Alkenes prototropic rearrangements

There exist early examples of this transformation [507, 508], but due to the symmetric structure of the alkene part, only isotope labeling, etc., allowed the exclusion of a prototropic rearrangement. Furthermore, due to the high reaction temperatures of 340 °C and above, several different products are formed. A low-temperature version (77 K) of this reaction via the radical cation has been reported [509]. The chirality transfer has been studied and a detailed mechanistic investigation has been conducted [510] typical experiments in that context were the reactions of substrates such as 155 and 157 (Scheme 1.70). 

This reaction, for which the term prototropic rearrangement is sometimes used, is an example of electrophihc substitution with accompanying allyhc rearrangement. The mechanism involves abstraction by a base to give a resonance-stabilized carbanion, which then combines with a proton at the position that will give the more stable alkene ... 

As mentioned before, allenes can be formed by prototropic rearrangement of alkynes or, if an appropriate hydrogen is present in the allylic position, by direct elimination. The bromovinyl ether yields, in a trans elimination, the alkyne ether (Scheme 39), but the other isomer, where trans elimination is not possible, gives both the alkynyl ether and the allenyl ether (Scheme 40). This corresponds to the problem of Hofmann and Zaitsev orientation in alkene synthesis. 

Scheme 7.19). Prototropic shift of the initial adduct to produce ROCHCl2 and, subsequently, the formate ester is a less favourable pathway. Alternatively, the carbon monoxide-separated ion-pair can lose a proton leading to an alkene, or cycloadducts derived from further reaction with the carbene. The formation of rearranged products from the reaction of 1 -hydroxymethyladamantane suggests that a relatively unencumbered carbenium cation can also be generated, which leads to a Nametkin rearrangement of the system [4]. 

The reaction of the dimethyl-derivative (27) with butoxide ion might be expected to produce the chlorocyclopropene (28) however, in practice two eliminations occur to produce (31) and the carbene (30), which can be trapped by an added alkene. Both products may be derived from (28), by a 1,4- or a formal 1,2-elimination respectively a study using a 14C-label at C-l of (27) showed that the carbene (30) was formed with the label exclusively at C-l, suggesting elimination via (29)32). However, in a related study, the isolated cyclopropene (28) labelled with 12C at C-l has been shown to react with methyl lithium to produce the carbene (30) labelled only at C-2 this suggests either that the reaction of (28) with butoxide follows a completely different course to that with methyl lithium, or that (28) is not involved in the reaction of (27) with base33). In a similar reaction the dichloride (32) has been shown to react with t-butoxide in DMSO to produce the allene (33) the product may be explained in terms of initial elimination to produce (34), followed either by rearrangement to the alkyne (35) and then elimination or by direct 1,4-elimination as in (36), followed in either case by a prototropic shift. Whatever the mechanism, a 12C-label at Ca in (32) is found at Ca in (33) 33). 

A number of interesting and useful organic reactions involve isomerizations of substances having one or more carbon-carbon double bonds. This chapter deals with the kinetics of reactions of alkenes, cycloalkenes and substituted alkenes which involve migration of carbon-carbon double bonds, with or without structural alteration of the carbon skeleton of the starting materials. These reactions include prototropic and anionotropic rearrangements, several concerted unimolecular isomerizations such as the Cope and Claisen rearrangements, and a number of non-concerted thermal isomerization reactions. 

Volume 9 deals with the majority of addition and elimination reactions involving aliphatic compounds. Chapter 1 covers electrophilic addition processes, mainly of water, acids and halogens to olefins and acetylenes, and Chapter 2 the addition of unsaturated compounds to each other (the Diels-Alder reaction and other cycloadditions). This is followed by a full discussion of a-, y- and S-eliminations (mainly olefin and alkyne forming) and fragmentation reactions. In Chapter 4 carbene and carbenoid reactions, and in Chapter 5 alkene isomerisation (including prototropic and anionotropic, and Cope and Claisen rearrangements), are discussed. 

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