Benzylic stabilization, ketone

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

Uber 7 bildet sich thermisch das Keton 8. Der Benzyl-phenolather 9 erweist sich jedoch als stabil, da offenbar die gleichzeitige Stoning zweier aromatischer Systeme nicht erzwungen wird. 

Because of its high crystallinity, the hnmopolymer (PVDV) is insoluble in most solvents at room temperature. However, since the regularity of repeating units in the chain is decreased by copolymerization, Saran is soluble in cyclic ethers and aromatic ketones. This copolymer (100 g) is plaslicized by die addition of a-melhyl-benzyl ether (5 g), stabilized against ultraviolet light degradation by 5-chloro-2-hydroxybenzophenone (2.0 g) and heat stabilized by phenoxypropylene oxide (2.0 g). 

Which of the two R—( ) bonds cleave in the Norrish Type I reaction of an unsymmetrical ketone depends on the relative stabilities of the two possible R-radicals. Diaryl ketones do not react at all and 53 cleaves to give a benzyl rather than an aryl radical. From ethyl methyl ketone, an ethyl radical is formed considerably more frequently than a methyl radical.86... 

The neutral 1,4- and 1,2-quinone methides react as Michael acceptors. However, the reactivity of these quinone methides is substantially different from that of simple Michael acceptors. The 1,6-addition of protonated nucleophiles NuH to simple Michael acceptors results in a small decrease in the stabilization of product by the two conjugated 7T-orbitals, compared to the more extended three conjugated 7T-orbitals of reactant. However, the favorable ketonization of the initial enol product (Scheme 1) confers a substantial thermodynamic driving force to nucleophile addition. By comparison, the 1,6-addition of NuH to a 1,4-quinone methide results in a large increase in the -stabilization energy due to the formation of a fully aromatic ring (Scheme 2A). This aromatic stabilization is present to a smaller extent at the reactant quinone methide, where it is represented as the contributing zwitterionic valence bond structure for the 4-0 -substituted benzyl carbocation (Scheme 1). The ketonization of the product phenol (Scheme 2B) is unfavorable by ca. 19 kcal/mol.1,2... 

Under similar conditions, even benzylic C-H bonds of some hydrocarbons (xanthene and fluorene) are converted to the ketones (e.g., fluoren-9-one). Notwithstanding the parallel activities of the two catalysts, different mechanisms were tentatively proposed (277-279). Thus, for R.UC0AI-LDH-CO3, it was postulated that the presence of Co in the structure facilitated the formation of high-valent Ru(V)=0 species. In contrast, for the Ru-HAP catalyst, it was proposed that after coordination of the reactant to Ru as an alcoholate, the carbonyl compound was eliminated, leaving a Ru-H compound, which in a next step is reoxidized by O2. The basic nature of the HAP (or LDH) support may actually favor the latter route, with formation of an alcoholate. Filtration tests and elemental analyses confirm the stability of the supported species in both catalysts. 

Shi and coworkers found that vinyl acetates 68 are viable acceptors in addition reactions of alkylarenes 67 catalyzed by 10 mol% FeCl2 in the presence of di-tert-butyl peroxide (Fig. 15) [124]. (S-Branched ketones 69 were isolated in 13-94% yield. The reaction proceeded with best yields when the vinyl acetate 68 was more electron deficient, but both donor- and acceptor-substituted 1-arylvinyl acetates underwent the addition reaction. These reactivity patterns and the observation of dibenzyls as side products support a radical mechanism, which starts with a Fenton process as described in Fig. 14. Hydrogen abstraction from 67 forms a benzylic radical, which stabilizes by addition to 68. SET oxidation of the resulting electron-rich a-acyloxy radical by the oxidized iron species leads to reduced iron catalyst and a carbocation, which stabilizes to 69 by acyl transfer to ferf-butanol. However, a second SET oxidation of the benzylic radical to a benzylic cation prior to addition followed by a polar addition to 68 cannot be excluded completely for the most electron-rich substrates. 

A particular case of oxidative functionalization is the formation of bibenzyls. This occurs with medium to good yield when the benzyl radical is highly stabilized (e.g., benzhydryl, xanthyl) using aromatic ketones as the sensitizers [19-21]. Better results are obtained with titanium dioxide, particularly when starting form benzylsilanes, in view of the easy fragmentation of the radical cation [239]. 

The similar tertiary benzylic a-alkoxyorganolithium 108 has some configurational stability at -78 °C in THF, but it is chemically unstable and after 2 h has formed the ketone 109.54 This ketone is formed in 20% ee, but without knowledge of the timescale of the dimerisation it is not possible to assess the half-life for racemisation of 108 in THF... 

Aromatic aldehydes and aromatic ketones also can be reduced to hydrocarbons in a completely different manner, namely via the so-called ionic hydrogenation followed by an ionic hydrogenolysis. This kind of reduction is possible only if it can proceed via resonance-stabilized cationic intermediates. This resonance stabilization is readily achieved in a benzylic position, and it is therefore advantageous to employ aromatic carbonyl compounds in this kind of reduction. The carboxonium ion A, formed... 

Selenium-stabilized carbanions behave as excellent nucleophiles and react with primary alkyl bromides or iodides, allylic and benzylic bromides, epoxides, oxeta-nes, disulfides, trialkylsilyl chlorides, aldehydes, ketones, carbon dioxide, dime-thylformamide, acid chlorides or alkyl chloroformates. With conjugated enones, in the presence of HMPA as cosolvent, the 1,4-addition product is essentially obtained. 

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