Electrocyclic ionization

The Friedel-Crafts alkylation of aromatics with ro-haloalkyl fluorides always favors the formation of ru-haloalkylarenes (9, 10). Symbiosis determines the selective C—X bond severance. Similarly, the indene synthesis by alkylation of benzene with 1-bromo-l-fluorocyclopropanes is shown to proceed via an electrocyclic ionization of the hard F (11). This behavior is opposite to the trend of the uncatalyzed thermal heterolysis of halocyclopropanes (see Chapter 11, Section 11.4). 

A number of studies using the same or closely related methodology deal with lower homologues of the pentadienes and hexadienes. They will only be mentioned here briefly. For isomerization of ionized butadienes by electrocyclic reactions, see Section m.D. 

A variety of different precursors have been shown to rearrange to divinyl ketones or equivalent species under highly ionizing acidic conditions. Equivalents of both cross-conjugated and linear dienones have been generated and cyclize to cyclopentenones in situ. The structural diversity of substrates in this class is remarkable and testifies to the utility of these ring-forming reactions which are unified by the propensity for cationic electrocyclization. 

Ring expansion can be drawn in various ways. There is a direct route from the neutral amine, or s anion, that doesn t look very convincing, or you can ionize one of the chlorides first and open the Practical heterocyclic chemistry, wclopropyl cation in an electrocyclic process. This is a good method to make 3-substituted Academic Press, London, 1968, oradines. 

Electrospray ionization mass spectrometry was applied to various hexa-(65) and penta- (66) para-substituted phenoxy cyclotriphosphazenes. In an attempt to understand the fragmentation patterns given by the various phosphazenes, pathways have been proposed that involve ring closure reactions upon the loss of phenoxide. Scheme 15. Intially, loss of phenoxide is noted. Following this loss is proposed a electrocyclic ring closure reaction that ultimately leads to the loss of an additional phenoxide. Hydrolysis... 

The acetolysis of isomeric cyclopropyl tosylates (45-47) at 100 °C takes place by concerted electrocyclic ring opening and ionization. The loss of tosyloxy group in ionization step is assisted by the electron density of the developing p-orbitals that are trans to the leaving group. Hence, the isomer 47 undergoes much faster acetolysis than the other isomers [27]. 

Bromo-l-aryl-substituted indenes have been reported to be synthesized from 1,2-diaryl-substituted gem-dibromocyclopropanes via a domino sequence involving silver(I)-promoted ionization and 2a -disrotatory electrocyclic ring-opening followed by a 4a -conrotatory electrocyclic ring-closing reaction of the allylic carbocation intermediate (Scheme 46). ... 

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