Nucleophilic solvent molecule

For silylium ions, one cannot expect a similar independence of the medium, since Si easily extends its coordination sphere by forming hypercoordinated compounds with five or six ligands. [79] In solution, nucleophilic solvent molecules S (or counterions X ) will be suitable coordination partners (Scheme 1). 

Specific complexation of R3Si+ by nucleophilic solvent molecules... 

Cyclic "onium intermediates of the type (2) or (3) are believed to survive only until attacked by an anion or nucleophilic solvent molecule. The reaction leads specifically to the trans product. This is a consequence of the extra stability... 

The presentation of the rearranging intermediates as mesoionic ground-state species (102 and 106) has gained wide acceptance as a tool for interpretation and prediction of structural changes in this field. The intervention of such intermediates is suggested by the facile incorporation of nucleophilic solvent molecules in the course of the photochemical transformation of dienones, and by a number of acid-catalyzed non-photolytic reactions which either parallel the phototsomerizations or correlate photoproducts from reactions in protic and aprotic media. 

A similar system with additional strain on the cyclopropane ring is quadricyclane. Its reaction with benzeneselanyl chloride in apolar solvents gave quantitatively addition products with preference for the formation of stereoisomeric 3-chloro-2-phenylselanylbicyclo[2.2.1]hept-5-enes 4 and 5. In polar or protic solvents such as acetonitrile, methanol or acetic acid, the competing opening of only one of the cyclopropane rings became dominant. The benzeneselanyl cation cleaved the most strained C — C bond in cyclopropane with concomitant or subsequent reaction with a nucleophilic solvent molecule. ... 

The oxidation occurs at relatively low potentials due to the electron-donating mesityl groups. Apparently, axial shielding of the coordinatively unsaturated metal center by the bulky mesityl groups stabilizes the rare mononuclear Pt center toward dimerization or other aggregation, but more importantly also toward attack of nucleophilic solvent molecules or electrolyte ions at the very... 

When a scale of YC1 values (17) was used together with N0Ts values within equation 2, an l value of 0.30 was determined for tert-butyl chloride solvolysis. An Sn2 (intermediate) mechanism, involving both an intermediate and covalent interaction by a nucleophilic solvent molecule, was proposed... 

Comparison of the calculated nucleophilic accelerations of 9, 17, and 106 to the degree of inversion (72, 74, and 87%, respectively) for Vla-c shows that even though the kinetic effects of solvent assistance are significant, this fact does not lead to the complete inversion characteristic of the SN2 reaction as found in acetolysis of 2-octyl tosylate (45). The best description of the first intermediate in the solvolysis of Vla-c would thus appear to be the solvated ion pair shown in Scheme II. In this species, the solvent is involved in nucleophilic solvation of the central carbon as well as the remainder of the carbocation and also participates in electrophilic solvation of the anion. Numerous solvent molecules are involved, and no strong interaction of a single nucleophilic solvent molecule at the central carbon leading to exclusive inversion occurs. 

The positively charged carbon atom in a carbocation is an extremely electron-dehcieni (electrophilic) carbon. As such, its behavior is dominated by a need to obtain an electron pair from any available source. The Sn I reaction illustrates the most obvir>"s fate of a < nr .- -t on c a.bination with an external Lewis base, forming a new bond to carbon. However, the electron deficiency of cationic carbon is so great that even under typical SnI solvolysis conditions, surrounded by nucleophilic solvent molecules, some of the cations won t wait to combine with external electron-pair sources. Instead, they will seek available electron pairs within their own molecular structures. The most available of the.se are electrons in carbon-hydrogen bonds one carbon removed from the cationic center (at llic so-called carbon) ... 

In protic solvents, the allylic carbonium ion formed by acid-catalyzed alkyl carbon-oxygen bond fission can recombine either with the carboxylic acid molecule or with a solvent molecule. The electrostatic attraction between the carbonium and carboxylate ions, which is a major factor in isomerization of allylic esters by ion-pair internal return during solvolysis, is absent in the acid-catalyzed reaction. The more numerous, usually more nucleophilic, solvent molecules in the solvation shell of the carbonium ion should compete effectively with the departed carboxylic acid molecule and solvolysis rather than isomerization should be the predominant reaction. For example, in the presence of 0.05 M perchloric acid, solvolyses of cis- and //- //7.s-5-methyl-2-cyclohexenyl p-nitrobenzoates are not only very much faster than in the absence of the acid, but polarimetric and titrimetric rates of solvolysis of optically-active esters were identical within experimental error. For these esters, the acid-catalyzed solvolysis was not accompanied by a detectable amount of isomerization. Braude reported, on the basis of indirect evidence, that isomerization accompanies acid-catalyzed hydrolysis of a-ethynyl-y-methylallyl acetate in aqueous dioxane. It was shown that, under some experimental conditions, the spectrophotometrically determined rate of appearance of the rearranged 1 -yne-3-ene chromophore exceeds the titrimetrically determined rate of hydrolysis,... 

The solvent effect on the reactions of organotin compounds may also be explained by the fact that the nucleophilic solvent molecules solvate the transition complex formed in the course of the reaction [Na 72, Pe 73a, Pe 73b]. 

The cationic intermediate suspected in the reaction can be trapped by added nucleophiles (including nucleophilic solvent molecules) if its lifetime is long enough for diffusion to bring the two species together. 

If we simply continue the process of ionization, we have the pure SnI reaction. If we let a nucleophilic solvent molecule interact with the rear of the departing bond and ultimately form a bond to carbon, we have the Sn2 reaction with inversion (Fig. 7.68). 

Decarboxylation reactions may be induced (depending on the acid) in a variety of ways thermally, bacteriologically, photochemically, or even by electrolysis as in the anodic reaction of the Kolbe synthesis. Thermally induced decarboxylation of many carboxylic acids in solution proceeds by a bimolecular mechanism involving addition of a nucleophilic solvent molecule to an electrophilic carbon atom on the root molecule - preferably at a carbon adjacent to (a) or one removed from (P) the carboxyl carbon (Fraenkel et al. 1954 Clark 1958, 1969). An electrophile-nucleophile pair is formed in the transition state, which subsequently undergoes heterolytic fission (i.e., decomposition of a molecule into two ions of opposite charge) to yield CO2, a proton, and a carbanion the latter two species are reactive intermediates, which then combine rapidly (Brown 1951). The solvent molecule departs unaffected and in this sense the solvent may be considered... 

The kinetics of decarboxylation of dicarboxylic acids appear to be relatively unaffected by the nature of the containment vessel, suggesting that it proceeds via a homogeneous reaction involving interaction with a nucleophilic solvent molecule. Consistent with this assignment of a common mechanism is the existence of similar isokinetic relationships for this class of acid. 

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