Arene ligand reactivity complexes

Loss of Coordinated Arene. We previously stated that the arene ligand in ruthenium(II)-arene complexes is relatively inert towards displacement under physiological conditions. While this is generally true, there are a few exceptions to this rule and this type of reactivity can be used to advantage. Weakly bound arenes, for instance, can be thermally displaced, a property convenient for the synthesis of ruthenium-arene complexes that are not readily available through more common synthetic routes. This way, the reaction of a precursor dimer, [RuCl2(etb)]2 (etb, ethylbenzoate) (68), with either 3-phenyl-1-propylamine or... 

A new imidazole-functionalized calix[4]arene ligand, able to form a dinuclear Cu2+ complex, has been reported to hydrolyze HPNP and ethyl p-nitropheny lphosphate [70]. The dinuclear complex was found to be 22-and 330-fold more reactive than the corresponding monomer towards the above substrates, respectively. Dinuclear Cu2+ complexes of linked triazacyclononane ligands are reported to promote the hydrolysis of the monoribonucleotide GpppG, a model for the 5 -cap structure of mRNA [71]. The dinuclear complexes offer some 100-fold higher reactivity compared to the mononuclear Cu2+-triazacyclononane system. 

The isolation of the first halobenzene complex, (q6-chlorobenzene)tricarbonylchromium(0), allowed a test for a direct analog of classical SNAr reactivity.15 The activating effect of the Cr(CO>3 unit was found to be comparable to a single p-nitro substituent in reaction with methoxide in methanol and the substituted arene ligand was detached with mild oxidation (equation 2). 

The complexes are air stable indeed, a limitation is the need for powerful oxidizing agents, such as Jones reagent CrVI, to detach the arene ligand.38 They are highly reactive toward nucleophiles. This limits the number of compatible synthesis manipulations that can be carried on in the presence of the [(arene)Mn(CO)3] unit but broadens the scope of effective nucleophiles. 

The complexation of an arene to the tricarbonylchromium unit promotes the addition of nucleophiles to the arene ring due to the strong electron-withdrawing ability of the Cr(CO)3 group. Other effects of the coordination of the metal on the reactivity of the arene ligand have been well-documented in the literature [1] and concern (Scheme 1) (i) the stabilization... 

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]. 

Mori M (2005) Synthesis and Reactivity of Zirconium-Silene Complexes. 10 41 -62 Muniz K (2004) Planar Chiral Arene Chromium (0) Complexes as Ligands for Asymetric Catalysis. 7 205-223 Murai S see Kakiuchi F (1999) 3 47-79... 

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. 

A widely observed reaction of cationic arene complexes is the addition of hydride, alkyl, or aryl anions to the arene ligand giving cyclohexa-dienyl derivatives 51, 154, 185, 243, 251). Other nucleophiles such as CN , OMe , and Ng" have been employed, but the products are generally less stable 185, 432). The following order of reactivity of TT-complexes toward nucleophiles has been noted cycloheptatriene > arene > C Ph > C5H5 103). However, the order will depend to some extent on the system 17). 

Bis(trichlorosilyl)nickel(II) complex 4, having an r/ -arene hgand,is prepared by reaction of hexachloro disilane with highly reactive, vaporized nickel in the presence of toluene (Eq. 2) [12]. Worthy of note is that the arene ligand is displaced by three molecules of carbon monoxide to give 5 [13]. 

Since more reactive alkenes, such as vinyl arenes or sterically strained polycycles, react more readily in the hydroamination reaction, several asymmetric hydroami nation reactions utilizing these substrates have been disclosed. Weakly basic anilines can react with vinyl arenes to give the Markovnikov addition products 6 and 7 with good yields and enantioselectivities in the presence ofa chiral phosphine ligand Pd complex as demonstrated by Hartwig (Eq. 11.3) [13] and later by Hii (Eq. 11.4) [14]. 

In this section, we will review the nature and chemical reactivities of several kinds of ruthenium complexes under the following headings ruthenium carbonyl complexes, dichlororuthenium complexes, chlorohydrido complexes, dihydridoruthenium complexes, ruthenium complexes with chiral ligands, ruthenium complexes with cyclopentadienyl ligands, and ruthenium arene/diene complexes. 

Catalytic HPNP-tranesterification reactivity (Fig. 40, bottom has been reported for zinc complexes assembled using calix[4]arene ligands (Fig. 65).172,248 251 Complex 9 induces a 23,000-fold rate enhancement over the uncatalyzed intramolecular cyclization reaction of HPNP.248 Kinetic and mechanistic studies indicate strong binding of the HPNP substrate to the complex (K— 5.5 x 10-4M-1) and a kc ll value of 7.7 x 10 4s 1 at pH = 7.0 in acetonitrile/20 mM HEPES (1 1) at 25 °C. As 9 is 50 times more active for HPNP transesterification than 8, which contains only one zinc center, cooperativity between metal centers occurs in the former complex. Notably, the monozinc calix[4]arene complex 8 is 6 times more reactive than 7. This result provides evidence that the hydrophobic calix[4]arene moiety plays an important role in the reaction. 

Coordination of an aromatic ring tp a transition metal can reverse the usual reactivity of the ring, from nucleophilic to electrophilic. Nucleophilic reagents can add to f/ -arene ligands to produce t -cyclohexadienyl complexes ... 

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