Carbanions ortho

Bunnett (1991) expresses doubts that the aryldiazene can be formed at all under these strongly basic conditions, after Huang and Kosower (1968) showed that phenyldiazene is destroyed in water at pH 13.8 (25 °C) with a half-life of <10s, doubtless via C6H5-N2. In addition, Broxton s proposal (Scheme 8-53) does not provide a satisfying explanation of why the ortho halogen has such a strong effect in favor of the carbanion mechanism. 

Ordinary ketones are generally much more difficult to cleave than trihalo ketones or p-diketones, because the carbanion intermediates in these cases are more stable than simple carbanions. However, nonenolizable ketones can be cleaved by treatment with a 10 3 mixture of t-BuOK—H2O in an aprotic solvent such as ether, dimethyl sulfoxide, 1,2-dimethoxyethane (glyme), and so on, or with sohd t-BuOK in the absence of a solvent. When the reaction is applied to monosubstituted diaryl ketones, that aryl group preferentially cleaves that comes off as the more stable carbanion, except that aryl groups substituted in the ortho position are more readily cleaved than otherwise because of the steric effect (relief of stain). In certain cases, cyclic ketones can be cleaved by base treatment, even if they are enolizable. " OS VI, 625. See also OS VH, 297. 

In such cases, the more acidic hydrogen is removed. Since acidity is related to the field effect of Z, it can be stated that an electron-attracting Z favors removal of the ortho hydrogen while an electron-donating Z favors removal of the para hydrogen. The second factor is that the aryne, once formed, can be attacked at two positions. The favored position for nucleophilic attack is the one that leads to the more stable carbanion intermediate, and this in turn also... 

The latter reagent also methylates certain heterocyclic compounds (e.g., quinoline) and certain fused aromatic compounds (e.g., anthracene, phenanthrene). The reactions with the sulfur carbanions are especially useful, since none of these substrates can be methylated by the Friedel-Crafts procedure (11-12). It has been reported that aromatic nitro compounds can also be alkylated, not only with methyl but with other alkyl and substituted alkyl groups as well, in ortho and para positions, by treatment with an alkyllithium compound (or, with lower yields, a Grignard reagent), followed by an oxidizing agent such as Bra or DDQ (P- 1511). 

Carbanions of a-chloroalkyl phenyl sulfones react with nitrobenzenes to effect direct nucleophilic replacement of hydrogens located ortho and para to the nitro group (Eq. 9.24).38 A very important feature is that VNS of hydrogen usually proceeds faster than conventional SNAr of halogen located in equally activated positions (Eq. 9.25).38 The rule that VNS of... 

Phenyl aryl cyclopropenones16 were cleaved by methanolic KOH to a mixture of cis aryl cinnamic acids (318/319 R = phenyl, R = aryl), whose rates of formation gave rise to a linear Hammett-type correlation with a values in the range of -0.268 to +0.373 and p = 0.75. This also indicates that cleavage yielding the more stable carbanion is preferred. Interestingly, ortho-substituted aryl phenyl cyclopropenones gave only a-phenyl-0-aryl acrylic acids (319 R = phenyl, R = aryl), which was explained in terms of steric interactions. 

The ortho-ring junction that converts the triphenylmethyl structure into that of the ion LX increases the stability of the carbanion but decreases that of the carbonium ion. It will be recalled that this structural modification of the triphenylcarbonium ion had about the same effect as the introduction of one to two nitro groups. 

In some reactions of carbanions or organometallic compounds with very weak acids the base has a choice of protons and can give more than one salt. An example is the nuclear metallation of a substituted benzene in which the product may be oriented ortho, meta, or para. The actual results of a few such reactions are shown in Table XII. 

