Nucleophilic and electrophilic attack

The N-oxide function has proved useful for the activation of the pyridine ring, directed toward both nucleophilic and electrophilic attack (see Amine oxides). However, pyridine N-oxides have not been used widely ia iadustrial practice, because reactions involving them almost iavariably produce at least some isomeric by-products, a dding to the cost of purification of the desired isomer. Frequently, attack takes place first at the O-substituent, with subsequent rearrangement iato the ring. For example, 3-picoline N-oxide [1003-73-2] (40) reacts with acetic anhydride to give a mixture of pyridone products ia equal amounts, 5-methyl-2-pyridone [1003-68-5] and 3-methyl-2-pyridone [1003-56-1] (11). 

The attack of HF is thought to proceed by a similar mechanism in which there is simultaneous nucleophilic and electrophilic attack on the network silicon and oxygen atoms, respectively, according to ... 

Imidazo[l,5-tf]pyridines are aromatic systems in which the bridgehead nitrogen N-4 contributes to the aromaticity with its lone pair. Therefore, this nitrogen atom is not nucleophilic and electrophilic attacks occur at the N-2 position. SgAr occurs at C-l, but also sometimes at C-3, depending on the conditions used (Figure 3). 

Condensed-to-atom Fukui functions for nucleophilic and electrophilic attacks on H2CO estimated from Equation 12.10 ... 

Condensed-to-atom softness values for nucleophilic and electrophilic attacks can easily be estimated by multiplying the respective FF values by global softness value. 

It is to be noted that/(r) is normalized to unity. Due to discontinuity problem in the number of electrons [13] in atoms and molecules, the right- and left-hand side derivatives at a fixed number of electrons introduces the concepts of EF for nucleophilic and electrophilic attack, respectively. Introducing the finite difference approximation and the concept of atom condensed Fukui function (CFF) [14], the working equations are... 

A majority of the compounds encountered in this class of heterocycles exist as A -dioxides. The nomenclature for such compounds is somewhat ambiguous as the terms sultam, sulfonamide, and 1,2-thiazine 1,1-dioxide have all been used to describe these molecules. The reactivity of 1,2-thiazine 1,1-dioxides 4 (as the 22/-tautomer) has been thoroughly investigated and much of this work has been described in Chapter 6.06 of CHEG-II(1996) <1996CHEC-II(6)349>. This S(vi)-oxidized compound 4 undergoes both nucleophilic and electrophilic attack, which are often non-regioselective. 

Figure 6.28 Approach trajectories for nucleophilic and electrophilic attack on the molecular clip 6.48.

The second class of TAM RE AC s inventory includes the reactions between the coordinated ligands and external organic reagents. We divide these reactions into nucleophilic and electrophilic attacks and consider them as acid-base interactions. Table III presents their general description. The nucleophilic attacks are either addition reactions to unsaturated coordinated ligands (Reactions 44-46) or abstraction reactions (usually a proton abstraction, Reactions 47-50). The electrophilic attacks are similarly addition reactions (Reactions 51 and 52) and abstraction reactions (usually a hydride abstraction, Reactions 53-59). Reactions 60 to 63 represent some other intermolecular reactions. 

Although a simple mechanism can be drawn for this transformation, it is only useful as a hook-keeping exercise to ensure that the correct structure is drawn for the product. In reality the reaction is a concerted process and the usual considerations of nucleophilic and electrophilic attack do not apply. 

Both nucleophilic and electrophilic attack on trigonal carbon can take place by two pathways (Fig. 5.4)—direct attack on one of the cr bonds attached to the double bond (path a), or by attack on the n bond (path b), with the formation of an ionic intermediate, followed by the loss, respectively, of a nucleofugal group X or an electrofugal group M. There are also formally unimolecular pathways, SN1 and SeI, with ionisation followed by capture by the nucleophile or electrophile, but the former is rare and the latter unknown. 

We now turn to the stereochemistry governed by a ring system, and we shall look at both nucleophilic and electrophilic attack, since usually they have similar stereochemical preferences rather than contrasting preferences. In addition to several reactions that are straightforwardly electrophilic attack, we shall see several which can be described as electrophilic in nature, like the reactions of alkenes with osmium tetroxide, with peracids, with some 1,3-dipoles, and with boranes, and of dienes with dienophiles in Diels-Alder reactions. Some of these reactions are pericyclic, the pericyclic nature of which we shall meet in Chapter 6. For now, it is only their diastereoselectivity that will concern us. 

Diastereoselective Nucleophilic and Electrophilic Attack on Double Bonds Free of Steric Effects... 

Allyl complexes are susceptible to nucleophilic and electrophilic attack. A typical reaction of an allyl complex with a nucleophile is illustrated in Eq. (5) [17]. Nucleophilic attack at one of the terminal carbons of an allyl ligand most often occurs on the face of the allyl group opposite the metal, as shown in Eq. (5) however, the nucleophile may bind to the metal center initially if there is a vacant site available and, then, transfer to the endo face of the allyl group. 

Fig. 2. Positional selectivity in substitution reactions of azines. Black arrows electrophilic substitutions white arrows nucleophilic substitutions. Dashed and solid light arrows refer to second best positions for nucleophilic and electrophilic attack, respectively.

U. Belluco, R. A. Michelin, P. Uguagliati, B. Crociani, Mechanisms of nucleophilic and electrophilic attack on carbon bonded palladium(II) and platinum(II) complexes, J. Organomet. Chem. 250 (1983) 565. 

The number of studies of inorganic reaction mechanisms by theoretical methods has increased drastically in the last decade. The studies cover ligand substitution reactions, insertion reactions oxidative addition, nucleophilic and electrophilic attack as well as metallacycle formation and surface chemistry, in addition to homogeneous and heterogeneous catalysis as well as metalloenzymes. We can expect the modeling to increase further both in volume and in sophistication [173],... 

Nucleophilic and Electrophilic Attack by One it Bond on Another. A combination of nucleophilic and electrophilic attack on double bonds is the core of the aldol reaction, where both the nucleophile and the electrophile are n bonds.479 The ideas we have seen in the previous two sections can be combined to understand the transition structure 5.63 calculated for this reaction in the gas phase.480 This transition... 

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