Functional group equivalents carbanions

A retrosynthetic analysis of an a,/J-unsaturated aldehyde or ketone involves an initial functional group interconversion into a /1-hydroxycarbonyl compound, followed by a disconnection into the carbocation (12) and the carbanion (13) synthons. The reagent equivalents of these two synthons are the corresponding carbonyl compounds. 

The most vital and crucial aspect of construction reactions are essentially comprise of such reactions which help in developing the basic carbon-carbon single bonds (perhaps on which the rest of the pyramid is made subsequently). Therefore, such reactions primarily need a carbon nucleophile in order to make available the electrons for the bond formation besides, a carbon electrophile to accept them appropriately. In usual practice, the nucleophiles are typified by carbanions or their equivalent substitutes and also the jr-bonds of benzene rings (aromatic) or alkenes (aliphatic). Likewise, the electrophiles are examplified by electron-deficient carbon-atoms commonly attributed by three t3rpes of entities, such as carbonyls conjugated carbonyls and C-atoms that rapidly become electron-deficient on being deprived of an attached functional group. 

Vinyl sulfones are reactive as dienophiles. The sulfonyl group can be removed reductively with sodium amalgam (see Section 5.6.2). In this two-step reaction sequence, the vinyl sulfone functions as an ethylene equivalent. The sulfonyl group also permits alkylation of the adduct, via the carbanion. This three-step sequence permits the vinyl sulfone to serve as the synthetic equivalent of a terminal alkene.68... 

Examples of this approach to the synthesis of ketones and carboxylic acids are presented in Scheme 1.6. In these procedures, an ester group is removed by hydrolysis and decarboxylation after the alkylation step. The malonate and acetoacetate carbanions are the synthetic equivalents of the simpler carbanions lacking the ester substituents. In the preparation of 2-heptanone (entries 1, Schemes 1.5 and 1.6), for example, ethyl acetoacetate functions as the synthetic equivalent of acetone. It is also possible to use the dilithium derivative of acetoacetic acid as the synthetic equivalent of acetone enolate.29 In this case, the hydrolysis step is unnecessary, and decarboxylation can be done directly on the alkylation product. 

The addition of unstabilized a-nitrile carbanions to a,3-unsaturated carbonyl acceptors affords predominantly 1,2-addition products, 56 while lithiated acetonitrile derivatives having a-alkoxy, a-aromatic, a-dialkylamino, a-phenylselenyl, a-phenylthio or a-trimethylsilyl substituents afford 1,4-adducts. However, some of these are acyl anion equivalents (Section 1.2.2.3.2) so this discussion is limited to a-stabi-lized nitriles in which the nitrile function is retained after removal of the activating group. Notable examples are trimethylsilylacetonitrile (208),157 phenylthioacetonitriles (209),158a b phenylselenylacetoni-... 

The reduction equivalents required for conversion of N2 to NH3 are obtained from the carbanionic phenyl groups bonded to nickel in 14 (biphenyl formation). The Ni atoms, which are still present as Ni(0), i.e., in the form of metallic nickel after hydrolysis, only function as agents for the transfer of electrons from C6H5 to N2. In this respect, the hydrolysis of... 

Other examples of functionalization at C-2 via phosphorus ylides and phosphonate carbanions are described in Section 4.12.11. Utilization of 2-non-phosphorus-containing carbanions was also exemplified. Thus, 2-silicon-substituted 1,3-benzodithioles were synthesized via deprotonation of benzo-l,3-dithiole 245 with -BuLi and subsequent treatment of the resulting anion with trimethylsilyl chloride (TMSCl) to give 2-(trimethylsilyl)-benzo-l,3-dithiole 246 (Scheme 29). The second silyl group was introduced by further deprotonation of 246 ( -BuLi) followed by the reaction with an additional equivalent of TMSCl. Tin-substituted benzo-l,3-dithioles were synthesized in a similar way but the deprotonation of the monostannyl derivative was carried out with LDA (Scheme 29) <1996CL171>. 

This reaction type differs from die three-component reaction reported by Grigg et al. Thus, Grigg et al. [53] (Scheme 7) immobihzed 3-iodo-4-(N-acetyl-N-(2-methyl-2-propenyl)amino)benzoate (36) onto a sohd support. In the presence of suitable Pd salts, Pd substituted the iodide function of the aromatic. The proximal isopropyhdene group trapped the resulting metalated species in an intramolecular Heck reaction. The resulting alkyl palladium species (37) could then react with a suitable carbanion equivalent. The authors used vinylstaimanes or boronates for this purpose, which they obtained in situ from alkynes by hydroboration or hydro-starmylation. The latter procedure allowed them to attach the same vinylic species via its terminal carbon (boronate) (41) and its subterminal carbon (stannane) (39). 

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