Allyhc hydrogens

Protons on carbons adjacent to a carbonyl group are deshielded slightly more than allyhc hydrogens... 

Isoprene is highly reactive both as a diene and through its allyhc hydrogens, and its reactions are similar to those of butadiene (qv) (8). Apart from polymerisation, the most widely investigated isoprene reactions are the formation of six-membered rings by the Diels-Alder reaction ... 

Reaction Mechanism. High temperature vapor-phase chlorination of propylene [115-07-17 is a free-radical mechanism in which substitution of an allyhc hydrogen is favored over addition of chlorine to the double bond. Abstraction of allyhc hydrogen is especially favored since the allyl radical intermediate is stabilized by resonance between two symmetrical stmctures, both of which lead to allyl chloride. 

Vulcanization oi cuting is accomplished via chemical cioss-linking reactions involving allyhc hydrogen or halogen sites along the polymer backbone to... 

The recent development of using polymerizable surfactants in microemulsion polymerizations has enabled the production of transparent sohd polymers with some nanostructure. Randomly distributed bicontinuous nanostructures of water channels and polymer domains in sohd polymers can be readily obtained from the polymerization of bicontinuous microemulsions consisting of various types of vinyl monomers and polymerizable surfactants with no allyhc hydrogen. 

The presence of acrolein for both Ag catalysts indicates that still the double bond may he attacked first upon adsorption on the catalyst instead of the activation of the allyhc hydrogens. 

Figure 6.10. A representation of a hydrocarbon fragment with vinylic and allyhc hydrogens. The localized allylic C-H bond is shown as coplanar with the locahzed n bond to suggest partial orbital overlap. The representations were created using the MOViewer in WebMO version 6.0.002p.

Only a few reactions belong to the class of group transfer reactions. Most common is the ene reaction, where allyhc hydrogens are transferred to an electrophilic alkene, referred as an enophile. For example,... 

First, the attack of hydroxide creates a double bond on VDF units in the VDF-HFP diad. Then, a fluoride ion rearrangement of the initial double bond occurs. The resulting allyhc hydrogen is abstracted by fluoride, followed by an ehmination of a second fluoride. So, a bifluoride and a formally conjugated non-coplanar diene are formed. Then, a nucleophihc attack by the hydroxide on the diene forms an enone and a subsequent attack of the fluoride ion onto the highly acidic hydrogen of the tertiary carbon atom. The final product is the dienone [32,33]. 

Two types of NMR absorptions are typically found in alkenes vinyl absorptions due to protons directly attached to the double bond (4.5-6.5 ppm) and allylic absorptions due to protons located on a carbon atom adjacent to the double bond (1.6-2.6 ppm). Both types of hydrogens are deshielded due to the anisotropic field of the r electrons in the double bond. The effect is smaller for the allyhc hydrogens because they are more distant from the double bond. A spectrum of 2-methyl-1-pentene is shown in Figure 3.38. Note the vinyl hydrogens at 4.7 ppm and the allyhc methyl group at 1.7 ppm. 

The sphtting patterns of both vinyl and aUylic hydrogens can be quite complex due to the fact that the hydrogens attached to a double bond are rarely equivalent and to the additional complication that allyhc hydrogens can couple to ah of the hydrogens on a double bond, causing additional splittings. These situations are discussed in Chapter 5, Sections 5.8-5.9. 

Allylic compounds (CH2=CHCH2X) are usually reluctant to homopolymerization, due to the activation of the allyhc hydrogen atom towards abstraction. The generated allylic radical is highly stabilized by both the substituent X and by delocalization of the free electrons into the double bond. These radicals add to monomer very slowly and perform side reactions that in turn lead to retardation. 

Scheme 10.8 outlines the application of rhodium-catalyzed allyhc amination to the preparation of (il)-homophenylalanine (J )-38, a component of numerous biologically active agents [36]. The enantiospecific rhodium-catalyzed allylic amination of (l )-35 with the lithium anion of N-benzyl-2-nitrobenzenesulfonamide furmshed aUylamine (R)-36 in 87% yield (2° 1° = 55 1 >99% cee) [37]. The N-2-nitrobenzenesulfonamide was employed to facilitate its removal under mild reaction conditions. Hence, oxidative cleavage of the alkene (R)-36 followed by deprotection furnished the amino ester R)-37 [37, 38]. Hydrogenation of the hydrochloride salt of (l )-37 followed by acid-catalyzed hydrolysis of the ester afforded (i )-homophenylalanine (R)-3S in 97% overall yield. 

Hexafluoroacetone and hydrogen peroxide in buffered aqueous solution epoxidize alkenes and allyhc alcohols.81 82 A/yV-Dialkylpiperidin-4-one salts are also good catalysts for epoxidation.83 84 The quaternary nitrogen enhances the reactivity of the ketone toward nucleophilic addition and also makes the dioxirane intermediate more reactive. 

Amide-Based Sulfonic Acids. The most important amide-based sulfonic acids are the alkenylamidoalkanesulfonic acids. These materials have been extensively described in the literature. A variety of examples are given in Table 5. Acrylamidoalkanesulfonic acids are typically prepared using technology originally disclosed by Lubrizol Corporation in 1970 (80). The chemistry involves an initial reaction of an olefin, which contains at least one allyhc proton, with an acyl hydrogen sulfate source, to produce a sulfonated intermediate. This intermediate subsequendy reacts with water, acrylonitrile, and sulfuric acid. 

This allyhc oxidation can be suppressed by working with the hpophihc catalyst C. Hydrophobic olefins such as cydooctene are expected to interact favorably with the aromatic p-tosylate on the peroxoW-LDH surface. The transfer of the electrophihc oxygen to the olefin then takes place in an essentially hydrophobic compartment of the reaction system, as in the Venturello-Ishii reactions. In such an environment the olefin is enriched with respect to hydrogen peroxide and this improves the effidency of the oxidant use. Another potential advantage of our system C is that there is no need for a chlorinated solvent to constitute a second, apolcir hquid phase. 

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