Reactions rearrangements involving

Adenosylcobalamin (coenzyme B 2) is required in a number of rearrangement reactions that occurring in humans is the methylmalonyl-Co A mutase-mediated conversion of (R)-methylmalonyl-Co A (6) to succinjl-CoA (7) (eq. 1). The mechanism of this reaction is poorly understood, although probably free radical in nature (29). The reaction is involved in the cataboHsm of valine and isoleucine. In bacterial systems, adenosylcobalamin drives many 1,2-migrations of the type exemplified by equation 1 (30). 

All the examples quoted in this section concerning fragmentations or rearrangements involve photochemistry. An interesting thermal reaction has been described (72TL2235) in which the pyrolysis of indazole between 700 and 800 °C leads to a mixture of (197) and (198 Scheme 15). A mechanism involving the 3// tautomer and the carbene seems reasonable. 

Special reactions of side-chain substituents rearrangements involving three-atom side chains... 

This particular polymer is a fibre-forming material (Perlon U). Although in many respects this reaction resembles the formation of polyesters and polyamides it is not a condensation reaction but involves a transfer of hydrogen atoms and thus may be considered as an example of rearrangement polymerisation. 

In radical reactions not involving bromine or chlorine on the substrate, rearrangements are much rarer One example is the fluorination of di-tert butyl ketone which produces perfluormated / rt-buty isobutyl ketone [J5] Although isolated yields are poor only the rearranged ketone could be isolated This is perhaps only the second example of a 1,2-acyl shift Low fluorine substrate ratios show that this rearrangement occurs after monofluorination... 

Silver trifluoroacetate is a suitable catalyst for various cationic rearrangements involving multiple carbon-carbon bonds [49 5(1] In the presence of silver trifluoroacetate, 2 propynyl acetates rearrange to the butadienyl acetates to give dienes that are useful in Diels-Alder reactions [49] (equation 22)... 

The structures in the (1,0) and (0,1) comers are not necessarily stable species, they may correspond to hypothetical structures. In the Cope rearrangement it appears that the reaction only involves a single TS, independently of the number and nature of substituents. The reaction path may change from B A C depending on the system, but there are no intermediates along the reaction coordinate. 

The Cornforth rearrangement involves the thermal interconversion of 4-carbonyl substituted oxazoles, with exchange between the C-C-O side-chain and the C-C-O fragment of the oxazole ring. These reactions generally involve compounds where a heteroatom (-OR, -SR, -Cl) is attached to the 5-position (R2) of the starting oxazole. 

Tile 1,3-dithietaiie 11a, when heated, rearranged quantitatively into a disulfide 111. Tlie reaction may involve a dithietane 112 as an intermediate, although a [3,3] sigmatropic rearrangement of 113 could equally explain the formation of 111 (90JOC2421). 

Many variations of the reaction can be carried out, including halogenation, nitration, and sulfonation. Friedel-Crafts alkylation and acylation reactions, which involve reaction of an aromatic ling with carbocation electrophiles, are particularly useful. They are limited, however, by the fact that the aromatic ring must be at least as reactive as a halobenzene. In addition, polyalkylation and carbocation rearrangements often occur in Friedel-Crafts alkylation. 

The Curtius rearrangement, like the Hofmann rearrangement, involve migration of an -R group from the G-O carbon atom to the neighboring nitro gen with simultaneous loss of a leaving group. The reaction takes place on heat ing an acyl azide that is itself prepared by nucleophilic acyl substitution of m acid chloride. 

Isoborneol (Problem 27.37) is converted into camphene on treatment with dilute sulfuric acid. Propose a mechanism for the reaction, which involves a carbocation rearrangement. 

Because a [1,5] sigmatropic rearrangement involves three electron pairs (two ir bonds and one cr bond), the orbital-symmetry rules in Table 30.3 predict a suprafacial reaction. In fact, the 1,5] suprafacial shift of a hydrogen atom across... 

The following thermal rearrangement involves two pericyclic reactions in sequence. Identify them, and propose a mechanism to account for the observed result. 

On the basis of all these experiments various mechanisms have at some stage been advanced for the Fries rearrangement involving the free acylium ion or as a tightly bound ion pair, Ji-complexes and cyclic intermediates. It is clearly impossible to reconcile all the experimental data by one reaction mechanism. It is probable that many such mechanisms are possible, each one operative under a certain set of conditions. 

Treatment of (—)-(S)-276 with allyl Grignard reagents gives optically active allylic sulphoxides 288. This reaction, however, involves an allylic rearrangement via transition state 289 as evidenced by Mislow and his collaborators362 (equation 160). 

An exception to the above are fatty acid methyl esters, which, due to the reaction mechanism involving molecular rearrangements with excess S03, have to be sulfonated at a slightly higher mole ratio of S03 to methyl esters (namely, 1.15-1.20/L). Outside the reaction tubes, in the reactor jacket, cooling water is circulated to control the liquid-film temperature and removing the reaction heat. 

Rearrangement of polyhalobenzenes can also be catalyzed by very strong bases for example, 1,2,4-tribromobenzene is converted to 1,3,5-tribromobenzene by treatment with PhNHK." This reaction, which involves aryl carbanion intermediates (Sgl mechanism), has been called the halogen dance. 

The Curtius rearrangement involves the pyrolysis of acyl azides to yield isocy-anates. " The reaction gives good yields of isocyanates, since no water is present to... 

In order to understand the regiochemistry of HX additions to alkenes, we focused our attention on the intermediate carbocation. We argued that the reaction would proceed via the more stable carbocation. This all-important principle will also help explain why some reactions will involve a rearrangement. For example, consider the following reaction ... 

Clearly, we must be able to predict when to expect a carbocation rearrangement. There are two common ways for a carbocation to rearrange either through a hydride shift or through a methyl shift. Your textbook will have examples of each. Carbocation rearrangements are possible for any reaction that involves an intermediate carbocation (not just for addition of HX across an alkene). Later in this chapter, we will see other addition reactions that also proceed through carbocation intermediates. In those cases, you will be expected to know that there will be a possibility for carbocation rearrangements. 

Gentisate is formed by a strain of Bacillus sp. in an unusual rearrangement from 4-hydroxybenzoate (Crawford 1976) that is formally analogous to the formation of 2,5-dihydroxyphenylacetate from 4-hydroxyphenylacetate by Pseudomonas acidovorans (Hareland et al. 1975). Similarly, the metabolism of 4-hydroxybenzoate by the archaeon Haloarcula sp. strain D1 involves the formation of 2,5-dihydroxybenzoate (Fairley et al. 2002). All these reactions putatively involve an NIH shift. 

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