Alkenes reduction with alkali metals

Open-chain acetylenes yield pure trans alkenes on reduction with alkali metal in liquid ammonia. However, reduction of cyclodecyne (111) with sodium in liquid ammonia led to a mixture of cis- (112, > 90%) and /rn/w-cyclodecene (113, > 4%y -The formation of the cis isomer (112) was attributed to the reduction of 1,2-cyclo-decadiene (114) formed by a rapid isomerization of the starting cycloalkyne, while the formation of the trans isomer (113) was ascribed to the direct reduction of 111. 

Control of cis-trans-geometry is well illustrated by catalytic hydrogenation or alkali metal reduction of alkynes, as discussed in Sections 4.8.B and 4.9.C, respectively. The Lindlar catalyst (sec. 4.8.B) allows selective reduction of alkynes to the cis-alkene, as in the conversion of 23 to 24 in Kaiser s synthesis of niphatoxin B. This contrasts sharply with treatment of an alkyne with alkali metals (sec. 4.9.C) to give the... 

Cobalt has an odd number of electrons, and does not form a simple carbonyl in oxidation state 0. However, carbonyls of formulae Co2(CO)g, Co4(CO)i2 and CoJCO),6 are known reduction of these by an alkali metal dissolved in liquid ammonia (p. 126) gives the ion [Co(CO)4] ". Both Co2(CO)g and [Co(CO)4]" are important as catalysts for organic syntheses. In the so-called oxo reaction, where an alkene reacts with carbon monoxide and hydrogen, under pressure, to give an aldehyde, dicobalt octacarbonyl is used as catalyst ... 

Very little work has been done on reactions involving nucleophiles formed from hydrocarbons.124-142 The limitation on basicity of the carbanion, so that it does not react with solvent, has led to use of conjugated hydrocaibons, such as dienes or alkenes conjugated with aromatic rings. When initiated by dissolving alkali metal in liquid ammonia, complex mixtures are often produced on account of reduction processes,124 and regiochemistry and multiplicity of arylation in conjugated systems also create prob-... 

Dihalocydopropanes readily undergo reductive dehalogenation under a variety of conditions. Suitable choice of reagents and reaction conditions will allow the synthesis of monohalocyclopropanes or the parent cyclopropanes.19 " The ease of reduction follows the expected order I > Br > Cl > F. In general, complete reduction of dibromo and dichloro compounds is accomplished by alkali metal in alcohol,99-102 liquid ammonia103 or tetrahydrofuran (equations 28 and 29).104 The dihalocydopropanes can be reduced conveniently with LAH (equation 30).105 LAH reduction is particularly suited for difluoro compounds which are resistant to dissolving metal reductions.19 106 It is noteworthy that the sequence of dihalocar-bene addition to an alkene followed by the reduction of the dihalocyclopropyl compounds (equation 31) provides a convenient and powerful alternative to Simmons-Smith cyclopropanation, which is not always reliable. 

Sodium amalgam is effective for promoting defluorination (Scheme 14), to produce an interesting series of dienes (38), (40) and (42), especially (38) [7, 8, 36], but tetrakis(dimethylamino)ethene (TDAE) (43) (Scheme 15) is a much safer system to use and, consequently, with the latter reagent, the process may be scaled up [36]. TDAE (43) is successful because its donor capacity is high, i.e. it has been variously described as being similar to that of zinc [37] or to alkali-metals [38] and will react, therefore, with most perfluorinated alkenes or -cycloalkenes. Furthermore, the fact that the dienes (38), (40) and (42) may be isolated derives from the fact that the dienes have more CF= sites than the starting alkenes and consequently their respective reduction potentials vary by... 

This reductive coupling involves two steps. The coupling is induced by single electron transfer to the carbonyl groups from alkali metal, followed by deoxygenation of the 1,2-diol with low-valent titanium to yield the alkene. 

McMurry developed a reduction procedure that is used for alkenation of carbonyl compounds in the presence of low-valent titanium (LVT) reagent. The reagent (thought to be a mixture of Ti(0) and Ti(II) species) is formed by the reduction of TiCU or TiCb with a suitable reducing agent (Zn-Cu alloy, LiAlH4 or alkali metal are the most commonly used). 

Birch reduction of aromatic compounds involves reaction with an electron-rich solution of alkali metal lithium or sodium in liquid ammonia (sometimes called metal ammonia reduction). Usually a proton donor such as tert-butanol or ethanol is used to avoid the formation of excess amount of LiNH2 or NaNH2. The major product is normally a 1,4-diene. This reaction is related to the reduction of alkynes to frans-alkenes ° (section 6.2.2). 

Red ReOs, which can be obtained by CO reduction of the heptaoxide at 175 280 °C, is a good electrical conductor with a metal-like temperatme dependence the d electron is clearly delocalized. It has octahedral Re and easily forms nonstoichiometric bronzes in which an alkali metal donates an electron to the metal-0x0 lattice and enters the stmetme as a cation. Supported ReOs is important in alkene metathesis catalysis. No Tc analog is known. Blue Re20s, brown Re02, and black Rc203 are also known. For Tc, the only lower oxide is TCO2. 

Both disubstituted alkynes (Chapter 3.3, this volume) and isolated terminal double bonds may be reduced by alkali metals in NH3, but isolated double bonds are usually stable to these conditions. However, 16,17-secopregnanes (10 equation 8) afford mixtures of cyclization products (11) and (12) in 61% to 80% yield with Na naphthalenide-THF, Na-NHs-THF, Na-THF or Li-NHs-THF. With Na-NHa-THF-r-butyl alcohol, a 91% yield of a 72 28 mixture of (11) (12) (R = Me) is obtained. This type of radical cyclization of alkenes and alkynes under dissolving metal reduction conditions to form cyclopentanols in the absence of added proton donors is a general reaction, and in other cases it competes with reduction of the carbonyl group. Under the conditions of these reactions which involve brief reaction times, neither competitive reduction of a terminal double bond nor an alkyne was observed. However, al-lenic aldehydes and ketones (13) with Li-NHs-r-butyl alcohol afford no reduction products in which the diene system survives. ... 

Isolated carbon-carbon double bonds are not normally reduced by dissolving metal reducing agents. Reduction is possible when the double bond is conjugated, because the intermediate anion can be stabilized by electron delocalization. The best reagent is a solution of an alkali metal in liquid ammonia, with or without addition of an alcohol - the so-called Birch reduction conditions. Under these conditions conjugated alkenes, a,p-unsaturated ketones and even aromatic rings can be reduced to dihydro derivatives. 

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