Sulfonates, substitution leaving groups

For electrophiles such as Me3SiCH2X strong ground-state destabilization has been observed for X = 4-nitrobenzoate[130]. For X= halide, on the other hand, this ground-state destabilization is significantly smaller, and it may therefore be advisable to choose carboxylates or sulfonates as leaving groups when alkylations with a-silyl-substituted electrophiles are to be performed. 

SuIfona.tlon, The sulfonic acid group is used extensively in the dyes industry for its water-solubilizing properties, and for its ability to act as a good leaving group in nucleophilic substitutions. It is used almost exclusively for these purposes since it has only a minor effect on the color of a dye. 

In addition to protons, other electrofugic leaving groups such as SO3 (i. e., anions of sulfonic acids), Cl, Br, I, C02, and others can also be displaced in azo coupling reactions with aromatic substrates. The mechanism of such substitutions is in principle the same as that of dehydrogenation (see Fischer and Zollinger, 1972). 

Although halides are common leaving groups in nucleophilic substitution for synthetic purposes, it is often more convenient to use alcohols. Since OH does not leave from ordinary alcohols, it must be converted to a group that does leave. One way is protonation, mentioned above. Another is conversion to a reactive ester, most commonly a sulfonic ester. The sulfonic ester groups tosylate, brosylate, nosylate, and mesylate are better leaving groups than... 

Recently, the scope of the allylic substitution has been extended to sulfinate salts 84 to obtain allylic sulfones 85. Due to solubility problems of both nucleophile 84 and carbonate leaving group, a polar solvent mixture of DMF and 2-methoxyethanol had to be employed, which limits the reaction to the use of a phosphine ligand. Thus, various aryl sulfinates 84 and functionalized carbonates 81 could be converted to the corresponding allylic sulfones 85 with good to excellent yields and regioselectivites and complete retention of stereochemistry (eq. 2 in Scheme 20) [65]. 

Polar C=Y double bonds (Y = NR, O, S) with electrophilic carbon have been added to suifinic acids under formation of sulfones. As in the preceding section one must distinguish between carbonyl groups and their derivatives on the one hand, and carboxylic acids (possessing leaving groups at the electrophilic carbon) on the other. Aldehydes " of sufficient reactivity—especially mono-substituted glyoxals - —and their aryl or arylsulfonyl imines have been added to suifinic acids (in a reversible equilibrium) to yield a-hydroxy or a-amino sulfones the latter could also be obtained from the former in the presence of primary amines (equation 26). 

Nucleophilic substitution of leaving groups is probably the most important area in pyrimidine reactivity and, in particular, the differential reactivity of C-2 and C-4 is the most investigated topic. The displacement of 2- and 4-sulfide and sulfone groups is referred to in the synthesis section. The selective hydrolysis of 4-amino-2-chloropyrimidines under acidic conditions has been studied in great detail by a process research group <06OPRD921>. 

Earlier (Table 6, p. 119) we saw data on the reactivity of various nucleophiles toward an aryl sulfinyl sulfone in (139), a substitution that also involves an arenesulfinate as the leaving group, but one in which the substitution takes place at a sulfinyl ( S=0) rather than a sulfenyl ( S) sulfur. In Section 9 we present data on the rates of reaction of the same nucleophiles in an analogous substitution at a sulfonyl sulfur, Nu- + PhS02S02Ph - PhS02Nu + PhS02. At that point we will discuss how changing the oxidation state of the sulfur atom at which the substitution occurs... 

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