Electrophilic Substitution at

A most interesting reaction was found when the tetrafluoroborate 221 in acetonitrile was treated with water or with weak aqueous bases. Instead of the expected hydroxymethyl derivative 230, only the 4.4 -bis-(l. l-dimethoxy-2.6-diphenyl- 

A -phosphorin)-methane 224 m. p. 141 -142 °C, in 86% yield, could be isolated. This reaction proceeds by an electrophilic attack of the cation 221 on the 4-hy-droxymethyl-X -phosphorin intermediate 230, CH20H being split off. Indeed, formaldehyde can be isolated 

This type of X -phosphorin reaction opens up new possiblities of preparing 4-substituted X -phosphorins by other electrophilic substitutions. They are now being studied by us. 

Analogous reactions of electron-rich aromatic compounds can be found in the literature When 4-substituted-N.N-dimethylanilines 231 are treated with dia-zonium salts or some other electrophiles, position 4 is substituted by the electrophile group to 4-substituted dimethylanilines  

It seemed possible that treatment of cation 233a with a strong proton abstracting base could led to a resonance stabilized carben 234a (R = H). Pohl by 

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SuIfona.tlon, Sulfonation is a common reaction with dialkyl sulfates, either by slow decomposition on heating with the release of SO or by attack at the sulfur end of the O—S bond (63). Reaction products are usually the dimethyl ether, methanol, sulfonic acid, and methyl sulfonates, corresponding to both routes. Reactive aromatics are commonly those with higher reactivity to electrophilic substitution at temperatures > 100° C. Tn phenylamine, diphenylmethylamine, anisole, and diphenyl ether exhibit ring sulfonation at 150—160°C, 140°C, 155—160°C, and 180—190°C, respectively, but diphenyl ketone and benzyl methyl ether do not react up to 190°C. Diphenyl amine methylates and then sulfonates. Catalysis of sulfonation of anthraquinone by dimethyl sulfate occurs with thaHium(III) oxide or mercury(II) oxide at 170°C. Alkyl interchange also gives sulfation. 

Results for the neutral pyrazole molecule show a considerable spread. The tt-electron and total (Tr-l-cr) densities predict electrophilic substitution at the 4-position as found. Results for thiazole also agree with experimentally determined electrophilic and nucleophilic reactivity. 

The most complete discussion of the electrophilic substitution in pyrazole, which experimentally always takes place at the 4-position in both the neutral pyrazole and the cation (Section 4.04.2.1.1), is to be found in (70JCS(B)1692). The results reported in Table 2 show that for (29), (30) and (31) both tt- and total (tt cr)-electron densities predict electrophilic substitution at the 4-position, with the exception of an older publication that should be considered no further (60AJC49). More elaborate models, within the CNDO approximation, have been used by Burton and Finar (70JCS(B)1692) to study the electrophilic substitution in (29) and (31). Considering the substrate plus the properties of the attacking species (H", Cl" ), they predict the correct orientation only for perpendicular attack on a planar site. For the neutral molecule (the cation is symmetrical) the second most reactive position towards H" and Cl" is the 5-position. The activation energies (kJmoF ) relative to the 4-position are H ", C-3, 28.3 C-5, 7.13 Cr, C-3, 34.4 C-5, 16.9. 

Nitrates have only been prepared from saturated equatorial 3-hydroxyl groups by reaction with concentrated nitric acid and acetic anhydride at low temperature.Electrophilic substitution at C-6 precludes the satisfactory formation of nitrates from A -3j5-ols. 

No example of electrophilic substitution at ring carbon atoms has been reported. 

Electrophilic substitution at ring nitrogen atoms has been limited to protonation and iV-alkylation of the anion derived from a pyrido-pyrimidinone.i - Thus, the sodium salt of pyrido-[2,3-d]pyrimidine-2,4-(l//,3ir)-dione and dimethylsulfate yield the 1,3-dimethy] derivative (176). 

Only one electrophilic substitution at the pyrylium ring has been described, namely the j8-deuteration of 2,4,6-trimethylpyrylium or 2,4,fi-triphenylpyrylium perchlorates on prolonged reflux in CHjCOOD possibly, however, this reaction proceeds via the pseudo base. The nitration of 2,4,6-triphenylpyrylium does not affect the pyrylium ring, but attacks the 2- and 6-phenyl groups in the meta-, and the 4-phenyl group in the am-position, as was proved by... 

