Agent, electrophilic transfer

Most biotin-dependent carboxylations (Table 18.5) use bicarbonate as the carboxylating agent and transfer the carboxyl group to a substrate earbanion. Bicarbonate is plentiful in biological fluids, but it is a poor electrophile at carbon and must be activated for attack by the substrate earbanion. 

Sulfonic acids themselves are unfit for electrophilic transfer of sulfonyl groups because of the poor nucleofugality of the hydroxide anion. However, the high acidity obviously leads to an equilibrium between the acids and their anhydrides and water, from which water can be removed either by special reaction conditions (i.e., azeotropic distillation with appropriate solvents) or chemically with anhydride forming agents (equation 63). sulfonic acid anhydride sulfonylations are compiled in Table 10. 

The first step in the reaction of HNCI2 and NCI3 is the formation of tetrachlorohydrazine from the reaction of NCI3 with the nucleophile NCL, which is formed from HNCI2. Here NCI3 is an electrophilic amination agent that transfers NCI2+. 

The usual oxidizing agents transfer oxygen (or halogens and related species with subsequent hydrolysis) stepwise to the sulfur of thioethers Rates of step A compared with those of step B are faster with electrophilic oxidation agents (peroxy acids) inversely, rates of step B compared with those of step A are faster with nucleophilic oxidation agents (peroxy anions)339-341. 

Finally, the reaction of 19b with potassium fluoride in the presence of a crown-ether phase-transfer agent to yield the sulfonyl fluoride 67 and diphenylacetylene belongs to the same category in which a nucleophile (F in this case) attacks the electrophilic sulfur of the sulfone group (equation 19). 

As useful as tetranitromethane is as a charge-transfer nitrating agent, it is generally too unreactive to effect the comparable electrophilic nitration of most aromatic donors, except the most electron-rich ones. Thus in order to make the direct comparison between the photochemical and thermal nitration of the same ArH, we now turn to the A-nitropyridinium acceptor (PyNOj) as the alternative nitrating agent (Olah et al., 1965, 1980). For example, PyNO can be readily prepared as a colourless crystalline salt, free of any adventitious nitrosonium impurity, and used under essentially neutral conditions. Most importantly, the electrophilic reactivity of this... 

In order to examine the effect of the nitrating agent, anisole can be also treated with Me02CPyN02, PyN02 and MeOPyN02. Control experiments carried out simultaneously in the dark establish the absence of any electrophilic (thermal) component to anisole nitrations under all charge-transfer conditions (Kim et al., 1993 ). The photochemical nitrations of... 

In contrast to the processes 1-4 where organohypervalent iodine compounds behave as the electrophilic species, other approaches deal with reactions where these reagents either act as oxygen transfer agents (nucleophilic or radical) or undergo radical processes. 

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