Solvent assisted dissociation

Figure A. Solvent assisted dissociation, or SAD mechanism X-solvent molecule or other ligand in second coordination shell.

This is the mechanism which has been called the solvent assisted dissociative mechanism, or the SAD mechanism. I think the name interchange mechanism might be a good one, perhaps better than the SAD mechanism. 

In 1958, as I remember it, Fred Basolo and I, Arthur Adamson and Henry Taube, and H. R. Hunt came out with this mechanism basically, stressing slightly different features. Then Dr. Tobe joined the bandwagon with this mechanism in 1959, and the term solvent assisted dissociation was coined by Wallace and his group in Canada in 1961. That is the story as I see it at the present time... 

The so called solvent assisted dissociation mechanism in which reagent Y (which may or may not be solvent) slips in when the bond between the metal and... 

Usually the kx term corresponds to solvent-assisted dissociation, the term to direct nucleophilic attack by L, and both paths make comparable contributions to the overall mechanism of substitution. There are, however, exceptions to this general picture. Thus, if the solvent is non-co-... 

The novel feature in this investigation is the use of non-polar solvents, in several of which the value of is the same. The first term in the rate law is said to correspond not to solvent-assisted dissociation, as in the usual polar solvents, but rather to simple dissociation. While the determination of rates and the deduction of rate laws for substitution at palladium(ii) is fairly common, determination of activation parameters is rare. These are, however, reported for aquation of rra 5-[PdCl2(NH3)2] and of trans-[PdCl(N02)(NH3)2]. ... 

A non-aqueous analogue of the above examples of base hydrolysis in aqueous solution is solvolysis in acetic acid-acetate solutions. This reaction has been studied for tra/i -CCoCOgC-Rja engl+j where R = o-tolyl or p-tolyl, whose rates of solvolysis are found to decrease with increasing concentration of added acetate. This behaviour is in direct contrast to that of the aqueous system. In acetic acid the V lcb mechanism appears not to operate, and the mechanism must be solvent-assisted dissociation, with ion-pairs less reactive than the free cation. ... 

The ki path corresponds to solvent-assisted dissociation. Formation of this pen-tacyanoimidazolatoferrate(III) complex follows a simple second-order rate law ... 

As for solvents, liquid ammonia or dimethylsulfoxide are most often used. There are some cases when tert-butanol is used as a solvent. In principle, ion-radical reactions need aprotic solvents of expressed polarity. This facilitates the formation of such polar forms as ion-radicals are. Meanwhile, the polarity of the solvent assists ion-pair dissociation. This enhances reactivity of organic ions and sometimes enhances it to an unnecessary degree. Certainly, a decrease in the permissible limit of the solvent s polarity widens the possibilities for ion-radical synthesis. Interphase catalysis is a useful method to circumvent the solvent restriction. Thus, 18-crown-6-ether assists anion-radical formation in the reaction between benzoquinone and potassium triethylgermyl in benzene (Bravo-Zhivotovskii et al. 1980). In the presence of tri(dodecyl)methylammonium chloride, fluorenylpi-nacoline forms the anion-radical on the action of calcium hydroxide octahydrate in benzene. The cation of the onium salts stabilizes the anion-radical (Cazianis and Screttas 1983). Surprisingly, the fluorenylpinacoline anion-radicals are stable even in the presence of water. 

Generally, in bromine addition to carbon-carbon double bonds, bromine bridging, solvent dependent dissociation of the ionic intermediates, steric interactions between the counteranion and the first bonded halogen during the nucleophilic step, the possibility of carbon-carbon rotation in the carbenium ion intermediate, preassociation phenomena and nucleophilic assistance determine the stereochemical behavior of the reaction . Several of these factors have been invoked to explain the stereochemistry of bromine addition to dienes, although others have been completely ignored or neglected. Bromine addition to cyclopentadiene, 1,3-cyclohexadiene, 2,4-hexadienes and 1,3-pentadienes has been examined repeatedly by Heasley and coworkers and the product distribution has been... 

Similar to the bipyrimidine palladium complex, Bercaw and coworkers demonstrated that hydroxy-bridged dimers [ (diimine)Pd(OH) 2]2+ 11 could also effect activation of the allylic C-H bond [25]. Mechansitic studies revealed that the solvent (trifluoroethanol or MeOH) assisted dissociation of dimer 11 to monoca-tionic monomer 12, followed by the rate-limiting -coordination of indene to palladium center, and finally by fast C-H activation (Scheme 9). 

H-shift (Reaction (3.15)) was considered feasible based on (i) the analogy to the well-known, solvent-assisted 1,2-H-shift within alkoxy radicals [19] and (ii) the exo-thermicity based on the homolytic bond dissociation energies (BDEs) of the N-H (406 kj mol ) and the C-H bond (363 kj mol ) (representative values for the Gly anion [47]). However, both pulse radiolysis and y-radiolysis experiments concluded that the 1,2-H-shift in aminyl and amidyl radicals derived from amino acids and peptides must be rather slow (kis 1.2 x 10 s ) [37, 40]. 

The proposed catalytic cycle is shown in Figure 1.34. ° The catalytic cycle starts with intermediate a, obtained from [Pd(OCOCF3)2 (P)-BINAP)] via dissociation of one OCOCF3. The reaction of intermediate a with molecular H2 (possibly solvent-assisted) affords monohydride intermediate b. Substrate (2-methylindole in Figure 1.34) enters the cycle in the proton-ated form (from organic acid presented at stoichiometric amount) of iminium cation a, which forms hydrogen-bonded adduct c. After rate-and enantio-determining hydride transfer, catalyst-product complex d is formed. The cycle restarts after product dissociation into solution. 

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