Cyclization reaction of acetylene

A [2+2+2] cyclization reaction of acetylenes and nitriles leads to the formation of furo[3,4-r]pyridines under cobalt-catalyzed conditions (Scheme 11) <2005SL1188>. 

C. Cyclization Reaction of Acetylenic Allenyl Lithiums, Vinyllithiums and... 

Oxo carbon anions that have aromatic character are derived from the hydroxy acids (7-VIa-d). They are made relatively easily. Electrochemical reduction of CO, which proceeds via radical ions gives C403" interaction of K and CO at 170°C gives K C606. Cyclization reactions of acetylenic ethers, for example, Bu OC=COBu by Co2(CO)8, and treatment of the resulting butoxides with CF3C02H also gives the anions. 

The results presented in the previous sections demonstrate the importance of point defects at the surface of oxide materials in determining the chemical activity of deposited metal atoms or clusters. A single Pd atom in fact is not a good catalyst of the cyclization reaction of acetylene to benzene except when it is deposited on a defect site of the MgO(lOO) surface. A detailed analysis of the reaction mechanism, based on the calculation of the activation barriers for the various steps of the reaction, and of a study of the preferred site for Pd binding, based on the MgO/Pd/CO adsorption properties, has shown that the defects which are most likely involved in the chemical activation of Pd are the oxygen vacancies, or F centers, located at the terraces of the MgO surface and populated by two (neutral F centers) or one (charged paramagnetic F centers) electrons. 

The cyclization reaction of acetylene at ambient temperatures to form benzene on Pd(lll) has recently been reinvestigated in an attempt to gain insights into its mechanism. It has been claimed that, at low exposures of acetylene, oifly ethylidyne is formed. At higher exposures, benzene is formed, albeit at low coverages, as indicated by appearance of the V4 band of benzene at 720 cm . ... 

The study of the cyclization reaction of acetylene to benzene on Pd atoms supported on MgO allows one to clarify some of the experimental aspects and to draw some general conclusions about the role of defects on oxide surfaces. The results show that only in the presence of surface defects does a single Pd atom become an active catalyst for the reaction, in fact, an isolated Pd atom is not capable of adding and activating three acetylene molecules, an essential step for the process. The change in the electronic structure of supported Pd is connected to the electron donor ability of the substrate, which does not simply act as an inert substrate. The... 

Fig. 2.14. Energy profile for the cyclization reaction of acetylene to benzene occurring on a Pd atom supported on an Fbc center, Pdi/Fsc (DFT-BP resnlts). Reproduced from [223], Copyright 2000 American Chemical Society...

The study of the cyclization reaction of acetylene to benzene on Pd atoms supported on MgO allows one to clarify some of the experimental aspects and to draw some general conclusions about the role of defects on oxide surfaces. The results show that only in the presence of surface defects a single Pd atom... 

Lowinger, T.B. and Weiler, L. (1992) Stereoselective Formation of - orZ-Exocyclic Alkenes via Radical Cyclization Reactions of Acetylenic Esters, J. Org. Chem. 57, 6099-6101. 

Wolf LB, Tjen KCMF, Rutjes FPJT (1998) Pd-catalyzed cyclization reactions of acetylene-containing a-amino acids. Tetrahedron Lett 39 5081-5084... 

C. Palladium-Catalyzed Cyclization Reactions of Acetylene Containing Amino Acids... 

Reactions of acetylene and iron carbonyls can yield benzene derivatives, quinones, cyclopentadienes, and a variety of heterocycHc compounds. The cyclization reaction is useful for preparing substituted benzenes. The reaction of / fZ-butylacetylene in the presence of Co2(CO)g as the catalyst yields l,2,4-tri-/ f2 butylbenzene (142). The reaction of Fe(CO) and diphenylacetylene yields no less than seven different species. A cyclobutadiene derivative [31811 -56-0] is the most important (143—145). 

