molecules Review Recent Advances in the Reactions of Cyclic Carbynes Qian Su, Jipeng Ding, Zhihui Du, Yunrong Lai, Hongzuo Li, Ming-An Ouyang, Liyan Song * and Ran Lin * Key Laboratory of Biopesticide and Chemical Biology (Ministry of Education), College of Plant Protection, Fujian Agriculture and Forestry University, Fuzhou 350002, China; [email protected] (Q.S.); [email protected] (J.D.); [email protected] (Z.D.); [email protected] (Y.L.); [email protected] (H.L.); [email protected] (M.-A.O.) * Correspondence: [email protected] (L.S.); [email protected] (R.L.) Academic Editors: Evamarie Hey-Hawkins, Santiago Gómez-Ruiz and Goran Kaluderovi´c¯ Received: 6 October 2020; Accepted: 28 October 2020; Published: 30 October 2020 Abstract: The acyclic organic alkynes and carbyne bonds exhibit linear shapes. Metallabenzynes and metallapentalynes are six- or five-membered metallacycles containing carbynes, whose carbine-carbon bond angles are less than 180◦. Such distortion results in considerable ring strain, resulting in the unprecedented reactivity compared with acyclic carbynes. Meanwhile, the aromaticity of these metallacycles would stabilize the ring system. The fascinating combination of ring strain and aromaticity would lead to interesting reactivities. This mini review summarized recent findings on the reactivity of the metal–carbon triple bonds and the aromatic ring system. In the case of metallabenzynes, aromaticity would prevail over ring strain. The reactions are similar to those of organic aromatics, especially in electrophilic reactions. Meanwhile, fragmentation of metallacarbynes might be observed via migratory insertion if the aromaticity of metallacarbynes is strongly affected. In the case of metallapentalynes, the extremely small bond angle would result in high reactivity of the carbyne moiety, which would undergo typical reactions for organic alkynes, including interaction with coinage metal complexes, electrophilic reactions, nucleophilic reactions and cycloaddition reactions, whereas the strong aromaticity ensured the integrity of the bicyclic framework of metallapentalynes throughout all reported reaction conditions. Keywords: cyclic carbynes; reactivities; metallabenzynes; metallapentalynes 1. Introduction The alkyne (carbon-carbon triple bond), which is a fundamental functional group in organic chemistry, is involved in a large number of reactions in organic chemistry [1–5]. Carbyne complexes, i.e., transition metal complexes with metal-carbon triple bonds, can be described as analogs of alkynes, whereas a transition metal replaces one of the sp carbons. They have attracted considerable attention because of their remarkable features and their significance as catalysts or reagents for various types of organic transformations [5–11]. Part of the reactivities of carbynes paralleled that of their organic parents, such as nucleophilic reactions, electrophilic reactions, photochemistry, oxidation and reduction, reactions with chalcogenides, reactions with unsaturated organic substrates (cycloadditions) and substitution on the α carbon (Figure1). However, the incorporation of metal into the triple would lead to fascinating properties, which were quite different from organic alkynes. For example, late-transition-metal carbyne species were applied as catalysts in alkyne metathesis or alkyne polymerization reactions. Carbynes might be coupled or reactive to other metal complexes to afford bi- or multi-metallic complexes. Ligand substitution is one of the fundamental reactions in organometallic chemistry, which is certainly involved in the reactions of carbynes. In addition, carbyne ligands might be cleaved from metal centers to give various organic products (Figure1). Molecules 2020, 25, 5050; doi:10.3390/molecules25215050 www.mdpi.com/journal/molecules Molecules 2020, 25, x 2 of 24 Molecules 2020,, 25,, xx 2 of 24 addition, carbyne ligands might be cleaved from metal centers to give various organic products addition, carbyne ligands might be cleaved from metal centers to give various organic products addition,Molecules(Figure 20201). carbyne , 25, 5050 ligands might be cleaved from metal centers to give various organic products2 of 24 (Figure 1). Figure 1. Reactions of acyclic carbynes. Figure 1. Reactions of acyclic carbynes. The alkyne moiety is normally linear due to the sp hybridization of acetylenic carbon. Ring The alkynealkyne moietymoiety is is normally normally linear linear due due to theto the sp hybridizationsp hybridization of acetylenic of acetylenic carbon. carbon. Ring strainRing strainThe would alkyne be moietyraised fromis normally the introduction linear due of to al thekyne sp moiety hybridization into an oforga acetylenicnic cycle. carbon. As a result, Ring wouldstrain would be raised