Dicarbonyl compounds monoanions

Alkylation of dianions occurs at the more basic carbon. This technique permits alkylation of 1,3-dicarbonyl compounds to be carried out cleanly at the less acidic position. Since, as discussed earlier, alkylation of the monoanion occurs at the carbon between the two carbonyl groups, the site of monoalkylation can be controlled by choice of the amount and nature of the base. A few examples of the formation and alkylation of dianions are collected in Scheme 1.7. In each case, alkylation occurs at the less stabilized anionic carbon. In Entry 3, the a-formyl substituent, which is removed after the alkylation, serves to direct the alkylation to the methyl-substituted carbon. Entry 6 is a step in the synthesis of artemisinin, an antimalarial component of a Chinese herbal medicine. The sulfoxide serves as an anion-stabilizing group and the dianion is alkylated at the less acidic a-position. Note that this reaction is also stereoselective for the trans isomer. The phenylsulfinyl group is removed reductively by aluminum. (See Section 5.6.2 for a discussion of this reaction.)... 

The process is assumed to take place by a chemoselective attack of the dianion 2-223 at the bromomethyl group of 2-221 and subsequent nucleophilic attack of the resultant monoanion 2-224 onto the epoxide moiety to give 2-225. Use of the sodium-lithium-salt 2-223 of the dicarbonyl compound 2-220, the reaction temperature as well as the Lewis acid LiC104, are crucial. The reaction seems to be quite general, since various 1,3-dicarbonyl compounds can be converted into the corresponding furans. 

Alkylation reactions of dianions occur at the more basic carbon. This technique allows alkylation of 1,3-dicarbonyl compounds to be carried out cleanly at the less acidic position. Because, as discussed earlier, alkylation of the monoanion occurs at the carbon between the two carbonyl groups, the site of monoalkylation can be controlled by choice of the amount and nature of the base. A few examples of the formation and alkylation of dianions are collected in Scheme 1.8. 

Dianions from (3 dicarbonyl compounds react quite well through the terminal carbon site, under irradiation [21,39]. Monoanions of (3-dicar-bonyl compounds do not react with 2-bromopyridine [40], 2-chloroquinoline... 

As previously indicated, monoanions of / -dicarbonyl compounds fail to react with phenyl halides however, the 1,3-dianions are suitable nucleophiles and react quite well through the terminal carbanion site185. 

Pentanedione (acac) and related jS-dicarbonyl compounds are an extremely important class of hgands that have been studied widely for many years. In general, jS-dicarbonyl compounds exist as mixtures of tautomeric keto and enol forms (equation 15). These compounds are usually easily deprotonated to form monoanions, which form the basis for a large class of coordination compounds, encompassing vir-tuaUy every element. In addition, coordination compounds of the dianions and trianions of jS-diketones have been observed, as well as complexes of the neutral molecules. ... 

The monoanions of /3-dicarbonyl compounds also form organometallic species, usually involving the central carbon atom in the M-C bond (14). Such species are known for a number of metals, particularly those of the platinum group. This mode of binding leaves the two carbonyl oxygen atoms free, allowing the formation of bimetalhc species (15). ry -Allylic (16) and terminal (17) carbon bonding have also been observed. 

A facile approach to polysubstituted chiral dihydrofiirans starts from pyranose triflates and monoanions of 1,3-dicarbonyl compounds <94CC173S>. Treatment of cyclopropylsulfides of type 9 with ammonium cerium nitrate (CAN) gives furans by a tandem oxidative ring cleavage-cylization reaction <94CC1S29>. 

The application of dianion chemistry in synthesis is not confined to 7-alkylation of p-dicarbonyl compounds. Dianions derived from p-keto sulfoxides can be alkylated at the 7 -carbon atom. Nitroalkanes can be deprotonated twice in the a-position to give dianions 7. In contrast to the monoanions, the dianions 7 give C-alkylated products in good yield (1.19). ... 

The monoanions of 1,3-dicarbonyl compounds react smoothly with the cis-oriented epoxy triflate 1 to give the intermediate I which, after further base treatment, leads to a dihydrofuran system [20] (Scheme 7). After treatment of 1 with dianions of 1,3-dicarbonyl compounds, tetrahydrofurylidine formation is observed under kinetic conditions [20] (Scheme 7). 

In the presence of a very strong base, such as amide ion or an organolithium reagent, it is possible to convert dicarbonyl compounds to their dianions. Subsequent alkylation of such dianions leads to alkylation at the more strongly basic enolate site, rather than at the carbon atom between the two carbonyl carbons. The more acidic methylene group activated by two carbonyl substituents is the preferred site in the monoanion, as discussed earlier. The ability to determine the site of monoalkylation by choice of the amount and nature of the basic catalyst has significantly expanded the synthetic utility of enolate alkylations. Scheme 1.7 gives some examples of formation and alkylation of dianions. 

Fig. 41. Coordination modes for j6-dicarbonyl compounds as the molecule, monoanion, and dianion in metal complexes.

Biaryls were obtained by C-alkylation using 2,6-di-tert-butyl phenolate as nucleophile with/rara-chlorobenzonitrile and 2-chloro-5-cyanopyridine676. Bromobenzonitriles and bro-mocyanopyridines have been employed in a study in which it was demonstrated that these compounds can be successfully used to arylate monoanions of / -dicarbonyl compounds677. 

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