Incorporation of additional donor functionality into the periphery of phosphinomethanide ligands also has dramatic consequences for the structures of their alkali metal complexes. The complex [Li C(SiMe2Ph)(PMe2)2 ]3 (49) crystallizes as solvent-free cyclic tri-mers (Fig. 18), in which each lithium is primarily coordinated by two P atoms from one ligand and the carbanion center of an adjacent ligand (138, 139). This is supplemented by an essentially -interaction with the ipso and an ortho-carbon of the phenyl ring associated with the carbanion bonded to lithium. Each Li is thus bound by two P atoms, two aryl carbons, and a central carbon of the phosphinomethanide ligands. 

Vicarious nucleophilic substitution of nitroarenesThe carbanion (2, NaOCH,) of 1 reacts with p-chloronitrobenzene to give, after acidic workup, 5-chloro-2-nitrobenzyl p-tolylsulfone (3). The reagent 2 is typical of a number of carbanions which can undergo nucleophilic substitution ortho or para to nitro-... 

A variety of localized lithiated carbanions, such as aryllithiums and sulfur- and silicon-substituted alkyllithiums, have been found, by application of C, Li, and Li NMR techniques, to form triple ions in THF-HMPA solution. Thus, change to triple ion structures (18a-g) could be discerned as HMPA (2-5 equiv.) was added to solutions of monomeric structures (17a-g) in 4 1 THF-diethylether. The amount of triple ion is sensitive to ortho substitution monomeric (17a) and (17b) form 65-80% triple ion in presence of 1-3 equiv. HMPA whereas (17c) and (17e) form less than 20% at 5 equiv. HMPA. Pyridylthio-substituted carbanion (19) forms bis-chelated triple ion (20). 

It was realized that the mechanism of Birch reduction involves protonation of the anion-radical formed by the addition of one electron to the reacting aromatic compound. This is followed by rapid addition of a second electron and protonation of the forming carbanion to yield nonconjugated alicyclic products. Protonation of the anion-radical by added alcohol is the rate-limiting stage. Recent calculations show that the ortho and meta positions in anisole are most enhanced in density by electron introduction. The para position is not appreciably affected (Zimmerman and Alabugin 2001 Scheme 7.9). 

Recently, Behiman and coworkers discussed the mechanism of the Elbs oxidation reaction and explained why the para product predominates over the ortho product in this oxidation. According to the authors, semiempirical calculations show that the intermediate formed by the reaction between peroxydisulfate anion and the phenolate ion is the species resulting from reaction of the tautomeric carbanion of the latter rather than by the one resulting from the attack by the oxyanion. This is confirmed by the synthesis of the latter intermediate by the reaction between Caro s acid dianion and some nitro-substituted fluorobenzenes. An example of oxidative functionalization of an aromatic compound is the conversion of alkylated aromatic compound 17 to benzyl alcohols 20. The initial step in the mechanism of this reaction is the formation of a radical cation 18, which subsequently undergoes deprotonation. The fate of the resulting benzylic radical 19 depends on the conditions and additives. In aqueous solution, for example, further oxidation and trapping of the cationic intermediate by water lead to the formation of the benzyl alcohols 20 (equation 13) . ... 

Construction of the closely related NSAID bromefenac (46-8) depends on the Gassman indolone synthesis [46] for incorporation of the acetic acid chain. That reaction involves an anion-initiated electrocyclic rearrangement related conceptually to the little-known Hauser ortho substitution rearrangement. The simplest example of the latter depends on the formation of a carbanion by abstraction of one of the acidic protons from a benzyltrimethyl quaternary salt to give I (the... 

Replacement of the pyridine ring by a more strongly basic ethylpiperidine moiety leads to the antiarrhythmic dmg disobutamide (55-5). The synthesis of this compound also involves successive carbanion alkylation reactions. Thus, reaction of the anion from ortho-chloroacetonitrile (55-1) with A-(2-chloroethyl) piperidine gives the intermediate (55-3) alkylation of the anion from this leads to (55-4). Hydrolysis with sulfuric acid completes the preparation of disobutamide (55-5) [56]. 

PhO " is an ambident anion with negative charge on both O and the ortho C atoms of the ring. Attack by O gives the ether attack by the ortho carbanion gives o-allylphenol. 

---