Benzyl resonance

The spectrum of styrene in toluene was compared with that in toluene containing an equimolar quantity of Zr (benzyl) 4 at 0.06 M using a sweep of 2500 Hz. The methylene doublets of styrene were in identical positions (1115 and 1230 Hz) in these spectra. Experiment (b) (Table XIX) was repeated at — 60°C using 250-Hz expansion on a 100-MHz spectrometer. The benzyl resonance was observed to shift approximately 4 Hz (relative to toluene) upheld. The lack of splitting in the latter indicates equilibrium (if any occurs) is very rapid. Finally, the effect of temperature on the systems Zr (benzyl) 4 in styrene and Zr (benzyl) 4 toluene were examined and the results are given in Table XX. They show that no specific interaction of styrene with Zr (benzyl) 4 occurs. The interaction of toluene would probably be of the type (XXIV) whereas styrene would interact similarly or in a manner shown in (XXV), both interactions would affect the environment of the benzyl protons in Zr (benzyl) 4 if they occurred to any significant... 

Fig. 2.22. Non-planarity of trityl cations compromise between maximum benzylic resonance (planarizing effect) and minimum aryl/aryl repulsion (twisting effect).

In terms of the mechanism, the activation energy for dj-methyl cinnamate (3, Table 10) should be lower than that of m-2-butene by about one benzyl resonance (/ benzyl — 13 kcal.mole ) and one carbonyl allyl resonance (5.8 kcal.mole" ). The corrected activation energy, obtained from the observed rate constant at T and the estimated z4-factor, is in reasonably good agreement with this expectation. Therefore, although the experimental parameters for m-methyl cinnamate are low, the rate coefficients are reasonably reliable. Such is not the case... 

If Aff°(-COOH) = -52.4 kcal.mole is used along with the corrected activation for the diphenylacetic acid decomposition, one estimates a resonance energy in the transition state (and in the (C6H5)2CH- radicals) of about 25.2 kcal.mole . This is just twice the benzyl resonance energy as might be expected... 

The observation that the coalescence temperature is dependent on the concentration of 14 points to a bimolecular process. Furthermore, it appeared that addition of LiCl to a solution of 14 lowers the coalescence temperature of the NMe2 and benzylic resonances. Therefore, ionic species, most probably resulting from initial dimerization of 14, are proposed to be intermediates in the inversion of configuration at tin. 

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). 

Benzylic resonance Resonance stabilization of the phenylinethyl (benzyl) radical, CaH.sCHs- (also cation and anion). 

In a second [1 + 2] self-sorting experiment, bis(dibenzylammonium) axle IS " and wheels 10 and la were mixed in a 1 2 2 ratio in CDCI3. The corresponding NMR spectrum clearly showed ent/o-benzyl resonances in the 4—6 ppm region corresponding to (r,r)-18 cla2 pseudo[3]rotaxane (Fig. 30.15) ruling out the... 

The product of this step is an alcohol, which can be readily converted into the corresponding carbocation by acid (Section 9-2), partly because the ensuing charge is resonance stabilized by the adjacent benzene ring benzylic resonance, Section 22-1, related to allylic resonance, Sections 14-1 and 14-3). The cation subsequently enables the second electrophilic aromatic substitution step to provide DDT. Another more advanced mechanism is the subject of Problem 66. 

A final note You may have recognized that the allylic anion discussed above is also benzylic and hence enjoys additional resonance stabilization by conjugation with the benzene ring. Benzylic resonance will be discussed in detail in Section 22-1. For more practice with Robinson annulations, go to Problems 59, 63, and 64. 

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

What explains the ease of benzylic halogenation The answer lies in the stabilization of the phenyhnethyl (benzyl) radical by the phenomenon called benzylic resonance (Figure 22-1). As a consequence, the benzylic C-H bond is relatively weak (DH° = 87 kcal moF, 364 kJ moF ) its cleavage is relatively favorable and proceeds with a low activation energy. 

Reminiscent of the effects encountered in the corresponding allylic systems (Section 14-3), benzylic resonance can affect strongly the reactivity of benzylic halides and sulfonates in nucleophilic displacements. For example, the 4-methylbenzenesulfonate (tosylate) of 4-methoxyphenylmethanol (4-methoxybenzyl alcohol) reacts with solvent ethanol rapidly via an SnI mechanism. This reaction is an example of solvolysis, specifically ethanolysis, which we described in Chapter 7. 

Styrene can insert into the M-M bond of [Rh(OEP)]2 (OEP = octaeth-ylporphyrin) via initial M-M bond homolysis to give the 15e metalloradical [Rh(OEP)]. This adds to the alkene to give [PhCH( )CH2Rh(OEP)], stabilized by benzylic resonance, followed by the sequence of Eq. 7.24. [Rh(OEP)]2 also initiates radical photopolymerization of CH2=CHCOOR, where the intermediate C radicals add repetitively to acrylate rather than recombine with a metalloradical as in Eq. 7.24. 

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