Thermal Rearrangement Reactions

Not surprising, the most prevalent synthetic utility is the assembly of the pyrrolidine ring. N-Chloroamine 27 was obtained by treatment of N-methyl-2-cyclopentylethylamine (26) with N-chlorosuccinimide. Under classic Hofmann-Loffler-Freytag reaction conditions, 27 was rearranged either thermally or by UV irradiation in sulfuric acid to bicyclic amine... 

When additional substituents are introduced in the 2- and/or 4-position, the thermal rearrangement gives products in which the substituents that were originally located in the 1- and 5-position of the quadricyclane are then located at C4 and C5 of the oxepin 7.30,123 In order to trap intermediates of this rearrangement reaction by intramolecular cycloaddition vinyl and acetylene groups were linked with different spacer groups to C2 of quadricyclane.123 In this manner two different intramolecular cycloadducts were isolated in addition to oxepin derivatives.123... 

Some thermal rearrangement reactions of 1-benzoxepins show the participation of the solvent in the formation of stable products, e.g. 6172,247 and 7.177 The synthesis of methoxy-substituted 1-benzoxepins by O-methylation of the anions generated by the deprotonation of the respective oxo derivative with ferf-butoxide is often limited by the rapid aromatization to methoxy-substituted naphthalenes, e.g. 816 and 9,173 under the reaction conditions.16,173... 

In contrast to the above rearrangement reaction, the analogs 1 rearrange to 5-[(ethoxycarbonyl)-amino]-2-phenylpyridines 2 under thermal conditions.29 30... 

The potential for solvent-free synthesis is relatively large, with examples of many well-known reaction types proceeding quite well under this type of regime these include transesterification, condensation and rearrangement reactions. Many workers have moved away from conventional thermal sources for providing the energy needed for these reactions -... 

Chapter 6 looks at concerted pericyclic reactions, including the Diels-Alder reaction, 1,3-dipolar cycloaddition, [3,3]- and [2,3]-sigmatropic rearrangements, and thermal elimination reactions. The carbon-carbon bond-forming reactions are emphasized and the stereoselectivity of the reactions is discussed in detail. 

Due to some stability concerns with the N-Cbz group of 8 at high temperatures, compound 25 was used as a model substrate for the reaction. Substrate 25 was irradiated for 2 min (internal temperature reached 185 °C) in a variety of solvents and all thermal reactions reached >95% conversion (Table 6.1). Both aprotic polar solvents (entries 6 and 9) and protic polar solvent (entry 7) gave poor assay yields of product 26. With nonpolar solvents (entry 10) such as o-xylene and xylenes, the rearrangement reaction provided the highest assay yield and proved to be the best solvent choice [9e],... 

Tetralin has been shown to undergo thermal dehydrogenation to naphthalene and rearrangement to methyl indan in either the absence or presence of free radical acceptors [ 1, 2]. The presence of free radical acceptors usually accelerates the rearrangement reaction. Even with alkylated Tetralins>... 

L abbe has studied the rearrangement reactions of 1,2,3-thiadiazoles to differently substituted 1,2,3-thiadiazoles <1983CC588>. He also studied many 5-azido-l,2,3-thiadiazoles 33 that rearranged to 1,2,3,4-thiatriazoles 34 (Equation 5) <1988BSB163>. He even found that l,2,3-thiadiazole-4-carboxaldehydes 35 upon treatment with amines underwent thermal rearrangement to 1,2,3-triazoles 36 (Equation 6) <1993J(P1)1719>. 

Step 10 is a thermal cyclization reaction not requiring a catalyst. The participation of a radical intermediate in the rearrangement reaction was verified by radiotracer studies (94a). 

Often, the observed species may undergo rearrangement either thermally or photochemically. The end products of such isomerizations tend to have singlet ground states, rendering IR spectroscopy the most powerful tool for elucidating these processes. If the end product is the result of a thermal reaction, then due to the low temperature, not many pathways are available. This facilitates identification and also allows for the possibility of carrying out kinetic measurements. 

The mechanisms (a)-(c) can also be discussed in connection with the thermal decomposition and rearrangement reactions of organic polysulfides. For example, dimethyl tetrasulfide when heated to 80 °G for several hours disproportionates to the corresponding tri-, penta- and hexasulfides. On prolonged heating small amounts of disulfide are formed in addition... 

In summary, it should be pointed out, however, that the exact mechanism of the thermal rearrangement of sulfur rings and chains is still unknown and that further investigations are necessary in this connection. For any further discussion it may also be interesting to take into account the recent results on the molecular rearrangement reactions of cyclic selenium sulfides and of elemental selenium which take place at considerably lower temperatures compared with elemental sulfur and for which — as far as solid selenium is concerned — interesting ionic mechanisms have been proposed... 

A quantitative solid-state ene addition between two ligands of the platinum complex 397 gives the rearranged platinum complex 398 upon extensive heating to 140 °C [123]. This rearrangement reaction (cf. Sect. 24) is treated here, as it is the only known quantitative solid-state ene addition to date. Further quantitative solid-state [4-1-4] additions and higher vinylogs, as well as ene additions also of the intermolecular type, await detection both as thermal and photochemical reactions. 

In oxidation studies it has usually been assumed that thermal decomposition of alkyl hydroperoxides leads to the formation of alcohols. However, carbonyl-forming eliminations of hydroperoxides, usually under the influence of base, are well known. Of more interest, nucleophlic rearrangements, generally acid-catalyzed, have been shown to produce a mixture of carbonyl and alcohol products by fission of the molecule (6). For l-butene-3-hydroperoxide it might have been expected that a rearrangement (Reaction 1) similar to that which occurs with cumene hydroperoxide could produce two molecules of acetaldehyde. 

The factors that control if and how these cyclization and rearrangement reactions occur in a concerted manner can be understood from the aromaticity or lack of aromaticity achieved in their cyclic transition states. For a concerted pericyclic reaction to be thermally favorable, the transition state must involve An + 2 participating electrons if it is a Hiickel orbital system, or 4 electrons if it is a Mobius orbital system. A Hiickel transition state is one in which the cyclic array of participating orbitals has no nodes (or an even number) and a Mobius transition state has an odd number of nodes. 

Cyclopentanones may also be synthesized from a,/ -unsaturated ketones and diiodo-methane. The ketone is converted to the O-silyl enol, and carbene is added to the enol double bond using the Simmons-Smith reaction (see p. 74f.). Thermal rearrangement of the resulting 1-siloxy-l-vinylcydopropane and add-catalyzed hydrolysis of the silyl enol ether leads to cyclopentanones in excellent yields (C. Girard, 1974). Very high temperatures, however, are needed, and this obviously limits the generality of this rearrangement reaction. 

The rates of both the thermal and the catalytic rearrangement reactions drastically depend on the electronic nature of the substituents in the aryl ring. Thus, with Ar = C6H4OMe-4, the reaction proceeds at room temperature leading to 21 in quantitative yield. When Ar = C6H4Me-4, the reac-... 

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