Lithium metal alkenes

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... 

Subsequently, Julia, Uguen and Callipolitis104 105 used both lithium metal in ethyl-amine and sodium amalgam in ethanol to effect reductive cleavages of j8-hydroxysulphones or of allylic sulphones. The latter reaction is part of a synthetic sequence for the construction of alkenes that has been used with some considerable success... 

The use of lithium amides to metalate the a-position of the N-substituent of imines generates 2-azaallyl anions, typically stabilized by two or three aryl groups (Scheme 11.2) (48-62), a process pioneered by Kauffmann in 1970 (49). Although these reactive anionic species may be regarded as N-lithiated azomethine ylides if the lithium metal is covalently bonded to the imine nitrogen, they have consistently been discussed as 2-azaallyl anions. Their cyclization reactions are characterized by their enhanced reactivity toward relatively unactivated alkenes such as ethene, styrenes, stilbenes, acenaphtylene, 1,3-butadienes, diphenylacetylene, and related derivatives. Accordingly, these cycloaddition reactions are called anionic [3+2] cycloadditions. Reactions with the electron-poor alkenes are rare (54,57). Such reactivity makes a striking contrast with that of N-metalated azomethine ylides, which will be discussed below (Section 11.1.4). 

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

Mechanism Lithium metal donates an electron to the carbon-carbon triple bond. The radical anion A accepts a proton to give a vinylic radical B. Transfer of another electron gives a vinylic anion C (trans-vinylic anion is more stable than the cis-vinylic anion), which on protonation gives trans-alkene (Scheme 6.5). 

Silanes also add to alkenes under radical conditions (using AIBN) with high anti-Markovnikov selectivity. An alternative route to alkylsilanes reacted an alkene with lithium metal in the presence of 3 equivalents of chlorotrimethylsilane. 

Finely divided Ti(0) metal particles produced by reduction of TiClg with potassium or lithium metal or with a zinc-copper couple " reductively couple aldehydes and ketones at elevated temperature to produce alkenes. 

AlkyWthium reagents. 1-Alkenes can be converted into these reagents in two steps anti-Markownikoff addition of thiophenol followed by cleavage of the resulting alkyl phenyl sulfides with a form of lithium metal (equation I). The... 

Metallated alkenes such as alkenyllithium or Grignard species undergo addition reactions with various electrophiles. Reaction with primary alkyl bromides or iodides is possible. Wurtz self-coupled products can be avoided if the alkenyllithium species is generated by tin-lithium exchange, or by insertion of lithium metal into... 

Alkenes can be obtained from aldehydes or ketones on reductive dimerization by treatment with a reagent prepared from titanium(III) chloride and zinc-copper couple (or L1A1H4), or with a species of active titanium metal formed by reduction of titanium(III) chloride with potassium or lithium metal. This McMurry coupling reaction is of wide application, but in intermolecular reactions generally affords a mixture of the E- and Z-alkenes (2.99). 

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. 

Reaction of alkenes with diiodomethane in the presence of a zinc/copper couple produces cyclopropanes by methylene transfer from an organozinc species. This is the established method of synthesising cyclopropanes. However, the procedure requires activation of the zinc by use of Zn/Cu, Zn/Ag couples, I2 or lithium metal. The first of these is most commonly used, but the sensitivity of the system to air makes the results irreproducible and erratic. Furthermore, the reaction which follows has an induction period of indeterminate length and can be extremely violent. Both these factors contribute to making large scale reactions fickle and dangerous. 

Aldehydes (RCHO) react with 1-diazo-l-lithioacetone to give a-diazo- -hydroxyketones (48). These latter compounds can then be converted to P-diketones by the addition of rhodium(il) acetate a procedure which has been applied in the synthesis of -damascone. 1,6,6A -Trithiapentalene and a metal salt catalyse the lithiation of unactivated alkenes to alkenyl-lithiums by lithium metal, constituting a very useful method for preparing such compounds since in the past the preparation of alkenyl-lithiums from lithium metal has been confined to alkenes containing relatively acidic protons. Finally, the allenyl-lithium reagent (49) can be converted to various functionalized allenes by simple electrophiles. ... 

The following reactions of olefins have also been studied formation of dialkyl adducts (517) from vinyl monomers and dienes with lithium metal and alkyl bromides in THF alkylsulphonium salts (518) from thioethers and protonated alkenes hydroformylation of styrene and a-methylstyrene in the presence of bis-(iV-a-methylbenzylsalicylaldiminato)cobalt(n) to give 2-phenylpropanal (optical purity 1.9%) and 3-phenylbutanal (optical purity 2.9%) various electron-rich olefins of the type (519) with primary amines... 

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. ... 

There are a wide variety of methods for introduction of substituents at C3. Since this is the preferred site for electrophilic substitution, direct alkylation and acylation procedures are often effective. Even mild electrophiles such as alkenes with EW substituents can react at the 3-position of the indole ring. Techniques for preparation of 3-lithioindoles, usually by halogen-metal exchange, have been developed and this provides access not only to the lithium reagents but also to other organometallic reagents derived from them. The 3-position is also reactive toward electrophilic mercuration. 

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