Rearrangements, thermal

Chemical Properties. Reactions of quaternaries can be categorized iato three types (169) Hoffman eliminations, displacements, and rearrangements. Thermal decomposition of a quaternary ammonium hydroxide to an alkene, tertiary amine, and water is known as the Hoffman elimination (eq. la) (170). This reaction has not been used extensively to prepare olefins. Some cycHc olefins, however, are best prepared this way (171). Exhaustive methylation, followed by elimination, is known as the Hoffman degradation and is important ia the stmctural determination of unknown amines, especially for alkaloids (qv) (172). 

Pyridazinones may undergo ring contraction to pyrroles, pyrazoles and indoles, the process being induced either by an acid or base. The structure of the final product is strongly dependent on the reaction conditions. For example, 4,5-dichloro-l-phenylpyridazin-6(lFT)-one rearranges thermally to 4-chloro-l-phenylpyrazole-5-carboxylic acid (12S), while in aqueous base the corresponding 4-hydroxy acid (126) is formed (Scheme 40). 

Alkylthiothiazoles rearrange thermally into the 3-alkylthiazoline-2-thiones in the imidazole series a thermal equilibrium is reached. 

A surpnsing feature of the reactions of hexafluoroacetone, trifluoropyruvates, and their acyl imines is the C-hydroxyalkylation or C-amidoalkylaOon of activated aromatic hydrocarbons or heterocycles even in the presence of unprotected ammo or hydroxyl functions directly attached to the aromatic core Normally, aromatic amines first react reversibly to give N-alkylated products that rearrange thermally to yield C-alkylated products. With aromatic heterocycles, the reaction usually takes place at the site of the maximum n electron density [55] (equaUon 5). 

In some cases, when difluorocyclopropyl derivatives have the appropriate geometry, they rearrange thermally in a concerted manner, and the energy required IS lower than that required by radical formation [133, 134] (equations 31-33)... 

When uradiated, fluonnated isomers of Dewar benzene yield pnsmane derivatives that rearrange thermally to benzene Photolysis of hexakis(mfluororaethyl)benzvalene ozonide gives tetrakis(tnfluoromethyl)cyclobutadiene and its dimer [J47]... 

The intramolecular Diels-Alder reaction has become one of the most promising methods for the construction of complex cyclohexene moieties 104). In view of the fact that cyclobutenes rearrange thermally and conrotatary to butadienes (Eq. (9))8l l05) and their willingness to undergo Diels-Alder reactions with dienophiles (Eq. (10))... 

This reaction can be used to effect a [3 + 2] or a [3 + 2 + 2] annelation to a cyclopentenone ketal or a cycloheptadienone ketal, respectively. Thus the adduct of a vinylcuprate with 1 rearranges thermally (240-290°) to 3-cyclopentenone ketals (3) in 50-80% yield (equation I). [3 + 2 + 2]Annelation involves preparation of... 

Di-iodocyclopropanes are unstable and cannot be isolated readily [133]. Other dihalocyclopropanes rearrange thermally during work up, or in the presence of the base, to produce ring-opened allylic dihalides or vinyl halides [e.g. 15, 81, 87, 96, 100, 103, 152, 157] (Scheme 7.4). Thermal rearrangement of the dihalocarbene adducts from halocycloalkenes leads to ring expanded products or methylene derivatives [e.g. 87], whereas 1, l-dichlorocyclopent-3-enes produce halobenzenes in high yield [96]. 

As noted with the reactions between terpenes and dihalocarbenes, mono-insertion adducts at the more electron-rich sites can be isolated from the reaction of non-conju-gated acyclic and cyclic dienes although, depending on the reaction conditions, the bis-adducts may also be formed. Norbomadiene produces both 1,2-endo and 1,2-exo mono-insertion adducts with dichlorocarbene, as well as a 1,4-addition product (Scheme 7.4) [67]. The mono adduct produced from the reaction with dimethylvinylidene carbene rearranges thermally to yield the ring-expanded product (Scheme 7.4) [157] a similar ring-expanded product is produced with cyclo-hexylidene carbene [149]. 

