Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Vinyllithiums unsaturated

Vinyllithium [917-57-7] can be formed direcdy from vinyl chloride by means of a lithium [7439-93-2] dispersion containing 2 wt % sodium [7440-23-5] at 0—10°C. This compound is a reactive intermediate for the formation of vinyl alcohols from aldehydes, vinyl ketones from organic acids, vinyl sulfides from disulfides, and monosubstituted alkenes from organic halides. It can also be converted to vinylcopper [37616-22-1] or divinylcopper lithium [22903-99-7], which can then be used to introduce a vinyl group stereoselectively into a variety of a, P-unsaturated systems (26), or simply add a vinyl group to other a, P-unsaturated compounds to give y, 5-unsaturated compounds. Vinyllithium reagents can also be converted to secondary alcohols with trialkylb o r ane s. [Pg.414]

Removal of the carbonate ring from 7 (Scheme 1) and further functional group manipulations lead to allylic alcohol 8 which can be dissected, as shown, via a retro-Shapiro reaction to give vinyl-lithium 9 and aldehyde 10 as precursors. Vinyllithium 9 can be derived from sulfonyl hydrazone 11, which in turn can be traced back to unsaturated compounds 13 and 14 via a retro-Diels-Alder reaction. In keeping with the Diels-Alder theme, the cyclohexene aldehyde 10 can be traced to compounds 16 and 17 via sequential retrosynthetic manipulations which defined compounds 12 and 15 as possible key intermediates. In both Diels-Alder reactions, the regiochemical outcome is important, and special considerations had to be taken into account for the desired outcome to. prevail. These and other regio- and stereochemical issues will be discussed in more detail in the following section. [Pg.660]

A simple two-step protocol for the generation of a terminal diene is to add allyl magnesium bromide to an aldehyde or a ketone and subsequent acid or base catalysed dehydration (equation 34)72. Cheng and coworkers used this sequence for the synthesis of some indole natural products (equation 35)72a. Regiospecific dienones can be prepared by 1,2-addition of vinyllithium to a,/l-unsaturated carbonyl compounds and oxidative rearrangement of the resulting dienols with pyridinium dichromate (equation 36)73. [Pg.378]

The /S-bromodihydropyran (652) on halogen-metal exchange with f-butyllithium affords a useful organometallic (653) that can function as the equivalent of a 5-hydroxy aldehyde enolate (Scheme 151) (79TL67). The vinyllithium species adds to ketones and aldehydes at -110 °C in good yield and also reacts in a conjugate mode with a,/S-unsaturated carbonyl compounds in the presence of copper(I) ions. On hydrolysis and oxidation, the dihydropyran component (654) can be transformed into a substituted 5-lactone (655). [Pg.474]

Waak 13-15) was the first to attempt anionic initiation by means of an unsaturated lithiumorganic compound, namely vinyllithium. This reagent can be obtained by an exchange reaction between tetravinylzinc and butyllithium ... [Pg.7]

Vinyllithium initiates the polymerization of styrene and of similar monomers. However, the rate of initiation is fairly low, and the molecular weights exhibited by the polymers obtained are by one order of magnitude higher than the expected ones. This is not the only drawback of the method. A macromolecule obtained by anionic polymerization using vinyllithium carries an allylic double bond at the chain end, and this type of unsaturation is not adequate for the subsequent radical copolymerization because of its low reactivity towards radicals and of isomerization that can take place. [Pg.7]

Waak 15) also used other unsaturated lithium organic initiators (such as allyl-lithium, crotyllithium) for styrene polymerization. Though more efficient than vinyllithium, these initiators exhibit the same disadvantages. Therefore, the same conclusions apply. [Pg.7]

Addition of an acyl anion or its equivalmt to a,p-unsaturated ketones is an important synthetic mediod for 1,4-dicaibonyl compounds. In the palladium method for 1,4-dicartxHiyl compounds, a vinyl Grignard or vinyllithium reagent is used as a synthetic equivalent of the acetyl anion. Reaction of lithium divinylcuprate with 2-cyclohexenone (44) afforded 3-vinylcyclohexanone (45). which was oxidized to the 1,4-diketone (46 Scheme 12). ° ... [Pg.457]

Thies ° has shown that the 1,3-oxy-Cope rearrangement can be used to advantage as a two-carbon ring expansion method (equation 9). ° Addition of vinyllithium to a cyclic p. -unsaturated ketone (6), followed by pyrolysis of the TMS ether (7) of the resulting alcohol, gives predominandy the 1,3-rearrangement product (8). [Pg.787]

