Chemical Society Reviews Silver -Catalysed Reactions of Alkynes: Recent Advances Journal: Chemical Society Reviews Manuscript ID: CS-REV-01-2015-000027.R2 Article Type: Review Article Date Submitted by the Author: 02-Jun-2015 Complete List of Authors: Fang, Guichun; Northeast Normal University, Department of Chemistry Bi, Xihe; Northeast Normal University, Page 1 of 48Chem Soc Rev Chemical Society Reviews Dynamic Article Links ► Cite this: DOI: 10.1039/c0xx00000x www.rsc.org/ csr CRITICAL REVIEW Silver-Catalysed Reactions of Alkynes: Recent Advances Guichun Fang,a Xihe Bi*a,b Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x 5 Silver is a less expensive noble metal. Superior alkynophilicity due to π-coordination with the carbon- carbon triple bond, makes silver salts ideal catalysts for alkyne-based organic reactions. This review highlights the progress in alkyne chemistry via silver catalysis primarily over the past five years (ca. 2010–2014). The discussion is developed in terms of the bond type formed with the acetylenic carbon (i.e. , C–C, C–N, C–O, C–Halo, C–P and C–B). Compared with other coinage metals such as Au and Cu, 10 silver catalysis is frequently observed to be unique. This critical review clearly indicates that silver catalysis provides a significant impetus to the rapid evolution of alkyne-based organic reactions, such as alkynylation, hydrofunctionalization, cycloaddition, cycloisomerization, and cascade reactions. alkynylation, cycloaddition, cycloisomerization of functionalized 1. Introduction alkynes (enynes, multiynes, propargyl compounds, etc. ), and hydrofunctionalization.9 Moreover, in addition to the activation Alkynes and their derivatives are among the most valuable 50 of carbon-carbon triple bonds, other functional groups, such as 15 chemical motifs, because of their abundance and versatile 1 imines and carbonyls are also activated through coordination with reactivities. These fundamental chemicals can serve as molecular silver, providing a useful and important method for facilitating building blocks in designing novel organic reactions and 2 many different organic transformations with high atom assembling functional materials. In particular, as verified by the 10 efficiency. recent advances in transition metal-catalysed transformations, 20 they can be used to rapidly access complex molecular architectures.3 Silver, which possesses the electronic configuration [Kr] 4d 10 5s 1 in group 11, the so-called “coinage metals”, is inexpensive compared to gold. The outer orbital 5s 1 electronic configuration of silver allows it, either alone or in 25 combination with other transition metals, to form a series of silver(I) salts with a variety of counter anions. Similar to gold(I) 55 salts that have been shown to be powerful catalysts for alkyne Fig. 1 Activation of the carbon-carbon triple bond by silver catalyst. transformations, 4 silver(I) salts also function as σ- and/or π-Lewis acids.5 Silver exhibits special properties towards alkyne A survey of the literature related to the topic of silver-catalysed 10 30 activation due to its d electronic configuration, favouring reactions of alkynes revealed a considerable number of alkyne- interactions with the carbon-carbon π-bond of alkynes, referred to based reactions catalysed by this noble metal, particularly over as alkynophilicity; therefore, silver can be considered to be one of 60 the past decade (Fig. 2). A number of excellent reviews have the most powerful activators of a carbon-carbon triple bond. 6 As incorporated the advances in the silver-catalysed reactions of shown in Fig. 1, upon coordination to the carbon-carbon triple alkynes from different aspects, particularly the special issue of 11 35 bond of alkynes, silver salts lead to the formation of a silver-π- “Coinage Metals in Organic Synthesis ” in Chem. Rev. (2008), complex, facilitating the formation of C–X bonds (X = C, N, O, “The Organic Chemistry of Silver Acetylides ” by Pale et al . 12 Halo, P, etc .) by nucleophilic attack on this activated multiple 65 (2007), and a book titled “Silver in Organic Chemistry ” edited bond. For a terminal or silylated alkyne, the reaction pathway by Harmata (2010). 13 However, a review that focuses on the could involve a different conversion into silver acetylide via specific topic of silver-catalysed reactions of alkynes remains 7 14 40 deprotonation/desiliconization in the presence of bases, which elusive. Moreover, rapid development in this field since 2010 itself reacts either as a nucleophile to be trapped by electrophiles has been witnessed. Therefore, this review is timely in or by engaging in cross-coupling reactions through the 70 highlighting these advances to the chemical community. transmetalation process.8 This ability of silver to induce π- activation is not the only factor responsible for its activity and 45 effectiveness; additionally, the transformation exhibits good functionality in a number of important reactions such as This journal is © The Royal Society of Chemistry [year] [journal] , [year], [vol] , 00–00 | 1 Chemical Society Reviews Page 2 of 48 hierarchic reaction pathway that is fundamentally different from the classical coupling schemes in wet chemistry. The reaction is initiated by covalent coupling between two molecules rather than by single-molecule dehydrogenation. The resulting dimer 45 undergoes two subsequent dehydrogenation processes, which are expected to be rate-limiting according to the comparatively large barriers. Notably, the Ag(111) surface is more efficient compared with Au(111) and Cu(111). The on-surface coupling reaction is formally reminiscent of classic Glaser−Hay coupling schemes, 50 but is essentially different; this can be interpreted as a basic step of surface-confined acetylide chemistry. This reaction presents a Fig. 2 A statistical analysis is made of the silver-catalysed reactions of new approach towards the realization of two-dimensional carbon- alkynes through the survey of the articles published in ACS , RSC , Wiley , rich or all-carbon polymers. Elsevier , etc . during the past fourteen years. 5 Scope and Organization This review presents an overview of the transformations of alkynes under silver catalysis over the past five years (ca. 2010– 2014). A comprehensive review is an overwhelming task. Thus, this text will cover silver(I)-catalysed organic reactions of 10 alkynes and their important derivatives via functionalization of the C–C triple bond and the formation of new C–C, C–N, C–O, C–Halo, C–P and C–B bonds (Fig. 3). To fully profile some specific reactions, a brief description of the background will be provided. Unless necessary, reactions with silver(I) salts as 55 15 cocatalysts together with other metal salts have not been included Scheme 1 in this review. The differences in reaction pathways resulting from diverse reagent combinations will be addressed with an More recently, the Wen group reported a convenient AgNO 3- emphasis on discussing the reaction mechanism, aiming to inspire catalyzed efficient homocoupling of (hetero)aryl/alkyl alkynes new ideas for the further design and development of novel using PPh 3 as a ligand, which afforded a wide range of 1,3-diynes 20 19 reactions. 60 3 in excellent yields (Scheme 2). Scheme 2 Fig. 3 Classification of the reactions by the types of bonds formed with acetylenic carbon. Formation of Csp 2–Csp bonds: The transition metal-catalysed Sonogashira coupling of terminal alkynes with aryl and alkenyl 2. Formation of Carbon−Carbon Bonds 65 halides has become one of the most efficient and straightforward methods to form Csp 2–Csp bonds in organic synthesis. This 20 25 2.1 Alkynylation coupling reaction was first established in the 1970's. So far, a great number of modifications for palladium catalyst systems Formation of Csp –Csp bonds: Glaser−Hay coupling, a well- have been developed to overcome the disadvantages of the known named reaction, proceeds identically through the 70 reaction such as homocoupling products caused by CuI, dehydrogenation of a terminal alkyne with a Cu(I) ion and expensive palladium complexes, and ugly smell of amines. establishment of a Cu–acetylide intermediate. 15 Likewise, a However to date, only one report related to the silver(I)-catalysed 30 related silver acetylide can be attained using a silver ion. Sonogashira-type coupling of terminal alkynes with aryl iodides However, research findings on Ag-related syntheses of butadiyne or bromides 4 has been described by Wang and co-workers in moieties remain scarce. Recently, the research groups of 75 2006 (Scheme 3). This reaction proceeded in the presence of AgI Klappenberger 16 and Studer and Fuchs 17 successively reported (10 mol%), PPh (30 mol%) and K CO (2 equiv) in DMF at the homo-coupling of terminal alkynes 1 on the noble metal 3 2 3 100 °C for 8–12 h, affording the corresponding internal alkynes 5 35 surface (Scheme 1). Alkyne homo-coupling occurs on the in 62–99% yields. The mechanism for this silver-based Ag(111) metal surface in ultra -high vacuum under mild Sonogashira reaction is not fully clear. 21 conditions, leaving volatile H 2 as the sole by-product. The mechanism for the surface-assisted covalent coupling of terminal alkynes on Ag(111) was elucidated using density functional 18 40 theory (DFT)-based transition state calculations, suggesting a 2 | Journal Name , [year], [vol] , 00–00 This journal is © The Royal Society of Chemistry [year] Page 3 of 48 Chemical Society Reviews 45 concentration of CO 2 and the purity of the combustion gas had significant impacts on the yields because the directly introduced gas did not work. Furthermore, the aqueous MEA solution could be continuously recycled without any decrease in the CO 2 capture and release efficiency, thus demonstrating the practicality of this Scheme 3 50 protocol for CO 2 capture. Carbon dioxide (CO 2) is one of the most abundant C1 feedstocks on earth. 22 In this respect, a high-energy active reagent, 5 powerful catalyst, extra energy or high CO 2 pressure is generally required for successful CO 2 incorporation.