Lithium metal alkynes

An alternative method for the conversion of an alkyne to an alkene uses sodium or lithium metal as the reducing agent in liquid ammonia as solvent. This method is complementary to the Lindlar reduction because it produces... 

O Lithium metal donates an electron to the alkyne to give an anion radical. .. ... 

Dissolving-Metal Reduction of Aromatic Compounds and Alkynes. Dissolving-metal systems constitute the most general method for partial reduction of aromatic rings. The reaction is called the Birch reduction,214 and the usual reducing medium is lithium or sodium in liquid ammonia. An alcohol is usually added to serve as a proton source. The reaction occurs by two successive electron transfer/proto-nation steps. 

A first structural characterization of a cyclobutadiene dianion was performed by Boche and coworkers, who generated the dilithium salt of the 1,2-diphenylbenzocyclobutadiene dianion (143) (by deprotonation with n-butyllithium in the presence of TMEDA) (Figure 17) . Nevertheless, the molecular structure of 143 resembles more the structures of dilithiated alkenes, synthesized by reaction of the corresponding alkynes with metallic lithium. In that class of compounds, carbon-carbon bonds, capped by two lithium centres, are the structural motif (see Section II. E). 

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 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°. 

Ammonia, NH3 Used as a solvent for the reduction of alkynes by lithium metal to yield trans alkenes (Section 8.5). 

Nanoparticles of Cu(0) powder that undergo oxidation in the presence of an amine hydrochloride lead to active Cu(I). The amine is presumed to play several roles reduction of Cu(II) to Cu(I) stabilization of Cu(I) as ligand and enhancement of solubility of copper in organic media. Nanosize clusters also catalyze click reactions, in this case in H2O//-BUOH at 25 °C without salts. A simplified route to copper nanoparticles (CuNPs) relies on CuC, lithium metal, and catalytic amounts of 4,4 -di-t-butylbiphenyl (DTBB) in THF at ambient temperatures.They exist as a range of nanospheres mainly between 1 and 6 nm, as analyzed by transmission electron microscopy (TEM). Terminal alkynes and a variety of azides can be cyclized in THF (88-98% isolated yields) between room and refluxing temperatures. Cycloadditions take place within 10-30 minutes, and simple filtration suffices to remove the catalyst. Recycling of CuNPs, however, is not an option. 

A ketone or aldehyde is reduced to an alcohol by reaction with sodium or lithium metal in liquid ammonia, in the presence of ethanol. This is called a dissolving metal reduction and it proceeds by an alkoxy-radical known as a ketyl 21, 22, 46, 49. Alkynes are reduced to E-alkenes under dissolving metal conditions. Benzene is reduced to a cyclohexadiene under the same conditions 23, 28, 33, 37, 41, 48. 

Sodium or lithium metal in ammonia causes a dissolving metal reduction of alkynes to give a trans alkene product. This reduction does not work for terminal alkynes, but other metals are available (such as Zn-Cu) to accomplish this transformation. 

Alkynes can also be reduced to alkenes by using either sodium or lithium metal in liquid ammonia or in low-molecular-weight primary or secondary amines. The alkali metal is the reducing agent and, in the process, is oxidized to M, which dissolves as a metal salt in the solvent for the reaction. Reduction of an alkyne to an alkene by lithium or sodium in liquid ammonia, NH3(Z), is stereoselective it involves mainly anti addition of two hydrogen atoms to the triple bond. 

Alkenes with perfluoroalkyl substituents have been prepared by the reaction of perfluoroalkyl iodides with terminal alkynes in the presence of ultrasonically dispersed zinc and Cul. A general procedure for the formation of organocopper reagents from alkyl and aryl halides and lithium metal in the presence of Cul or l-pentynylcopper(I) under ultrasonic irradiation has also been described. ... 

A useful alternative to catalytic partial hydrogenation for converting alkynes to alkenes IS reduction by a Group I metal (lithium sodium or potassium) m liquid ammonia The unique feature of metal-ammonia reduction is that it converts alkynes to trans alkenes whereas catalytic hydrogenation yields cis alkenes Thus from the same alkyne one can prepare either a cis or a trans alkene by choosing the appropriate reaction conditions... 

The alkynylation of estrone methyl ether with the lithium, sodium and potassium derivatives of propargyl alcohol, 3-butyn-l-ol, and propargyl aldehyde diethyl acetal in pyridine and dioxane has been studied by Miller. Every combination of alkali metal and alkyne tried, but one, gives the 17a-alkylated products (65a), (65c) and (65d). The exception is alkynylation with the potassium derivative of propargyl aldehyde diethyl acetal in pyridine at room temperature, which produces a mixture of epimeric 17-(3, 3 -diethoxy-T-propynyl) derivatives. The rate of alkynylation of estrone methyl ether depends on the structure of the alkyne and proceeds in the order propar-gylaldehyde diethyl acetal > 3-butyn-l-ol > propargyl alcohol. The reactivity of the alkali metal salts is in the order potassium > sodium > lithium. 

We see from these examples that many of the carbon nucleophiles we encountered in Chapter 10 are also nucleophiles toward aldehydes and ketones (cf. Reactions 10-104-10-108 and 10-110). As we saw in Chapter 10, the initial products in many of these cases can be converted by relatively simple procedures (hydrolysis, reduction, decarboxylation, etc.) to various other products. In the reaction with terminal acetylenes, sodium acetylides are the most common reagents (when they are used, the reaction is often called the Nef reaction), but lithium, magnesium, and other metallic acetylides have also been used. A particularly convenient reagent is lithium acetylide-ethylenediamine complex, a stable, free-flowing powder that is commercially available. Alternatively, the substrate may be treated with the alkyne itself in the presence of a base, so that the acetylide is generated in situ. This procedure is called the Favorskii reaction, not to be confused with the Favorskii rearrangement (18-7). ... 

The analogous dimerization of alkynes over Fe(C0)5 is not applicable, so clearly a different route towards alkynylated derivatives of 25 was needed. Comparison of 25 to cymantrene suggests that metallation of the hydrocarbon ligand should be the route of choice for the synthesis of novel substituted cyclobutadienes. In the literature, addition of organolithium bases (MeLi, BuLi) to the CO ligands with concomitant rearrangement had been observed [25]. But the utilization of LiTMP (lithium tetramethylpiperidide, Hafner [26]) or sec-BuLi as effectively non-nucleophilic bases led to clean deprotonation of the cyclobuta-... 

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