Other Rearrangement Reactions

The rearrangements 67 — 70, 71 — 72 and 74 — 75 include the transformation of conjugated dienes to cumulenes. Nevertheless, these reactions take place with very high yields in some cases, because either an irreversible step of hydrolysis such as 69 — 70 is involved or the very exothermic transformation from cyanates to isocyanates is used. Comparison of the energies, calculated by ab initio methods [121], shows that, for example, the energy of methyl isocyanate is lower than that of methyl cyanate by 26.8 kcal mol-1 and that of vinyl isocyanate is lower than that of vinyl cyanate by 28.1 kcal mol-1. 

The photolysis of the furan derivatives 78 yielded the butadienals 79 as the main products [123], Further isomerizations leading to allenic esters used the radiation of a cyclopropene-1 -carboxylic acid ester [124] or applied flash vacuum pyrolysis to 3 -ethoxy cyclobut- 2-en-l-one[125]. 

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This radical mechanism can be exploited for cascade catalysis if the rate of radical cychzation (or other rearrangement reactions) for R " is significantly faster than the rate of R attack on nickel. Cardenas and coworkers [53] have proved this concept... 

In this part, some other rearrangement reactions displaying certain features that are either not discussed or not emphasized in the previous parts will be presented, and they include the following. 

Table 8.4. Examples of other rearrangement reactions (continued).

Other Rearrangement Reactions.—In addition to the above-mentioned ring expansion and contraction reactions, miscellaneous rearrangements such as the photochemical [1,3] sigmatropic shift have been used in synthesis. An early example was in the synthesis of model corrins the formation of the vinylogous amide (141) occurred in 50% yield by irradiation of the N-acyl-enamide (140) in cyclohexane.3-Acetylcyclohexanone (143) was similarly obtained by irradiating the vinyl alcohol (142) in benzene. ... 

In turn the oxazoline-containing polymer may then react very rapidly (e.g. at 240°C) with such groups as carboxyls, amines, phenols, anhydrides or epoxides, which may be present in other polymers. This reaction will link the two polymers by a rearrangement reaction similar to that involved in a rearrangement polymerisation without the evolution of water or any gaseous condensation products (Figure 7.14). 

Over the years many blends of polyurethanes with other polymers have been prepared. One recent example is the blending of polyurethane intermediates with methyl methacrylate monomer and some unsaturated polyester resin. With a suitable balance of catalysts and initiators, addition and rearrangement reactions occur simultaneously but independently to give interpenetrating polymer networks. The use of the acrylic monomer lowers cost and viscosity whilst blends with 20% (MMA + polyester) have a superior impact strength. 

It was pointed out earlier that the low nucleophilicity of fluoride ion and its low concentration in HF solutions can create circumstances not commonly observed with the other halogen acids. Under such conditions rearrangement reactions either of a concerted nature or via a true carbonium ion may compete with nucleophilic attack by fluoride ion. To favor the latter the addition of oxygen bases, e.g., tetrahydrofuran, to the medium in the proper concentration can provide the required increase in fluoride ion concentration without harmful reduction in the acidity of the medium. 

On the other hand, reaction of the S-protected intermediate 261 with 2,5-difluoronitrobenzene (258) provided better yields (61%) of the same product 260 as the reaction without the protection that gave 45% yield of this compound (Scheme 40). The fact that the same product was formed in both cases suggests that the Smiles O-S rearrangement took place during the reaction (98JHC699). 

Rearrangements and other side-reactions are rare. The ester pyrolysis is therefore of some synthetic value, and is used instead of the dehydration of the corresponding alcohol. The experimental procedure is simple, and yields are generally high. Numerous alkenes have been prepared by this route for the first time. For the preparation of higher alkenes (> Cio), the pyrolysis of the corresponding alcohol in the presence of acetic anhydride may be the preferable method." The pyrolysis of lactones 9 leads to unsaturated carboxylic acids 10 ... 

If the deuterium isotope effect on the rearrangement rate ( H/ D3)r is larger than unity and is approximately equal to that on the rate of dediazoniation ( H/ D3)S, it can be concluded that the ion-molecule pair 8.13 is the more likely intermediate for the rearrangement reaction. On the other hand, an isotope effect on the rearrangement rate that is smaller than or equal to unity would indicate the involvement of the benzenespirodiazirine cation 8.17 as an intermediate. 

In Volume 13 reactions of aromatic compounds, excluding homolytic processes due to attack of atoms and radicals (treated in a later volume), are covered. The first chapter on electrophilic substitution (nitration, sulphonation, halogenation, hydrogen exchange, etc.) constitutes the bulk of the text, and in the other two chapters nucleophilic substitution and rearrangement reactions are considered. 

The radical rearrangement reaction, serving as a timing device, has been called a free radical clock 2 It provides a means of evaluating the rate constant for reactions of this radical with other substrates. The example shows how the radical-chromium(II) rate constant can be determined. A number of other instances have been summarized.13... 

No matter how produced, RN2 are usually too unstable to be isolable, reacting presumably by the SnI or Sn2 mechanism. Actually, the exact mechanisms are in doubt because the rate laws, stereochemistry, and products have proved difficult to interpret. If there are free carbocations, they should give the same ratio of substitution to elimination to rearrangements, and so on, as carbocations generated in other SnI reactions, but they often do not. Hot carbocations (unsolvated and/or chemically activated) that can hold their configuration have been postulated, as have ion pairs, in which OH (or OAc , etc., depending on how the diazonium ion is generated) is the coun-... 

Direct elimination of a carboxylic acid to an alkene has been accomplished by heating in the presence of palladium catalysts.Carboxylic esters in which the alkyl group has a P hydrogen can be pyrolyzed, most often in the gas phase, to give the corresponding acid and an alkene. No solvent is required. Since rearrangement and other side reactions are few, the reaction is synthetically very useful and is often carried out as an indirect method of accomplishing 17-1. The yields are excellent and the work up is easy. Many alkenes have been prepared in this manner. 

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. 

Scheme 10.14 gives some other examples of Wolff rearrangement reactions. Entries 1 and 2 are reactions carried out under the classical silver ion catalysis conditions. Entry 3 is an example of a thermolysis. Entries 4 to 7 are ring contractions done under photolytic conditions. Entry 8, done using a silver catalyst, was a step in the synthesis of macbecin, an antitumor antibiotic. Entry 9, a step in the synthesis of a drug candidate, illustrates direct formation of an amide by trapping the ketene intermediate with an amine. 

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