Arene ligand reactivity nucleophilic substitution

Carbon nucleophiles of type (iii) add to the arene ligand and do not rearrange examples include the very reactive anions, such as 2-lithio-2-methyl-l,3-dithiane, and the less sterically encumbered anions, such as lithio acetonitrile and /-butyl lithioacetate. In these cases, the anion adds to an unsubstituted position (mainly ortho or meta to Cl, as in 22) and does not rearrange. Then iodine quenching, even after a long period at 25 °C, gives almost exclusively the products from formal substitution for hydrogen, as from (22) in Scheme 8. 

Such a charge transfer from the ligated arene can lead to (a) nucleophilic addition or substitution, (b) electron transfer, and (c) proton elimination/transfer, thus revealing the dose relationship between all of these processes. The reactivity of the arene ligands towards nudeophiles in (arene)ML complexes depends on the electrophilidty of the metal fragments [MLn], this increasing in the order [Cr(CO)3] < [Mo(CO)3] [FeCp]+ < [Mn(CO)3]+ [2]. For example, in (arene)FeCp+, which is widely used for synthetic purposes, a chloro or nitro substituent on the arene is readily substituted by such nudeophiles as amides, eno-lates, thiolates, alkoxides, and carbanions [45]. 

Intermolecular Nucleophilic Substitution with Heteroatom Nucleophiles. A patent issued in 1965 claims substitution for fluoride on fluorobenzene-Cr(CO)3 in dimethyl sulfoxide (DMSO) by a long list of nucleophiles including alkoxides (from simple alcohols, cholesterol, ethylene glycol, pinacol, and dihydroxyacetone), carboxylates, amines, and carbanions (from triphenyhnethane, indene, cyclohexanone, acetone, cyclopentadiene, phenylacetylene, acetic acid, and propiolic acid). In the reaction of methoxide with halobenzene-Cr(CO)3, the fluorobenzene complex is ca. 2000 times more reactive than the chlorobenzene complex. The difference is taken as evidence for a rate-limiting attack on the arene ligand followed by fast loss of halide the concentration of the cyclohexadienyl anion complex does not build up. In the reaction of fluorobenzene-Cr(CO)3 with amine nucleophiles, the coordinated aniline product appears rapidly at 25 °C, and a carefiil mechanistic study suggests that the loss of halide is now rate limiting. 

While carbon nucleophiles were suggested to be efficient in substitution for fluoride in the 1966 patent, the first examples in the primary literature appeared in 1974 i62,i69 is now clear that there are three reactivity classes of carbon nucleophiles (Scheme 39) (a) stabilized carbanions (from carbon acids with pA a < ca. 18) (b) more reactive carbanions (P a > 20), which give complete conversion to cyclohexadi-enyl addition products prior to slow equilibration via reversible anion addition and (c) more reactive carbanions (pATa > 20), which give irreversible addition to the arene ligand. ... 

In agreement with the above order of reactivities, Friedel-Crafts acylation of C6H6Cr(CO)3 does not proceed very easily. The reduced electron density on the arene ring compared with that in the free ligand, however, enhances its susceptibility to nucleophilic substitution ... 

The attachment of a transition metal to an unsaturated hydrocarbon ligand transforms the reactivity properties of the ligand. Whereas the classic chemistry of alkenes and arenes is that of electrophilic addition and substitution reactions, the classic reactivity of multihapto-complexes is their reactions with nucleophiles. Such complexes need not be cationic to be good electrophiles, but it helps, and not surprisingly, the most powerful electrophilic multihapto-complexes have positive charge stabilized by the metal. 

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