Nucleophile oxygen-containing

Alkyl halides are encountered less frequently than their oxygen-containing relatives alcohols and ethers, but some of the kinds of reactions they undergo—nucleophilic substitutions and eliminations—are encountered frequently. Thus, alkyl halide chemistry acts as a relatively simple model for many mechanistically similar but structurally more complex reactions found in biornolecules. We ll begin in this chapter with a look at how to name and prepare alkyl halides, and we ll see several of their reactions. Then in the following chapter, we ll make a detailed study of the substitution and elimination reactions of alkyl halides—two of the most important and well-studied reaction types in organic chemistry. 

The basicity of the carbon surface derives from the n electron decolization in graphite carbon layers and the antioxidant character of carbon forming oxygen-containing groups [180]. Delocalized n electrons are capable of nucleophilic attack giving Lewis basicity to the carbon [180,189]. 

Like the trichloromethyl peroxide radical, peroxothio compounds can perform even nucleophilic oxygenation of substrates that are inert to Oj" in aprotic solvents. For example, stilbene is not changed in dry benzene containing 18-crown 6-ether and KOj. In the presence of diphenylsul-fide, however, the interaction takes place and results in the formation of stilbene epoxide. According to Oae et al. (1981), stilbene initially gives PhCH(00 )CH Ph anion-radical adduct. Abstraction of O from the adduct leads to stilbene epoxide with 40% yield (Oae et al. 1981). 

Three years later, the same group showed that oxygen-containing nucleophiles can also be used (Scheme 35) [58], IV-Benzoylated aldimines 32 were treated with alcohols 92 in the presence of chiral BINOL phosphate R)-3m (5 mol%, R = 9-anthryl) to provide iV,0-aminals 93 in high yields (62-99%) and good enantiose-lectivities (65-95% ee). 

The rate of complex formation of dimethylsilylene with a variety of Lewis bases was found to be close to the diffusion limit in cyclohexane at room temperature.33,34 The results of Yamaji et al.34 indicate that the rate of this reaction is governed not so much by electronic factors such as the HOMO energy of the Lewis base as by steric hindrance around the heteroatom center. In contrast, Baggott et al.35 found a satisfying correlation between rate constants and ionization energies of the nucleophile for the reaction of dimethylsilylene with various oxygen-containing substrates in the gas phase. 

D is correct. Epoxides, oxygen-containing cyclic compounds, have much higher reactivity levels than other ethers. This is due to a highly strained three-member ring that can be opened by nucleophilic attack. Benzene is stabilized by electron delocalization, which is possible in aromatic compounds. 

The reaction is based upon the two components condensation between an aldehyde or ketone 6 (or their synthetic equivalents) and alcohol 95, which contains an allylsilane (or vinylsilane) moiety. The IMSC reaction is mediated by Lewis or Bronsted acids, which activate the carbonyl group of 6 towards nucleophilic attack. After addition of alcohol 95 on the activated carbonyl, the oxonium cation 96 is formed, which is intramolecularly captured by the pendant allylsilane function, leading to oxygen-containing rings 97 (Scheme 13.38). This process typically requires a stoichiometric (or more) amount of Lewis acid. 

In order to get a rapid and complete hydrolysis, an acid or a base catalyst may be used. In both cases the reaction occurs by a nucleophilic attack of the oxygen contained in water, to the silicon atom. Hydrolysis mechanisms were studied by Osterholz and Pohl,15 using alkyltrialkoxysilanes instead of tetra-alkoxysilanes. Both types of compounds are following the mechanism below. 

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