Electrophiles formation

The interaction of sulphones with other electrophiles—formation of... 

Deprotonation of allylic aryl sulfoxides leads to allylic carbanions which react with aldehyde electrophiles at the carbon atom a and also y to sulfur . With benzaldehyde at — 10 °C y-alkylation predominates , whereas with aliphatic aldehydes at — 78 °C in the presence of HMPA a-alkylation predominates . When the a-alkylated products, which themselves are allylic sulfoxides, undergo 2,3-sigmatropic rearrangement, the rearranged compounds (i.e., allylic sulfenate esters) can be trapped with thiophiles to produce overall ( )-l,4-dihydroxyalkenes (equation 24). When a-substituted aldehydes are used as electrophiles, formation of syn-diols 27 occurs in 40-67% yields with diastereoselectivities ranging from 2-28 1 (equation 24) . ... 

When more acidic nitriles such as PhCH2CN (pK21.9) are used (in DME) with in situ produced Ph as the EGB, esters with a-hydrogens can also be used as electrophiles. Formation of / -oxoesters may be carried out in a similar... 

Oxidation States of Alcohols and Related Functional Groups 467 11-2 Oxidation of Alcohols 469 11-3 Additional Methods for Oxidizing Alcohols 472 11-4 Biological Oxidation of Alcohols 474 11-5 Alcohols as Nucleophiles and Electrophiles Formation ofTosylates 476... 

A diastereoselective ewrfo-cyclization into an oxidatively generated oxocarbenium ion was a key step in a formal synthesis of leucascandrolide A. Exposing 56 to CAN provided cw-tetrahydropyran 57 in high yield and with excellent stereocontrol (Scheme 3.20). This transformation provides further evidence that oxidative electrophile formation is tolerant of several functional groups and can be applied to complex molecule synthesis. The synthetic sequence also utilized a Lewis acid mediated ionization reaction to form an oxocarbenium ion in the presence of the homobenzylic ether (58, 59), illustrating that two carbocation precursors that ionize through chemically orthogonal conditions can be incorporated into the same structure. 

Organic anions (Reactions with electrophiles formation of basic species)... 

Using C02 as an electrophile, formation of 5-alkyl-2-thiophenecarbox-ylic acids (52) from the corresponding borates (51) can also be realized (86MI1) (Scheme 20). Analogously, 3-alkyl derivatives of furan and thiophene (54) are formed from the corresponding borates (53) and iodine or bromine (81BCJ1587) (Scheme 21). 

A number of cyclopropanes can be obtained by replacing one or several groups attached to a cyclopropane ring with other atoms or substituents. Many of these substitution reactions are two-step or multistep processes, which may involve base-induced generation of cyclopropyl anions that are quenched by an electrophile, formation under basic conditions of cyclopropenes which are trapped by a nucleophile, or generation of a cyclopropyl radical that undergoes subsequent reactions under neutral conditions. Formal substitution also takes place when cyclopropanes are converted to cyclopropylidenes which suffer l,n insertion, n > 3. Substitution reactions therefore cover a variety of compounds and a wide range of reaction conditions. 

Some alkyl-substituted cyclopropanes have been obtained by treating cyclopropylidene-triphenyl-2 -phosphane with various electrophiles. Formation of a C-C bond to the ring at the expense of a C-P bond takes place very efficiently when a phosphorus ylide is treated with both cyclohexene oxide and styrene oxide. The products obtained, 9,9-ethylene-8,8,8-triphenyl-... 

Markovnikov s rule can be explained by realizing that two different carbocations can be formed from the reaction between an unsymmetrical alkene and an electrophile. Formation of the more stable carbocation will predominate and wiU lead to the product predicted by Markovnikov s rule. 

Deprotonation of the a-carbon of the ketone by ethoxide to make an enolate whose oxygen does an Sj 2 displacement of chloride from chloroacetonitrile, followed by intramolecular electrophilic formation of the epoxide ring in structure E. 

In many types of S Ar reactions, cationic electrophile formation requires one or more steps after functional group protonation or activation (Fig. 1.2). Alcohols and related functional groups are protonated, and with subsequent cleavage of C—O bond, the carbocation electrophile (11) is formed. In a similar respect, a common method of nitration involves the use of HNOj with H SO. The nitronium ion electrophile (NO, 12) is formed by protonation of nitric acid and subsequent loss of water by cleavage of the N—O bond [8]. The nitrosonium ion electrophile (NO ) may be generated by an analogous transformation from nitrous acid, HNO [9]. Likewise, M-acyhminium ion electrophiles (i.e., 13) may be formed by ionization of A-hydroxymethylamides [10]. 

In addition to the mechanisms of electrophile formation, another critical consideration relates to electrophile strength. There has been a vast amount of work done to characterize electrophile strengths [19]. Although much of the work relates to chemistry with n-type nucleophiles and nonaromatic tc-nucleophiles, some studies have sought to estimate electrophile strengths in SgAr reactions. Relative electrophile strengths became apparent as the synthetic S Ar reactions were developed. While the nitronium ion (NO ) salts react with benzene under mild conditions, carboxonium ions such as protonated formaldehyde (CH2=OH+) are weaker electrophiles and consequently do not react with benzene. 

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