Organolithium compounds, reactions with dienes

Barluenga et al. have demonstrated that the reaction of organolithium compounds 45 with zirconocene methyl chloride in THF, followed by addition of different vinyl bromides and further heating to +65 °C, led to dienes 46 and 47 in different ratios (Scheme 19) [50]. The latter was demonstrated to be dependent on the structure of the starting organolithium compound and of the vinyl bromide used. Thus, with the use of nonsubstituted vinyl bromide 48, a mixture of regioisomeric dienes 49 and 50 was obtained, the branched one being the major isomer (Scheme 20). A reverse ratio was obtained for the trans-/3-bro-... 

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

The electropositive nature of alkali metals allows for insertions into alkali metal-carbon bonds. Thus, the reaction of an organoaUcali metal reagent with an unsaturated organic molecule produces a more complex organoaUcali metal compound. In particular, organolithium compounds will react in this way with conjugated dienes, even under mild conditions. For example, 1,3-butadiene reacts with ferr-butyl lithium to give an allyl lithium compound (equation 15). 

The base catalyzed decomposition of arylsulfonylhydrazones of aldehydes and ketones to provide alkenes is called the Bamford-Stevens reaction. When an organolithium compound is used as the base, the reaction is termed the Shapiro reaction. The most synthetically useful protocol involves treatment of the substrate with at least two equivalents of an organolithium compound (usually MeLi or BuLi) in ether, hexane, or tetramethylenediamine. The in s/ft formed alkenyllithium is then protonated to give the alkene. The above procedure provides good yields of alkenes without side reactions and where there is a choice, the less highly substituted alkene is predominantly formed. Under these reaction conditions tosylhydrazones of a,(3-unsaturated ketones give rise to conjugated dienes. It is also possible to trap the alkenyllithium with electrophiles other than a proton. 

The chemical reactivities of the alkali metal organometallic compounds (RM) vary widely depending on metal M, basicity of the solvent systems used, and steric and electronic properties of the organic group R. In many reactions an important factor is the stabilization resulting from formation of a delocalized carbanion system as in the polymerization of dienes or aromatic substituted ethylenes, and in Reactions 3, 4, 5, and 10 in Table I. It is primarily with these delocalized carbanion systems that this review is concerned although saturated organolithium compounds are discussed briefly. 

The functionalization of polymeric organolithium compounds with 3,4-epoxy-1-butene (EPB) provides the potential to prepare a polymer molecule with dual functionality as well as a potential precursor to a diene-functionalized macromonomer. It has been shown that the reaction of EPB with methyllithium results in three modes of addition to the... 

The important conclusion is that the carbonation reaction of polymeric organolithium compounds in hydrocarbon solution with gaseous carbon dioxide can be carried out in essentially quantitative yield by adding sufficient quantities of Lewis bases such as THF or TMEDA prior to the functionalization reaction. It is particularly important to note that this procedure ensures that functionalized polydienes with high 1,4-enchainment can be prepared since the Lewis base is not present during the diene polymerization in hydrocarbon solution. ... 

Condensations of amines with 1-chlorocycloheptene in the presence of the complex base , sodamide and sodium t-butoxide, gave mixtures of enamines and cyclo-hepta-1,2-diene dimer. Excess n-butyl-lithium has been used to convert vinyl and aryl bromides into organolithium compounds which undergo useful reactions with, for example, benzophenone, dimethyl disulphide, diphenyl disulphide, trimethylsilyl chloride, and methyl iodide. Hence 1-bromo-cis-cyclononene was converted into 1-methylthio-cis-cyclononene via 1-lithio-cis-cyclononene the 1-methylthio... 

Many interesting and important synthetic applications of 1,1-diphenylethylene and its derivatives in polymer chemistry are based on the addition reactions of polymeric organolithium compounds with 1,1-diphenylethylenes. Therefore, it is important to understand the scope and limitations of this chemistry. In contrast to the factors discussed with respect to the ability of 1,1-dipheny-lalkylcarbanions to initiate polymerization of styrenes and dienes, the additions of poly(styryl)lithium and poly(dienyl)lithium to 1,1-diphenylethylene should be very favorable reactions since it can be estimated that the corresponding 1,1-diphenylalkyllithium is approximately 64.5kJ/mol more stable than allylic and benzylic carbanions as discussed in Sect. 2.2 (see Table 2). Furthermore, the exothermicity of this addition reaction is also enhanced by the conversion of a tt-bond to a more stable a-bond [51]. However, the rate of an addition reaction cannot be deduced from thermodynamic (equilibrium) data an accessible kinetic pathway must also exist [3]. In the following sections, the importance of these kinetic considerations will be apparent. 

The reactions of carbon dioxide and carbon disulfide with organoUthium reagents have attracted much attention and have been applied in various organic synthesis. Reaction of carbon dioxide with organolithium compounds normally affords carboxylic acids after hydrolysis [50]. The formation of unsymmetrical ketones was reported from the reaction of CO2 and two organolithium compounds via an intermolecular reaction pathway [51]. When 1,4-dilithio-l,3-dienes 1 was treated with CO2, cyclopentadienone derivatives 20 with various substituents could be prepared in high yields in one-pot within several minutes via cleavage of one of the C=0 double bonds (Scheme 10) [52]. The experimental results indicate that this intermolecular reaction pattern affords cyclopentadienones in the reaction mixture before hydrolysis. 

Due to its high ionic character, the carbon-lithium bond is very reactive and adds under mild conditions to ethylene or dienes and under more severe conditions to other alkenes. Some functionalized alkenes can be used, and high regio- and stereo-selectivity is usually observed in these carbolithiation reactions, especially if a precoordination of the lithium organometallic with the alkene is possible. Intramolecular carbolithiations of alkenes proceed under mild conditions and allow the preparation of several stereochemically well defined mono- and bi-cyclic compounds. Alkynes are too reactive, and can lead, with organolithium derivatives, to several side reactions, and seldom afford the desired carbolithiated product in good yield. 

Because the reactions of related in -cyclohexadienyl complexes are synthetically valuable, the reactions of this ligand have been studied extensively. An outline of how this chemistry can be conducted on the Fe(CO)j fragment is shown in Equation 11.51. A variety of cyclohexadienes are readily available from Birch reduction of substituted aromatics. Coordination and abstraction of a hydride, typically by trityl cation, leads to cationic cyclohexadienyl complexes. These cyclohexadienyl complexes are reactive toward organolithium, -copper, -cadmium, and -zinc reagents, ketone enolates, nitroal-kyl anions, amines, phthalimide, and even nucleophilic aromatic compounds such as indole and trimethoxybenzene. Attack occurs exclusively from the face opposite the metal, and exclusively at a terminal position of the dienyl system. This combination of hydride abstraction and nucleophilic addition has been repeated to generate cyclohexa-diene complexes containing two cis vicinal substituents. The free cyclohexadiene is ttien released from the metal by oxidation with amine oxides. ... 

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