Methyl carbanions

The haloform reaction of unsymmetrical perfluoroalkyl and co-hydroper-fluoroalkyl trifluororaethyl ketones gives the alkane corresponding to the longer alkyl chain [54] (equation 53) If the methyl group contains chlorine, the reaction can take different pathways, leading to loss of chlorine (equation 54), because of the variable stability of the chlorine-substituted methyl carbanions in alkali. 

A form of vitamin B12 can produce methyl carbanion, a reactive species that is responsible for methylation of Hg (see Figure 8.2, Craig 1986, and IAEA Technical Report 137). Methyl carbanion acts as a nucleophilic agent toward Hg ions. 

Methyl arsenic, like methyl mercury, is generated from inorganic forms of the element by methylation reactions in soils and sediments. However, the mechanism is evidently different from that for mercury, depending on the attack by a methyl car-bonium ion rather than a methyl carbanion (Craig 1986, Crosby 1998). Methylation... 

Methylation of [Co(tmt)]2+ with Mel leads to the potent methyl carbanion donor trans-[Co(tmt)Me2]+ (186). Reaction of this complex with variety of methyl-lead(IV) compounds in MeCN is rapid, leading to the same monomethylcobalt(III) product, but resulting in different methylated Pb derivatives depending on the reaction stoichiometry and Pb compound.839 The same complex rapidly transfers Me groups to Zn2+ and Cd2+ in MeCN,840 or Pb2+ and Sn2+ in water.302,841 The kinetics of Co—C bond formation in the reactions with primary alkyl and substituted primary alkyl radicals has been found to be influenced more by the structure of the macrocycle than by the nature of the radicals.842... 

The methyl carbanion, CH3, has bond angles close to that in a tetrahedral arrangement of atoms, 109°, indicating 4 electron groups around the central C atom and sp3 hybridization. The lone pair of electrons exerts significant repulsive force on the electrons in bonding orbitals and must be counted as an electron group. 

High-level ab initio calculations have provided more precise structural details, and relative stability estimates, for members of the 7-norbornyl anion series (12-15). Far from being classical carbanions, each of the ions is stabilized by delocalization of the negative charge into accessible LUMOs of anti-parallel C—C bonds of the molecular framework and each is more stable than methyl carbanion. Consequently, it is unlikely that solution studies of the unsaturated systems will reveal any bishomo-antiaromatic character. 

Preparation of a somewhat more complex leukotriene antagonist begins by aldol condensation of the methyl carbanion from quinoline (29-1) with meta-phthalalde-hyde (29-2) to give the stilbene-like derivative (29-3) dimer formation is presumably inhibited by the use of excess aldehyde. Reaction of that product with A,A-dimethyl-3-mercaptopropionamide in the presence of hexa-methylsilazane affords the silyl ether (29-4) of the hemimercaptal. Treatment of that intermediate with ethyl 3-mercaptopropionate leads to the replacement of the silyl ether by sulfur and the formation of the corresponding thioacetal (29-5). Saponification of the ester group leads to the carboxylic acid and thus to verlukast (29-6) [33]. 

Model reactions have contributed significantly to our understanding of biological processes. Both pyridoxal phosphate (vitamin B6) and Bi2-coenzymes have proved useful in mechanism studies. Methyl transfer reactions to various metals are of environmental significance. In 1968 it was shown that methylcobalamin could transfer a methyl carbanion to mercury(II) salts in aqueous solutions. Recent research on interaction between B12-coenzymes and platinum salts has shown that charged Ptn salts labilize the Co—-C bond. Secondly, the B12-coenzymes are unstable in the presence of platinum salts this observation correlates with the fact that patients who have received cw-platin develop pernicious anemia. 

Deprotonation seems to be a very slow process at low temperature in ether as a solvent so that the attack of the methyl carbanion at the ar-... 

Electronegativity influences the basic strength of the compound. If we compare the fluoride ion, hydroxide ion, amide ion and the methyl carbanion, then the order of basicity is as shown in the following figure ... 

