Elimination intramolecular

The above reaction is a convincing example of an intermolecular hydrogen abstraction leading essentially to the same result as obtained in the pyrolysis of alkyl-substituted thiirane oxides through an intramolecular /(-elimination of hydrogen. 

Fig. 29 EM-profiles for competing intramolecular elimination and substitution from o- 0C6H40(CH2) 4Br [1] in 99% Me2SO as a function of the size n of the cyclic transition states. The point for the elimination reaction where n = 6 is an estimate for the upper reactivity limit. (Reproduced with permission from Dalla Corte/ al., 1983)...

The intramolecular elimination of water from alcohols, which constitutes the usual method for the preparation of olefines, does not proceed so simply, when concentrated acids are used, as the equation... 

When 1,2-diols are subjected to the same reaction conditions required for the formation of sulphonic esters, oxiranes are produced [27]. Presumably, the mono ester is initially formed and, under the basic conditions, intramolecular elimination occurs to produce the oxirane. Partial hydrolysis and ring-closure of a,p-di(tosyloxy) compounds under basic phase-transfer catalytic conditions provides a convenient route to carbohydrate oxiranes [e.g. 28, 29]. Oxiranes have been produced by an analogous method via carbonate esters from partially protected carbohydrates [30],... 

By Dehydration of a-Diketone Hydrazone Oximes Another general method of synthesis of 2i/-triazoles involves intramolecular elimination of water from adjacent hydrazone and oxime functions (Scheme 32). The usual reagent is acetic anhydride, but others, including phosphorus pentachloride and urea, have been used. 

It should be noted that, although it is not important in this particular case, there is an additional path available for some alkoxy radicals formed in the oxidation of certain halogenated organics, that is, the intramolecular elimination of HC1 from a-monochloro-alkoxy radicals. For example, the alkoxy radical CH2C1CHC10 both reacts with 02 and eliminates HC1,... 

A 1,4-addition mechanism was postulated50 for the addition of cyclo-hexylamine to 72b in the presence of nitromethane. An intramolecular elimination of bromine afforded the (unisolated) epimino-ketonucleo-side 100, which underwent further attack by nitromethane, to give 7-(3,4-cyclohexylamino-3,4-dideoxy-2-C-nitromethyl-a-L-allopyrano-syl)theophyl ine (101). 

Recently, an interesting method involving an intramolecularly assisted dchydrolluorination, mediated by (benzotriazol-l-yloxy)tris(dimethylamino)phosphonium hexafluorophosphate (BOP), was described.37 The reaction proceeds via a hydrobenzotriazole ester intermediate followed by concerted intramolecular elimination of hydrogen fluoride. 

Reaction XI. Intramolecular Elimination of Water from certain Molecules. (A., 227, 242.)—When phenyl isocrotonic acid is heated, water is eliminated and a-naphthol is formed. This synthesis, which is of theoretical interest, was discovered by Fittig. It is one of the proofs of the structure of naphthalene. 

Photocyclization arising by intramolecular elimination of HC1, HBr, or HI has been extensively used in the synthesis of nitrogen-containing heterocycles, but much less so in oxygen and sulfur systems. The mechanism of these transformations is not always clear some proceed by way of an initial carbon-halogen bond homolysis, whereas others involve cyclization and... 

A study of the decomposition of /f-hydroxy-/V-chloroamines in aqueous medium has established that pre-equilibrium formation of the conjugate alcoholate is a prerequisite feature of the competing fragmentation and intramolecular elimination paths (Scheme 13).98 A very high effective molarity (EM = 2 x 105 M) has been estimated for the intramolecular process, which cannot occur in the case of (/V-chloro)butylethanolaminc. For reaction of (A-c h I o ro )ct hy I cth an o I am i nc kmln/k g = 6.1 and the solvent isotope effect ( oh- /koo- )obs = 0.68 is consistent with pre-equilibrium deprotonation followed by a unimolecular reaction in which there is no participation by solvent. 

Hofmann s Rule is valid for all intramolecular eliminations and for the Hofmann Elimination. Most bimolecular eliminations will follow Saytzeff s Rule. 

As a further stereoselective organic synthesis [40-47] using reactive sp2 carbon-centered radicals, eq. 10.23 shows the preparation of chiral 4-te/7-butylcyclohexene (49) from the optically pure o-bromophenyl sulfoxide (48) through 1,5-H shift by sp2 carbon-centered radical, followed by (3-elimination. This reaction looks like a thermal concerted intramolecular elimination reaction (Ei). 

The relatively stable intermediate C12C=S=0 was isolated (44). The main difference between these two reactions is the HCI elimination from the postulated X3CSOH. If X = F an intermolecular reaction takes place and for X = Cl an intramolecular elimination occurs. 

There is another, complementary version of the Peterson reaction that uses base to promote the elimination. The starting materials are the same as for the acid-promoted Peterson reaction. When base (such as sodium hydride or potassium hydride) is added, the hydroxyl group is deprotonated, and the oxyanion attacks the silicon atom intramolecularly. Elimination takes place this time via a syn-periplanar transition state—it has to because the oxygen and the silicon are now bonded together, and it is the strength of this bond that drives the elimination forward. 

The intramolecular elimination of water from 2 with phosphorus pentoxide22 yields 1,2,4-trioxolans (6)22-24 (Rieche s ozonide synthesis without ozone 22). 

An alternative elimination in solution, i.e., nitrile formation from the 2,4-dimethoxy-syn-benzaldoxime pseudosaccharyl ether was found to be first order.43 The modest influence of the solvent on the rate of reaction was interpreted254 in terms of its stabilizing effect on an all planar conformation.14 If the mechanism is similar to type (ii) a coplanar transition state would be suited for an intramolecular elimination reaction. 

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