Benzylic resonance stabilization

Propagation involves addition of the propagating species to styrene monomer (Eqs. 22.33 and 22.34). Head-to-tail addition, the normal mode, retains the negative charge at a benzylic, resonance-stabilized position. 

Of the known triplet states, the cross-conjugated non-Kekule diradicals are classified as delocalized triplet diradicals (e.g., 14). In contrast, the radical centers in localized triplet diradicals (e.g., 5-7) are not joined through a n system. For the localized 1,3-diphenyl-substitutcd cyclopentane-1,3-diyl triplet diradicals 6b and c, a planar five-membered ring with coplanar arrangement of the aryl groups is expected to allow the maximum contribution of the benzylic resonance stabilization, which is worth about 50-54 kJ mol 1 [28], For such aryl-substituted triplet diradicals (e.g., 9-12), the magnetic dipole interaction of the two uncoupled spins is given by Eq. (6). 

REACTIVITY AT THE PHENYLMETHYL (BENZYL) CARBON BENZYLIC RESONANCE STABILIZATION... 

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. 

Instability at the chain end could then arise via the benzylic hydrogen atoms due to the possible resonance stabilization of the resulting radical [Eq. (18)] ... 

We saw in Section 6.9 that the stability order of alkyl carbocations is 3° > 2° > 1° > —CH3. To this list we must also add the resonance-stabilized allvl and benzyl cations. Just as allylic radicals are unusually stable because the... 

Because of resonance stabilization, a primary allylic or benzylic carbocation is about as stable as a secondary alkyl carbocation and a secondary allylic or benzylic carbocation is about as stable as a tertiary alkyl carbocation. This stability order of carbocations is the same as the order of S l reactivity for alkyl halides and tosylates. 

Figure 16.20 A resonance-stabilized benzylic radical. The spin-density surface shows that the unpaired electron (blue) is shared by the ortho and para carbons of the ring.

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. 

The amount of the resonance stabilization is similar to that for the benzyl radical. In radicals formed from monomers having C=0 or C=N groups conjugated with the carbon-carbon double bonds, the corresponding resonance structures... 

II), and its formation therefore is more probable. If the substituent X possesses unsaturation conjugated with the free radical carbon, as for example when X is phenyl, resonance stabilization may be fairly large. The addition product (I) in this case is a substituted benzyl radical. Comparison of the C—I bond strengths in methyl iodide and in benzyl iodide, and a similar comparison of the C—H bond strengths in methane and toluene, indicate that a benzyl radical of type (I) is favored by resonance stabilization in the amount of 20 to 25 kcal. 

That resonance stabilization of intermediate biradicals is important in determining the efficiency of decarbonylation follows from the following examples yielding benzyl radicals upon loss of carbon monoxide(57) ... 

A corresponding correlation is obtained for the rate constants of a,a -phenyl substituted alkanes 26 (R1 = C6H5, R2 = H, R3 = alkyl) (see Fig. 1 )41). It has, however, a different slope and a different axis intercept. When both correlations are extrapolated to ESp = 0, a difference of about 16 kcal/mol in AG is found. This value is not unexpected because in the decomposition of a,a -phenyl substituted ethanes (Table 5, no. 22—27) resonance stabilized secondary benzyl radicals are formed. From Fig. 1 therefore a resonance energy of about 8 kcal/mol for a secondary benzyl radical is deduced. This is of the expected order of magnitude54. ... 

The results of ab initio calculations provide evidence that Me2NC(S)-[14+] is stabilized by resonance electron donation from the a-thioamide group (A, Scheme 12) and by covalent bridging of sulfur to the benzylic carbon (B, Scheme 12).96 Direct resonance stabilization of the carbocation will increase the barrier to the nucleophile addition reaction, because of the requirement for the relatively large fractional loss of the stabilizing resonance interaction (A, Scheme 12) at the transition state for nucleophile addition to a-substituted benzyl carbocations.8,13,28 91-93 If the solvent adds exclusively to an open carbocation that is the minor species in a mixture of open and closed ions, then... 

Estimating stability it is possible to apply criteria commonly used in organic chemistry. Tertiary alkyl carbocation is more stable than the secondary one which is in its turn more stable than the primary one. For the carbon ions of this type the row of the stability is reversed. Allyl and benzyl cations are stable due to the resonance stabilization. The latter having four resonance structures may rearrange to be energetically favorable in the gas phase tropilium cation possessing seven resonance forms (Scheme 5.3). 

Due to the benzylic p-Tc resonance stabilization the C+-C,pso bond has partial double bond character and the ortho, ortho and meta, meta methine groups syn and anti to the silyl group are non-equivalent. The effect pf the a-silyl group on the positive charge in benzyl cations can be estimated by comparison of the NMR spectroscopic data of the 1 -phenyl-1 -(trimethylsilyl)ethyl cation 1 with those for the 1-phenylethy 1 cation 5 (P) and the cumyl pation 3 (15, 16, 17) (Table 1). 

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