Palladium(0)-catalysed coupling of non-conjugated dienes, aryl iodides and stabilized carbon nucleophiles has been developed468. An incredibly high yield (86%) of pentacycle 343 has been obtained from a Pd(0)-catalysed zipper reaction of acetylenic pentaene 342. The reaction is triggered off by a Pd-catalysed cyclization of acetylenic bond and the first olefinic bond469. 

Nickel catalysts for the syntheses of cyclic compounds were first successfully utilized by Reppe, who was able to prepare cyclooctatetraene from acetylene (65). This eight-membered ring synthesis, and also the preparation of cyclic products from strained olefins (e.g., bicycloheptene and norbornadiene) and acrylonitrile, have been adequately reviewed elsewhere (7) and will therefore not be considered further. A short account of the cyclization reactions of butadiene using nickel-containing catalysts has appeared previously in this series (/). The discovery of new synthetic possibilities and a deeper understanding of the mechanism of these reactions justify a more extensive treatment. 

On the other hand, the cyclization reaction of a vinyllithium onto an acetylenic unit provides an efficient route to five- and six-membered bis-exocyclic 1,3-dienes, which react stereoselectively with a wide range of dienophiles157. The 5-exo carbolithiation reaction of vinyllithiums 369, derived from the corresponding vinyl bromides, is syw-stereospecific giving, after hydrolysis, the /(-isomer of five-membered outer-ring dienes 370 and tolerates aryl-, silyl- or alkyl-substituents at the distal acetylenic carbon (Scheme 97). However, the alkyl-substituted alkynes are far more resistant to rearrangement than the aryl- or silyl-substrates and the addition of TMEDA and longer reaction times are needed for the latter... 

Obrecht D (1989) Acid-catalyzed cyclization reactions of substituted acetylenic ketones a new approach for the synthesis of 3-halofurans, flavones, and styrylchromones. Helv Chim Acta 72 447 56... 

The formation of 66 by reaction of acetylenes with (f/ -CsHs) (PhjP)Co(acetylene) has been investigated in detail. Kinetic studies indicate the intermediacy of ( / -C5H5)Co(acetylene)2 (Scheme 24), which cy-clizes to a coordinatively unsaturated cobaltacyclopentadiene by a spontaneous oxidative coupling. Regioselectivity of the cyclization process is controlled by steric rather than by electronic character of substituents. 

KIO montmorillonite efficiently catalyzes the cyclization reaction of alkyl 3-(2-benzimidazolylthio)acrylate (8). 4-Oxo-4H-[l,3]thiazino[3,2-a]benzimidazole (9) was isolated in almost quantitative yield, by contrast the yield was only 24 % in diphenyl ether at 260 (Table 3). Reaction of 2-mercaptobenzimidazole (7) with diethyl acetylene-dicarboxylate (10) afforded only cyclized products (11 and 12). 2-(Carboethoxymethylene)-3-oxo-2H,3H-thiazolo[3,2-a]-benzimidazole (11) formed initially, rearranges to ethyl 4-oxo-4H-[l,3]thiazino[3,2-a]benzimidazole-2-carboxylate (12) (Table 4). 

The more complex [2 -f 2 + 2] cycloisomerization reaction of acetylene units is also catalyzed by transition metal-alkyne n complexation and can be readily utilized for the synthesis of a variety of polysubstimted benzene derivatives in a straightforward manner (10, 352, 353). Recently, this methodology has been applied to the cyclization of 15-membered, nitrogen-containing di- and triacetylenic macrocycles. Upon coordination with Pd(0) to the triacetylenic macrocycle at ambient temperature, the ti-coordinated Pd(0) complex results. Subsequent refluxing of this species in toluene promotes cycloisomerization to the hexasubstituted arene (354) (Scheme 28). The Rh(I) [e.g., RhCl(CO)(PPh3)2] complex also catalyzes these same transformations in high (>80%) yields. 

