Resonance benzyl anion

In addition reactions to chiral carbonyl compounds, the stereochemical course taken by resonance-stabilized alkali metals or magnesium benzyl anions resembles that taken by localized carbanions of similar bulk. Thus, conditions can be delineated which lead to either the steric approach or chelation control the following serve as examples. 

FIGURE 3.3 Deprotection of functional groups by reduction. Hydrogenolysis of benzyl-based protectors.1 Attack by electrons liberates the protector as the benzyl anion because the latter is stabilized by resonance. This is a simplified presentation of the reaction. 

Eq. (5.63)] initiates the transformation.235 The next step, the attack of anion 36 on the alkene [Eq. (5.64)], is the rate-determining step since it results in the transformation of a resonance-stabilized anion (36) into anions (37 and 38) that are not stabilized. Transmetalation of 37 and 38 forms the end products and restores the benzylic carbanion [Eqs. (5.65) and (5.66)] ... 

This is similar to postulating the various resonance forms for the benzyl anion but does not so easily permit para products. Takegami found that this isomerization was strongly promoted by cobalt hydrocarbonyl. It would be interesting to know if this is also true of the isomerization of aliphatic acylcobalt carbonyls. 

AUyhc and benzylic anions are about 59 kj mol (14kcal mol ) more stable than their non-allylic and nonbenzyUc counterparts. There are two reasons for the stabilities of these anions. The first is resonance stabilization and second is the polar effect of the double bond (Scheme 3.6). 

A methoxy substituent on a benzene ring has an effect that is opposite that of a nitro substituent, so the benzyl anion is a model for the MOs of anisole. Excitation removes an electron from and places it in ij/s, which produces a charge difference suggested by the resonance representation shown in Figure 12.52. These simple models help us understand the photochemically induced solvolysis in which m-nitrophenyl trityl ether (103) does not react in the dark but does readily undergo photosolvolysis, as shown in Figure 12.53. ° ... 

The benzyl anion 4 is more stable than the methyl anion 2 because it is stabilised by resonance - the negative charge is delocalised on to the 2, 4 and 6 positions of the ring. 

Allyl and benzyl cations are the prototype delocalized carbenium ions (look at Figures 1.21 B and 1.22). Conventional it delocalization does not convert a carbenium ion to a carbo-nium ion in this case. No hypervalent atoms are involved in any of the resonance structures for allyl or benzyl. The same orbitals that carry the negative charge in allyl and benzyl anion carry the positive charge in allyl and benzyl cation. 

Piperidine functions as a base and deprotonates the benzylic proton, giving an aromatic, resonance-stabilized anion... 

FIGURE 3.6. The benzyl anion in terms of resonance theory. 

The r-bond order of the CN bond in aniline is therefore low. This means that its resonance integral will be small and this in turn implies an increase in the associated NBMO coefficient [cf equation (3.36)] and so in the Tc-electron density at nitrogen. In other words, aniline is much more like a classical structure with a localized CN bond than is the benzyl anion. The same effect is seen in other cases, too. Thus the n energy of union of ammonia and butadiene to form pyrrole is much less than that for the corresponding carbanion. 

Frontier Molecular Orbitals of Benzyl Cation, Radical, and Anion At the molecular orbital level, the most important MO for benzyl cation is the lowest unoccupied molecular orbital (LUMO), for benzyl radical it is the singly occupied molecular orbital (SOMO), and for benzyl anion the highest occupied molecular orbital (HOMO). Figure 11.13 shows the LUMO for benzyl cation where it is clear that the carbon 2p atomic orbitals that contribute the most are those of the benzylic carbon and the ring carbons that are ortho and para to it. These are the same carbons that, according to resonance, share the positive charge. The SOMO of benzyl radical and HOMO of benzyl anion are not shown, but are virtually identical to the LUMO of benzyl cation, in keeping with their respective resonance descriptions. 

Resonance in benzylic anions makes benzylic hydrogens relatively acidic... 

In Summary Benzylic radicals, cations, and anions are stabilized by resonance with the benzene ring. This effect allows for relatively easy radical halogenations, SnI and Sn2 reactions, and benzylic anion formation. 

Carbanions, like carbocations, may be stabilized by resonance delocalization the allyl and benzyl anions are relatively stable. The enolate anion, 4.77, is even more so, since in one of the resonance forms, the negative charge is located on the electronegative oxygen atom. The stability of carbanions will be very important to our discussions of the values for organic compounds in Chapter 8. 

Hthiated 4-substituted-2-methylthia2oles (171) at -78 C (Scheme 80). Crossover experiments at—78 and 25°C using thiazoles bearing different substituents (R = Me, Ph) proved that at low temperature the lithioderivatives (172 and 173) do not exchange H/Li and that the product ratios (175/176) observed are the result of independent metala-tion of the 2-methyl and the C-5 positions in a kinetically controlled process (444). At elevated temperatures the thermodynamic acidities prevail and the resonance stabilized benzyl-type anion (Scheme 81) becomes more abundant, so that in fine the kinetic lithio derivative is 173, whereas the thermodynamic derivative is 172. 

The absolute rate of dissociation of the radical anion of /i-nitrobenzyl chloride has been measured as 4 x 10 s . The w-nitro isomer does not undergo a corresponding reaction. This is because the meta nitro group provides no resonance stabilization of the benzylic radical. 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

Decarboxylation of 1,3-dimethylorotic acid in the presence of benzyl bromide yields 6-benzyl-1,3-dimethyluracil and presumably involves a C(6) centered nucleophilic intermediate which could nonetheless have either a carbene or ylide structure. Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to explore the gas-phase reactions of methyl nitrate with anions from active methylene compounds anions of aliphatic ketones and nitriles react by the 5n2 mechanism and Fco reactions yielding N02 ions are also observed nitronate ions are formed on reaction with the carbanions derived from toluenes and methylpyridines. 

This addition is energetically favored because a more stable primary carb-anion is formed from a less stable tertiary carbanion. The addition step in the side-chain alkylation reaction is probably not energetically favored because a primary carbanion is formed from a resonance-stabilized benzylic carbanion. However, as the rapid and energetically favored transmetalation reaction following it restores the benzylic carbanion, the over-all process takes place readily. An alternative radical combination mechanism has also been proposed by Morton and Ward 39) for the alkylation reaction. 

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