Ethyne alkylation

Strong bases readily eliminate thiol from vinylic sulfides. 1,2-Bis(alkyl- or arylthio)ethynes for example, react with two equivalents of BuLi or alkali amide to give metallated alkyl- or arylthioacetylenes and thiolates [24] ... 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

What is common to all these results is that the adsorption of ethyne on Pd (as on Ni) leads to gradual self-hydrogenation to give first alkenyl and then to give alkyl-type surface species. For chemical balance these must coexist with hydrogen-deficient carbonaceous species. 

Room-temperature spectra of species on Ni and Pd evaporated films that were also deposited at room temperature showed spectra from a mixture of ethyne-like species, probably 7r-bonded, and alkyl species with soft rCH modes indicating agostic C-H-M interactions. 

Addition of the anion generated from a monosubstituted alkyne (or ethyne) to an alkylborane followed by iodination places an alkyl group from the alkylborane on the alkyne (Equation B4.5). 

The addition of alkoxycarbonylcarbene derived by catalysed decomposition of methyl diazoacetate to several simple, and in particular terminal, alkynes leads to low yields S7), but the reaction with 1 -trimethylsilylalkynes proceeds reasonably efficiently subsequent removal of the silyl-group either by base or fluoride ion provides a route to l-alkyl-3-cyclopropenecarboxylic acids. In the same way 1,2-bis-trimethylsilyl-ethyne can be converted to cyclopropene-3-carboxylic acid itself58 . The use of rhodium carboxylates instead of copper catalysts also generally leads to reasonable yields of cyclopropenes, even from terminal alkynes 59). 

The benzannulation reaction tolerates a range of alkyl and aryl allcynes, which may bear additional functionalities. The simultaneous presence of two bulky substituents directly attached to the C=C bond, as for example in bis(trimethylsilyl)ethyne, however, blocks the final electrocyclization and causes the reaction to stop at the vinyl ketene stage [44]. Neither very electron-rich nor very electron-poor allcynes can undergo benzannulation. Strongly electron-deficient allcynes such as hexafluorobut-2-yne cannot adequately compete with car-... 

Upon treatment of bis(dimethylsilyl)ethyne 62 with trialkylborane, 1,1-organoboration took place with formation of 63, which by hydrosilylation of the C=C bond of the alkyl group resulted in the formation of the new seven-membered heterocycle 64 (Scheme 13). The presence of the SiH—B bridge in 63 was confirmed unequivocally by IR, 111, 13C, and 29Si NMR spectroscopic evidence <1999AGE124>. Compound 64 was isolated by distillation as a colorless, extremely air-sensitive liquid. 

The simplest alkyne, HC = CH, named in the lUPAC system as ethyne, is more often called acetylene, its common name. The two-carbon alkyl group derived from acetylene is called an ethynyl group (HC=C-). Examples of alkyne nomenclature are shown in Figure 11.1. 

Alkylation of dilithium ethyneditellurolate with methyl iodide or ethyl iodide yielded bis[alkyltelluro]ethynes ... 

Some reagents are able to alkylate or acylate an hydroxy function, and the group is also subject to nucleophilic substitution. Thus, whereas ethyne with cadmium acetate catalyst at 230°C is able... 

Examples of some compounds that exhibit a high level of reactivity as dienophiles are shown in Scheme 10.2 (p. 844). Scheme 10.4 presents some typical uncatalyzed D-A reactions. Part A shows normal electron demand reactions. Each of the reactive dienophiles has at least one strongly electron-attracting substituent on the carbon-carbon double or triple bond. Part B shows several inverse electron demand D-A reactions. Ethene, ethyne, and their alkyl derivatives are poor dienophiles and react only under vigorous conditions. 

A primary alkyl halide can be prepared from a terminal alkene (using HBr in the presence of a peroxide). A terminal alkene can be prepared from a terminal alkyne, and a terminal alkyne can be prepared from ethyne and an alkyl halide with the appropriate number of carbons. 

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