Elimination reaction fragments

In an elimination reaction, fragments of a molecule are removed and form a new molecule. Elimination involving a free radical mechanism operates in many pyrolytic reactions. First, an initiation occurs by pyrolytic cleavage, followed by... 

Recently the first examples of complexes between the four-membered amidinato-Group 13 metal(l) heterocycles and transition metal fragments were reported. Complexes of the type CpFe(CO)2[M(X) But(NR)2 ] (M = Al, Ga, In X = Cl, Br R = Pri, Gy) were formed in salt-elimination reactions between Na[CpFe(CO)2] and [But(NR)2]MX2. A series of complexes between the four-membered amidinato-Group 13 metal(l) heterocycles and Group 10 metal(O) fragments have been prepared according to Scheme 35. ... 

Photochemical elimination reactions include all those photoinduced reactions resulting in the loss of one or more fragments from the excited molecule. Loss of carbon monoxide from type I or a-cleavage of carbonyl compounds has been previously considered in Chapter 3. Other types of photoeliminations, to be discussed here, include loss of molecular nitrogen from azo, diazo, and azido compounds, loss of nitric oxide from organic nitrites, and loss of sulfur dioxide and other miscellaneous species. 

Two approaches have been used in the synthesis of these types of compounds. Small boron-phosphorus ring compounds can serve as building blocks, and addition and elimination reactions with other main group elements can then extend the cage structure (see Schemes 23 and 24, Section 12.12.6.4.5). Alternatively, an unsaturated carbenoid fragment can be added to the bicyclic fragment as illustrated in Scheme 31 <1998IC490>. 

The observed differences in the fragmentation pathways of the isomeric pairs were further enhanced under chemical ionization conditions. Thus, elimination reactions became even more pronounced in the o/t/ o-nitro-substiluted compounds than in the para isomers83. 

R. E. Gawley, The Beckmann reactions Rearrangements, elimination-additions, fragmentations, and rearrangement-cyclizations , in Organic Reactions, Vol. 35 (Ed. A. S. Kende), Wdey-Intersdence, London, 1988, pp. 1-415. 

Reactions with alcohols proceed similarly, but as the products cannot eliminate any fragment, formation of alkoxy-carbenes is observed (Equation 1.14) ... 

This unexpected result may be related to the increase in TOC on fraction G3 and may be further evidence of the polymerization phenomenon discussed earlier. However, this hypothesis must be carefully considered because of our limited knowledge of pyrolysis mechanisms. The possibility of phenol formation during the thermal fragmentation process from elimination reactions followed by cycliza-tion of poly conjugated chains has been suggested by Bracewell (22) and should be investigated. 

While carbon and oxygen radicals add irreversibly to carbon-carbon double bonds, the fragmentation reaction is rapid (and often reversible) for elements like tin, sulfur, selenium and the halogens (Scheme 36). This elimination reaction can be very useful in synthesis if the eliminated radical Y- can either directly or indirectly react with a radical precursor to propagate a chain. Given this prerequisite, an addition chain can be devised with either an allylic or a vinylic precursor, as illustrated in Scheme 37. Carbon radicals are generated by the direct or indirect reaction with Y- and are removed by the -elimination of Y-. Selectivity is determined by the concentration of the alkene acceptor and the rate of -elimination... 

Michael addition. Fluoride ion can catalyze Michael additions.4 In reactions that are slow owing to steric hindrance, or where elimination or fragmentation competes, use of high pressure5 may prove expeditious. 

Another type of elimination reaction favoured under plasma conditions is the decarboxylation. Carbocyclic acids easily lose carbon dioxide to form the parent hydrocarbons. In acid anhydrides decarboxylation is followed by a decar-bonylation. Cyclic or bicyclic anhydrides fragment forming unsaturated compounds, a reaction which has been studied with phthalie anhydride 24>. This anhydride decomposes to dehydrobenzene which, in the absence of other compounds, dimerizes, trimerizes or polymerizes. Orientation experiments indicated similar results for aliphatic acid anhydrides. 

H027o2 -Elimination Reactions and -Fragmentation of Oxyl Radicals... 

Most of the positive-ion fragments can be rationalized as having similar structures whether the charge-bearing species is a proton or an alkali metal cation, most commonly a sodium ion. However, while B ions derived from protonated molecules can be considered to have the structure of an oxonium ion (Fig. 29a), B ions from sodiated molecules are proposed to be produced by an elimination reaction which generates a C- l-C-2 double bond, with the sodium ion imparting the charge157 (Fig. 29b). 

R. E. Gawley, The Beckmann Reactions Rearrangements, Elimination-Additions, Fragmentations, and Rearrangement-Cyclizations, Org. React. 1988, 35, 1-420. 

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