reaction of a diazo 12 mass spectroscopy.[17,18] Two years later, our group reported the compound, originally ethyl diazoacetate (1), with an arene, 13 first formation of gold(I) carbenes in solution via retro-cyclo- such as benzene, to form a cycloheptatriene (Scheme 3). This 14 propanation reaction.[19] Soon after, based on this initial finding, 15 a general methodology for the cyclopropanation of olefins 16 based on the retro-Buchner reaction of 7-aryl-1,3,5-cyclohepta- 17 trienes was developed.[20] This retro-cyclopropanation (decarbe- 18 nation) process represents a safe and general way of generating 19 gold(I) carbenes without the need of introducing stabilizing or 20 undesired functional groups. Scheme 3. Buchner ring expansion of benzene. 21 Following these initial results, a wide range of synthetic 22 methodologies were developed based on the generation of 23 gold(I) carbenes by retro-cyclopropanation. In this review, we 24 25 26 Mauro Mato was born in Ferrol (Spain) in the Institute of Organic Chemistry of the CSIC 27 1993. He completed his B.Sc. in Chemistry at in Madrid. In 1992 he returned to the UAM as 28 the University of A Corun˜a in 2016, and his a Professor of Organic Chemistry and in 2004 M.Sc. on Synthesis, Catalysis, and Molecular he moved to Tarragona as a Group Leader at 29 Design at the Rovira i Virgili University the Institute of Chemical Research of Catalonia 30 (Tarragona, Spain) in 2017, receiving in both (ICIQ). He has been Liebig Lecturer (Organic 31 degrees the Extraordinary Award (best aca- Division, German Chemical Society, 2006), 32 demic record of each year). Then he started Abbot Lecturer in Organic Chemistry (Univer- 33 his Ph.D. studies under the supervision of Prof. sity of Illinois at Urbana-Campaign, 2009), 34 Antonio M. Echavarren at the Institute of Schulich Visiting Professor (Technion, Haifa, Chemical Research of Catalonia (ICIQ), where 2011), Sir Robert Robinson Distinguished Lec- 35 he works on new methods for the generation turer (University of Liverpool, 2011), Novartis 36 of metal carbenes. Lecturer in Organic Chemistry (Massachusetts 37 Institute of Technology, 2015) and Kurt Alder Cristina Garcı´a-Morales was born in Moguer 38 Lecturer 2017 (University of Cologne). In 2012 (Huelva, Spain) in 1990. After a one-year he got a European Research Council Advanced 39 period as an Erasmus student at Strathclyde Grant. Prof. Echavarren is a member of the 40 University (Glasgow, United Kingdom), she International Advisory Board of Organic & received her B.S. degree in Chemistry from 41 Biomolecular Chemistry, Chemical Society Re- Universidad de Huelva (Huelva, Spain) in 2013. 42 views, Advanced Synthesis and Catalysis, and After earning her MSc at the Universitat Rovira 43 Organic Letters, member of the Editorial Board i Virgili (Tarragona, Spain) she has been a PhD of Chemistry European Journal, and Associate 44 student in the group of Prof. Antonio M. Editor of Chemical Communications. He is a 45 Echavarren at the Institute of Chemical Re- Fellow of the Royal Society of Chemistry. He 46 search of Catalonia (ICIQ), where she works on received the 2004 Janssen-Cylag Award in enantioselective gold(I) catalysis and the char- 47 Organic Chemistry and the 2010 Gold Medal acterization of highly reactive gold(I) carbenes. 48 of the Royal Spanish Chemical Society and an 49 Prof. Dr Antonio M. Echavarren was born in Arthur C. Cope Scholar Award from the ACS. 50 Bilbao (Spain) and received his PhD at the He is the President of the Spanish Royal 51 Universidad Auto´noma de Madrid (UAM, 1982) Society of Chemistry (RSEQ).
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