Alkenyllithium species

The Shapiro reaction is the conversion of ketone to an alkene via a tosylhydrazone. The tosylhydrazone is treated with two equivalents of strong base, typically n-BuLi or MeLi, to afford an alkenyllithium species which is quenched with an electrophile to form an alkene (see also chapter 5, 5.1.25 Shapiro reaction). Reviews (a) Chamberlin, A. R. Bloom, S. H. Org. React. 1990, 39,1-83. (b) Shapiro, R.H. Org. React. 1976,23,405. 

The elaboration of a polyunsaturated 5 9-fused ring system based on transannular addition of an alkenyllithium species to a triple bond following the Br—Li exchange of a bromoalkene is clean and elegant. 

Simple proton abstraction is more difficult in the case of alkenyl substrates (p/iTa 36, but see Scheme 1.110) and a more-common method for the formation of alkenyllithium (vinyllithium) species is halogen-lithium exchange with n- or tert-butyllithium. The exchange reaction is stereospecific, such that the stereochemistry of the starting alkenyl halide determines the stereochemistry of the product alkenyllithium species (1.114). Subsequent reaction with electrophiles also occurs with retention of configuration and this of course is important in order to prepare stereodefined alkenyl products. 

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... 

In contrast, treatment of the arylsulfonyl hydrazone with two equivalents of a strong base, such as BuLi or lithium diisopropylamide (LDA), effects the Shapiro reaction for the formation of alkenes. Double deprotonation of the hydrazone gives the dianion 40, which fragments to the alkenyllithium species 41 (2.36). Addition of an electrophile leads to the alkene product. The second deprotonation takes place syn to the ArSOiN" (aryl sulfonamido anion) group and the less-substituted alkene is typically formed. Thus phenylacetone provides 3-phenylpropene and not the isomeric styrene (2.37). Protonation of the alkenyllithium species 41 to give a 1,2-disubstituted alkene generally leads to predominantly the Z-isomer. 

An alternative to tosyl or trisyl hydrazones is the use of phenylaziridinyl hydra-zones (e.g. 42). If the protonated alkene product is desired, then the base LDA can be used as a catalyst. LDA is regenerated by reaction of the alkenyllithium species with the by-product, diisopropylamine (2.40). ... 

A review entitled a-heteroatom-substituted 1-alkenyllithium regents carbanions and carbenoids for C-C bond formation has addressed the methods of generation of such species, illustrated the carbenoid reactivity of a-lithiated vinyl halides and vinyl ethers, and emphasized the synthetic potential of the carbanion species in asymmetric synthesis of a-hydroxy- and a-amino-carbonyl compounds. ... 

They also showed that the gem-dimetal species from allylzinc and alkenyllithium lead to selective reactions with electrophiles as shown in Scheme 201 ,4 . 

Combining substrate-induced diastereoselection and mutual diastereoselectivity, as illustrated for the crotylzincation of the alkenyllithium derived from 209, led to excellent results as the gewt-dimetallic species 217 was obtained in a highly stereoselective fashion. The stereochemical outcome was explained by the addition of the kinetically reactive cisoid metallotropic form of the crotylzinc reagent anti to the propyl group in the chelated allyl alkenylzinc intermediate. After hydrolysis, compound 218 was obtained as a single diastereomer (equation 106)148,149. 

The mutual diastereoselection between the alkenyllithium and the zincated allylic ether turned out to be the same as for a crotylzinc species, and could be rationalized by kinetic addition of a cisoid -alkoxyallylic organozinc compound. 

Z-Alkenyllithium reacts with crotylmagnesium bromide and zinc dibromide in ether at — 50 °C to give the stable 1,1-dimetallic species via the postulated transition state involving a zinca-ene reaction. After acidic hydrolysis, anti -(35, 4/ )-dimethyl-l-nonene was obtained in 87% yield with a diastereoselection of 92 8 [101, 105] (Scheme 7-89). 

A useful method for the generation of 1-alkenyllithiums is founded on the N,C-lithiation of A -tosylhydrazones. The intermediate 260a enters into an equilibrium with the metallotropic species 260b from which lithium p-toluenesulfmate and nitrogen are eliminated. Applied to P-ionone, the Shapiro approach opens a convenient entry to 3-(2,6,6-trimethylcyclohex-l-enyl)-l-methylene-2-propenyllithium (Scheme 1-207). ... 

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