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Abstract
The first part of this research extends the phenylation of carbanions with diphenyliodonium salts and elucidates the mechanism of such phenylation. The second part of this research investigates the thermal reactions of 2-aryliodon iobenzoates: their rearrangement to aryl 2-iodobenzoates and their cleavage to benzyne and other reactive intermediates.
Part I: Phenylation of the anion of 2-phenyl-1,3-indandione with diphenyliodonium salts in t-butyl alcohol gave 2,2-diphenyl- 1,3-indandione in 86-93% yield. Also formed in the reaction were iodobenzene and, in small amount, 2,2'-diphenyl-2,2'-biindan-1,1', 3,3'-tetrone. The radical scavenger, oxygen, to which the anion of 2-phenyl-1,3-indandione is inert, reduced the yield of 2,2- diphenyl-1,3-indandi one to 53%, while styrene reduced the yield of 2,2-diphenyl-1,3-indandlone to 70% and gave polymers of low molecular weight containing carbonyl groups. When the phenylation was run in isopropyl alcohol, dehydrogenation of isopropyl alcohol to acetone occurred, with concomitant formation of benzene; the yield of phenylated product fell to 49%.
The following mechanism is proposed; electron transfer within an ion pair from a carbanion (R) to an iodonium ion (AriAr) to give a radical pair, the members of which largely react together, either by radical displacement of R on diphenyllodine or by coupling of R. and phenyl radicals. Some free radicals, formed by diffusing apart of the members of the radical pairs, later couple to give ArR, ArAr and RR.
Part II Acid-catalyzed condensation of 2-iodosobenzoic acid. with benzene and substituted benzenes has given 2-carboxydiphenyl- jodonium salts, which were converted to the corresponding belaines, 2-aryllodonlobenzoates. On heating, these betaines rearranged in part to aryl 2-iodobenzoates; this ester formation was repressed by alkoxyl groups and enhanced by nitro groups in the ring of betaine not bearing the carboxylate lon. It is suggested that the rearrangement Involves an Intramolecular nucleophilic displacement.
A competitive thermal reaction of the betaines, favored at higher temperatures, is cleavage to aryl iodides, carbon dioxide, benzyne and other reactive Intermediates, In the absence of benzyne- trapping reagents these intermediates may react to give xanthone and 3,4-benzocoumarin. Although a present choice between the various reaction paths for forming or utilizing benzyne is not possible, benzyne must be formed from the ring bearing the carboxylate group.
In the presence of benzyne-trapping reagents, addition products of the Diels-Alder type were formed. The efficiency of benzyne-trapping reagents increased as follows: anthracene <1,3- diphenylisobenzofuran<2,3,4,5-tetraphenylcyclopentadienone<2,5-bis- (p-dimethylaminophenyl)-3,4-diphenylcyclopentadienone<2,5-di-p- anisyl-3,4-diphenylcyclopentadienone.
Formation of benzyne was not repressed by electron-donating substituents on the phenyl ring, while formation of benzyne from betaines bearing the electron-withdrawing nitro group was de- pressed by the competitive ester formation.
Reactions of 2-phenyliodon lobenzoate with nucleophiles were found to be similar to those of other iodonium salts.
