Vinyllithium intermediate

When methylene bisphosphonate (169) is reacted in a Horner reaction with an aromatic aldehyde, the alkenyl phosphonate 170 is produced (Scheme 5.25). By metalation with LDA in THF, this is converted to the vinyllithium intermediate 171 that, with the ketone 172, affords a Baylis-Hillman reaction-type product, 173 on base treatment, this is converted to the arylallene 174 [67]. 

A tanden anh-vicinal difunctionalization of alkynes, involving the addition of a lithium organotellurolate to an activated alkyne, with subsequent trapping of the vinyllithium intermediate with electrophiles, was recently reported and named electrotelluration. ... 

Conjugated dienes can be prepared from certain ketones via their trisylhydrazones (386) by the Shapiro reaction (equation 102). This involves a reductive metallation to a vinyllithium intermediate, transmetallation, for example, with Cu(I) iodide, and oxidative coupling. ... 

Since an excess of n-BuLi has been inevitable, iron-ate complexes were proposed as catalytic relevant species, which generate a vinyllithium intermediate by carbometallation. The latter was verified by a quench with aldehydes and ketones with generation of the corresponding allylic alcohol. It is important to note that these reactions took place in low yields with alkyl-Grignard reagents. 

The issue of stereoselectivity of alkene formation from acyclic tosyl- or trisyl-hydrazones has been addressed in only a few cases. For straight chain ketones, there is a preference for m-alkene formation (see Scheme 67). A few other examples appear to give lower ratios, which is surprising in view of several reports of highly cis selective reactions in which the vinyllithium intermediate was trapped with electrophiles.Hydrocarbon branching in the a -position degrades the stereoselectivity considerably (Scheme 67). 

A novel class of chiral indenes (verbindenes) was prepared from enantiopure verbenone by K.C. Rupert and coworkers who utilized the Shapiro reaction and the Nazarov cyciization as the key transformations. The bicyclic ketone substrate was treated with triisopropylbenzenesulfonyl hydrazide to prepare the trisyl hydrazone that was then exposed to n-BuLi. The resulting vinyllithium intermediate was reacted with various aromatic aldehydes to afford the corresponding allylic alcohols. 

Further examples of useful vinyllithium intermediates which have been generated using dn-lithium exchange include the substituted butadiene precursors (38), (39) and (40), and the cyclohexadiene (41) all four intermediates react efficiently (60-86% yields) and directly with a range of primary alkyl bromides or iodides. Such alkylations can also be performed intramolecularly, as for example in a preparation of the hydrindane (43) from the vinylstannane (42). ... 

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). 

The two modifications described above have allowed for the efficient capture of vinyllithium intermediates generated in Shapiro reactions with a wide variety of electrophiles. For example, the alkyllithium reagent 20 prepared from treatment of trisylhydrazone 18 with n-BuLi was effectively trapped with benzaldehyde (62% yield, 21), 1 -bromobutane (58% yield, 22), and bromine from 1,2-dibromoethane (43% yield, 23)." Similarly, formation of 20 from tosylhydrazone 19 followed by trapping with CO2 afforded 24 (52% yield). The reaction of intermediate 20 (generated from 19) with cyclohexenone provided 25, the product of 1,2-addition, in 61% yield.13... 

The resulting vinyllithium intermediates may be trapped with a proton or some of the representative electrophiles shown in eqs 5-6. l ... 

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... 

Alpha alkylation of hydrazones and Shapiro reaction in a one-pot process provides a versatile route to tetrasubstituted alkenes. The addition of 2 equiv of butyllithium to a trisyUiydrazone generates a dianion, which may be trapped at low temperature with different electrophiles such as methyl iodide. An additional equivalent of butyllithium generates a dianion species that decomposes at room temperature to yield the vinyllithium intermediate of the Shapiro reaction. This t)q)e of compound can react with different electrophiles as commented before. In this example, the final addition of paraformaldehyde gave rise to the tetrasubstituted alkene represented in eq 22. ... 

When the Shapiro reaction is carried out on trifluoro-methylhydrazones, the vinyllithium intermediate undergoes defluorination prior to trapping to give 1,1-difluoroaUenes (eq 25). ... 

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