Ammonia, addition alkynes

High temperature, pressure, and catalyst are required to achieve addition of ammonia to alkynes. Acetylene and ammonia yield a complex mixture of heterocyclic nitrogen bases.311,312 Ethylideneimines, thought to form through the intermediate enamines, are the products of the reaction of acetylene with primary alkylamines in the presence of catalysts.313... 

The alternative reverse addition procedure can give incomplete reduction of the alkyne (33). An increase in the ratio of liquid ammonia to alkyne (34), the addition of co-solvents (23), the use of lithium rather than sodium, or the use of a higher temperature in an autoclave are advisable for the reduction of high molecular weight alkynes to overcome solubility problems which can also result in incomplete reduction. The resulting olefin is usually very pure ji isomer containing no detectable Z isomer. Use of an alcohol as a co-solvent and proton donor can accelerate the reduction, but the resulting olefin then contains a minor amount of the Z isomer. Polymer-bound alkynes can not be successfully reduced with sodium in liquid ammonia (35). 

An anti addition of hydrogen atoms to the triple bond occurs when alkynes are reduced with lithium or sodium metal in ammonia or ethylamine at low temperatures. 

The synthesis of alcohols, ethers, and ketones by metal-catalyzed addition of water or alcohols to alkenes and alkynes is a well-established reaction in organic chemistry. Many regio- and stereoselective modifications of these reactions are known. In contrast, the analogous addition of ammonia or primary and secondary amines to nonactivated alkenes and alkynes has not had a comparable development, in spite of extensive efforts. In this section, we summarize the recent results of amination to unsaturated compounds. 

Many 1-alkynes can be prepared in high yields by adding bromine to 1-alkenes and subsequently treating the adduct with three equivalents of alkali amide in liquid ammonia the 1-alkyne is liberated (after removal of the ammonia) by addition of water. 

Enyne ethers HC=CCH=CHOR are useful synthetic intermediates. They can be prepared by base-catalysed addition of alcohols to diacetylene. The required conditions are rather forcing and not very attractive for laboratory scale preparations. A much more convenient way to prepare the enyne ethers (in these cases more than 80 rel.% of the -isomer is obtained) consists in treatment of the easily accessible 1,4-dialkoxy-2-alkynes with two equivalents of alkali amide in liquid ammonia. The first step in this elimination is the (transient) formation of an "anion RO-fiH-C CCH OR, which eliminates ROH (143). The resulting cumulenic ether ROCH=C=C=CH2 is immediately converted into the metallaied enyne ether. 

Similarly, 1,1-dioxides are formed either by the cyclodehydration of diacylsulfones and ammonia (Scheme 115) (720S(52)135) or through the action of sodamide on bis(2-chlorovinyl) sulfones (255) which are themselves obtained by a two-step sequence from alkynes (Scheme 116) (72S311). A less general route involves the addition of the thiirene sulfone (256) to the mesoionic oxazolone (257). In this example the initial adduct (258) eliminates carbon dioxide spontaneously, thus affording the 1,4-thiazine (259) (75CL1153). 

In contrast to the reaction of alkenes, the addition of H—SR reagents to alkynes generally requires higher temperatures and the presence of a base.534 Terminal alkynes generally react to give anti-Markov-nikov products. Substituted alkynes, however, will provide terminal alkenes if the reaction is carried out in sodium and liquid ammonia (equation 305).554 Polyynes react with H2S to produce thiophenes (equation 306).555... 

The metal-ammonia reduction proceeds by addition of an electron to the alkyne to form a radical anion, followed by protonation to give a neutral radical. Protons are provided by the ammonia solvent or by an alcohol added as a cosolvent. Addition of another electron, followed by another proton, gives the product. 

Na, or Li in liquid ammonia, for example) to reduce aromatic rings and alkynes. The dissolving metal reduction of enones by lithium metal in liquid ammonia is similar to these reactions—the C=C bond of the enone is reduced, with the C=0 bond remaining untouched. An alcohol is required as a proton source and, in total, two electrons and two protons are added in a stepwise manner giving net addition of a molecule of hydrogen to the double bond. 

The best way of ensuring anti addition of hydrogen across any triple bond is to treat the alkyne with sodium in liquid ammonia. 

The preparation of pure isolated E olefins is readily accomplished by the reduction of an alkyne with metallic sodium or lithium in liquid ammonia (27,32). This reaction is preferably carried out by the addition of the alkyne in an ether to a mixture of sodium (or lithium) in liquid ammonia at -30°. 

The most general synthetic route to benzene oxides-oxepins is that initially developed by Vogel for 1. 1,4-cyclohexadienes (readily available from [2+4] cycloaddition of alkynes and butadienes, lithium-ammonia reduction of arenes, or dehydration of cyclohexenols) were converted to dibromoepoxides, the immediate precursors of benzene oxides. Modifications of this route have been used to prepare Ic and Id. Treatment of the monosubstituted arene oxide 43 (Figure 3) with (Et)4NF or thermal isomerization of 3-oxaquadricyclane provide additional synthetic routes to la. Similarly, the thermal (or photochemical) isomerization of the monoepoxide of Dewar benzene yielded la. ... 

Triple bonds can also be selectively reduced to double bonds with diisobutyla-luminum hydride (Dibal-H), ° with activated zinc (see 12-38), with hydrogen and Bi2B-borohydride exchange resin, or (internal triple bonds only) with alkali metals (Na, Li) in hquid ammonia or a low-molecular-weight amine. Terminal alkynes are not reduced by the Na NH3 procedure because they are converted to acetylide ions under these conditions. However, terminal triple bonds can be reduced to double bonds by the addition to the Na—NH3 solution of (NH4)2S04, which liberates the free ethynyl group. The reaction of a terminal alkyne with... 

When an alkyne is then added to the solution, an electron adds to the triple bond to yield an intermediate anion radical—a species that is both an anion has a negative charge) and a radical (ha.s an odd number of electrons). This anion radical is a strong base, which removes from ammonia to give a vinylic radical. Addition of a second electron to the vinylic radical gives a vinylic anion, which abstracts a second from ammonia to give trans alkene product. The mechanism is shown in Figure 8.4. 

Lithium or sodium in liquid ammonia reduces disubstituted alkynes to tran -alkenes. The reaction is carried out by addition of the alkyne in ether to a mixture of Na in NH3 (1), and the alkene produced is the (ii)-isomer. Overreduction and isomerization of the alkene are suppressed by addition of t-BuOH. ... 

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. ... 

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