Carbanion sources

Sodium borohydride and lithium aluminum hydride react with carbonyl compounds in much the same way that Grignard reagents do except that they function as hydride donors rather than as carbanion sources Figure 15 2 outlines the general mechanism for the sodium borohydride reduction of an aldehyde or ketone (R2C=0) Two points are especially important about this process... 

Carboxylation (Section 19 11) In the preparation of a car boxyhc acid the reaction of a carbanion with carbon diox ide Typically the carbanion source is a Gngnard reagent... 

The parent bis-benzene complex is hydrolytically sensitive [20] and must be used in its crude form. All the reactions with carbanions give ET products. First, the purple 19e mono-cation [Fe(C6Me6)2] + [25-28] is formed immediately at —90 °C. Then the black 20e complex Fe(C6Me6)2 is observed [25-29], However, recently, Zaworotko et al. have succeeded in making a C-C bond using AlEt3 as the carbanion source [30]. 

Grignard reaction and similar transformations allow C-C bond formation without a palladium catalyst. Grignard reagents and organolithium compounds are very versatile carbanion sources used in the synthesis of acyclic, heterocychc and carbo-cychc compounds. The esters, ketones and aldehydes are more stable when the reaction takes place on solid supports than in the hquid-phase, because this immo-bihzed components are not so sensitive towards water or oxygen. In the total synthesis of (S)-zearalenone (155) on solid supports the Grignard reaction is one of the key steps (Scheme 3.16) [120]. 

The structure in the box comes from the carbonyl compound (acceptor) the structure in the oval comes from the o-bromoester (carbanion source). 

Tangible evidence was found for the in situ production of perfluoro(2,3,4,5-tetrahydro-pyridines) 2, 3, and 4 during the transfer of F- from perfluoro-l-fluoropiperidine (1) to carbanionic sources.22 23 Perfluoro-l-fluoropiperidine (1) was employed to convert triphenyl-phosphane, -arsane, and -stilbane to the difluorides, triphenylphosphorus difluoride, triphenyl-arsenic difluoride, and triphenylantimony difluoride, respectively,24 and sodium phenoxide to 2- and 4-fluorophenol.25 Perfluoro-l-fluoropiperidine (1) reacts with 7V,/V-dimethylaniline, substituting the 0/7/m-hydrogen by fluorine. A similar reaction proceeds with /V,/V-diethylaniline.26... 

The work on stannacycloalkanes and -cycloalkenes up to 1972 has been reviewed (9), and again, though in less detail, in 1982 (10). Most of the early studies concern the formation of five- and six-membered rings, and claims for smaller systems in particular should be treated with caution. More recent efforts have been directed to the synthesis of both strained and expanded rings. The synthetic methodology, however, remains dominated by the use of difunctional carbanion sources (Grignard and lithium reagents) or the hydrostannation reactions of tin dihydrides. 

Carbanion source CH3N O CH3C=N CHCI3 CH3C=CH 9 H H Ph2CH2 ch3 ( j) no2 no2... 

Carbanion source CHjN 0 CHjC=N CHCI3 CH3C-CH 9 H H Ph2CH2 CH3 2>—NO2 N02... 

Imidazoles and benzimidazoles with a free NH group are weak acids, but sufficiently acidic to be able to form anions in the presence of strong bases <87AHC(4l)l87,88CHE469). This property becomes important in transformations sueh as 7V-alkylation and metallation. Whereas most metal salts hydrolyze very readily in aqueous media, they can be prepared and isolated from anhydrous aprotic solvents. Thus, Af-lithiated imidazoles, prepared under such conditions, react in turn with copper(I) cyanide and an alkyllithium to give cuprates [R(Im)Cu(CN)Li2] which are useful carbanion sources <92TL1041>. Some determined acid values appear in Table 6. 

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