The Grignard reagent in this reaction is methyl magnesium iodide (CH3MgI) and it is the source of a methyl carbanion (Following fig.). Actually, the methyl carbanion is never present as a separate ion, but the reaction proceeds as if it were present. The methyl carbanion is the nucleophile in this reaction and the nucleophilic centre is the negatively charge carbon atom. The aldehyde is the electrophile. Its electrophilic centre is the carbonyl carbon atom since it is electron deficient. 

The electronic nature of the groups attached to the anionic carbon also affects the proportion of C-alkylation. The amount of C-alkylation in the reaction of ArCH( -Bu)Cl with a series of a-methylated carbanions, having similar steric interactions, decreases when the stabilizing substituents on the carbanion are changed from cyano to alkoxycarbonyl and to acetyl. 

A carbanion can act as a leaving group if it is stabilized somehow. We have already seen several examples of this. For example, CX3 , a methyl carbanion stabilized by the inductive effect of three electronegative halogen atoms, leaves in the haloform reaction (see Section 20.2). 

A carbanion has a trivalent carbon atom that bears a negative charge. There are eight electrons around the carbon atom (three bonds and one lone pair), so it is not electron deficient rather, it is electron rich and a strong nucleophile (Lewis base). A carbanion has the same electronic structure as an amine. Compare the structures of a methyl carbanion and ammonia ... 

There is an inverse relationship between the acidity of an acid and the basicity of its conjugate base. That is, the more acidic the acid, the weaker the basicity of the conjugate base and vice versa. For example, if the acid is very weak, like methane, the conjugate base, the methyl carbanion, is a very strong base. On the other hand, if the acid is very strong, like sulfuric acid, the... 

It has been demonstrated that methylsulfanyl difluoromethyl phosphonate (438) has an excellent potential as a freon-free source of phosphonodifluoro-methyl carbanion. A simple preparative procedure involving sequential treatment of the former with butyl lithium and different electrophiles allowed the preparation of a wide range of new fluorinated building blocks (439) (Figure 80). 

This discussion of aliphatic carbanion structures has included mainly organolithium compounds simply because the structures of most aliphatic caibanions incorporate lithium as the counterion and also because this alkali metal cation is the most widely used by synthetic organic chemists. For comparison the entire series of Group la methyl carbanion structures, i.e. MeNa, MeK, MeRb and MeCs, have been determined. Methylsodium was prepared by reaction of methyllithium with sodium r-butoxide. Depending upon the reaction conditions, the products obtained by this procedure contain variable amounts of methyllithium and methylsodium (Na Li atom ratios from 36 1 to 3 1). Hie crystal structure of these methylsodium preparations resembles the cubic tetramer (38) obtained for methyllithium with the Na— Na distances of 3.12 and 3.19 A and Na—C distances of 2.58 and 2.64 A. 

A mixed Li /Mg+ aggregate corresponding to (213) is formed with either phenyl or methyl carbanions. An unusual lithium/magnesium acetylide is formed with stoichiometry Li2[(PhCsC)3Mg(TMEDA)]2 and is depicted as (214). The same authors also report the ion pair characterized as the mixed benzyllithium/magnesium TMEDA complex (215). ° A different mixed lithium/magnesium aggregate depicted as (216) is found for the THF-solvated anion of tris(trimethylsi-lyl)methyl carbanion. ... 

The intermediate acetyl derivative Nb(r -C5H5)2(COMe)CO, resulting from nucleophilic attack of the methyl carbanion on a coordinated carbonyl group, undergoes decarbonylation. 

Notice that the carbon atom has one lone pair of electrons and is involved in three single covalent bonds. This carbon species is called a methyl carbanion, and it bears a single negative charge, i.e. 4-8+V4(6)=-l. 

Methane may be deprotonated under extreme conditions to give the methyl carbanion, CH3. ... 

The methyl free radical CH3 has bond angles of about 120°, whereas the methyl carbanion... 

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