Complexes with electrophilic reactivity

Reactivity of dinitrogen in a complex with electrophiles, including the proton and organic free radicals35... 

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). 

Low reactivity of pyridine compared with benzene at carbon Reaction at nitrogen prevails Formation of stable complexes with electrophiles A-Nitro Heterocycles as Nitrating Agents... 

In general, the reactivity of the pyridine ring in nucleophilic substitution reaction decreases in the row C2 > C4 > C3. Consequently, more synthetic routes are reported for 4-fluoropyridines compared to 3-fluoropyridines. Pyridines can form cationic complexes with electrophiles resulting in activation of heterocyclic ring toward nucleophilic substitution. On the other hand, pyridines have signih-cantly reduced reactivity toward electrophiles and typically undergo electrophilic substitution reactions in the present of strong Lewis acids selectively in the position 3. ... 

This chapter will initially cover several aspects of dihapto-coordination of aromatic molecules, including the scope of the dearomatization agent and the aromatic substrate. The primary focus of this work, however, will be the fundamental organic reactions of these complexes with electrophiles and the subsequent reactions of those products. Several applications of this methodology will also be illustrated. Owing largely to its earlier discovery, the majority of the organic transformations reviewed will be with osmium(II), however, recent arene transformations promoted with rhenium(I) and molybdenum(O) will also be discussed, with an emphasis on differences in reactivity compared to those of osmium. 

Two resonance forms can be depicted for this type of complex as exemplified in Equation (50), and the contribution of the allylic form is higher than the contribution of the palladacyclobutanone. Interestingly, the reverse situation is found for the analogous platinum complexes. " The contribution of the Ty -allyl form can be enhanced in polar media, and its importance is manifest in the facile reactivity of these palladium complexes with electrophiles. " The reaction of a bis-imidazolium salt with Pd(OAc)2 affords another example where a zwitterionic neutral imidazolium fragment is bound to the metal in an j/ -oxatrimethylenemethane form 102. " ... 

Then, we studied the reactivity of different organogold complexes with electrophiles. While with (D" ), I", Br, and Cl" donors always a formation of a bond between carbon and these electrophiles was observed, with donors, always a C-C bond formation was induced, in this case a homodimerization (Scheme 9) [71] for a detailed full paper on this chemistry, see [72]. 

In view of the overall increased reactivity of furan compared with thiophene it would be anticipated that furan would be less regioselective in its reactions with electrophiles than thiophene. Possible reasons for the high regioselectivity of furan in electrophilic substitution reactions include complex formation between substrates and reagents and the ability of heteroatoms to assist in the stabilization of cationic intermediates (80CHE1195). 

There is another important factor in the low reactivity of pyridine derivatives toward electrophilic substitution. The —N=CH— unit is basic because the electron pair on nitrogen is not part of the aromatic n system. The nitrogen is protonated or complexed with a Lewis acid under many of the conditions typical of electrophilic substitution reactions. The formal positive charge present at nitrogen in such species further reduces the reactivity toward electrophiles. 

The validity of this statement is confirmed by the rates of IC1 additions (see Table 12). Because for these additions the formation of a cationic intermediate by direct attack of the electrophile on the double bond is rate determining, their order of rates is comparable to those of polymerizations. It is therefore understandable that the polymerization rates correlate much better with the reactivities of the monomers during an electrophilic addition of cationogenic agents (such as IC1) than with the relatively unspecific EDA complex formation. 

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... 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

The characteristic reactivity of neutral dg alkylidene complexes of Ru, Os, and Ir is with electrophilic reagents. The osmium methylene 47 reacts with the widest range of electrophiles, the most significant reactions being summarized in Scheme 2. 

Enantiomerically pure and racemic / -carbonyl sulfoximes were treated with diethylzinc to afford the corresponding ethylzinc enolates 148a-c in both racemic and optically active forms (Scheme 94).214 Despite their rather similar solution structures, these complexes exhibited markedly different solid-state structures and reactivities with electrophiles. 

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