Solvent nucleophilicity and

Dediazoniation in Highly Nucleophilic Solvents and in the Presence of Good Nucleophiles... 

Dihydropyrroles have recently become readily available by ring-closing metathesis. For this purpose, N-acylated or N-sulfonylated bis(allyl)amines are treated with catalytic amounts of a ruthenium carbene complex, whereupon cyclization to the dihydropyrrole occurs (Entries 6 and 7, Table 15.3 [30,31]). Catalysis by carbene complexes is most efficient in aprotic, non-nucleophilic solvents, and can also be conducted on hydrophobic supports such as cross-linked polystyrene. Free amines or other soft nucleophiles might, however, compete with the alkene for electrophilic attack by the catalyst, and should therefore be avoided. 

Dimer formation can be quenched by conducting the experiment in a nucleophilic solvent, and the product obtained is characteristic of radical cation trapping. The anti-Markovnikov addition of acetone across the C-C single bond of the methylated analogue, eq. 41 (116,117),... 

Regiocontrol in these useful reactions is achieved by a careful selection of alkylation or acylation reagent, base, nucleophile, solvent, and reaction temperature. Reactions of oxyazolium salts are useful for regioselective introduction of substituents both at ring positions and at lateral positions. The alkylation or acylation followed by reaction with nucleophile, base, or... 

Although the substitution inertness of these tetrahedral isomers might be a consequence of the chelation, it is more probable that either situation, preferred nucleophilic attack at the planar or at the tetrahedral isomers, might prevail depending on the nature of the substrates, nucleophiles, solvents, and so on. 

This viewpoint may well be correct. But the differences in stability between the various classes of carbonium ions are great enough that, by and large, reactions fall into three separated groups (a) for primary substrates, Sn2 (b) for tertiary substrates, SnL with an intermediate that approximates our idea of a simple (solvated) carbonium ion and (c) for secondary substrates, a two-step reaction that is SnI to the extent that there is a cationoid intermediate, but one formed with nucleophilic assistance and still encumbered with nucleophile (solvent) and leaving group. 

In summary, the observation of carbocations in superacids has provided a wealth of previously inaccessible information on stmcture and reactivity. The infinite lifetime of the cations precludes, of course, any stereochemical studies. On a scale of activation energies, the rearrangements studied in superacids overlap in part with those observed in more nucleophilic solvents and extend toward higher energy barriers. 

The change in solvent polarity thus leads to an appreciable variation in product composition, which is further changed under doping conditions. In the presence of LiClO4 the products indeed arise exclusively from skeletal rearrangements and incorporation of external nucleophiles, solvent and perchlorate anion. The formation of C1O4 -incorporated products can be increased by carrying out the reaction in non-nucleophilic solvents -. ... 

Secondary halides are borderline, and substitution or elimination may be favored, depending on the particular base/nucleophile, solvent, and temperature at which the reaction is carried out. Elimination is favored with strong bases/good nucleophiles—for example, hydroxide ion and ethoxide ion. Substitution is favored with weak bases/poor nucleophiles—for example, acetate ion. Table 7.7 summarizes these generalizations about substitution versus elimination reactions of haloalkanes. 

Yoshida and coworkers [27-29] have shown that it is possible to substantially extend the approach of electrogenerating a metastable intermediate and trapping with a nucleophile or electrophile by employing low temperatures and flow technology combined with the use of non-nucleophilic solvents, and have illustrated the approach with a wealth of examples. 

Since the immediate product of the cyclization step is a carbonium ion, it is often found that the product consists of a mixture of closely related compounds, all resulting from the same carbonium ion. These usually include products from the capture of the carbonium ion by nucleophilic solvent and a mixture of the various alkenes resulting from loss of a proton. 

The rearrangement of epoxides occurs in excellent yield and selectivity in many cases.Considering that there are many efficient protocols for a selective epoxidation, the two-step sequence epoxidation/rearrangement constitutes a powerful strategy to obtain complex molecules. The rearrangement is usually carried out using Lewis acids in a non-nucleophilic solvent, and it can be realized using di-, tri-, and tetrasubstituted epoxides as substrate. 

A different reaction takes place with acetonitrile as solvent. The initially formed contact ion pair (17,12) undergoes diffusive separation to the corresponding solvent-separated ion pair (SSIP). The Si-O bond is cleaved by assistance of the nucleophilic solvent, and the resulting a-carbonyl radical 22 combines with the chloranil radical anion 12 to yield the final product 23. 

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