Useful reviews on redox transformations of thiophen derivatives (179 references), on the stereochemistry of carbonyl derivatives of five-membered heterocycles (257 references), on synthetic approaches to dihydrothiophens (135 references), and on biosteric thiophens" have been published. Aspects of thiophen chemistry have been treated in reviews on the synthesis of heterocycles by thermal [2 + 2] cycloaddition reactions of acetylenes and on aspects and perspectives of organic heterocyclic chemistry. A review comparing the chemistry of thieno[2,3-h]- and thieno [3,2-i ]-thiophen with that of benzo[ft] thiophen and quinoline has been published. In Advances in Heterocyclic Chemistry, the development of benzo[6] thiophen from 1968 to 1980 and of selenophen from 1970 to 1980 was presented. Other aspects of thiophen chemistry are treated in chapters on Dewar Heterocycles," on Cyclizations under Vilsmeier Conditions, on Polyfluoroheteroaromatic Compounds, and on Reactions of Benzyne with Heterocyclic Compounds. " Several dissertations treating various aspects of thiophen chemistry have appeared. " ... 

A general cyclization reaction of 6-lithiated (trimethylsilyl)alkynes is exemplified by the conversion of the iodo compound 499 into the methylenecyclopentane 500 by treatment with -butyllithium . Heating a mixture of the acetylenic silane 501 and 2-(benzyloxymethyl)allylzinc bromide 502 resulted in the diene 503, which cyclized to the methylenecyclopentene 504 in the presence of tetrakis(triphenylphosphine)palladium ... 

Aryl and alkyl-substituted diynes and tetraynes have been synthesized in good yields (82-99%) by TBAF-promoted desily-lation and Cu-catalyzed oxidative dimerization of triisopropylsi-lyl (TlPS)-protected acetylenes (eq 38). Copper acetate was used as oxidant in this reaction. Aryl- and alkenyl alkynes were made under similar conditions (eq 39). Pd/C with TBAF was used in ligand- and copper-free, one-pot, domino Halex-Sono-gashira reactions. Similarly, TBAF promoted the synthesis of 2-substituted indoles by a tandem Sonogashira/cyclization reaction of 2-iodoanilines and terminal alkynes. ... 

Reppe [83] developed the Reppe reaction of carbonylation, vinylation, ethy-nylation and cyclization by reaction of acetylene with carbon monoxide, water, alcohols, amines, aldehydes and ketones, etc. In 1952, the reaction of olefins in place of acetylene, in the presence of metal carbonyl catalysts, produces alcohols. This process is called the new Reppe process, and butanol has been industrially produced by this process since around 1960. 

Coupling reactions with organic halides [62,71], cyclization addition of acetylenes with carbon dioxide, hydrogenations, reductions, oxidations, alkoxylations, ami-nations and insertions [100] have been reported as using nickel compounds. 

The cyclization additions of acetylenes with carbon dioxide are the reactions to form the bicyclic compounds such as alkylene and heterocyclic ring (e.g., 2-pyrone ring) compounds, one of the reactions being shown in eq. (19.57) [68,102,103]. 

The cyclization reaction of non-conjugated acetylenic phosphonate dimethyl esters (167 n = 2, 3, 4) in the presence of (bis-collidine) bromo- and iodohexafluorophosphates led to the formation of halo enol phostones (Scheme 58). This cyclization allowed the formation of 6 to 8-membered compounds, either by endo or exo pathways vide supra Scheme 56). 

Reyiews.—Recent reviews on areas of acetylenic chemistry include synthetic routes to average-ring-size cycloalkynes, a study of the bonding in metal-acetylene complexes, transition-metal complexes of acetylene, intramolecular cyclization reactions with acetylenic bond participation, oligomerization of acetylenes induced by metals of the nickel triad, an article on the handling of acetylenic compounds, and a book on preparative acetylenic chemistry. ... 

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