Phospholes and analogs offer a wide variety of coordination modes and reactivity patterns, from the ti E) (E = P, As, Sb, Bi) through ri -dienic to ri -donor function, including numerous and different mixed coordination modes. Electrophilic substitution at the carbon atoms and nucleophilic properties of the phosphorus atom are well documented. In the ri -coordinated species, group V heteroles nearly acquire planarity and features of the ir-delocalized moieties (heterocymantrenes and -ferrocenes). 

The mechanism for the conversion of the A -oxide (94) to the o-methylaminophenylquinoxaline (96) involves an initial protonation of the A -oxide function. This enhances the electrophilic reactivity of the a-carbon atom which then effects an intramolecular electrophilic substitution at an ortho position of the anilide ring to give the spiro-lactam (98). Hydrolytic ring cleavage of (98) gives the acid (99), which undergoes ready dehydration and decarboxylation to (96), the availability of the cyclic transition state facilitating these processes. ... 

The opposite case—reaction of an arenediazonium species with an aliphatic substrate —is possible if a sufficiently acidic C—H bond is present e.g. with /3-keto esters and malonic esters. The reaction mechanism is likely to be of the Sel-type an electrophilic substitution at aliphatic carbon ... 

Draw resonance structures of the intermediate carbocations in the bromillation of naphthalene, and account for the fact that naphthalene undergoes electrophilic substitution at Cl rather than C2. 

In accord with experimental observations, SCFMO calculations indicate that an /V-acyl function should deactivate 5f/-dibenz[/>,/]azepine towards electrophilic substitution at the carbocyclic rings, and that substitution should occur preferentially at the CIO position.32 The calculated dipole moment (0.98 D) for 5f/-dibenz[A,/]azepine compares favorably with the measured value (0.96 D), and is expected for a nonplanar ring system.58... 

Volume 8 Volume 9 Volume 10 Volume 12 Volume 13 Proton Transfer Addition and Elimination Reactions of Aliphatic Compounds Ester Formation and Hydrolysis and Related Reactions Electrophilic Substitution at a Saturated Carbon Atom Reactions of Aromatic Compounds Section 5. POLYMERISATION REACTIONS (3 volumes)... 

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

Much less work has been done on electrophilic aliphatic substitution mechanisms than on nucleophilic substitutions, and the exact mechanisms of many of the reactions in this chapter are in doubt. For many of them, not enough work has been done to permit us to decide which of the mechanisms described in this chapter is operating, if indeed any is. There may be other electrophilic substitution mechanisms, and some of the reactions in this chapter may not even be electrophilic substitutions at all. 

The reactions in this chapter are arranged in order of leaving group hydrogen, metals, halogen, and carbon. Electrophilic substitutions at a nitrogen atom are treated last. 

Many known color reactions involve electrophilic substitution at an electron-rich aromatic or heteroaromatic (cf. 4-(dimethylamino)-benzaldehyde - acid reagents and vanillin reagents ). Here aliphatic or aromatic aldehydes react in acid medium to yield polymethyne cations which are intensely colored di- or triarylcarbenium ions [4, 10]. 

Anisole has charge densities as shown in (85) and (86). This indicates that ground state anisole should undergo electrophilic substitution at the ortho... 

Benzo-fused pyridopyrrolizines can be prepared by an acid-induced cyclodehydration of the appropriately substituted hydroxypyrrolopyridines. In the case of 124 (Equation 7), this is best rationalized as an intramolecular electrophilic substitution at the o-carbon of the benzyl substituent <1988CC623, 1990J(P1)1757, 2001J(P1)1446>. 

Warren, S. and Williams, M.R., Electrophilic substitution at phosphorus dealkylation and decarboxylation of phosphinylformate esters, Chem. Com-mun., 180, 1969. 

The intramolecular arylation of sp3 C-H bonds is observed in the reaction of l-/ r/-butyl-2-iodobenzene under palladium catalysis (Equation (71)) 94 94a 94b The oxidative addition of Arl to Pd(0) gives an ArPdl species, which undergoes the electrophilic substitution at the tert-butyl group to afford the palladacycle. To this palladacycle, another molecule of Arl oxidatively adds, giving the Pd(iv) complex. 

Electrophilic substitutions at the pyrrole nucleus were described only with 1,2-dihydropyrrolizines which react with the same regioselectivity as would do equivalently substituted monocyclic pyrroles. 

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