Allyl dianion

The reductive elimination of a variety of )3-substituted sulfones for the preparation of di-and tri-substituted olefins (e.g. 75 to 76) and the use of allyl sulfones as synthetic equivalents of the allyl dianion CH=CH—CHj , has prompted considerable interest in the [1,3]rearrangements of allylic sulfones ". Kocienski has thus reported that while epoxidation of allylic sulfone 74 with MCPBA in CH2CI2 at room temperature afforded the expected product 75, epoxidation in the presence of two equivalents of NaHCOj afforded the isomeric j ,y-epoxysulfone 77. Similar results were obtained with other a-mono- or di-substituted sulfones. On the other hand, the reaction of y-substituted allylic sulfones results in the isomerization of the double bond, only. The following addition-elimination free radical chain mechanism has been suggested (equations 45, 46). In a closely related and simultaneously published investigation, Whitham and coworkers reported the 1,3-rearrangement of a number of acyclic and cyclic allylic p-tolyl sulfones on treatment with either benzoyl peroxide in CCI4 under reflux or with... 

The reaction of 1,3-dibromopropene with carbonyl compounds mediated by indium in water gives 3,3-disubsti-tuted propene 63 (Scheme 55). In the formation of 63, 1,3-dibromopropene acts as a w-allyl dianion synthon. Aromatic aldehydes generally have a higher selectivity than aliphatic ones in the product formation. Unsubstituted and electron-withdrawing group-substituted benzaldehydes give mainly 3,3-disubstituted propene 63. For electron-rich benzaldehydes, the formation of both 3,3- 63 and 1,3-disubstituted propene 64 is dramatically decreased and the selectivity is reversed completely to give diene 65 and homoallylic alcohol 66.219... 

Reactions alkylations, reactions with epoxides and aldehydes, conjugate additions Heterocyclic synthesis with allyl silanes Reactions with Co-stabilised cations An Allyl Dianion The Role of Tin in Anion Formation Halide Exchange with Chelation Indium Allyls Allyl Anions by Deprotonation The synthesis ofall-trans dienes The synthesis ofall-trans retinol... 

Dibromopropene acts as a gem-allyl dianion synthon. However, the reaction is not as clean as one might desire, due to competing reductive debromination of the 1 1 products., l-Dichloro-2-propene undergoes a y-selective addition process, affording syn-chlorohydrins, but the allylindium species derived from 3-bromo-l-trimethylsilylpropene attacks aldehydes with the a-carbon. ... 

The 1.3-allylic diacetate 135 can be used for the formation of the methy-lenecyclopentane 137 with the dianionic compound 136(86]. The cyclohexa-none-2-carboxylate 138 itself undergoes a similar annulation with the 1,3-allylic diacetate 135 to form the methylenecyclohexane derivative 139(90]. The reaction was applied as a key step in the synthesis of huperzin A[91]. On the other hand. C- and 0-allylations of simple J-dikctones or. 1-keto esters take place, yielding a dihydropyran 140(92]. 

The dianions derived from furan- and thiophene-carboxylic acids by deprotonation with LDA have been reacted with various electrophiles (Scheme 64). The oxygen dianions reacted efficiently with aldehydes and ketones but not so efficiently with alkyl halides or epoxides. The sulfur dianions reacted with allyl bromide, a reaction which failed in the case of the dianions derived from furancarboxylic acids, and are therefore judged to be the softer nucleophiles (81JCS(Pl)1125,80TL505l). 

Dianion formation from 2-methyl-2-propen-l-ol seems to be highly dependent on reaction conditions. Silylation of the dianion generated using a previously reported method was unsuccessful in our hands. The procedure described here for the metalation of the allylic alcohol is a modification of the one reported for formation of the dianion of 3-methyl-3-buten-l-ol The critical variant appears to be the polarity of the reaction medium. In solvents such as ether and hexane, substantial amounts (15-50%) of the vinyl-silane 3 are observed. Very poor yields of the desired product were obtained in dirnethoxyethane and hexamethylphosphoric triamide, presumably because of the decomposition of these solvents under these conditions. Empirically, the optimal solvent seems to be a mixture of ether and tetrahydrofuran in a ratio (v/v) varying from 1.4 to 2.2 in this case 3 becomes a very minor component. 

Carboxylic acids can be alkylated in the a position by conversion of their salts to dianions [which actually have the enolate structures RCH=C(0")2 ] by treatment with a strong base such as LDA. The use of Li as the counterion is important, because it increases the solubility of the dianionic salt. The reaction has been applied to primary alkyl, allylic, and benzylic halides, and to carboxylic acids of the form RCH2COOH and RR"CHCOOH. This method, which is an example of the alkylation of a dianion at its more nucleophilic position (see p. 458),... 

