Ethanol, alkyne elimination

Triazines react also with electron-rich dienophiles such as ethyl vinyl ether (401 R = Et) or vinyl acetate (401 R = Ac) in boiling dioxane to yield the pyridine derivatives (376). After the usual [4 + 2] cycloaddition and nitrogen elimination from the bicyclic compound (402), the dihydropyridines (403) eliminate ethanol or acetic acid to give the aromatic pyridines (376). The dienophiles (401) can therefore be used as alkyne equivalents (69TL5171). 

Intermediate V thus formed would allow for the transfer of the dimethylamino group to the alkyne by virtue of a push-pull effect of coordination of the triple bond with the catalytic proton and the electron releasing effect of the two ether oxygens. The discoverers of this reaction assumed that it proceeds by way of carbonium ions (VI and VII) which subsequently eliminate or add ethanol to give III and IV, respectively. 

Pentacarbonyl(carbene)chromium complexes with a primary amino group at the alkenyl terminus 32, readily obtained by addition of ammonia to alkynyl-substituted complexes, initially rearranged upon heating to pentacarbonylchromium-coordinated 1-azabuta-l,3-dienes 33, which subsequently underwent [4 + 2] cycloaddition of an alkyne to give coordinated 4-ethoxy-1,4-dihydropyridines the corresponding pyridines, e.g. 34, were formed by 1,4-elimination of ethanol. ... 

Finally, insertion of alkynes into dihydrogen mononuclear cationic rhenium complexes, stabilized by the triphos (= l,l,l-tris(diphenylphosphinomethyl)ethane) ligand, leads to vinylidene derivatives which react with water or ethanol eliminating silanols or methane, Fig. 18. Solvents were acetone or THF no chromatography was used. 

Other examples of elimination have been studied. A vicinal dibromide provides an alkyne when treated by ethanolic potassium hydroxide (Eq. 36). 

Elimination of halogen from a,P-dihalides in a variety of circumstances has been observed and can be used to prepare alkenes and alkynes from vicinyl dihalides. The process is usually of limited utility since the dihalides themselves are often prepared from the unsaturated compounds. As shown in Scheme 7.42, both erythro (meso) and threo ( )-2,3-dibromobutane diastereomers undergo stereospecific elimination of bromine to their respective (E)- (or trans) and (Z)- (or cis) 2-butenes on treatment with iodide anion (T) in ethanol (ethyl alcohol, CH3CH2OH). 

In the intramolecular method the organometallic reagent is attached to the alkyne in such a way that it cannot reach the silicon atom intramolecularly, yet it can easily reach the yS-carbon atom. The synthesis of 1-silaaUenes 571, 588, 589 by the dehalogenative intramolecular carbometalation-elimination (DICE) reaction is shown in equation 209. In the reaction of 2-bromo-2 [(fluorosilyl)]ethynyl]biphenyls 632-634 with 2 eq of f-butyllithium at 0 °C, the 1-silaUenes 571,588,589 wctc obtained in nearly quantitative yields Due to extreme steric congestions, 571 is stable at air and moisture and can be dissolved in boiling ethanol without observable transformation. ... 

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