Kinetics and Mechanism of the Pinacol Rearrangement. I. The Perchloric Acid-catalyzed Rearrangement of Benzopinacol and of Tetraphenylethylene Oxide in Acetic Acid Solution1
“…This well-known reaction is generally refered to as the pinacol rearrangement. The pinacol rearrangement can be promoted by both Bronsted and Lewis acids, and the reaction is general to many types pinacols or 1,2-diols 3a3b.…”
Section: Introductionmentioning
confidence: 99%
“…When benzopinacol ( 1a ) is reacted with acids of moderate strength, the pinacol rearrangement leads to 2,2,2-triphenylacetophenone ( 2 ) in high yield 3b. It is also well-known that appropriate epoxides with acids undergo similar ring-opening rearrangement to products such as 2 . 3c,d,f Recently, we found that benzopinacol ( 1a ) and substituted benzopinacols ( 1b , c , d , h , i ) react in triflic acid (TfOH) to yield 9,10-diphenylphenanthrene ( 3a ) and substituted 9,10-diarylphenanthrenes in excellent yield. , We now report studies on the general dehydrative condensation of aryl pinacols and related epoxides to condensed aromatics in superacids and their mechanism which involves superelectrophilic, dicationic intermediates. …”
Aryl pinacols and epoxides, respectively, are cleanly
and in high yield converted via superacidic
dehydrative cyclization to the corresponding condensed aromatics.
Dehydrative cyclization of
benzopinacol (1a), triphenylacetophenone (2), and
tetraphenylethylene oxide (9) give 9,10-diphenylphenanthrene (3a) as the major product in acidic
media stronger than H
o = −11.
Aryl
pinacol 12a forms the condensed aromatic 13a as
the major product in acidic media stronger than
H
o = −13.5. It is proposed that the
dehydrative cyclizations to provide aromatics 3a and
13a occurs
through dicationic intermediates. Substituted benzopinacols
1f, 1g, and 1j are prepared and
give
the corresponding phenanthrenes (3f, 3g, and
3j) in high yields. The regiochemistry of
the
cyclization of substituted benzopinacols is controlled by deactivating
substituents on the aryl rings.
Aryl pinacols (12a−d) derived from
acenaphthenequinone and pinacol 15 also give
condensed
aromatics (13a−d and 16, repectively) with
superacidic triflic acid.
“…Extensive experimental studies have been carried out for both aliphatic and aromatic diols. Several experimental methods, such as product analyses,1 stereochemical studies,2 tracer experiments,3, 4 and kinetic studies,5–9 have demonstrated that the reactions of aromatic diols proceed through carbocationic intermediates. Migratory aptitude is one of the key features in discussing the mechanism of pinacol rearrangement.…”
Section: Introductionmentioning
confidence: 99%
“…The fact that 14 C labeled ketones undergo scrambling of the label indicated that OH indeed migrates to the neighboring cationic carbon under strong acidic conditions 10, 11. The importance of OH migration and a possible involvement of epoxide during pinacol rearrangement have only been presented in a few reports,5, 7, 8 and there was a case, in which diol and epoxide were found to give different product ratios, and thus the reaction mechanisms of these two class of compounds were claimed to be different 9…”
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