Nucleophilic attack on coordinated

Numerous examples of nucleophilic attack on coordinated nitrile ligands are found in the literature, particularly when the transition metal is platinum(II).224 The nucleophilic attack of two equivalents of CIO I2CII20 on the electrophilic nitrile carbon atoms of both nitrile ligands in cis-or /r<7H.v-[PtC12(RCN)2] (R = Et, Prn, Pr1, Bu p-CF3C6H4, p- and o-MeC6H4) affords the corresponding A2-l,3-oxazoline complexes ((76) and (77), respectively), in which the heterocycle acts as a... 

J. E. Backvall, Nucleophilic Attack on Coordinated Alkenes , in Reaction of Coordinated Ligands (Ed. P. S. Braterman), Plenum Press, London 1986, pp. 679-731. 

John, G.R. and Mane-Maguire, A.P, Kinetics of nucleophilic attack on coordinated organic moieties. Part 7. Mechanism of addition of tertiary phosphines to tricarbonyl(dienyl)iron cations, /. Chem. Soc., Dalton, 873, 1979. 

Since thiolates are far more reactive than thiols (pK 8-9.5) in nucleophilic attack on coordinated NO, these rates are pH dependent [53]. The reaction of alkaline solutions of mercaptans with nitroprusside to yield reddish-purple solutions has long been used as a test for cysteine, glutathione or other thiol-containing compounds. However, if the solution is too basic, the nitroprusside/ hydroxide reaction becomes competitive [3]. 

Reactions illustrating nucleophilic attack on coordinated olefins and allyls are shown in Eqs 15.124 and 15.125. 

Commercially, the water-gas shift reaction is usually carried out over Fe304.173 However, current interest centers on homogeneous catalysts. Metal carbonyl complexes such as (HFe(C04)-. (RhtCO KJ-. and (Ru(bpy)2(CO)Cl] are effective and although all the mechanisms have not been worked out completely, the reactions may be viewed in general terms as beginning with a nucleophilic attack on coordinated carbon monoxide ... 

The synthetic utility of reactions of coordinated ligands is an important and varied subject. It is based on the enhancement in reactivity of organic ligands as a consequence of metal coordination. For example, the metal can act as a super add and cause enhanced nucleophilic attack on coordinated carbonyl and imine ligands. The metal ion can also enable the ligand itself to act as a nucleophile, sometimes by direct activation, sometimes by protecting other parts of the ligand and sometimes by a combination of both. 

A number of other nucleophiles are capable of nucleophilic attack on coordinated olefins. For example, when ethylene is oxidized by PdCl2 in alcoholic solvents, the corresponding vinyl ethers,... 

At a molecular level, the intimate mechanism for the water-gas shift reaction with 4.1 as the precatalyst is not known in any detail. In the conversion of 4.1 to 4.12, the evolution of hydrogen is probably indicative of the intermediacy of a hydrido complex. Similarly, in the conversion of 4.13 to 4.1, where HI and C02 are eliminated, nucleophilic attack on coordinated CO may be involved. Although these mechanistic conjectures are plausible, there is no direct evidence to support them. 

In situ IR spectroscopy shows the presence of 4.14 and 4.16 in the catalytic mixture. The presence of 4.15 is inferred from the known chemistry of metal carbonyls. Both nucleophilic attacks on coordinated CO by HO and decarboxylation of the resultant complex as shown in 4.8 are well-precedented reactions. 

The catalytic cycle proposed for ethylene to acetaldehyde is shown in Fig. 8.2. The tetrachloro palladium anion 8.1 is used as the precatalyst. Conversion of 8.1 to 8.3 involves substitution of two chloride ligands by ethylene and water. Nucleophilic attack on coordinated ethylene leads to the formation of 8.4. The latter then undergoes substitution of another Cl- ligand. Conversion of 8.5 to 8.6 involves /3-hydride abstraction and coordination by vinyl alcohol. Intramolecular hydride attack to the coordinated vinyl group leads to the formation of 8.7. The latter eliminates acetaldehyde, proton, and CF and in the process is reduced to a palladium complex of zero oxidation state. 

Ans. Ethylene to acetaldehyde, cyclohexane to adipic acid, p-xylene to tere-phthalic acid, and propylene to propylene oxide. For the first, redox chemistry of palladium and nucleophilic attack on coordinated ligand. For the second and third, radical chain, and for the fourth, oxygen atom transfer. 

Reactions involving nucleophilic attack on coordinated ligands are promoted by very polar solvents that stabilize anions. Metal carbonyls are also attacked by strong nucleophiles (RLi) to give the Fischer carbene complexes qv below. Section A 1.4),... 

The reaction occurs well below the temperature at which most of the parent metal carbonyls exchange with free CO and so is a direct nucleophilic attack on coordinated CO, although it may alternatively proceed via a prior electron path. The resulting acyl anions can be isolated as their [R4N] " or [ (C6H5)3P 2N] salts but are reactive and are used directly in subsequent alkylations with organic halides, acetylenes, a-/i-unsaturated carbonyls and alkyloxonium salts to form organic condensation products or metal-carbene complexes. 

Nucleophilic attack on coordinated CO followed by / -hydrogen elimination occurs with neutral clusters to produce anionic clusters ... 

Nucleophilic attack on coordinated ligands is a widely encountered type of reaction. For example, carbonyl complexes are readily attacked by various nucleophiles, including OH , OR , NR3, NR, and CH. A well-known example is the base reaction of carbonyl complexes (Eq. 2-74). 

Many additional routes to metal-alkyl complexes other than transmetallation and alkylation are discussed in later chapters of this text. For example, metal-alkyl complexes are generated by insertion of an olefin into a metal-hydride or or metal-hydrocarbyl species. Such insertion reactions are discussed in Chapter 9, but an example of the synthesis of a zirconium alkyl by olefin insertion into a zirconium hydride is shown in Equation 3.9. Metal-alkyl complexes are also generated by nucleophilic attack on coordinated olefins (Equation 3.10) or carbene ligands (Equation 3.11). These reactions are presented in detail in Chapter 11. 

Vinyl complexes are typically prepared by the same methods used to prepare aryl complexes. Vinyl mercury compounds, like aryl mercury compoimds, are easily prepared (by the mercuration of acetylenes), and are therefore useful for the preparation of vinyl transition metal complexes by transmetallation. The use of vinyl lithium reagents has permitted the s rnthesis of homoleptic vinyl complexes by transmetallation (Equation 3.35). Reactive low-valent transition metal complexes also form vinyl complexes by the oxidative addition of vinyl halides with retention of stereochemistry about the double bond (Equation 3.36). Vinyl complexes have also been formed by the insertion of alkynes into transition metal hydride bonds (Equation 3.37), by sequential electrophilic and nucleophilic addition to alkynyl ligands (Equation 3.38), and by the addition of nucleophiles to alkyne complexes (Equation 3.39). The insertion of alkynes into transition metal alkyl complexes is presented in Chapter 9 and, when rearrangements are slower than insertion, occurs by s)m addition. In contrast, nucleophilic attack on coordinated alkynes, presented in Chapter 11, generates products from anti addition. 

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