Benzylic radicals, stabilization

This (7,o-diradical lacks the benzylic radical stabilization found in 61, and therefore cyclization of 68 is less exothermic than that of 59. Musch and Engels note that the Schmittel cyclization (AG (CCSD(T)/cc-pVDZ) = 18.6 kcal mol ) of 68 is favored over the Myers-Saito cyclization (AG (CCSD(T)/cc-pVDZ) = 21.1 kcal mor ). This is opposite to the case for the open chain analog 69, where the barrier for Myers-Saito cyclization is 9.7 kcal mol below the barrier for the Schmittel cyclization. However, fusing a cyclopentane ring with the eneyne-butatriene (70) favors the Myers-Saito cyclization over the Schmittel cyclization, AG (UB3LYP/6-31G(d) = 22.2 versus 22.8 kcal mol . Since neocarzinostatin chromophore follows the Myers-Saito pathway exclusively, nature has carefully balanced many factors in creating this system. 

In orbital terms as represented m Figure 11 10 benzyl radical is stabilized by delo cahzation of electrons throughout the extended tt system formed by overlap of the p orbital of the benzylic carbon with the rr system of the ring... 

Section 11 10 Chemical reactions of arenes can take place on the ring itself or on a side chain Reactions that take place on the side chain are strongly influ enced by the stability of benzylic radicals and benzylic carbocations... 

The stabilizing effects of vinyl groups (in allylic radicals) and phenyl groups (in benzyl radicals) are very significant and can be satisfactorily rationalized in resonance terminology ... 

Allylic and benzylic radicals are also stabilized by both acceptor and donor substituents. As shown in Table 12.5, theoretical calculations at the MP2 level indicate that substituents at the 2-position are only slightly less elfective than 1-substituents in the... 

The same is true for decarbonylation of acyl radicals. The rates of decarbonylation have been measured over a very wide range of structural types. There is a very strong dependence of the rate on the stability of the radical that results from decarbonylation. For example, rates for decarbonylations giving tertiary benzylic radicals are on the order of 10 s whereas the benzoyl radical decarbonylates to phenyl radical with a rate on the order of 1 s . ... 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

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. 

According to these data, which structural features provide stabilization of radial centers Determine the level of agreement between these data and the radical stabilization energies given in Table 12.7 if the standard C—H bond dissociation energy is taken to be 98.8 kcal/mol. (Compare the calculated and observed bond dissociation energies for the benzyl, allyl, and vinyl systems.)... 

We attributed the decreased bond dissociation energy in propene to stabilization of allyl radical by electron delocalization. Similarly, electron delocalization stabilizes benzyl radical and weakens the benzylic C—H bond. 

The chain propagation step consists of a reaction of allylic radical 3 with a bromine molecule to give the allylic bromide 2 and a bromine radical. The intermediate allylic radical 3 is stabilized by delocalization of the unpaired electron due to resonance (see below). A similar stabilizing effect due to resonance is also possible for benzylic radicals a benzylic bromination of appropriately substituted aromatic substrates is therefore possible, and proceeds in good yields. 

Reaction occurs exclusively at the benzylic position because the benzylic radical intermediate is stabilized by resonance. Figure 16.20 shows how the benzyl radical is stabilized by overlap of its p orbital with the ring 77 electron system. 

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.

Problem 16.20 Refer to Table 5.3 on page 156 for a quantitative idea of the stability of a benzyl radical. How much more stable (in kj/mol) is the benzyl radical than a primary alkyl radical How does a benzyl radical compare in stability to an allyl radical ... 

A benzyl radical is more stable than a primary alkyl Tadical bv 52 kj/mol and is similar in stability to an ailyl radical. 

Benson17 has tried to collect some thermodynamic data based on a number of empirical rules for this class of radicals. He estimated heats of formation for HS02, MeSO 2) PhSO 2 and HOSO 2 as —42, —55, —37 and — 98kcalmor respectively. He also estimated a stabilization energy for the benzenesulfonyl radical of 14 kcal mol"1, which is very similar to that of the benzyl radical. However, recent kinetic studies18 (vide infra) have shown that arenesulfonyls are not appreciably stabilized relative to alkanesulfonyl radicals, in accord with the ESR studies. 

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) ... 

Figure 21.7 Stabilization of a benzylic radical by resonance with the unsaturated ring...

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. ... 

Radical attack on methylbenzene (toluene, 60) results in preferential hydrogen abstraction by Cl leading to overall substitution in the CH3 group, rather than addition to the nucleus. This reflects the greater stability of the first formed (delocalised) benzyl radical, PhCH2 (61), rather than the hexadienyl radical (62), in which the aromatic stabilisation of the starting material has been lost ... 

The principle product is derived from the combination of tolyl and benzyl radicals and is a consequence of the greater stability of the tolyl radical. When the monomer is present in large excess, the majority of the radicals are captured and the principle product in these circumstances is polymer ... 

In the absence of 1,4-CHD, the biradical 55e undergoes an intramolecular 1,5-hydrogen shift to form 57, making it possible for an intramolecular radical-radical coupling to occur to produce 58 (Scheme 20.13). The fact that 58 was produced from 54e lends support to the formation of the a,3-didehydrotoluene biradical 55e as a transient reaction intermediate. It is also worth noting that the benzylic radical center in 55 is a stabilized triarylmethyl radical. 

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