Solvents coordinated

TT-Aliylpalladium chloride reacts with a soft carbon nucleophile such as mal-onate and acetoacetate in DMSO as a coordinating solvent, and facile carbon-carbon bond formation takes place[l2,265], This reaction constitutes the basis of both stoichiometric and catalytic 7r-allylpalladium chemistry. Depending on the way in which 7r-allylpalladium complexes are prepared, the reaction becomes stoichiometric or catalytic. Preparation of the 7r-allylpalladium complexes 298 by the oxidative addition of Pd(0) to various allylic compounds (esters, carbonates etc.), and their reactions with nucleophiles, are catalytic, because Pd(0) is regenerated after the reaction with the nucleophile, and reacts again with allylic compounds. These catalytic reactions are treated in Chapter 4, Section 2. On the other hand, the preparation of the 7r-allyl complexes 299 from alkenes requires Pd(II) salts. The subsequent reaction with the nucleophile forms Pd(0). The whole process consumes Pd(ll), and ends as a stoichiometric process, because the in situ reoxidation of Pd(0) is hardly attainable. These stoichiometric reactions are treated in this section. 

The water of hydration of these complexes can be replaced with other coordinating solvents. For example, the ethanol and methanol solvates were made by dissolving the hydrates in triethyl and trimethyl orthoformate, respectively (81,82). The acetic acid solvates are made by treating the hydrates with acetic anhydride (83). Conductivity and visible spectra, where appHcable, of the Co, Ni, Zn, and Cu fluoroborates in A/A/-dimethylacetamide (L) showed that all metal ions were present as the MLg cations (84). Solvated fluoroborate complexes of, Fe +, Co +, , Cu +, and in diethyl... 

T Kakitam, N Mataga. Comprehensive study on the role of coordinated solvent mode played m electron-transfer reactions m polar solutions. J Phys Chem 91 6277-6285, 1987. 

By contrast, in the more weakly coordinating solvent Et20, Me bridges and /X3-OR bridges can... 

Similar observations hold for solubility. Predominandy ionic halides tend to dissolve in polar, coordinating solvents of high dielectric constant, the precise solubility being dictated by the balance between lattice energies and solvation energies of the ions, on the one hand, and on entropy changes involved in dissolution of the crystal lattice, solvation of the ions and modification of the solvent structure, on the other [AG(cryst->-saturated soln) = 0 = A/7 -TA5]. For a given cation (e.g. K, Ca +) solubility in water typically follows the sequence... 

Planar-octahedral equilibria. Dissolution of planar Ni compounds in coordinating solvents such as water or pyridine frequently leads to the formation of octahedral complexes by the coordination of 2 solvent molecules. This can, on occasions, lead to solutions in which the Ni has an intermediate value of jie indicating the presence of comparable amounts of planar and octahedral molecules varying with temperature and concentration more commonly the conversion is complete and octahedral solvates can be crystallized out. Well-known examples of this behaviour are provided by the complexes [Ni(L-L)2X2] (L-L = substituted ethylenediamine, X = variety of anions) generally known by the name of their discoverer I. Lifschitz. Some of these Lifschitz salts are yellow, diamagnetic and planar, [Ni(L-L)2]X2, others are blue, paramagnetic, and octahedral, [Ni(L-L)2X2] or... 

THF. Non-coordinating solvents sucli as toluene or mixed toluene/cliloroalkane solvent systems afford tlie higliest tt values. 

The processes of complex-ion formation referred to above can be described by the general term complexation. A complexation reaction with a metal ion involves the replacement of one or more of the coordinated solvent molecules by other nucleophilic groups. The groups bound to the central ion are called ligands and in aqueous solution the reaction can be represented by the equation ... 

Those in which solvent molecules are directly involved in formation of the ion association complex. Most of the solvents (ethers, esters, ketones and alcohols) which participate in this way contain donor oxygen atoms and the coordinating ability of the solvent is of vital significance. The coordinated solvent molecules facilitate the solvent extraction of salts such as chlorides and nitrates by contributing both to the size of the cation and the resemblance of the complex to the solvent. 

Sect. 2.1.1) and [3C+2S] cyclopentene derivatives. The product distribution can be controlled by choosing the appropriate reaction conditions [72]. Moreover, the cyclopentene derivatives are the exclusive products from the coupling of fi-pyrrolyl-substituted carbene complexes [72b,c] (Scheme 25). The crucial intermediate chromacyclobutane is formed in an initial step by a [2+2] cycloaddition. This chromacyclobutane rearranges to give the rf-complex when non-coordinating solvents are used. Finally, a reductive elimination leads to the formal [3C+2S] cyclopentene derivatives. 

An interesting strategy for the diastereoselective synthesis of five-membered carbocycles was achieved by the reaction of alkenylcarbene complexes and lithium enolates derived from simple methyl ketones [79]. The use of more or less coordinating solvents (THF or Et20) or the presence of cosolvents such as PMDTA allows the selective synthesis of one or the other diastereoisomer of the final cyclopentene derivative (Scheme 32). 

For the indene derivatives M two different reaction pathways have been discussed so far, starting from the ( )-metallatriene D. A strongly coordinating solvent may induce an electrocyclic ring closure yielding the metallacyclohexa-diene K, and the indene product is obtained after tautomerisation and reductive... 

The distribution of products obtained from the benzannulation reaction may be influenced by the concentration of alkyne substrate [18]. In strongly coordinating solvents the ratio of the phenolic benzannulation product over five-membered cyclisation products increases with the concentration of the alkyne (Scheme 10). 

We should note at this point, that the above reaction implicitly refers to aqueous solutions, and that, for convenience, we have explicitly excluded free and coordinated solvent molecules. Strictly, the above relationships should be written as in Eq. (8.2). 

The equilibrium between propargyl- and allenyl-tin compounds is not spontaneous, but it occurs in the presence of Lewis acids or coordinating solvents, and an ion-pair mechanism has been proposed (159). Substitution by iodine, or addition to chloral, occurs with propargyl/al-lenyl rearrangement (160, 161), analogous to the allylic rearrangement already mentioned. 

In addition to influencing the reactivity, the coordinating ability of the solvent affects the mechanism of CO insertion. Thus, when a poorly coordinating solvent is employed together with a good nucleophile L, the... 

Fig. 6. Hydrosilylation mechanism for chloroplatinic acid. L = unspecified ligand, presumably chlorine, hydrogen, or coordinated solvent.

Kinetic studies of the hydride cluster [W3S4H3(dmpe)3] with acids in a non-coordinating solvent, i.e., dichloromethane, under the pseudo-first-order condition of acid excess, show a completely different mechanism with three kineti-cally distinguishable steps associated to the successive formal substitution of the coordinated hydrides by the anion of the acid, i.e., Ch in HCl [37]. The first two kinetic steps show a second-order dependence with the acid concentration. 

---