be fromraised the from introduction the introduction of alkyne of moietyalkyne moiety into an into organic an orga cycle.nic As cycle. a result, As a limitedresult, strainlimited would insights be couldraised be from gained the introductioninto the reactivity of alkyne of organic moiety cyclic into analkynes organic due cycle. to the As instability a result, insightslimited insights could be could gained be into gained the reactivity into the ofreactivity organic cyclicof organic alkynes cyclic due alkynes to the instability due to the caused instability by the limitedcaused byinsights the high could ring be tension gained [1 2,13].into the The reactivity incorporation of organic of a metal cyclic center alkynes into due the acetylenicto the instability carbon highcaused ring by tension the high [12 ring,13]. tension The incorporation [12,13]. The ofincorporation a metal center of intoa metal the acetyleniccenter into carbon the acetylenic would stabilize carbon causedwould stabilizeby the high the ring strained tension cyclic [12,13]. carbon The cyclesincorporation containing of a alkynes.metal center The intocyclic the carbynes acetylenic obtained carbon thewould strained stabilize cyclic the carbon strained cycles cyclic containing carbon alkynes.cycles containing The cyclic alkynes. carbynes The obtained cyclic untilcarbynes now obtained could be woulduntil now stabilize could the be strained classified cyclic as six-memberedcarbon cycles containingmetallabenzynes alkynes. [14–21] The cyclic A and carbynes five-membered obtained classifieduntil now as could six-membered be classified metallabenzynes as six-membered [14–21 ] metallabenzynesA and five-membered [14–21] metallapentalynes A and five-membered [22–24] B untilmetallapentalynes now could be [22–24] classified B (Figure as six-membered 2). In addition, metallabenzynes the first metallapyridyne [14–21] A [25] and was five-membered documented (Figuremetallapentalynes2). In addition, [22–24] the first B (Figure metallapyridyne 2). In addition, [ 25] was the documented first metallapyridyne by Xia and [25] coworkers, was documented providing metallapentalynesby Xia and coworkers, [22–24] providing B (Figure one 2). precious In addition, example the offirst cyclic metallapyridyne carbynes. [25] was documented byone Xia precious and coworkers, example ofproviding cyclic carbynes. one precious example of cyclic carbynes. Figure 2. Strained metallacycles with carbyne bond. Figure 2. Strained metallacycles with carbyne bond. The difference difference between between acyclic acyclic carbynes carbynes and and cyclic cyclic carbynes carbynes might might be due be due to the to ring the ringstrain. strain. The The difference between acyclic carbynes and cyclic carbynes might be due to the ring strain. The Thechemical chemical properties properties of cyclic of cyclic carbynes carbynes are summar are summarizedized in Figure in Figure 3. Among3. Among the thelimited limited examples, examples, the chemical properties of cyclic carbynes are summarized in Figure 3. Among the limited examples, the thereaction reaction types types of cyclic of cyclic carbynes carbynes are aresimilar similar to thos to thosee of acyclic of acyclic carbynes carbynes except except that that in most in most cases cases the reaction types of cyclic carbynes are similar to those of acyclic carbynes except that in most cases the the“migratory “migratory insertion” insertion” [26] [26 would] would not not stop stop at atthe the carbene carbene complexes complexes and and lead lead to to the the transformation transformation “migratory insertion” [26] would not stop at the carbene complexes and lead to the transformation of aromatic metallacycles into cyclopentadienescyclopentadienes (Cp) complexescomplexes [[27].27]. of aromatic metallacycles into cyclopentadienes (Cp) complexes [27]. Figure 3. Reactions of cyclic carbynes. Figure 3. Reactions of cyclic carbynes. Carbynes are usually recognized as important intermediates in many transition-metal-catalyzed organic reactions [28,29]. The thorough investigation of the reactivities and properties might provide a Molecules 2020, 25, x 3 of 24 Carbynes are usually recognized as important intermediates in many transition-metal-catalyzed organic reactions [28,29]. The thorough investigation of the reactivities Molecules 2020, 25, 5050 3 of 24 and properties might provide a new perspective for the application of late-transition-metal carbyne species as prosperous catalysts in various organic reactions. The chemistry of metallabenzynes has newbeen perspectivefully illustrated for the and application nicely reviewed of late-transition-metal [14–21]. However, carbyne there is species no review as prosperous paper covering catalysts the inentirety various