Reactivity toward Electrophilic Reagents

Figure 30 Indazole reactivity towards electrophilic reagents...

Reactions of alkynes with electrophiles are generally similar to those of alkenes. Because the HOMO of alkynes (acetylenes) is also of n type, it is not surprising that there IS a good deal of similarity between alkenes and alkynes in their reactivity toward electrophilic reagents. The fundamental questions about additions to alkynes include the following. How reactive are alkynes in comparison with alkenes What is the stereochemistry of additions to alkynes And what is the regiochemistry of additions to alkynes The important role of halonium ions and mercurinium ions in addition reactions of alkenes raises the question of whether similar species can be involved with alkynes, where the ring would have to include a double bond ... 

The reaction of benzylmagnesium chlorides wnth thiophenealde-hydes and thienyl ketones has been used for the preparation of styrylthiophenes and 1,2,2-triarylethylenes, which are of biological interest. In stilbene and 1,2,2-triphenylethylene the reactivity toward electrophilic reagents is transferred with deactivation to the double bond. However, styrylthiophene is formylated and acylated... 

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. 

Metalated cyclic aldo-nitrones are characterized by high reactivity toward electrophilic reagents. Reactions with aldehydes and ketones afford satisfactory yields of a-hydroxymethyl substituted derivatives of nitrones (551). The reactions were also carried out with a number of aliphatic, aromatic, and hetero-aromatic aldehydes and ketones (Schemes 2.124 and 2.125). 

The 2,5-, 2,3- and 3,4-dihydropyridines have received very little study regarding their reactivity toward electrophilic reagents. This is undoubtedly due to their instability and the low number of authentic derivatives that are known. 

All are exceptionally reactive towards electrophilic reagents, particularly in alkaline solution, and all are readily oxidized. The 1,2- and 1,4-benzenediols, but not 1,3-benzenediol, are oxidized to quinones ... 

Condensed aromatic hydrocarbons are more reactive towards electrophilic reagents, and naphthalene, for example, may be brominated quite readily in solution in carbon tetrachloride without the need for a catalyst electrophilic attack takes place at the more reactive a-position to yield 1-bromonaphthalene (Expt 6.25). 

Benzoselenophene is generally more reactive toward electrophilic reagents, although, like 1-benzothiophene, it undergoes mainly p-substitution (Scheme 5). 

Tetrazoles exhibit qualities of acids, bases, acceptors of hydrogen bonds (cf. Section 6.07.4.5), and polydentate ligands (cf. Section 6.07.5.3.4). NH-LJnsubstituted tetrazoles behave both as substrates and intermediates in transacylation processes (cf. Section 6.07.5.4), etc. Tetrazolate anions (tetrazolides) possess high aromaticity and reactivity toward electrophilic reagents (cf. Sections 6.07.4.1 and 6.07.5.3.2). The thermal and photochemical decomposition of tetrazoles involves formation of nitrenes and other intermediates of high reactivity (cf. Sections 6.07.5.2 and 6.07.5.7) These properties provide a possibility of use tetrazoles as catalysts in chemical and biochemical reactions. 

Reactivity toward electrophilic reagents of this type of complex was evinced by partial chlorination, bromination, and acid-catalyzed proton exchange of the complex (B10H)0S)2Fe2 (21). 

Compared with the parent benzothiazinotropones, these oxides exhibit a greater reactivity toward electrophilic reagents. This is explained by the greater electron density on the rings due to the easier deprotonation of NH. Bromination takes place at C-9 and C-7, and nitration at C-9. 

Because of their high reactivity toward electrophilic reagents, Gilman14 described them as superaromatic molecules. This term, used afterward in many textbooks of organic chemistry, is misleading since all these nuclei are actually less aromatic than benzene. 

The effects of the cr—JT interaction on the ground-state properties of allyltrimethylmetal compounds are paralleled by the effect on reactivity towards electrophilic reagents. Mayr demonstrated that allyltrialkylsilanes, allyltrialkyl-germanes, and trialkylstannanes react with diphenylcarbenium ions at rates 105,5.6 x 105, and 109, respectively, relative to propene.158 The reaction rates were also found to be sensitive to the inductive effects of the other substituents attached to the metal. A theoretical evaluation of the factors determining the regiochemistry and stereochemistry of electrophilic addition to allylsilanes and other allyl systems is reported by Hehre et al.159 They predict a preference for electrophilic attack anti with respect to the silane substituent, a prediction that is supported by many experimental studies.82,160... 

Alkyl substituents in aromatic azoloazines are reactive towards electrophilic reagents in basic media. Basic reagents readily abstract protons from such alkyl groups yielding resonance stabilized carbanions. Thus, treatment of the methyl derivatives (243) with aldehydes gives the alkenes (245) (Scheme 21) <84H(22)174i). Ready formation of the resonance stabilized anions (244) is behind the activity of the methyl group. 

The 5-position of selenazoles is very reactive towards electrophilic reagents. Nitration of 2-alkylselenazoles affords 5-nitro derivatives under relatively mild conditions. 2-... 

Metal exchange. Lithiated ynamines 44 are. smoothly silylated to the corresponding silyl ynamines (84) 42>59 136). Such ynamines formally replace the very sensitive non-substituted ynamines but the increase in stability entails a loss of reactivity toward electrophilic reagents because silicon acts as a weak electron acceptor. 

There is rich experimental evidence showing that aryl positions adjacent to strained annelated rings exhibit reduced reactivity towards electrophilic reagents [2-7,98-101], These data are in harmony with the theoretical interpretation in terms of the bond fixation model presented above. It is possible that the ground state charge distribution in some particular fused molecular systems affects propensities of a and f3 atoms to undergo the electrophilic substitution reactions, but interpretation based solely on the rehybridization effect at carbon junction atoms offered by Siegel et al.[9] is obviously unjustified. 

Sulfonic, carboxylic, etc., groups present in the substrate may easily undergo substitution (Fig. 8) during Maimich synthesis, due to their reactivity toward electrophilic reagents ... 

Compound (10) has an enhanced reactivity towards electrophilic reagents, which has been ascribed to the ideal siting of the nitrogen atom to take part in nucleophilic... 

On comparison of simple 1,2,4-triazoles with benzene, replacement of CH=CH in the latter by NH enhances reactivity towards electrophilic reagents but two replacements of CH by N act in the opposite direction (see Section 4.02.1.1). Comparison with other azoles is of greater heuristic value still (B-76MI41200). 

An olefinic bonds conjugated with the 1 position of a diene complex is highly reactive toward electrophilic reagents because of the stability of the resulting complexed cation. These reagents include not only protons but the trityl cation and bromine. ... 

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