Reactivity patterns with electrophiles

The pattern of reactivity is similar to that discussed for the azolinones in Sections 4.02.1.1.4 and 4.02.3.7.1. A difference is the greater nucleophilicity of sulfur, and thus more reaction of the ambident anion with electrophiles occurs at sulfur. 

Examine atomic charges and electrostatic potential maps of these ions. Which ion has the most electron-poor electrophilic carbon Which has the least electrophilic carbon Is the variation in charge consistent with the observed reactivity patterns Explain. 

The familiar pattern of 2-amination with sodamide ( — 33°, 90% yield) occurs also with 1,5-naphthyridine. Greater reactivity at the 2-position is attributed, as before, to a cyclic transition state with electrophilic attack at a ring-nitrogen concomitant with nucleophilic attack adjacent to the cationic center thus formed. 

From the point of view of both synthetic and mechanistic interest, much attention has been focused on the addition reaction between carbenes and alkenes to give cyclopropanes. Characterization of the reactivity of substituted carbenes in addition reactions has emphasized stereochemistry and selectivity. The reactivities of singlet and triplet states are expected to be different. The triplet state is a diradical, and would be expected to exhibit a selectivity similar to free radicals and other species with unpaired electrons. The singlet state, with its unfilled p orbital, should be electrophilic and exhibit reactivity patterns similar to other electrophiles. Moreover, a triplet addition... 

As mentioned earlier, metal complexation not only allows isolation of the QM derivatives but can also dramatically modify their reactivity patterns.29o-QMs are important intermediates in numerous synthetic and biological processes, in which the exocyclic carbon exhibits an electrophilic character.30-33 In contrast, a metal-stabilized o-QM can react as a base or nucleophile (Scheme 3.16).29 For instance, protonation of the Ir-T 4-QM complex 24 by one equivalent of HBF4 gave the initial oxo-dienyl complex 25, while in the presence of an excess of acid the dicationic complex 26 was obtained. Reaction of 24 with I2 led to the formation of new oxo-dienyl complex 27, instead of the expected oxidation of the complex and elimination of the free o-QM. Such reactivity of the exocyclic methylene group can be compared with the reactivity of electron-rich enol acetates or enol silyl ethers, which undergo electrophilic iodination.34... 

Attack on alkenylsilanes takes place at the a carbon and results in replacement of the silicon substituent by the electrophile. Attack on allylic groups is at the y carbon and results in replacement of the silicon substituent and an allylic shift of the double bond. The crucial influence on the reactivity pattern in both cases is the very high stabilization which silicon provides for carbocationic character at the / -carbon atom. This stabilization is attributed primarily to a hyperconjugation with the C—Si bond.56... 

The anion 7 is quite nucleophilic and undergoes exclusive C-alkylation upon reaction with electrophiles including chlorotrimethylsilane °. These reactivity patterns have led to the suggestion that enolates of iron-acyl complexes may be considered to behave similarly to the organic dianion 910- u. 

Polyfluoroaroinatics show similar reactivity patterns, wherein the ease of electrophilic substitution generally decreases with increasing fluorine content Pentafluorobenzene undergoes substitution only under forcing conditions, and perfluoroaromatics completely resist substitution, which would require displacement of F, but instead give addition products when they react with electrophiles [777, 772,126]. 

This pattern of reactivity is reflected in most reactions of electrophiles with complexes containing aromatic ligands the rates of reaction are modified but the position of substitution is unchanged with respect to the free ligand. The reactivity of a range of quinoline complexes with electrophiles has been studied in some detail and the products have been shown to be substituted in exactly the same sites as the free ligands. For example, di(8-oxyquinolinato)copper(n) reacts with molecular bromine to yield di(5,7-dibromo-8-oxyquinolinato)copper(n) (Fig. 8-3). 

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