Protonation reaction, acetylene

A rather more complex amino alcohol side chain is accessible by a variation of the Mannich reaction. Taking advantage of the acidic proton in acetylenes, propargyl acetate (62) is condensed with formaldehyde and dimethylamine to give the acetylated amino... 

There are numerous theoretical calculations on the protonation reaction of acetylene [2,126-128]. The focus is mostly on the protonation affinities and the structures of vinyl cations using a variety of methods and less on the dynamics of the addition. It is now known... 

The proton-transfer reactions of the vinyl cation H3CC=CH2 have been studied in a flow tube and theoretically. The lifetimes and UV-VIS absorption spectra of many aryl-substituted vinyl cations are reported. The solvent effects on the protonation of acetylene and ethylene are the subject of continuum solvent quantum-chemical calculations. The structures chosen were the symmetrically bridged non-classical vinyl and ethyl cations apparently the different hydration energies of these structures affect the energetics of their protonation in water. ... 

The proton transfer reaction between H3O+ and acetylene (C2H2) is substantially endothermic and therefore cannot occur [32,33]. However, propyne (C3H4), which has a proton affinity greater than H2O, undergoes fast non-dissociative proton reaction with H3O+. 

The proton of terminal acetylenes is acidic (pKa= 25), thus they can be deprotonated to give acetylide anions which can undergo substitution reactions with alkyl halides, carbonyls, epoxides, etc. to give other acetylenes. 

Alkynyl anions are more stable = 22) than the more saturated alkyl or alkenyl anions (p/Tj = 40-45). They may be obtained directly from terminal acetylenes by treatment with strong base, e.g. sodium amide (pA, of NH 35). Frequently magnesium acetylides are made in proton-metal exchange reactions with more reactive Grignard reagents. Copper and mercury acetylides are formed directly from the corresponding metal acetates and acetylenes under neutral conditions (G.E. Coates, 1977 R.P. Houghton, 1979). 

Because acetylene is a far weaker acid than water and alcohols these substances are not suitable solvents for reactions involving acetylide ions Acetylide is instantly converted to acetylene by proton transfer from compounds that contain —OH groups... 

Acetylenic Grignard reagents of the type RC CMgBr are prepared not from an acetylenic halide but by an acid-base reaction in which a Grignard reagent abstracts a proton from a terminal aUcyne... 

Protonated /V-chloroalkyl amines under the influence of heat or uv light rearrange to piperidines or pyrroHdines (Hofmann-Lriffler reaction) (88). The free-radical addition of alkyl and dialkyl-/V-chloramines to olefins and acetylenes yields P-chloroalkji-, P-chloroalkenyl-, and 8-chloroalkenylamines (89). Various N-hiomo- and N-chloropolyfluoroaLkylarnines have been synthesized whose addition products to olefinic double bonds can be photolyzed to fluoroazaalkenes (90). 

Electronegatively substituted acetylenes, such as dimethyl acetylenedicar-boxylate, do not react under normal conditions but will add the elements of hydrogen fluoride by reaction with fluoride ion (e g, CsF or tetraalkylammonium dihydrogen trifluoride) and a proton source under phase-transfer conditions [49, 50] (equation 8)... 

In recent years, trimethylsilyl protection has often been used for the methine proton of the acetylenic group because of the mild reaction conditions for desilylation. As a rule, the starting pyrazole trimethylsilyl derivative is mixed up, at room temperature, with a 2 A aqueous solution of NaOH, potash, or methanol solution in ammonia. 

Thus, ynaminoketones with 1,2-diaminobenzene form benzodiazepines with retention of the dialkylamino group. The reaction occurs as a nucleophilic addition in the absence of catalysts. With Q, /3-acetylenic ketones 1,2-diaminobenzene reacts in the same manner, but under proton-catalyzed conditions (72LA24). At the same time, ynamines and enynamines furnish with 1,2-diaminobenzene substituted benzimidazoles as aresultof double attack at the acetylene bond(83ZOR926 84ZOR1648). 

Acetylene and terminal alkynes are CH-acidic compounds the proton at the carbon-carbon triple bond can be abstracted by a suitable base. Such a deprotonation is the initial step of the Glaser reaction as well as the Eglinton... 

A striking result of this reinvestigation (128, 129) is the observation that the ratio of the product ketone to the acetylene formed from a-bromo-p-aminostyrene is a function of the pH (Table Vll) but that the rate at which they are formed is not. As the pH increases from 3.9 to 13.1, the relative yield of acetylene increases from 16% to 85%. Therefore, the acetylene formation by elimination of a proton from the vinyl cation (path b in route D in Scheme XI) is more susceptible to an increase in base strength than is ketone formation via the enol (path a). This observation is a rare case of pH control over product composition in a 1-El reaction. 

Naumberg, Duong, and Gaudemer (729) have studied the stereochemistry of the addition of various acetylenes to [Co (DMG)2py] in methanol. They found, as already noted by Schrauzer and Windgassen 163), that the main product from the reaction with phenylacetylene in alkaline solution (pH > 10.5) was the jS-phenylvinyl complex (PhCH=CHCo), formed by the trans addition of the Co(I) anion and a proton, i.e.. 

Reduction of acetylenes can be done with sodium in ammonia,220 lithium in low molecular weight amines,221 or sodium in HMPA containing /-butanol as a proton source,222 all of which lead to the A-alkene. The reaction is assumed to involve successive electron transfer and protonation steps. 

Studies of the interaction of acetylenes with zinc oxide clearly provide a very interesting avenue for more detailed study. Results to date, though still very fragmentary, suggest that the view that reactions of unsaturated hydrocarbons over zinc oxide occur via proton abstraction to form a species with considerable anionic character has considerable merit. 

Acetylene is sufficiently acidic to allow application of the gas-phase proton transfer equilibrium method described in equation l7. For ethylene, the equilibrium constant was determined from the kinetics of reaction in both directions with NH2-8. Since the acidity of ammonia is known accurately, that of ethylene can be determined. This method actually gives A f/ acid at the temperature of the measurement. Use of known entropies allows the calculation of A//ac d from AG = AH — TAS. The value of A//acij found for ethylene is 409.4 0.6 kcal mol 1. But hydrocarbons in general, and ethylene in particular, are so weakly acidic that such equilibria are generally not observable. From net proton transfers that are observed it is possible sometimes to put limits on the acidity range. Thus, ethylene is not deprotonated by hydroxide ion whereas allene and propene are9 consequently, ethylene is less acidic than water and allene and propene (undoubtedly the allylic proton) are more acidic. Unfortunately, the acidity of no other alkene is known as precisely as that of ethylene. 

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