Allylic effect

Oxime ethers derived from 2-pyridinecarboxaldehyde and glyoxylic acid can be allylated effectively in water with functionalized allylic bromides promoted by indium (Scheme 67). When the metal is positioned in close proximity to flanking heteroatomic centers, chelation by indium is operative and affects both reactivity and stereochemistry... 

Figure 4.2.14 The allylic effect. Both conformers have about the same energy, because the Jt-electrons of the enolic double bond attract the 7C-electrons of the allylic oxygen atom.

Consideration of the anomeric equilibria of various acetylated, 2,3-un saturated, pyranoid sugars led to the conclusion that polar, allylic substituents favor the quasi-axial orientation (0.8 kcal.mole for the acetoxyl group). Such an allylic effect was found to exert significant control over the conformational tendency and configurational equilibria of unsaturated, pyranoid sugar derivatives. Thus, tri-O-acetyl-2-hydroxy-D-xylal (41) adopts the H (d) conformation. In... 

Solid magnesium must be absent to avoid the formation of diallyl via allyl magnesium bromide the insertion of a short plug of glass wool effectively removes any finely divided magnesium. 

Benzoic acid and naphthoic acid are formed by the oxidative carbonylation by use of Pd(OAc)2 in AcOH. t-Bu02H and allyl chloride are used as reoxidants. Addition of phenanthroline gives a favorable effect[360], Furan and thiophene are also carbonylated selectively at the 2-position[361,362]. fndole-3-carboxylic acid is prepared by the carboxylation of 1-acetylindole using Pd(OAc)2 and peroxodisulfate (Na2S208)[362aj. Benzoic acid derivatives are obtained by the reaction of benzene derivatives with sodium palladium mal-onate in refluxing AcOH[363]. 

The carbopalladation of allylamine with malonate affords the chelating complex 510, which undergoes insertion of methyl vinyl ketone to form the amino enone 511[463]. The allylic sulfide 512 has the same chelating effect to give the five-membered complex 513 by carbopalladation[463.464]. 

Several Pd(0) complexes are effective catalysts of a variety of reactions, and these catalytic reactions are particularly useful because they are catalytic without adding other oxidants and proceed with catalytic amounts of expensive Pd compounds. These reactions are treated in this chapter. Among many substrates used for the catalytic reactions, organic halides and allylic esters are two of the most widely used, and they undergo facile oxidative additions to Pd(0) to form complexes which have o-Pd—C bonds. These intermediate complexes undergo several different transformations. Regeneration of Pd(0) species in the final step makes the reaction catalytic. These reactions of organic halides except allylic halides are treated in Section 1 and the reactions of various allylic compounds are surveyed in Section 2. Catalytic reactions of dienes, alkynes. and alkenes are treated in other sections. These reactions offer unique methods for carbon-carbon bond formation, which are impossible by other means. 

The ligand effect seems to depend on the substrates. Treatment of the prostaglandin precursor 73 with Pd(Ph3P)4 produces only the 0-allylated product 74. The use of dppe effects a [1,3] rearrangement to produce the cyclopen ta-none 75(55]. Usually a five-membered ring, rather than seven-membered, is predominantly formed. The exceptionally exclusive formation of seven-membered ring compound 77 from 76 is explained by the inductive effect of an oxygen adjacent to the allyl system in the intermediate complex[56]. 

Although allylic brommations and chlormations offer a method for attaching a reactive functional group to a hydrocarbon framework we need to be aware of two important limitations For allylic halogenation to be effective m a particular synthesis... 

This chapter focused on the effect of a carbon-carbon double bond as a stabilizing substituent on a positively charged carbon m an allylic carbocation, on a carbon bearing... 

Me3SiCH2CH=CH2i TsOH, CH3CN, 70-80°, 1-2 h, 90-95% yield. This silylating reagent is stable to moisture. Allylsilanes can be used to protect alcohols, phenols, and carboxylic acids there is no reaction with thiophenol except when CF3S03H is used as a catalyst. The method is also applicable to the formation of r-butyldimethylsilyl derivatives the silyl ether of cyclohexanol was prepared in 95% yield from allyl-/-butyldi-methylsilane. Iodine, bromine, trimethylsilyl bromide, and trimethylsilyl iodide have also been used as catalysts. Nafion-H has been shown to be an effective catalyst. 

Allyl ethers can be prepared by reaction of a phenol and the allyl bromide in the presence of base. Several reagents have been used to effect their cleavage ... 

The allyl group was used to protect the nitrogen in a /3-lactam synthesis, but was removed in a four-step sequence. Whether a transition-metal-catalyzed isomerization to the enamide followed by hydrolysis is an effective cleavage procedure remains to be tested and warrants further study. ... 

Vulcanisation can be effected by diamines, polyamines and lead compounds such as lead oxides and basic lead phosphite. The homopolymer vulcanisate is similar to butyl rubber in such characteristics as low air permeability, low resilience, excellent ozone resistance, good heat resistance and good weathering resistance. In addition the polyepichlorohydrins have good flame resistance. The copolymers have more resilience and lower brittle points but air impermeability and oil resistance are not so good. The inclusion of allyl glycidyl ether in the polymerisation recipe produces a sulphur-curable elastomer primarily of interest because of its better resistance to sour gas than conventional epichlorhydrin rubbers. 

Frontier orbital theory also provides the basic framework for analysis of the effect that the symmetiy of orbitals has upon reactivity. One of the basic tenets of MO theory is that the symmetries of two orbitals must match to permit a strong interaction between them. This symmetry requirement, when used in the context of frontier orbital theory, can be a very powerful tool for predicting reactivity. As an example, let us examine the approach of an allyl cation and an ethylene molecule and ask whether the following reaction is likely to occur. 

Any structural effect which reduces the electron deficiency at the tricoordinate carbon will have flie effect of stabilizing the caibocation. Allyl cations are stabilized by delocalization involving the adjacent double bond. 

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