Elimination reactions general features

A general cheletropic reaction is shown in Figure 12.2. This reaction involves the addition to, or extrusion from, a conjugated system of a group bound through a single atom. The reaction usually involves the elimination of simple stable molecules such as SO2, CO, or N2. The atom to which there were two a bonds carries away a pair of electrons, usually in a spn hybrid orbital. The addition of a carbene to a simple olefin to form a cyclopropane is also a cheletropic reaction which, as discussed in Chapter 14, is not predicted to be concerted. Cheletropic reactions incorporate features of both cycloaddition and electrocyclic reactions. 

This complex functions as a safer and more convenient source of H2SiCl2. General features of the synthesis are short reaction times, high yields, simple apparatus, and a routine workup that produces very pure product. The major advantage, of course, is the elimination of the need to handle dichlorosilane. A synthesis of SiH2Ph2 from [SiH2Cl2 teeda] is included to show the utility of this new reagent. 

Much less information is available on the deamination and related reactions over solid catalysts than on some other elimination reactions but it suffices for comprehension of the general features. 

In DNA, the carbohydrate part has no free OH groups. Yet, salient aspects have been obtained by studying the free-radical chemistry of carbohydrates (von Sonntag 1984). A general feature in carbohydrate free-radical chemistry is the water elimination of 1,2-dihydroxyalkyl radicals [reactions (31)-(35)]. 

Palladium(0)-catalysed coupling reactions of haloarenes with alkenes, leading to carbon-carbon bond formation between unsaturated species containing sp2-hybridised carbon atoms, follow a similar mechanistic scheme as already stated, the general features of the catalytic cycle involve an oxidative addition-alkene insertion-reductive elimination sequence. The reaction is initiated by the oxidative addition of electrophile to the zero-valent metal [86], The most widely used are diverse Pd(0) complexes, usually with weak donor ligands such as tertiary phosphines. A coordinatively unsaturated Pd(0) complex with a formally d° 14-electron structure has meanwhile been proven to be a catalytically active species. This complex is most often generated in situ [87-91],... 

A wide range of materials is included in this class. The common feature is the use of chain extension reactions to provide products with acceptable commercial properties. The chain extension reaction effectively reduces the actual number of chain ends, thereby eliminating the generally poor properties observed when very low-molecular-weight polymers are cross-linked. The chain extension step involves the reaction of a difunctional polymeric polyol with difunctional organic isocyanates to give the polyurethane ... 

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