Nucleophilic and electrophilic

There are ways to plot data with several pieces of data at each point in space. One example would be an isosurface of electron density that has been colorized to show the electrostatic potential value at each point on the surface (Figure 13.6). The shape of the surface shows one piece of information (i.e., the electron density), whereas the color indicates a different piece of data (i.e., the electrostatic potential). This example is often used to show the nucleophilic and electrophilic regions of a molecule. 

As conjugated systems with alternating TT-charges, the polymethine dyes are comparatively highly reactive compounds (3). Substitution rather than addition occurs to the equalized TT-bond. If the nucleophilic and electrophilic reactions are charge-controHed, reactants can attack regiospeciftcaHy. 

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). 

General synthetic methods were developed after 1920 and extended to many new systems. Oxidative syntheses of dyes are primarily of historical interest (1), whereas nonoxidative syntheses are the most versatile and employ varied combinations of nucleophilic and electrophilic regents. One review Hsts references for the synthesis of dyes prepared before 1959 (12), and another review provides supplemental references to more recent compounds (13). Many nucleophilic and electrophilic reagents used to synthesize cyanine and related dyes are tabulated in Reference 16. 

The fourth of Pedersen s general methods is expressed as method Y. In this approach, a single unit may be both nucleophile and electrophile and react with the corresponding portions of its counterpart to yield a macrocycle. This is illustrated in Eq. (3.6). Note that there are really two possibilities here. The first of these is that two units will react as illustrated, but the other possibility is that the single unit will cyelize to afford a crown of half the size. It is precisely this approach which Pedersen used in the first synthesis of 18-crown-6 (see Sect. 3.2). 

In contrast, fluorinated ketones have been used as both nucleophilic and electrophilic reaction constituents The (Z)-lithium enolate of 1 fluoro 3,3-di-methylbutanone can be selectively prepared and undergoes highly diastereoselec-tive aldol condensations with aldehydes [7] (equation 8) (Table 4)... 

Cyanoformamidines having both nucleophilic and electrophilic capacity react with hexafluoroacetone to give five-membered heterocycles [82] (equation 12). 

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 ... 

Figure 5.1 Some nucleophiles and electrophiles. Electrostatic potential maps identify the nucleophilic (red negative) and electrophilic (blue positive) atoms.

If the definitions of nucleophiles and electrophiles sound similar to those given in Section 2.11 for Lewis acids and Lewis bases, that s because there is... 

Thomsons )W Click Organic Interactive to identify and characterize nucleophiles and electrophiles in organic reactions. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

In addition to nucleophilic and electrophilic substitution reactions, other reactions have also been used to prepare poly(arylene edier)s, especially those with special structures which otherwise could not be prepared. The following paragraph briefly reviews these reactions. 

In so far as the composition of the electrical effect is considered, the values of p given in Table XVll show that for both nucleophilic and electrophilic radicals, the resonance effect seems to predominate, probably in the case of the former and almost certainly in the case of the latter. 

In another application of the cyanohydrin acetonide method, cyanohydrin acetonide 64 (Fig. 2) was developed as a common precursor to both the nucleophilic and electrophilic components of a convergent coupling [30]. Orthogonal... 

A related strategy of orthogonal nucleophilic and electrophilic activation was later employed in the synthesis of the polyene macrolide roflamycoin [32]... 

Whenever one compound uses its electrons to attack another compound, we call the attacker a nucleophile, and we call the compound being attacked an electrophile. It is very simple to tell the difference between an electrophile and a nucleophile. You just look at the arrows and see which compound is attacking the other. A nucleophile will always use a region of high electron density (either a lone pair or a bond) to attack the electrophile (which, by definition, has a region of low electron density that can be attacked). These are important terms, so let s make sure we know how to identify nucleophiles and electrophiles. 

PROBLEMS In each of the foUowing steps, identify the nucleophile and electrophile,... 

Of course, it is important that we have a nucleophile present, but how much we have doesn t matter. So now we can understand the 1 in SnL The rate of the reaction is dependent only on the concentration of the electrophile, and not that of the nucleophile. The rate is dependent on the concentration of only one entity, and the reaction is said to be first order. We signify this by placing a 1 in the name. Of course, this does not mean that you only need the electrophile. You still need the nucleophile for the reaction to happen. You still need two different things (nucleophile and electrophile). The 1 simply means that the rate is not dependent on the concentration of both of them. The rate is dependent on the concentration of only one of them. 

As is the case for aldol addition, chiral auxiliaries and catalysts can be used to control stereoselectivity in conjugate addition reactions. Oxazolidinone chiral auxiliaries have been used in both the nucleophilic and electrophilic components under Lewis acid-catalyzed conditions. (V-Acyloxazolidinones can be converted to nucleophilic titanium enolates with TiCl3(0-/-Pr).320... 

The positional specificity of these nucleophilic and electrophilic photosubstitutions can be readily understood by examination of the calculated charge densities of some of these molecules in their ground and first excited singlet states. For example, 4-nitrocatechol has the charge densities shown in (82) and (83).<139> Thus on this basis one would predict that 4-nitroveratrole... 

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