Benzylic stabilization

In ion D, in which the phenyl group would be expected to be coplanar with the cationic center to maximize delocalization, the observed angle is 25-30°. This should permit effective benzylic stabilization. The planes of the cyclopropyl groups in both structures are at 85° to file plane of file trigonal carbon, in agreement with expectation for the bisected... 

The results are explained as indicating that the addition of triplet arylcarbenes to intramolecular double bonds is accelerated by factor of 300-800 relative to inter-molecular addition. The intramolecular addition reactions of singlet arylcarbenes exhibit much smaller rate enhancements. The most stable planar conformer of singlet (102) cannot interact with the n bond of an allyl group attached to the ortho position. Rotation about the bond connecting the divalent carbon to the ring must occur in order for an electrophilic approach to take place. This rotation will result in the loss of benzylic stabilization. In marked contrast, the first step of the triplet addition can take place with no rotation of the divalent carbon. 

From the kinetic data the authors also deduced that the magnitude of the allylic stabilization in the vinyIsilyl radical is less than 7 kcal mol"1. This can be compared to the benzylic stabilization energy of about 2 kcal mol 1 which was derived for the phenylsilyl radical previously studied191. 

An aqueous base developable negative resist for use in 193 nm lithography was developed by combining NBHFA and norbornene bearing a pendant vic-diol (Fig. 118) [352]. One problem in the design of 193 nm resists is that the benzylic stabilization effect cannot be utilized to drive the acidolysis reactions and two kinds of ketone can be formed in this case. 

With l-phenyl-l,3-butadiene, the addition of HCl is exclusively at the 3,4-double bond. This reflects the greater stability of this product, which retains styrene-type conjugation. Initial protonation at C(4) is favored by the fact that the resulting carbocation benefits from both allylic and benzylic stabilization. 

With aryl benzyl ketones, indanols can be formed.This is a particularly favorable case because of the benzylic stabilization of both radical sites. 

Horton et al. studied the FAB activation chemistry of a sterically open, Me2Si-bridged bis(diamido)-zirconium dibenzyl as well as tridentate diamide zirconium dibenzyl and dimethyl complexes. NMR investigations revealed y -anion coordination to Zr for the former (60) and y -benzyl stabilization for the latter (61). 

If the anion is well-separated from the cation, other types of interactions may be involved in cation stabilization. Coordination modes often observed include solvent (aromatic) (cf., 26, 45) 91.i43.i87.i88 neutral metallocene alkyl (cf., 24, 43, 53) 7.90.137.138.143 nd multihapto benzyl interactions (cf., 25, 27, 36, 48, 55, 61) 143-144.192.202.311 Benzylmetallocene precursors often form more stable cationic complexes after activation, especially with noncoordinating anion-based activators such as Ph3C+B(C6F5)4, as a result of 7 -benzyl stabilization of the cation (cf., 25, 27, 36, 48, 55,... 

Figure 10.7 (a) Loss of a stabilizing stereoelectronic effect (i.e. benzylic stabilization of the vinyl radical) imposes additional penalty on the 5-endo-dig cyclization. (b) The penalty can be removed via conformational constraints in the reactant. 

The basic features of the reaction may be summarized as follows (Scheme 3-92). Nickel 7t-allyl dimers are often used as pre-catalysts. Treatment with monodentate phosphines and Lewis acids likely generates a cationic nickel hydride species, which serves as the active catalyst for the reaction. Regioselective hydrometallation of the styrene generates intermediate 26, with the regiochemistry being driven by benzylic stabilization. Insertion of ethylene followed by p-hydride elimination produces the hydrovinylation product and regenerates the active cationic nickel hydride species. 

The kinetic stability of a radical is largely controlled by steric factors. When the radical center is crowded, the radical becomes less reactive and persists longer under normal conditions (it has a longer life-time). Aromatic compounds that can form allylic radicals show similar benzylic stabilization. If the radical center is sterically crowded by bulky tertiary butyl substituents, the allylic radical intermediates formed by hydrogen transfer have kinetic stability that imparts important antioxidant properties (see Chapter 9). Thus, when phenolic compounds contain three bulky tertiary butyl substituents, they form persistent radicals after hydrogen donation and inhibit lipid oxidation by intermpting the propagation of free radicals (see Chapter 9). 

Elimination of Benzylic Phenyl Thioethers. That C-S bond activation by CuOTf is not limited to substrates that can generate sulfur-stabilized carbocation intermediates is illustrated by a C-C connective synthesis of trans-stilbene (eq 56). The elimination of thiophenol under mild conditions is favored by benzylic stabilization of a carbocation intermediate or an E2 transition state with substantial carbocationic character. 

As shown in the following mechanism, reaction is initiated by heterolytic cleavage of the carbon-chlorine bond to form a 2° carbocation, which rearranges to a considerably more stable 3° carbocation by shift of a hydrogen with its pair of electrons (a hydride ion) from the adjacent benzylic carbon. Note that the rearranged carbocation is not only tertiary (hyperconjugation stabilization) but also benzylic (stabilization by resonance delocalization). 

We know that a phenyl grou p stabilizes an adjacent radical due to resonance (benzyl, for example), and three phenyls should provide more stabilization than two. This is indeed the case trityl radical is certainly stabilized. However, it is not possible for all three aromatic rings of trityl to be in perfect conjugation with the radical center. Simple model building will convince you that a fully planar structure would experience severe steric clashes. The trityl radical adopts the nonplanar, propeller-like structure shown in Figure 2.5. In this structure, each ring stabilizes the radical considerably, but the overall stabilization is not three times the benzyl stabilization. 

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