Electrophilic additions Subject

In the synthesis in Scheme 13.46, a stereoselective aldol addition was used to establish the configuration at C(2) and C(3) in Step A. The furan ring was then subjected to an electrophilic addition and solvolytic rearrangement in Step B. 

Figure 1.11 Tyrosine residues are subject to nucleophilic and electrophilic reactions. The unprotonated phe-nolate ion may be alkylated or acylated using a variety of bioconjugate reagents. Its aromatic ring also may undergo electrophilic addition using diazonium chemistry or Mannich condensation, or be halogenated with radioactive isotopes such as 12iI.

O - P h e n y 1 - /V - e r y t h r o s y 1 nitrone (336), as a Ci,C i-bis-electrophile, when subjected to the double addition of Grignard reagents (in a domino style), leads to acyclic hydroxylamine (338) via the formation of open-chain nitrone (337 ). The reaction proceeds at 0°C with variable diastereoselectivity ranging from medium to good, depending on the organometalic reagent used (Scheme 2.140) (564). 

It seems obvious that electron-withdrawing groups enhance nucleophilic addition and inhibit electrophilic addition because they lower the electron density of the double bond. This is probably true, and yet similar reasoning does not always apply to a comparison between double and triple bonds.70 There is a higher concentration of electrons between the carbons of a triple bond than in a double bond, and yet triple bonds are less subject to electrophilic attack and more subject to nucleophilic attack than double bonds.71 This statement is not universally true, but it does hold in most cases. In compounds containing both double and triple bonds (nonconjugated), bromine, an electrophilic reagent, always adds... 

Substituted 1,2,3-triazole 1-oxides 448 have been reported to undergo electrophilic and nucleophilic aromatic substitution and are subject to debromination, proton-metal exchange, and halogen-metal exchange followed by electrophilic addition. Transmetallation and cross-coupling have not been described. 3-Substituted 1,2,3-triazole 1-oxides 448 can be proton-ated or alkylated at the O-atom and they can be deoxygenated and deal-kylated. The individual reactions are described in Section 4.2.7.1-4.2.7.14. 

Eberson and co-workers have recently discussed the probability that the interaction of ion-radicals with nucleophiles and electrophiles is subject to orbital symmetry constraints.31,32 This follows the observation that with perylene the cation-radical (18) the preferred course of reaction with halide ions is electron transfer rather than nucleophilic addition, whereas with the phenothiazine cation-radical (19) nucleophilic attack by Cl" and Br occurs. 

Silicon and tin are both subject to ipra-replacement by electrophiles, via an electrophilic addition/metal elimination mechanism analogous to other aromatic substitutions, but at a much faster rate than the corresponding replacement of hydrogen. /pra-substitutions also take place on heterocycles and, in the case of electron-rich systems, probably via the same type of mechanism. 

The Mo-Sx bonds (x = 1, 2, 4) within the Mo(OXSx) units react readily with SO2 to give various products that contain thiosulfite, thiosulfate, sulfite, or sulfate ligands (Tables V and VI). These reactions are best described as electrophilic additions to the Mo-S bond and in many instances may be subject to the effects of the spectator Mo=0 group (see Section III,B). 

The mechanisms of electrophilic addition reactions to unsaturated systems has been the subject of a recent monograph for this reason it is not proposed to discuss the criteria for adopting a mechanism in any detail in the present work, although it must not be thought that all such proposed mechanisms have been demonstrated incontravertibly. 

Halonium ions are another group of intermediates that have been characterized by spectral methods. Peterson et al. have shown that five- and six-membered cyclic halonium ions are reasonably stable species capable of being isolated, stored, and subjected to reaction with a variety of nucleophiles. Three-membered cyclic halonium ions (ethylenehalonium ions) are not so stable, yet because of their long-postulated involvement in electrophilic addition reactions with alkenes and in the nucleophilic displacement of j8-substituted leaving groups, they have attracted a significant amount... 

Coordination to the metal has a number of profound effects on the arene (Figure 10.4) the ring becomes electrophilic and subject to nucleophilic attack, the acidity of the ring protons increases and the acidity of any benzylic protons also increases. In addition, the formation of benzylic cations is facilitated, as the metal can stabilize them. Finally, the metal provides huge steric bulk on one face of the arene. 

Initial mechanistic analysis of the Strecker reaction catalyzed by a urea-based organocatalyst (Scheme 3.18) revealed that the catalytic activity is provided by the urea functionality of structurally complex catalyst 1. However, further studies revealed a bifunchonal character of urea and thiourea-based catalysts " as well as the possibility of multiple mechanistic pathways in catalysis of nucleophile-electrophile addition reactions. " Simplified but sufficiently effective (thio)urea catalysts 4a and 4b were used in the hydrocyanation reaction (Scheme 3.19) that was subjected to a combined experimental and computational study. °... 

In adding hydrogen halides and halogens to the >C=C< double bond of 1,2-PB, the functionalization degree of the polymer is mostly determined by the reactivity of the electrophilic agent. Relatively low degree of polydiene hydrochlorination (10-15%) at interaction of HCl and syndiotactic 1,2-PB [16, 39, 40] is caused by insufficient reactivity of hydrogen chloride in the electrophilic addition reaction by the double bond (Table 3.2). Due to this, more electron-saturated >C=C< bonds in 1,4 units of butadiene polymerization are subjected to modification. 

In Summary Halonium ions are subject to stereospeciflc and regioselective ring opening in a manner that is mechanistically very similar to the nucleophilic opening of protonated oxacyclopropanes. Halonium ions can be trapped by halide ions, water, or alcohols to give vicinal dihaloalkanes, haloalcohols, or haloethers, respectively. The principle of electrophilic additions can be applied to any reagent A-B containing a polarized or polarizable bond. 

We begin by introducing the chemistry of enolates and enols. Especially important is a reaction between enolate ions and carbonyl compounds, called the aldol condensation. This process is widely used to form carbon-carbon bonds both in the laboratory and in nature. Among the possible products of aldol condensation are a,/3-unsaturated aldehydes and ketones, which contain conjugated carbon-carbon and carbon-oxygen tt bonds. As expected, electrophilic additions may take place at either bond. However, more significantly, a, -unsaturated carbonyl compounds are also subject to nucleophilic attack, a reaction that may involve the entire conjugated system. 

The addition of LiCI also strongly enhances the rate of zinc insertion into alkyl halides, making possible a very simple and versatile preparation of functionalized alkylzinc reagents of type 247, which can be in situ trapped with various electrophiles or subjected to cross-coupling reactions (Scheme 2-98). ... 

Reactants with internal nucleophiles are also subject to cyclization by electrophilic sulfur reagents, a reaction known as sulfenylcyclization.92 As for iodolactonization, unsaturated carboxylic acids give products that result from anti addition.93... 

A diverse group of organic reactions catalyzed by montmorillonite has been described and some reviews on this subject have been published.19 Examples of those transformations include addition reactions, such as Michael addition of thiols to y./bunsatu rated carbonyl compounds 20 electrophilic aromatic substitutions,19c nucleophilic substitution of alcohols,21 acetal synthesis196 22 and deprotection,23 cyclizations,19b c isomerizations, and rearrangements.196 24... 

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