Assistance, electrophilic

It should be recognized that superelectrophilic reactions can also proceed with only electrophilic assistance (solvation, association) by the superacids without forming distinct depositive intermediates. Pro-tosolvolytic activation of electrophiles should always be considered in this context. 

It is claimed that a measures electrophilic assistance by the solvent, b measures nucleophilic assistance, and that at least three, and sometimes four, parameters are required to perform a dissection into these separate effects. These workers also decomposed Y into ir and a contributions, and N into tt and p contributions. Abraham et al. have compared the performance of Eqs. (8-79) and (8-80) and find that they are about equally successful in correlating data. 

The stereochemistry of the carboxylation of 4-substituted ( + )-(/ S)-fra ,v-1-(4-mcthylphcnyl-sulfinylmethyl)cyclohexane after metalation with methyllithium and quenching with carbon dioxide was reported64. The results listed in Table 1 show that the d.r. of around 75 25 under kinetic control changes to 25 75 under thermodynamic control. This is the result of the equilibration of the two diastereomeric metalated species. As shown by the experiment in hexamethylphosphoric Iriamide (IIMI A) (d.r. = 57 43 under kinetic control) an electrophilic assistance of the lithium cation to the electrophilic approach is probably involved. 

The presence of a large excess of lithium salt decreases the stereoselectivity when the reaction is performed under kinetic control. These results and those reported for deuteration (see Section D. 1.1.1.5.) show that this effect is only observed when electrophilic assistance of the lithium cation is involved. 

These observations lend some support to the suggestion of Capon and Rees32. It is, however, questionable that an amine or hydroxide ion would supply significant electrophilic assistance for separation of the leaving group, and acetate ion would be effective only in its enolic form, J... 

Solladie and coworkers545 confirmed the earlier result of Nishihata and Nishio546 that the carbonation of the a-sulphinyl carbanion proceeds under kinetic control with retention of configuration at the metallated carbon atom. However, they also found that the stereochemical outcome of this reaction depends on other factors. They observed that 90% of asymmetric induction may be achieved under kinetic control (reaction time < 0.5 min) by using a base with low content of lithium salts, a result consistent with an electrophilic assistance by the lithium cation (equation 286)545. 

Metal cations can lend electrophilic assistance to weaken the Pd—X bonds in the intermediate R-Pd —X. Either full fission of this bond, leading to the realization of a polar mechanism, or partial polarization, might take place. Soft Lewis acids (the cations of Cu, Ag, Tl) are used most often (see Chapter 9.8 for a discussion of how metal ions act as Lewis-acid catalysts). 

The Zn11 cation may lend soft electrophilic assistance for transmetalation. The most efficient such assistance is in the intramolecular chelating mode, which is exemplified by cross-coupling of... 

Values of mBr in alkene bromination nucleophilic and electrophilic assistance by protic solvents 272... 

Kinetic solvent isotope effect as a measure of electrophilic assistance to bromide ion departure limiting values rate data in ethanol, methanol and their aqueous mixtures using Bentley s TBr scale its decrease corresponds to the involvement of nucleophilic assistance. R = (/caqhtOII//cAcoH)r as a measure of nucleophilic solvent assistance. Model for a limiting bromination mechanism. Ruasse et al. (1991). /Ruasse and Zhang (1984). 9Argile and Ruasse (to be published). Modro et al. (1979). 

In addition to electrostatic medium effects and to electrophilic assistance described by Yx, nucleophilic solvents can assist positive charge development in heterolysis of secondary, and sometimes tertiary, substrates via an SN2 (intermediate) mechanism (Bentley et al., 1981). Equation (55), involving... 

VALUES OF mBr IN ALKENE BROMINATION NUCLEOPHILIC AND ELECTROPHILIC ASSISTANCE BY PROTIC SOLVENTS... 

The most significant difference between brominations in protic and non-protic solvents concerns the kinetic law. Whereas in protic media the reaction is first-order in bromine, in halogenated media it is second-order (Bellucci et ai, 1980). CTC ionization is electrophilically assisted by hydrogen bonding by a protic solvent to the leaving bromide and leads to a bromonium-bromide ion pair. In non-protic media, assistance to the bromination step is provided by a second bromine molecule, leading to a bromonium-tribromide ion pair. In other words, in protic media bromination is solvent-assisted (56) while in halogenated media it is bromine-catalysed (57). 

Potassium nitroacetate 53a reacts with Me3SiCl in aprotic solvents to give SENA (51a) in moderate yield. At the same time, the introduction of yet another electron-withdrawing group (N02 or CC>2Me) stabilizes the anion of salt (53) to an extent that it does not react with Me SiCl by the Sm mechanism without electrophilic assistance. Hence, K or Na salts 53b, C are inert with respect to halosilanes, and silver or mercury salts are required for the preparation of the corresponding nitronates. The latter salts are much safer to use as diox-anate complexes. These complexes react with halosilanes in inert aprotic solvents... 

The first step is usually the formation of a halogen-olefin charge transfer complex30,70, which rapidly evolves to an ionic intermediate. Protic solvents can electrophilically assist... 

Calculations were then refined by introducing electrophilic assistance by or Li", solvation of H by water, and optimization of the angle of nucleophilic attack. These conections are introduced either separately or simultaneously. It is found that the stereochemical influence of anion solvation is negligible, compared with that of the other two factors. In all cases, the shapes of the curves are only slightly modified, as can be seen by comparing Fig. 5 and 6 with Fig. 3 and 4. 

Electrophilic assistance (i.e. complexation of the carbonyl group by Li ) favours the wrong stereochemistry, when perpendicular attacks of the nucleophile are assumed compare entries 1 and 3, 6 and 8. 

Electrophilic assistance reduces the values of the optimal angles of attack (entries 2 and 5, 7 and 9). 

The electrophile-induced cyclization of heteroatom nucleophiles onto an adjacent alkene function is a common strategy in heterocycle synthesis (319,320) and has been extended to electrophile-assisted nitrone generation (Scheme 1.62). The formation of a cyclic cationic species 296 from the reaction of an electrophile (E ), such as a halogen, with an alkene is well known and can be used to N-alkylate an oxime and so generate a nitrone (297). Thus, electrophile-promoted oxime-alkene reactions can occur at room temperature rather than under thermolysis as is common with 1,3-APT reactions. The induction of the addition of oximes to alkenes has been performed in an intramolecular sense with A-bromosuccinimide (NBS) (321-323), A-iodosuccinimide (NIS) (321), h (321,322), and ICl (321) for subsequent cycloaddition reactions of the cyclic nitrones with alkenes and alkynes. 

Scheme 10 Electrophile assisted O2 insertion into tantalum alkyl...

Very often the configuration of the organometallic reagent used is unknown. But even if it is known, predictions are difficult to make, since the bonding character or the ion-pair structure of organometallic compounds, and therefore the selectivity, is not only influenced by the character of the metal but also by important external factors such as solvent, complexation and temperature24. In addition, the nature of the electrophile itself influences the mode of reaction. Complexation of the electrophile at the metal atom (electrophilic assistance) in general is believed to favor metalloretentive attack. 

A concerted addition of this kind, involving simultaneous intramolecular electrophilic assistance by the magnesium bound to oxygen, and intermolecular nucleophilic attack by... 

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