Benzyl cations steric effects

The cumyl cation (4) has been the subject of an X-ray crystallographic study, as its hexafluoroantimonate salt at —124 °C.31 It is nearly planar (8 ° twist), with a short bond between the C+ and the ring (1.41 A), consistent with benzylic delocalization. The Me—C+ bonds are also shortened, indicative of hyperconjugative interaction.31 However, calculations are taken to show that hyperconjugation is not important in isolated benzyl cations e.g. structures such as (6) are not important contributors to the overall structure of (5).32 The stabilization provided by alkyl groups would thus be because of their polarizability, and the Baker-Nathan effect would be due to steric hindrance to solvation.32 The heats of formation of some a-mcthylbcnzyl cations indicate that the primary stabilization in these species comes from the a-substitucnts, and that the stabilization provided by the aromatic ring is secondary.33... 

It seems clear that for reactions of carbocations with nucleophiles or bases in which the structure of the carbocation is varied, we can expect compensating changes in intrinsic barrier and thermodynamic driving force to lead to relationships between rate and equilibrium constants which have the form of extended linear plots of log k against log K. However, this will be strictly true only for structurally homogeneous groups of cations. There is ample evidence that for wider structural variations, for example, between benzyl, benzhydryl, and trityl cations, there are variations in intrinsic barrier particularly reflecting steric effects which lead to dispersion between families of cations. 

Radicals are species with at least one unpaired electron, which, in contrast to organic anions and cations, react easily with themselves in bond-forming reactions. In the liquid phase, most of these reactions occur with diffusion-controlled rates. Radical-radical reactions can be slowed only if radicals are stabilized by electronic effects (stable radicals) or shielded by steric effects (persistent radicals). However, these effects are not strong enough to prevent diffusion-controlled recombination of, for example, benzyl radicals or tert-butyl radicals.1 Only in extreme cases are the radical or di-tert-butylmethyl radical recombination rates low.2 While the recombination rates of the triphenyl-methyl radical is reduced due to both steric and radical stabilizing effects, the steric effect alone slows the recombination of the di-/t>/-/-butyl methyl radical. Since neither of the radicals have C-H bonds (I to the radical centre, disproportionation reactions, in which the hydrogen atom is transferred, cannot occur. 

Another possible explanation of the results invokes steric effects in the intermediate allyl anion (Figure 3). The steric interaction between the bulky chelated cation and the benzyl group probably pushes the... 

In addition to steric effects, there are other important substituent effects that influence both the rate and mechanism of nucleophilic substitution reactions. As we discussed on p. 302, the benzylic and allylic cations are stabilized by electron delocalization. It is therefore easy to understand why substitution reactions of the ionization type proceed more rapidly in these systems than in alkyl systems. Direct displacement reactions also take place particularly rapidly in benzylic and allylic systems for example, allyl chloride is 33 times more reactive than ethyl chloride toward iodide ion in acetone." These enhanced rates reflect stabilization of the Sjv2 TS through overlap of the /2-type orbital that develops at carbon." The tt systems of the allylic and benzylic groups provide extended conjugation. This conjugation can stabilize the TS, whether the substitution site has carbocation character and is electron poor or is electron rich as a result of a concerted Sjv2 mechanism. 

Electron-withdrawing substituents in anionic polymerizations enhance electron density at the double bonds or stabilize the carbanions by resonance. Anionic copolymerizations in many respects behave similarly to the cationic ones. For some comonomer pairs steric effects give rise to a tendency to altemate. The reactivities of the monomers in copolymerizations and the compositions of the resultant copolymers are subject to solvent polarity and to the effects of the counterions. The two, just as in cationic polymerizations, cannot be considered independently from each other. This, again, is due to the tightness of the ion pairs and to the amount of solvation. Furthermore, only monomers that possess similar polarity can be copolymerized by an anionic mechanism. Thus, for instance, styrene derivatives copolymerize with each other. Styrene, however, is unable to add to a methyl methacrylate anion, though it copolymerizes with butadiene and isoprene. In copolymerizations initiated by w-butyllithium in toluene and in tetrahydrofuran at-78 °C, the following order of reactivity with methyl methacrylate anions was observed. In toluene the order is diphenylmethyl methacrylate > benzyl methacrylate > methyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > t-butyl methacrylate > trityl methacrylate > a,a -dimethyl-benzyl methacrylate. In tetrahydrofuran the order changes to trityl methacrylate > benzyl methacrylate > methyl methacrylate > diphenylmethyl methacrylate > ethyl methacrylate > a-methylbenzyl methacrylate > isopropyl methacrylate > a,a -dimethylbenzyl methacrylate > t-butyl methacrylate. 

Coordination to the metal has a number of profound effects on the arene (Figure 10.4) the ring becomes electrophilic and subject to nucleophilic attack, the acidity of the ring protons increases and the acidity of any benzylic protons also increases. In addition, the formation of benzylic cations is facilitated, as the metal can stabilize them. Finally, the metal provides huge steric bulk on one face of the arene. 

Dimerization of arylamine cation-radicals is a prominent example of the product s steric hindrance effects on the reaction course. Although stable in AN, the A,A-dimethyl-p-toluidine dimerizes if its uncharged counterpart is present in the solution (Goto et al. 2002, Oyama and Goto 2003). The cation-radical loses a proton, and the parent amine reacts as a base, accepting this proton. Coupling of benzylic radicals is an understandable result of the following reaction ... 

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