Hexaaza-1,5-dienes RN=NNRNRN=NR, derivatives of 15 [96], are unusual high-energy molecules. Very recently, Cowley, Holland, and co-workers [101] fairly well stabilized the dianion RN R "16 as a ligand in a transition metal complex. These species are stabilized by such conjugations as those in allyl anions, which are special conjugations of the n-tr conjugations. 

Monoanions derived from nitroalkanes are more prone to alkylate on oxygen rather than on carbon in reactions with alkyl halides, as discussed in Section 5.1. Methods to circumvent O-alkylation of nitro compounds are presented in Sections 5.1 and 5.4, in which alkylation of the a.a-dianions of primary nitro compounds and radial reactions are described. Palladium-catalyzed alkylation of nitro compounds offers another useful method for C-alkylation of nitro compounds. Tsuj i and Trost have developed the carbon-carbon bond forming reactions using 7t-allyl Pd complexes. Various nucleophiles such as the anions derived from diethyl malonate or ethyl acetoacetate are employed for this transformation, as shown in Scheme 5.7. This process is now one of the most important tools for synthesis of complex compounds.6811-1 Nitro compounds can participate in palladium-catalyzed alkylation, both as alkylating agents (see Section 7.1.2) and nucleophiles. This section summarizes the C-alkylation of nitro compounds using transition metals. 

In contrast to the allyl system, where the reduction of an isolated double bond is investigated, the reduction of extensively delocalized aromatic systems has been in the focus of interest for some time. Reduction of the systems with alkali metals in aprotic solvents under addition of effective cation-solvation agents affords initially radical anions that have found extensive use as reducing agents in synthetic chemistry. Further reduction is possible under formation of dianions, etc. Like many of the compounds mentioned in this article, the anions are extremely reactive, and their intensive studies were made possible by the advancement of low temperature X-ray crystallographic methods (including crystal mounting techniques) and advanced synthetic capabilities. 

Thioamide dianions (generated by the highly efficient reaction of iV-benzyl thioamides with 2 equiv. of BuLi) take place alkylation, allylation and silylat-ion selectively at the carbon atom adjacent to the nitrogen atom of the thioamide dianions (Scheme 38).77... 

It was considered of interest to utilize dianion 26 <89PHC(1)1> as a potential three carbon allyl anion fragment to react with 6 with a view to develop a new synthesis of 2,3-substituted and fused carbazoies 28 involving protection, activation and deprotection of the... 

Claisen rearrangementThe allyl ether 1 when heated rearranges and cyclizes slowly and in low yield to the dihydrobenzofuran 2. However, the dianion (NaH) of 1 rearranges in refluxing DMF mainly to the isomeric dihydrobenzofuran 3, a precursor to the antibiotic ( )-atrovenetin (4). 

Z)-.S -Allylic ketene aminothioacetals underwent thio-Claisen rearrangement at room temperature to give iVjV-dimethyl /1-hydroxy a-allylic thioamides96. /3-I Iydroxy-/V,/V,-dimethylthioamides were deprotonated with LDA to afford a chelated dianion with Z-configuration. Alkylation of this dianion gave the corresponding Z a-hydroxy S-allylic... 

The phase-transfer catalysed reaction of nickel tetracarbonyl with sodium hydroxide under carbon monoxide produces the nickel carbonyl dianions, Ni,(CO) 2- and Ni6(CO)162, which convert allyl chloride into a mixture of but-3-enoic and but-2-enoic acids [18]. However, in view of the high toxicity of the volatile nickel tetracarbonyl, the use of the nickel cyanide as a precursor for the carbonyl complexes is preferred. Pretreatment of the cyanide with carbon monoxide under basic conditions is thought to produce the tricarbonylnickel cyanide anion [19], as the active metal catalyst. Reaction with allyl halides, in a manner analogous to that outlined for the preparation of the arylacetic acids, produces the butenoic acids (Table 8.7). 

Methyl (2E,4E)Senoates. The dianion (2) of 1 can be prepared by treatment with I.DA and then sec-butyllithium in THF at —78°. The dianion is alkylated selectively at the a-allylic position to give 3 as the major product. Treatment of 3 with LDA induces a [2,3]sigmatropic rearrangement to 4. Remaining steps to the diunsaturated ester (6) are methylation, oxidation, and dchydrosulfcnylation.1... 

When 5-methylisoxazole (474) was exposed to two equivalents of LDA in THF at -10 °C the dianion of acetoacetonitrile was generated (78TL4221). Reaction of this dianion (475) with 1.1 equivalents of allyl bromide gave the alkylated product (476) in 60% yield (Scheme 105). The reaction of the dianion with nitriles and aldehydes was also studied. 

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