Methyl anions

Fig. 1.9. Total energy as a function of distortion from planarity for methyl cation, methyl radical, and methyl anion. [Reproduced from J. Am. Chem. Soc. 98 6483 (1976) by permission of the American Chemical Society.]...

The substituent stabilization effects calculated for the methyl cation and the methyl anion refer to the gas phase, where no solvation effects are present, and therefore are substantially larger, in terms of eneigy, than would be the case in solution, where solvation contributes to stabilization and attenuates the substituent effects. 

Table 1.14. Calculated Stabilization of Methyl Anion by Substituents...

Draw Lewis structures for methyl anion, ammonia and hydronium cation. How many electrons are left over in each after all bonds have been made Display and compare electron density surfaces for methyl anion, ammonia and hydronium cation. Which is the smallest molecule Which is the largest Rationalize your observation. (Hint Compare the number of electrons in each molecule, and the nuclear charge on the central atom in each molecule.)... 

Isobe and coworkers407 found an interesting diastereoselective heteroconjugate addition of methyllithium to 314. The stereochemical control was considered to be determined at the stage of the intermediate 315. Since methyllithium is considered to be coordinated strongly with the methoxyethoxymethoxyl (OMEM) group, the methyl anion would attack the -carbon of the olefin only from one side, as shown below. 

The methyl anion (CH3) has been observed in the gas phase and reported to have a pyramidal structure. If this is a general structure for carbanions, then any... 

The methyl group migrates with its pair of electrons, as a methyl anion, CH3 (a methanide ion). 

However, other reaction directions via the formation of the phosphorane structure with migration of the methyl anion from one phosphorus atom... 

The utilization of the dibromides 315 permitted the generation of 311 by MeLi. Since MeLi is less reactive as a nucleophile than wBuLi, the ring opening of 311 by addition of the methyl anion analogous to the formation of 314 (Scheme 6.67) was of no importance. However, in the case of 315 (R = CH2Ph), the intramolecular insertion of the transient carbenoid or carbene, leading to 312, was the main reaction and hence the cause of low yields of the respective cycloadducts 316, 317, 319 and 320. Whether such an insertion has to be blamed for the very modest yields of 316 and 317 with R= Me could not be proved [154]. 

Vinyl acetate fails to react with dihalocarbenes, but reacts with the trihalo-methyl anion precursor to produce the l,l,l-trihalo-2-acetoxypropane [33, 166-168], In contrast, where the a-position is substituted by an alkyl group, normal cyclopropanation occurs. Cyclic enol acetates behave in a similar fashion to the enol ethers [12, 88], e.g. bicyclo[3,2,l]oct-2-enyl acetate and its 3-isomer... 

However, phenyllithium does not substitute fluorine or methoxy and methyl-lithium affords a mixture of fluorinated and methylated anions (Scheme 14) ... 

Before we move on from the hybrid orbitals of carbon, we should take a look at the electronic structure of important reactive species that will figure prominently in our consideration of chemical reactions. First, let us consider carbanions and carbocations. We shall consider the simplest examples, the methyl anion CHs and the methyl cation CH3+, though these are not going to be typical of the carbanions and carbocations we shall be meeting, in that they lack features to enhance their stability and utility. 

This is the methyl anion, and carries one negative charge. Carbon has four valence electrons, and in this structure the number of assigned electrons is... 

The TMEDA and THE complexes of the very sterically crowded methyllithium derivative tris(trimethylsilyl)methyUithium (9, Scheme 1) were studied by multinuclear solid state NMR spectroscopy in combination with solution NMR spectroscopy and X-ray crystallography to reveal the strucmre and dynamic behaviour of the complexes . In the solid state, this complex crystallizes as an ate-complex, with one lithium cation interacting with two substituted methyl anions while the other lithium cation is complexed by two TMEDA ligands. In solution, the afe-complex is partly transformed into solvated monomers or aggregates, depending on the experimental conditions. 

Strong base treatment of the spiro salt 49 gives a benzyne (107) from which the isolated products were produced by further reaction. For example, with n-butyllithium and furan in tetrahydrofuran, 108 is produced after hydrogenation and acid treatment via 109. Reaction with phenyllithium gives 110 (R == Ph and Me) by subsequent addition of phenyl or methyl anion to the benzyne, respectively, and 110 (R = I) by subsequent reaction with iodine anion. Similarly the 9,9-diphenyl salt 111 gives 112 with phenyllithium. Pyrolysis of the spiro salt 49 gives 50. 

Electron density surfaces can also be used to uncover trends and build qualitative descriptions. For example, size surfaces for the isoelectronic molecules, methyl anion, ammonia and hydronium cation show a marked decrease in overall size. 

Das allgemeine Prinzip der Esterkondensationen, daB sich aus zwei elektroneutralen Komponenten imter dem EinfluB einer Base inuner das im System mogliche am schwachsten basische Anion bildet, ist auch bei diesen Kondensationen gewahrt Das Athoxyl-Anion ist schwacher basisch als das Triphenyl-methyl-Anion das entstehende Carbeniat-Anion des an sich elektroneutralen a,a-disubstituierten -Ketocarbonsaureesters ist nur Nebenprodukt dieses Saure-Basen-Austausches. 

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