“…The cyclization of the dianion 23 of 1,1-diphenylallene with nitriles has been reported to result in the addition of up to four nitrile molecules onto the allene and formation of substituted imidazoles. 26a,b For example, the reaction of 23 with benzonitrile afforded a separable mixture of the imidazoles 24 and 25 (Scheme ). 12 Cyclization Reaction of the Dianion of 1,1-Diphenylallene with Benzonitrile…”
“…The cyclization of the dianion 23 of 1,1-diphenylallene with nitriles has been reported to result in the addition of up to four nitrile molecules onto the allene and formation of substituted imidazoles. 26a,b For example, the reaction of 23 with benzonitrile afforded a separable mixture of the imidazoles 24 and 25 (Scheme ). 12 Cyclization Reaction of the Dianion of 1,1-Diphenylallene with Benzonitrile…”
“…The formation of the products can be explained by acid-mediated extrusion of water The reaction of 10c with 4.5 equivalents of benzonitrile afforded the unexpected product 25 in 51% yield (Scheme 15). 17,18 In the course of this reaction, four molecules of benzonitrile were consumed and incorporated into the final product. Therefore, the reaction represents a domino process and it is termed a 'multiple anion capture reaction'.…”
Section: Scheme 13 Synthesis Of Compounds 14a-ementioning
The present personalized account highlights reactions of the dianion of 1,1-diphenylacetone and its applications. From a study directed towards the regioselectivity of such reactions a new allene synthesis via allene dianions was found by serendipity. In addition, reactions of allene dianions with various electrophiles have been studied.1 Introduction2 Cyclizations of the Dianion of 1,1-Diphenylacetone2.1 Reaction with Bis(cyclopentadienyl)zirconium Dichloride2.1 Reactions with Dichlorosilanes2.1 Reactions with Difluorosilanes2.1 Reactions with Ketones and Dielectrophiles3 Regioselectivity of Dianion Reactions4 Allene Dianions from Silyl Enol Ethers5 Scope of the Reactions of Allene Dianions6 Reactions of Allene Dianions with Ketones7 Reactions of Allene Dianions with Nitriles8 Conclusions
“…Further useful dianions can be generated from ketones (C=C(OM)CM or C=C(OM)CCM) [138][139][140][141][142], carboxylic acids (C=C(OM) 2 ) [143][144][145], succinic acid derivatives ((C=C(OM)OR) 2 ) [146], alkynes (MC"CCM, MC"CC-OM) [147][148][149][150][151][152][153][154][155], imidazoles [156], thiophenes [157,158], b-alanine derivatives (MN-CC=C(OM)X) [159], 3-nitropropanoates [160,161], 3-hydroxypropanoates [162,163], 2-hydroxyethylsulfones [164,165], arenes [166][167][168], allenes [169], thioamides [170], and sulfonamides (MC-SO 2 NM [171]; R 2 NSO 2 -CM 2 [172]). If the nucleophilic sites in the polyanion have different chemical hardness, regioselective alkylation can be achieved by selecting a soft or a hard electrophile (second reaction, Scheme 5.13).…”
The deprotonation of organic compounds by strong, non-nucleophilic bases followed by C-alkylation with a suitable electrophile has become one of the most predictable and straightforward methods for formation of C-C bonds. The popularity of this chemistry is also a consequence of the often-seen close resemblance of feasible structural modifications by a deprotonation/alkylation strategy to an unsophisticated retrosynthetic analysis of a given target compound. Other reactions, which involve, for instance, substantial rearrangement of the carbon framework (e.g. Cope rearrangement, fragmentations, ring contractions or enlargements) are usually more difficult to take into account during retrosynthetic analysis.LDA and related, sterically hindered, lithium dialkylamides, first investigated by Levine [1], have completely replaced the more nucleophilic sodium amide, which had been the base of choice for many years [2]. Because the reactivity of an organometallic compound depends to a large extent on its state of aggregation (i.e. on the solvent and on additives) and on the metal, transmetalation of the lithiated intermediates and the choice of different solvents and additives emerged as powerful strategies for fine-tuning the reactivity of these valuable nucleophiles.In this chapter the alkylation of carbanions with simple carbon electrophiles will be discussed, with special emphasis on the structure-reactivity relationship of the carbanion and on side reactions. In this context the term "carbanion" refers to an intermediate prepared by in-situ deprotonation of an organic compound, followed by optional cation exchange (transmetalation), which tends to be alkylated at carbon by soft electrophiles such as MeI or PhCHO. This type of reactivity is characteristic of enolates with an ionic M-O bond or for organometallic compounds with a strongly polarized or ionic M-C bond, M typically being an alkali metal, Mg, Cu, or Zn. Carbanions can, alternatively, also be prepared by halogen-metal exchange [3-8], sulfoxide-or sulfone-metal exchange [9-13], or by transmetalation of stannanes. The most suitable reagents for performing such metalating exchange reactions are organometallic compounds with a metal-bound secondary or tertiary alkyl group, such as tBuLi, sBuLi, iPr 2 Zn [14, 15], or iPrMgHal [6]. These reagents are thermodynamically less stable than organometallic compounds with primary alkyl 5 146 Scheme 5.1. Approximate relative rates of proton transfer in water at thermoneutrality (DpK a = 0) [35, 39, 51]. 147 Scheme 5.2. The effect of fluorine on the acidity of organic compounds [24, 62-65]. 148 Scheme 5.3. Dependence of the reactivity of carbanions on their basicity [68, 72-74].
Regioselectivity of Deprotonations and AlkylationsThe organic chemist is occasionally confronted with the problem that a compound has several differrent X-H groups of similar pK a . If only one of these is to be alkylated, one option would be to perform a regioselective deprotonation. This can sometimes be achieved by exploiting small differences be...
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