Electrophilic additions sulfones

In the first step an S03 molecule is inserted into the ester binding and a mixed anhydride of the sulfuric acid (I) is formed. The anhydride is in a very fast equilibrium with its cyclic enol form (II), whose double bonding is attacked by a second molecule of sulfur trioxide in a fast electrophilic addition (III and IV). In the second slower step, the a-sulfonated anhydride is rearranged into the ester sulfonate and releases one molecule of S03, which in turn sulfonates a new molecule of the fatty acid ester. The real sulfonation agent of the acid ester is not the sulfur trioxide but the initially formed sulfonated anhydride. In their detailed analysis of the different steps and intermediates of the sulfonation reaction, Schmid et al. showed that the mechanism presented by Smith and Stirton [31] is the correct one. 

Vinyl trifluoromethanesulfonates (triflates) are a new class of compounds, unknown before 1969, that have been used most extensively in solvolytic studies to generate vinyl cations.2,3,812 Three methods have been used to prepare these sulfonic esters. The first, involving the preparation and decomposition of acyltriazines,4 requires several steps to prepare the acyltriazines and is limited to the preparation of fully substituted vinyl triflates. The second method involves the electrophilic addition of trifluoromethanesulfonic acid to acetylenes5,8,15 and, consequently, is not applicable to the preparation of trisubstituted vinyl triflates and certain cyclic vinyl triflates. However, this second procedure is relatively simple and often gives purer products in higher yield than the subsequently discussed reaction with ketones. Table I lists vinyl triflates that have been prepared by this procedure. ... 

Electrophilic substitution of thianthrene takes place at C-2. No examples of even minor amounts of 1-mono-substituted product have been reported. Disubstitution gives 2,7- (usually) or 2,8-products. In a few cases, 2,6-derivatives have been claimed. The presence of a sulfoxide or sulfone unit greatly reduces the susceptibility of either ring to electrophilic substitution. Carbon-centered electrophilic addition to sulfur to produce 5-R-thianthrenium salts has been described rarely most examples of the formation of such salts have involved the thianthrene radical ion(l-t-). Treatment of thianthrene with alkyl/aryllithiums produces the 1-lithio-species, and these organometallic derivatives allow the introduction of substituents at this position. 

In contrast to the ease of N-functionalization, shown in Scheme 1, the triazolopyridine nucleus is resistant to direct nuclear oxidation or electrophilic additions. Electrophilic additions will occur on aryl substituents for example, nitration of l-phenyltriazolo[4,5-c]pyridine (26) and sulfonation of 2-phenyl-2i/-triazolo[4,5-6]pyridine (28) occur exclusively in the para position of the phenyl ring <34LA(514)279, 38MI 710-01). Nuclear functionalization was observed when l-( -butyl)-5-methyl-tri-azolo[4,5-c]pyridinium iodide (30) was treated with potassium ferricyanide to afford triazolopyridin-4-one (31), as shown in Scheme 2. Similarly, the iodide (30) is converted by either phosphorus oxychloride-phosphorus pentachloride, or bromine or nitric acid to 7-substituted triazolopyridin-4-ones (32) <37LA(529)288>. 

Electrophilic additions to vinyl sulfoxides and sulfones allow for further functionalization of these compounds at the a-position. Deprotonation of the a-hydrogen, under appropriate conditions, gives rise to a-vinyl anions which can be trapped by electrophiles. This protocol has been employed in the synthesis of a diverse range of compounds, such as 34 and 37, 40 and 43, and 46 (see Schemes 9, 10, and 11, respectively). Both enantiomers of optically pure propargylic alcohols (115) were conveniently prepared by the reaction of the a-vinyl anion of 2-(trimethylsilyl) vinyl... 

The efficient activation of oxime sulfonates by organoaluminum reagents enables the intramolecular cyclization of alkenic oxime mesylates, which involves the electrophilic addition of the intermediate ni-trilium ion to the double bond. This results in the direct formation of a wide variety of structurally diverse carbocyclic and heterocyclic systems. Four distinct cyclization modes, i.e. endo(B)-endo, endo(B)-exo, exo(B)-endo and exo(B)-exo are possible, as shown in Scheme 4P The values in parentheses refer to the yields obtained using SnCU. 

