Electrophilic reagents, definition

Markovnikov orientation the 1,2-addition of an unsymmet-rical electrophilic reagent to an alkene with attachment of the more positive moiety of the reagent (H in the original definition) to the less substituted carbon Michael addition conjugate addition addition of a nucleophile to the /3-position of a carbon-carbon double or triple bond that is conjugated to a carbonyl or other electron-withdrawing group... 

Aromatic iodides (3,287). The definitive paper on the synthesis of aromatic iodides by the reaction of arylthallium dilrifluoroacetates with potassium iodide has been published. Four procedures have been developed. I) Thallalion is carried out as usual and then an aqueous solution of potassium iodide is added directly. 2) The intermediate arylthallium ditrifluoroacctatc is isolated and then treated with potassium iodide. 3) For acid-sensitive. substrates solid TTFA in acetonitrile is used for thallalion. 4)These methods are unsuccessful with highly reactive compounds such as naphthalene and diphenyl. In such cases molecular iodine is used as the electrophilic reagent and TTFA is used as oxidant for the hydrogen iodide formed in the reaction. 

As discussed in the Introduction, Huckel s rule is an electronic criterion for aromaticity, and is based on the configuration of the rr electrons. Another characteristic of aromatic compounds is a relatively large HOMO-LUMO gap, which indicates the absence of high-energy, reactive electrons, in agreement with the reduced reactivity of aromatic compounds to electrophilic reagents. This facet of electronic configuration can be expressed in terms of hardness (see p. 96 for the definition of hardness in terms of DFT theory). ... 

This might be an appropriate time to remind the reader of important definitions in chemistry—electrophilic and nucleophilic compounds. An electrophilic reagent is a compound that in the course of its reaction with another compound acquires electrons or seeks to share electrons belonging to some other atom or ion. The nucleophilic reagent is one that transfers electrons or shares its electrons with another atom. Hydroxyl groups are thus nucleophilic.)... 

In the first instance, acidity influences the acid-base equilibria of the reactants. The amine is a Bronsted base. Aniline, a typical substrate, has pKa = 4.6, which means that the protonation shown in Scheme 3-11 is almost complete under normal conditions of diazotization (pH < 1). The base is definitely a much better reagent than the anilinium ion for nitrosation because the latter is an electrophilic substitution. One expects — simply on the basis of the equilibrium shown in Scheme 3-11 — that the rate of diazotization should decrease linearly with increasing acid concentration or, at higher acidities, with the Hammett acidity function h0 (for acidity functions see Rochester, 1970 Cox and Yates, 1983). 

Thus, by definition, electrophiles are electron-pair acceptors and nucleophiles are electron-pair donors. These definitions correspond closely to definitions used in the generalized theory of acids and bases proposed by G. N. Lewis (1923). According to Lewis, an acid is any substance that can accept an electron pair, and a base is any substance that can donate an electron pair to form a covalent bond. Therefore acids must be electrophiles and bases must be nucleophiles. For example, the methyl cation may be regarded as a Lewis acid, or an electrophile, because it accepts electrons from reagents such as chloride ion or methanol. In turn, because chloride ion and methanol donate electrons to the methyl cation they are classified as Lewis bases, or nucleophiles ... 

Although no definitive study of scavengers in photoaffinity labeling experiments has yet been made, the present indications are that thiols are the most suitable reagents. It is expected that as strong nucleophiles they should react with most photogenerated species which are electrophiles as well as with free radicals which can abstract hydrogen atoms from the... 

Regarding the possible mechanism, notably, toluene, ethylbenzene and terf-butylbenzene are less reactive than benzene, which is not consistent with the expected order for an electrophilic aromatic substitutions, such as that found with the classic Fenton reagent. There are also other differences with respect to the Fenton chemistry. In particular, under biphase conditions the reaction is definitely more selective although comparisons are difficult due to the huge amount of data, sometimes inconsistent, on the Fenton system (for which most of the data have been obtained with the iron used in stoichiometric amounts) it seems that selectivities dose to those observed under biphase conditions are only attained at a conversion around of 1%. Furthermore, in the biphase system, only a negligible amount (<1%) of biphenyl was detected among secondary products, whereas in the classic Fenton oxidation this compound is formed by radical dimerization of hydroxycydohexadienyl radicals in typical yields ranging from 8 to 39%. 

Brown (1959) has presented a charge transfer model of the transition state for electrophilic reactions which dififers appreciably from that proposed by Fukui and his collaborators and leads to the definition of a new reactivity index termed the Zvalue . The model is based on a more conventional formulation of the charge transfer mechanism, which avoids the complete transfer of electrons associated with v = 0,1,2 in Fukui s model. There is no dependence on the formation of a pseudo tt orbital in the transition state, nor is hyperconjugation invoked. A wave function c.t. for charge transfer complex is written as a linear combination of a wave function describing the unperturbed ground state of the molecule under attack, and a function 0 which dififers from in fhe replacement of one of its orbitals by the valence orbital of the reagent. Thus the wave function... 

As described in the definition of Buchwald indoline synthesis, this reaction involves the formation of a zirconocene-stabilized benzyne complex that intramolecularly inserts to an A-allyl group to give a tricyclic indoline zirconacycle. In the presence of oxidating reagent or electrophiles, the indoline zirconacycle decomposes into indoline derivatives. ... 

The Lewis definitions of acid-base interactions are now over a half a century old. Nevertheless they are always useful and have broadened their meaning and applications, covering concepts such as bond-formation, central atom-ligand interactions, electrophilic-nucleophilic reagents, cationic-anionic reagents, charge transfer complex formation, donor-acceptor reactions, etc. In 1923 Lewis reviewed and extensively elaborated the theory of the electron-pair bond, which he had first proposed in 1916. In this small volume which had since become a classic, Lewis independently proposed both the proton and generalized solvent-system definitions of acids and bases. He wrote ... 

To be accurate, the definition should be restricted to asymmetric reactions catalyzed by a combination of l,r-binaphthalene-2,2 -diol (BINOL, 4) and Ti(0 -Pr)4. Nonetheless, this chapter will give some background on non-chiral Lewis acid promoters, and include other asymmetric catalytic systems. We will not discuss the allylations that are promoted by Lewis bases, which are reviewed elsewhere, nor cover the reactions with other electrophiles. Excellent reviews already exist on "Selective Reactions Using Allylic MetaM and Catalytic Enantioselective Addition of Allylic Organometallic Reagents to Aldehydes and Ketones , as well as in the comprehensive monograph "Modern Carbonyl Chemistry. The use of BINOL-based catalysts in other fields of organic synthesis has also been reviewed. ... 

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