When the vinyllithium intermediate (190) is treated with water, the procedure provides a useful synthetic method for the conversion of ketones to alkenes (Scheme 79). The method is illustrated by the conversions of the tosylhydrazones of phenyl isopropyl ketone (194) and dipropyl ketone (195) to the alkenes (196) and (197), respectively (Scheme 79). In this method, experiments have demonstrated that the hydrogen is derived from the water, as indicated in Scheme 79, and thatTMEDA is an excellent solvent. The vinyllithium intermediate (190) may be trapped by other electrophiles thus, with carbon dioxide and DMF, the reaction affords ,[i-unsaturated carboxylic acids and aldehydes like (198) and (199) (Scheme 80). [Pg.218]

Some typical reactions of 1,1 -difluoroethene with nucleophiles are summarized in Scheme 2.18. Alkoxides [3], trialkylsilyl anion [4], ester enolates [5], and diphenylphosphinyl anion [6] attack the gem-difluorinated carbon of 5. However, it is noteworthy that nucleophilic substitution and proton abstraction are in some cases competitive, and thus s -butyl lithium abstracts the (3 -vinylic proton predominantly to generate vinyllithium. The lithium species can be trapped with an aldehyde, providing difluoroallyl alcohol, which is then hydrolyzed to a, (3-unsaturated carboxylic ester (11) [ 7 ] (Scheme 2.19). Some synthetically useful examples are shown in Schemes 2.20 and 2.21. Tetrathiafulvalene derivative (14) is prepared from difluorinated derivative (13) [8]. An elegant intramolecular version was demonstrated by Ichikawa, which provided a range of cyclized compounds (17), including dihydrofurans, thiophenes, pyrroles, and cyclopentenes, and also corresponding benzo derivatives (20) [2]. [Pg.113]

Trapping reactions of vinyllithium derivatives generated from tosylhydrazones with DMF in TMEDA produce a,/ -unsaturated aldehydes in 10-65% yields as illustrated in equations 66 and 6754,58. [Pg.426]

Or you might be more concerned about which uses the cheaper, or more readily available, starting materials—this was probably also a factor in the choice of methylmagnesium chloride and the unsaturated aldehyde in the second example. Both can be bought commercially, while the alternative route to this secondary alcohol would require a vinyllithium or vinylmagnesium bromide reagent that would have to be made from a vinyl halide, which is itself not commercially available, along with difficult-to-dry acetaldehyde. [Pg.193]

Direct carbonation of vinylalanes as a route to a, j8-unsaturated carboxylic acids is not efficient, but high yields can be obtained by a simple modification. Addition of methyllithium to a vinylalane produces an ate complex that reacts with carbon dioxide either directly, or, more likely, after decomposing to a vinyllithium and trialkylalane, to afford a,-unsaturated acids in high yield and stereospedficity ... [Pg.119]

Several investigators have attenuated the potent nucleophilic-ity of vinyllithium intermediates by transmetalation or trapping. Vinylstannanes, chromium-carbene complexes, and particularly vinylsilanes have been prepared. Alkenyllithiums react faithfully to give 1,2-addition products with a, -unsaturated ketones, and mixed cuprate reagents have also been prepared with some success by trapping vinyllithiums with phenylthiocopper for 1,4-addition (eq... [Pg.624]

General Discussion. (Z)-(Trimethylsilyloxy)vinyllithium (1) has essentially been used for a one-pot vinylogation of carbonyl compounds. Condensation of (1) with aldehydes and ketones (2) occurs readily at low temperature (—70 °C) then treatment of the reaction mixture with an acidic solution in mild conditions (IN hydrochloric acid, 0 °C to rt) produces the Q ,/3-unsaturated aldehydes (3), without double bond migration. The ( 0 isomers are obtained alone (from aldehydic compounds) or very predominantly (fromketonic compounds). The intermediate adducts, y-hydroxy enol ethers (4), can be isolated by slightly basic mild hydrolysis (eq l).l... [Pg.694]


See other pages where Vinyllithiums unsaturated is mentioned: [Pg.354]    [Pg.412]    [Pg.354]    [Pg.412]    [Pg.275]    [Pg.64]    [Pg.354]    [Pg.121]    [Pg.123]    [Pg.414]    [Pg.162]    [Pg.273]    [Pg.1022]    [Pg.145]    [Pg.1115]    [Pg.723]    [Pg.723]    [Pg.254]    [Pg.1393]    [Pg.1022]    [Pg.301]    [Pg.427]    [Pg.279]    [Pg.57]    [Pg.723]    [Pg.30]   


SEARCH



Vinyllithium

Vinyllithiums

© 2024 chempedia.info