Bixchler Napiralski, Dieckmann cyclization [15], Suzuki reaction [48], Wittig reaction, ozonolysis, condensation, esterification, nucleophilic substitution [49], Henry reaction, 1.3-dipolar cyclo-addition, electrophilic addition [50], oxidation chloride -> aldehyde [50], sulfide —> sulfone [51], alcohol —> ketone, Arbuzov reaction (phosphine-phosphorox-ide) [52], reduction hydration [45], ester -> alcohol [49, 53]... 

Imidazole, and 1-alkyl-imidazoles, are brominated with remarkable ease at all free nuclear positions. It is, at first sight, somewhat surprising that such relatively mild conditions allow bromination of imidazole at C-2, but it must be remembered that the neutral imidazole, not its protonic salt (cf. nitration and sulfonation), is available for attack, thus electrophilic addition of bromine to imine nitrogen, then addition of bromide at C-2 and dually elimination of hydrogen bromide may be the key to the 2-bromination. 

Benzene s aromaticity causes it to undergo electrophilic aromatic substitution reactions. The electrophilic addition reactions characteristic of alkenes and dienes would lead to much less stable nonaromatic addition products. The most common electrophilic aromatic substitution reactions are halogenation, nitration, sulfonation, and Friedel-Crafts acylation and alkylation. Once the electrophile is generated, all electrophilic aromatic substitution reactions take place by the same two-step mechanism (1) The aromatic compound reacts with an electrophile, forming a carbocation intermediate and (2) a base pulls off a proton from the carbon that... 

Sulfonation is generally a detoxification pathway whereby the conjugated product has greater water solubility and is therefore excreted more readily from the body. However, several chemicals have been shown to form mutagenic and carcinogenic reactive electrophiles upon sulfonation (Glatt, 2000). Additionally, it is the sulfonated forms of minoxidil and cholecystokinin that elicit biologic activity (Weinshilboum et al., 1997). 

An alternative strategy is to establish more robust covalent bonds between the G sheet and the sulfonic group. This can be achieved by attaching benzene sulfonate groups to the G sheet with the creation of robust, non-hydrolizable C-C bonds by electrophilic addition of suitable precursors containing benzene sulfonate moieties into C=C double bonds on the G sheet. In this way acid catalysts that can have activity for biomass transformation and particularly for cellulose depolymerization can be obtained [36]. 

The oxidative addition of organic electrophiles (halides, sulfonates, and related activated compounds) to palladium(O) is the first step in the cross-coupling and Heck reactions. Much work has been done on the mechanisms of the oxidative addition reactions of aryl and alkenyl halides and triflates (C(sp )-X electrophiles) [65], the most common organic electrophiles in cross-coupling reactions. 

Nucleophilic Addition. Sulfonation of the nitrogen atom of the C=N bond of imines enhances the electrophilicity of the azomethine carbon. Addition of sodium diethylphosphonate (15) to )V-(diphenylmethylene)-4-methylbenzenesulfonamide (2)... 

As in most electrophilic reactions, the abiUty to stabilize the positive charge generated by the initial addition strongly affects the relative rates. MX reacts faster than OX and PX because both methyl groups work in conjunction to stabilize the charge on the next-but-one carbon. Sulfonation was, at one time, used to separate MX from the other Cg aromatic isomers. MX reacts most rapidly to form the sulfonic acid which remains in the aqueous phase. The sulfonation reaction is reversible, and MX can be regenerated. 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

However, when the bulky substituent is no longer present at the electrophilic carbon atom, the addition of the olefin to the morpholine enamine of cyclohexanone leads largely to the tetrasubstituted isomer. For instance the reaction of this enamine with phenyl vinyl sulfone gave a 1 3 mixture of... 

Many variations of the reaction can be carried out, including halogenation, nitration, and sulfonation. Friedel-Crafts alkylation and acylation reactions, which involve reaction of an aromatic ling with carbocation electrophiles, are particularly useful. They are limited, however, by the fact that the aromatic ring must be at least as reactive as a halobenzene. In addition, polyalkylation and carbocation rearrangements often occur in Friedel-Crafts alkylation. 

---