Cycloaddition Reactions of Acetylenes

Ene and 2 + 2 cycloaddition reactions of acetylenic esters (7, 7 8 8, 13). In the early studies on the reaction of acetylenic esters with alkenes, Snider used A1C1( as the Lewis acid catalyst. The presently preferred catalyst is C2H5A1C12, which is a Lewis acid and also serves as a proton scavenger by reaction with HC1 to give ethane and AIC13. It is generally used in amounts close to 1 equivalent for a neutral Hlkene. 

Benzyne probably forms from acetylene by cycloaddition reaction of acetylene and diacetylene, concerted or stepwise (Woodward and Katz, 1959 Woodward and Hoffmann, 1965a, b) ... 

FIGURE 5. Non-concerted, asymmetric [2 + 2] cycloaddition reaction of acetylene on a Si(100)-2x1 surface. All atoms, except the C atoms, are Si atoms... 

For reviews of carbene complexes and transition metal mediated cycloaddition reactions of alkynes in organic synthesis see a) Dotz KH (1984) Angew Chem 96 573 Angew Chem Int Ed Engl 23 587 b) Schore NE (1988) Chem Rev 88 1081 c) Casalnuova JA, Schore NE (1995) Organomettalic cycloaddition reactions of acetylenes. In Stang PJ, Diederich F (eds) Modern acetylene chemistry, VCH, Weinheim, p 139 d) de Meijere A (1996) Pure Appl Chem 68 61... 

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 convenient synthesis of 2,5-bis(trifluoromethyl)tliiophaie 167 involved the [4+2]-cycloaddition reaction of acetylenes to 2,5-bis(trifluoroniethyl)-l,3,4-tliiadiazole 165 and subseqnent eUmination of nitrogen. The reaction proceeded under sufficiently mild conditions and led to 2,5-bis(trifluoromethyl)tliiophene 167 in high yield [88]. 

The cycloaddition reactions of acetylenes with annulenes and azulenes has been studied and can lead to a variety of products (Scheme 80). ... 

Such syntheses are difficult by classical methods [30-32], Chemical modifications of triple carbon-carbon bonds in reactions such as nucleophilic addition and cycloaddition lead to a wide range of new heterocycles, that cannot be synthesized by other routes [33-37], This area of chemistry of acetylene compounds is extremely promising since it complements earlier methods. New methodology for the formation of cyclic systems based on cycloaddition reactions of acetylene units has been described in the recent literature works published in the years 2000-2012 are considered in detail in Chapter 2. 

Zeller, E. A., B. Gartner, and P. Hemmerich 4a,5-Cycloaddition Reactions of Acetylenic Compounds at the Flavoquinone Nucleus as Mechanisms of Flavo-protein inhibitors. Z. Naturforsch. 27b, 1050 (1972). 

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