Thermal-mediated rearrangement

A detailed description of the numerous examples of vinylcyclopropropanes used in transition metal mediated organic synthesis is far beyond the scope of this section and can be found in several reviews. Prominent examples are conversions to open-chain products, as well as formation of four-, five-, six- and seven-membered rings via ring expansion or incorporation of other substrates such as carbon monoxide, alkenes or alkynes. Thus divinylcyclopropanes, obtainable via transition metal catalyzed cyclopropanation reactions, undergo a facile thermal Cope rearrangement to form cycloheptadienes. ... 

Synthesis of (—)-Tricycloillicinone. An elegant synthetic route to the neurotrophic (—)-tricycloillicinone employs a sodium amalgam mediated reductive elimination of a fS-alkoxy sulfone obtained by a thermal Claisen rearrangement (Eq. 159).272... 

Allyl A -phenylimidates are known to undergo thermal rearrangement to y.d-unsaturated anilides via their ketene A 0-acetal isomers250,231. The palladium(II)-mediated rearrangement preferentially leads to T -double-bond geometry, but decreased selectivities are observed in the case of an electron-donating substituent in position 2 of the pericyclic system, as in the rearrangement of 1361 and 2126. 

Lee and colleagues studied the ionic-liquid-mediated Beckmann rearrangement under microwaves (equation 96). They screened several ionic liquids and investigated the amount of sulfuric acid needed as catalyst. The microwave methodology was found to be superior to analogous conventional heating. All the ionic liquids tested worked well in the microwave and only 5 mol% sulfuric acid was needed. The BMIM ionic liquids ([BMIM]PFg, [BMIM]Bp4, [BMIMJSbFg and [BMIMJOTf) were found to be thermally stable. 

The cobalt(I)-mediated [2 + 2 + 2] cycloaddition of 1,5-diynes with mono-alkynes provides access to benzocyclobutene derivatives (Scheme 24). Thermal rearrangement of benzocyclobutenes into o-quinodimethane and subsequent Diels-Alder reaction with an alkene moiety allow the formation of a tricyclic compound. 

Since sUyl ynol ethers have an electron-rich triple bond, they are useful for Lewis acid catalyzed synthetic reactions. Lithium ynolates 175 are silylated by TIPSCl or TIPSOTf and TBSCl to afford the corresponding silyl ynol ethers 176 and 177, which are thermally stable and isolable, but sensitive toward acids (equation 71) . See also equations 9 and 10 in Section ll.C. An experimentally improved procedure for the purification of 176 derived from Kowalski s method is described. Lithium ynolate derived from Julia s method is also used for the preparation of 176. TMSCl and TESCl provide silyl ketenes 179, however, by C-silylation. These small silyl chlorides primarily gave the silyl ynol ethers 178, but, upon warming the reaction mixture, isomerization to the more stable silyl ketenes takes place. The soft electrophilic silyl chlorides like PhsSiCl afford silyl ketenes. Disi-lyl ynol ethers, prepared from ynolate dianions, are rearranged to disilylketenes mediated by salts . 

Rearrangement processes of alkyltitanocene dichlorides that occur under electron impact have been investigated using deuterium labelling. A novel type of zirconium-mediated coupling reaction of alkynes with vinyl bromide to afford 2,3-disubstituted dienes has been reported (see Scheme 105), and an inter-intramolecular reaction sequence has been proposed for the observed formation of vinylcyclohexadienes and/or methylenecycloheptadienes from the copper-catalysed reaction of zirconacyclo-pentadienes with allylic dichlorides. The essential step in these processes appears to be transmetallation of the zirconium-carbon bond of the zirconacyclopentadiene to produce a more reactive copper-carbon bond. New phosphorus heterocycles, e.g. (417), have been constructed by the thermal rearrangement of a [l,4-bis(trimethylsilyl)->/ -cyclooctatetraene]- ,3,5-triphospha-7-hafhanorbomadiene complex (416). 

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