Electrophiles special

Despite the fact that solutions of acetyl nitrate prepared from purified nitric acid contained no detectable nitrous acid, the sensitivity of the rates of nitration of very reactive compounds to nitrous acid demonstrated in this work is so great that concentrations of nitrous acid below the detectable level could produce considerable catalytic effects. However, because the concentration of nitrous acid in these solutions is unknown the possibility cannot absolutely be excluded that the special mechanism is nitration by a relatively unreactive electrophile. Whatever the nature of the supervenient reaction, it is clear that there is at least a dichotomy in the mechanism of nitration for very reactive compounds, and that, unless the contributions of the separate mechanisms can be distinguished, quantitative comparisons of reactivity are meaningless. 

Chemical Properties. The chemistry of ketenes is dominated by the strongly electrophilic j/)-hybridi2ed carbon atom and alow energy lowest unoccupied molecular orbital (LUMO). Therefore, ketenes are especially prone to nucleophilic attack at Cl and to [2 + 2] cycloadditions. Less frequent reactions are the so-called ketene iasertion, a special case of addition to substances with strongly polarized or polarizable single bonds (37), and the addition of electrophiles at C2. For a review of addition reactions of ketenes see Reference 8. 

Aniline and its derivatives are so reactive in electrophilic aromatic substitution that special strategies are usually necessary to carry out these reactions effectively. This topic is discussed in Section 22.14. 

The cyclic diazo compounds (diazirines 65) are very unreactive compounds. Specially noticeable is the absence of the reactivity toward electrophilic reagents which is characteristic of the linear isomers. Acids or aldehydes which react smoothly with the aliphatic diazo compounds are without action on the cyclic diazo compounds. Iodine does not attack the cyclic diazo compounds. 

If these considerations and investigations are taken into account, the reaction scheme, Eqs. (5), (6), and (7), can be deduced for the methylation of tautomeric and potentially tautomeric compounds by diazomethane (itself a special case of a reaction of ambifunctional compounds with electrophilic reagents). A vital step in this scheme... 

It has also been stated that the 5-position of selenazoles is more reactive toward electrophilic substitution than that of thiazoles. Such reactivity is still further increased by substituents in the 2-position of the selenazole ring, which can have an —E-effect. Simultaneously, however, an increasing tendency toward ring fission was observed by Haginiwa. Reactions of the selenazole ring are thus limited mainly to the 5-position which, specially in the 2-amino-and the 2-hydrazino-selenazoles, is easily substituted by electrophilic reagents. However, all attempts to synthesize selenazole derivatives by the Gattermann and by the Friedel-Crafts methods... 

The mechanisms of the electrophilic substitutions in the isoxazole nucleus have not yet been studied. They should not differ fundamentally from those usually accepted for the substitution of aromatic systems but the structural specificity of the isoxazole ring might give rise to some peculiarities, as recently specially discussed.One important point is that isoxazole shows a clearcut tendency to form coordination compounds. Just as pyridine and other azoles, isoxazoles coordinate with halogens and the salts of heavy metals, for example of cadmium,mercury,zinc. Such coordination... 

Ttansmetalation of tliioetliets to organocopper compounds can also be performed in some special cases. Tluis, tteatment of the ester 119 with MeyCuLi-LiCN provides the copper reagent 120, which can be treated successfully witli several electrophiles such as allyl bromide ot acid chlorides to afford the expected products such as 121 iScbeme 2.54) [115, 116]. 

Aromaticity (Chapter 15 introduction) The special characteristics of cyclic conjugated molecules. These characteristics include unusual stability, the presence of a ring current in the 1H NMR spectrum, and a tendency to undergo substitution reactions rather than addition reactions on treatment with electrophiles. Aromatic molecules are planar, cyclic, conjugated species that have An + 2 7T electrons. 

Vinyloxirancs and vinyl acetals constitute a special subset of allylic electrophiles. The product of Sn2 displacement of vinyloxiranes is an allylic alcohol, while the SN2 product from vinyl acetals is a vinyl ether. 

Sulfonic acids themselves are unfit for electrophilic transfer of sulfonyl groups because of the poor nucleofugality of the hydroxide anion. However, the high acidity obviously leads to an equilibrium between the acids and their anhydrides and water, from which water can be removed either by special reaction conditions (i.e., azeotropic distillation with appropriate solvents) or chemically with anhydride forming agents316 (equation 63). sulfonic acid anhydride sulfonylations are compiled in Table 10. 

The special substituent constants for + R para-substituents are denoted by a, and those for — R para-substituents are denoted by a+ 54. They are based respectively on the reaction series discussed above. Selected values are given in Table 1. Characteristic a or a+ values are sometimes distinguished for meta-substituents also, but only for a minority of substituents which show very marked + R or — R effects do these differ significantly from ordinary a values. The range of applicability of the Hammett equation is greatly extended by means of a and cr+, notably to nucleophilic (by a ) and to electrophilic (by cr+) aromatic substitution. 

Wanzlick showed that the stability of carbenes is increased by a special substitution pattern of the disubstituted carbon atom [12-16]. Substituents in the vicinal position, which provide n-donor/a-acceptor character (Scheme 2, X), stabilize the lone pair by filling the p-orbital of the carbene carbon. The negative inductive effect reduces the electrophilicity and therefore also the reactivity of the singlet carbene. 

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. 

Heterolytic cleavage of the tin-carbon bond is reviewed in references (94-96). Cleavage by electrophiles (e.g, HgXj or halogen) is dominated by electrophilic attack at carbon, and cleavage by nucleophiles principally involves nucleophilic attack at tin. Much of the interest in these processes centers on the intermediate mechanisms that may exist between these extremes, in which electrophilic attack is accompanied by some nucleophilic assistance, and vice versa. Allylic, al-lenic, and propargylic compoimds show a special reactivity by a special (Se2 or SE2y) mechanism. 

Both allenes141 and alkynes142 require special consideration with regard to mechanisms of electrophilic addition. The attack by a proton on allene might conceivably lead to the allyl cation or the 2-propenyl cation. 

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