Reactions of the Relatively Persistent Carboxylic Acid Enol2,2-Ditipylethene-1,1-diol. The Reversibility of Ketene Hydration¹

Abstract

The reactions of solutions of 2,2-ditipylethene-1,1-diol (tipyl ) 2,4,6-triisopropylphenyl) (1), which is the enol of ditipylacetic acid, were studied. Oxidation with (p-BrC 6 H 4 ) 3 N •+ SbCl 6 -gave a benzofuranone, i.e., a five-membered lactone (5) by cyclization via the oxygen on the ring, and ketonization with loss of aromaticity of the ring. Bromination also gave 5, presumably via an initial oxidation of the oxygen, as well as a bromine-containing six-membered lactone, a benzopyranone (9), formed by cyclization on an o-i-Pr group and ring bromination. No product resulting from reaction of bromine with the double bond was formed. This is ascribed to shielding of the double bond by steric crowding. Attempted etherification of the enolic OH groups did not give the ketene trimethylsilyl or methyl acetals. With diazomethane a bicyclic trienone 13 was formed, presumably by cyclopropanation of the ditipylketene (2) which is in equilibrium with 1, followed by a vinyl cyclopropanone rearrangement to 13. The formation of 1 by addition of water to 2 was found to be reversible as shown by isolation of a 18 O-labeled-2 from hydration of 2 in H 2 18 O. Several diarylketenes react with H 2 18 O to give several isotopomeric diarylacetic acids, and the formation of Ar 2 CH 18 O 2 H indicates the generality of the reversibility of the ketene hydration for these species.

1,1‐Ethenediol: The Long Elusive Enol of Acetic Acid

“…Frey and Rappoport demonstrated the generation of kinetically stabilized but short‐lived enols (Tip 2 =C(OH) 2 , Tip 2 =C(OH)NMe 2 , tip=2,4,6‐triisopropylphenyl) using bulky substituents by addition of water and dimethylamine to the corresponding ketene; the enols were observed and analyzed by NMR spectroscopy . The p K Enol values were experimentally estimated and computed; these are much higher than those of aldehydes and ketones.…”

The Photoelectric Response of Gold‐Nanoparticle Monolayers

“…Recent studies on the preparation of these enols, especially of amides, involve two approaches. (1) Addition of nucleophiles (e.g., H 2 O, RR′NH) to ketenes;4–14 this mostly gives short‐lived enols,4–14 although several of them are kinetically stabilized by bulky aryl groups15–23 and are sufficiently long‐lived to be detected by 1 H and 13 C NMR spectroscopies 15–23. (2) Addition of organic isocyanates RNCO to CH 2 YY′ (where Y and Y′ are electron‐withdrawing groups (EWGs)) gives the amides 2 , and/or their enols 1 , X = NRR′ or 1 / 2 mixtures 24–32.…”

1,1‐Ethendiol – Das lange Zeit schwer fassbare Enol der Essigsäure

“…O'Neill und Hegarty haben erstmals Enole vom Typ Ar 2 C=C(OH) 2 (Ar=Mesityl und Pentamethylphenyl) mittels Hydratisierung der entsprechenden Ketene dargestellt. Frey und Rappoport wiesen mittels NMR‐Spektroskopie die Bildung der sterisch anspruchsvollen und damit kinetisch stabilisierten aber kurzlebigen Enole (Tip 2 =C(OH) 2 und Tip 2 =C(OH)NMe 2 , tip=2,4,6‐Triisopropylphenyl) durch die Addition von Wasser und Dimethylamin an die entsprechenden Ketene nach . Die p K Enol ‐Werte wurden experimentell abgeschätzt und berechnet, wobei diese sehr viel höher ausfallen als vergleichbare Werte von Aldehyden oder Ketonen.…”

“…Frey und Rappoport wiesen mittels NMR-Spektroskopie die Bildung der sterisch anspruchsvollen und damit kinetisch stabilisierten aber kurzlebigen Enole (Tip 2 = C(OH) 2 und Tip 2 = C(OH)NMe 2 , tip = 2,4,6-Triisopropylphenyl) durch die Addition von Wasser und Dimethylamin an die entsprechenden Ketene nach. [10,11] Die pK Enol -Werte wurden experimentell abgeschätzt und berechnet, wobei diese sehr viel hçher ausfallen als vergleichbare Werte von Aldehyden oder Ketonen. Der pK Enol -Wert für das Stammsystem CH 3 COOH/CH 2 =C(OH) 2 wurde zu 18-21 [14,15] bestimmt, was gut mit den berechneten Werten von 19-25 übereinstimmt.…”