It is suggested that the overall equilibrium is controlled by the last step all work on dihydrodiazepines shows that equilibria between the two tautomeric forms lies almost completely on the side of the conjugated form. In support of this it was shown that the bisanil of tra 5-2,3-diamino-1,1-diphenylcyclopropane does not rearrange thermally into a dihydrodia-zepine (65CB2701). In this instance, the last step is prevented by the presence of two phenyl groups at the 6-position. 

The oxatriazine (178) rearranges thermally or under acidic conditions to give triazole (179) (Equation (67)) <93H(36)455>. 

A 5-allyl-5-vinyl-substituted 1,3-dioxan-2-one was used for a Cope rearrangement. Thermal treatment of 197 at 120-150 °C gave 5-alkylidene-l,3-dioxan-2-one in 75% yield (Equation 68) <2001JOC4447>. 

Thus in the N-silyl substituted series, 17 and 18, which rearrange thermally to the corresponding diazo compounds, the stability increases through the series R=Me, Ph, i-Pr. As discussed below, these compound undergo the usual cycloaddition and electrocyclization reactions of nitrile imines and are not simply overstabilized curiosities. The usefulness in synthesis of those with P—C bonds is probably limited since these bonds are not easily broken, but products derived from those with C—Si and C—B bonds (e.g., 21 and 22) should be capable of further... 

When the photooxide of 9-phenylanthracene (113, R = Ph) is treated with aqueous acetic acid, 3-(o-hydroxyphenyl)-l-phenylbenzo[c]furan (114, R = Ph) is obtained in 22% yield. The mechanism of Scheme 6 has been given for this rearrangement. Thermal decomposition of 113(R = Me, Ph), which might occur as shown in Scheme 7, also gives 114 (R = Me, Ph) in these cases the benzo[c]furans have not been isolated. They could be trapped, however, with A-methylmaleimide. ... 

The Af-methoxy compounds 87 (Scheme 32) rearrange thermally to the 3-methoxy isomers 88.123... 

Quinoxaline-2-carboxylic acid with thionyl chloride gives the acid chloride (79%), which undergoes the expected reactions.166 The gas-phase pyrolysis of quinoxaline-2,3-dicarboxylic anhydride (157) over a Nichrome coil gives o-phthalonitrile (158) (72%), probably via 2,3-quinoxalyne (159) which ring-cleaves, and the diisocyanide subsequently rearranges thermally to the dinitrile.167... 

A-Alkyl groups in neutral azoles can rearrange thermally to carbon. For example, 2-alkylimidazoles can be prepared in this way in a reaction which is irreversible, uncatalyzed, intramolecular and does not involve radicals (80AHC(27)24l). 

The cycloaddition potential of compound (47) is also demonstrated by its reaction with alkynes leading to isolable 1-Dewar pyridines (50). The latter were rearranged thermally or by acid catalysis into pyridines (49). The photochemical rearrangement of (50) led to azaprismane derivative (51) (89T3115). 

Triarylmethyl azides (13) rearrange thermally (170-190°) or photochemically (room temperature) to benzophenone anils... 

Diels-Alder reaction of onitrosobenzamide and 1,3-DIBF (no mp and spectral data given, only HRMS 93BSF101). Taylor and co-workers reported Diels-Alder type reactions of 1,3-DIBF and nitrosopyridines [82JOC552]. A Diels-Alder adduct of 1,3-DIBF and an azetidinone rearranges thermally to a ring opened product (92CJC2792). Whereas... 

JCS(P1)621). The reaction of 1-ethoxycarbonylpyrrole with dimethyl 2,5-bis(diazo)-3,4-dioxoadipate also results in heteroarylation of the pyrrole ring to yield (100), which rearranges thermally to give dimethyl l-ethoxycarbonyl-4,6-dihydroxyindole-5,7-dicar-boxylate (76T269). 

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