Transition metal complexes reaction with electrophiles

A number of transition metal complexes react with alkenes, alkynes and dienes to afford insertion products (see Volume 4, Part 3). A general problem is that the newly formed carbon-metal bond is usually quite reactive and can undergo a variety of transformations, such as -hydride elimination or another insertion reaction, before being trapped by an electrophile.200 Usually, a better stability and lower reactivity is observed if the first carbometallation step leads to a metallacycle. It is worthy to note that the carbometallation of perfluorinated alkenes and alkynes constitutes a large fraction of the substrates investigated with transition metal complexes.20015... 

An appreciable number of monographs and reviews deal with the meth-Q rsi,275,329-333 jjjg j j.gg gj jpunt of experimental work that has been performed provides a possibility for establishing favorable conditions of epoxidation with regard to the roles of the catalyst, the organic hydroperoxide, the structure of the olefin, and the medium. Simitar to the hydrogen peroxide-transition-metal complex reaction, this is an electrophilic reaction (Eq. 30). ... 

A related transformation to the previous carbene transfer reaction involves a nitrene ligand bonded to the metal center, in a metallonitrene intermediate in situ generated upon the appropriate selection of the catalyst and the nitrene precursor. As shown in Scheme 17, some transition metal complexes react with such a precursor to generate an unsaturated intermediate, generally electrophilic in nature, which might react with olefins or C—H bonds affording aziridines or amines in a catalytic manner. The most employed nitrene sources are hypervalent I(III) compounds such as PhI=NTs, chloramine-T or organic azides. 

Complex 13 undergoes electrophilic substitution with aromatic substrates. Thus, treatment with benzene in dichloromethane at ambient temperature results in the formation of the diphenyl complex 15 (Scheme V. Reaction of 13 with pyridine (5-6 equivs) in dichloromethane affords a new complex that is the result of pyridine a-CH activation. The NMR data clearly show two chemically equivalent coordinated pyridines and pyridine that has lost one of the a-hydrogens. Structure 16 is proposed from the preliminary data. The formation of 15 and 16 was quantitative by NMR monitoring, but these compounds are reactive and have not been isolated as pure solids. While main group Lewis acids are well known to undergo aromatic substitutions (e.g., mercurations, thallations, etc.) (33), relatively little is known about the ability of transition metal complexes to undergo electrophilic aromatic substitution (34). 

To replace the aforementioned acyl-main group and acyl-transition metal complexes, the natural course of events was to search for a stable and easy-to-handle acyl-metal complex that reacts as an unmasked acyl anion donor. Thus, the salient features of acylzirconocene chlorides as unmasked acyl anion donors remained to be explored. In the following, mostly carbon—carbon bond-forming reactions with carbon electrophiles using acylzirconocene chlorides as acyl group donors are described. 

Transition metal-catalyzed reactions of ct-diazocarbonyl compounds proceed via electrophilic Fischer-type carbene complexes. Consequently, when cr-diazoketone 341 was treated, at room temperature, with catalytic amounts of [ RhiOAcbh, it gave the formation of a single NH insertion product, which was assigned to the enol stmcture 342. At room temperature, in both solid state and in solution, 342 tautomerizes to give the expected 1-oxoperhydropyr-rolo[l,2-c]oxazole derivative 343 (Scheme 50) <1997TA2001>. 

The most important synthetic access to acceptor-substituted carbene complexes is the reaction of ylides with electrophilic, coordinatively unsaturated transition metal complexes (Figure 4.1 see also Section 3.1.3). 

The key success of these metal-catalyzed processes lies in the replacement of an unachievable carbozincation by an alternative carbometallation involving the transition metal catalyst, or another pathway such as an alkene-alkene (or alkyne) oxidative coupling promoted by a group IV transition metal complex, followed by transmetallation. An organozinc is ultimately produced and the latter can be functionalized by reaction with electrophiles. 

For successful reaction of [Fe2S2(NO)4]2 at transition metal centers a strongly electrophilic metal is required thus while no reaction was observed (29) with [(fy5-C5H5)Fe(CO)2Br], the more electrophilic cationic complex [( 5-C5H5)Fe(CO)2(THF)]BF4 reacted to provide a 90% yield of the air-stable [Fe2 SFe(CO)2( /5-C5H5) 2(NO)4]. 

The carbon dioxide molecule exhibits several functionalities through which it may interact with transition metal complexes and/or substrates. The dominant characteristic of C02 is the Lewis acidity of the central carbon atom, and the principle mode of reaction of C02 in its main group chemistry is as an electrophile at the carbon center. Consequently, metal complex formation may be anticipated with basic, electron-rich, low-valent metal centers. An analogous interaction is found in the reaction of the Lewis acid BF3 with the low-valent metal complex IrCl(CO)(PPh3)2 (114). These species form a 1 1 adduct in solution evidence for an Ir-BF3 donor-acceptor bond includes a change in the carbonyl stretching frequency from 1968 to 2067 cm-1. 

REACTIONS OF V-DIENE TRANSITION METAL COMPLEXES WITH ELECTROPHILES 697... 

S3 REACTIONS OF V TRIENE AND V-TETRAENE TRANSITION METAL COMPLEXES WITH ELECTROPHILES... 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

C-H activation with transition-metal complexes will open a new chemistry of catalytic carbon-carbon bond formation because of its potent ability to generate reactive carbon-metal complexes. The design of a catalytic reaction which involves C-H activation followed by reaction with a reagent such as an electrophile is particularly important to provide an environmentally friendly non-salt process, which proceeds under neutral conditions. 

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