Kinetics and mechanisms (excepting cycloadditions)

Generation and Detection of a Relatively Persistent Carboxylic Acid Enol2,2-Bis(2‘,4‘,6‘-triisopropylphenyl)ethene-1,1-diol

“…The unequivocal structural assignment of the hydration product of ketene 20 as the 1,1-enediol 25 and the similar, only much less studied, behavior of other diaryl ketenes, shows that hydration of these ketenes is initiated by addition of water to the CO bond. This has a bearing on the mechanistic argument concerning the details of ketene hydration. ,, A relevant mechanistic point is that hydration of diarylketenes is reversible as shown in detail for the 20/25 system . This and reactions of 1,1-enediol 25 which involve several reaction sites of 25 are discussed in the following paper ( J.…”

“…The difference in spatial proximity of the ketene's carbons and oxygen and the ortho methyls (in 13 ) or the isopropyls (in 20 ) could have been the reason for the substantial lower hydration rate of 20 relative to 13 by an in-plane nucleophilic attack at the ketene's C α . The nonbonded distances between the ortho methyls and the oxygen are 3.46−5.05 Å in 13 and 3.52−4.94 Å between the o- i-PrC-H carbon and the oxygen in 20 .…”

“…For simple enols conjugation of the enolic moiety to aromatic rings reduces the ketonization rate . Ketene hydration proceeds through an in-plane nucleophilic attack of water on the carbonyl carbon, initially generating a zwitterion and afterwards a carbanion whose charges are resonatively delocalized on C β ,17d, (eq 3). Consequently, electron withdrawing C β substituents will stabilize the transition state of the hydration and accelerate the reaction.…”

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

“…Reversibility of Other Diarylketenes Hydration. Both the hydration and the hydrolysis of ketenes become slower with the increased bulk of the ketene substituents. , Hence, attempted isolation of labeled ketene from the reaction mixtures formed in the hydration of less bulky and more reactive ketenes with H 2 18 O may not be easy, and a more convenient evidence for the reversibility of the hydration is desirable. Such evidence will be the observation of a partial formation of a di- 18 O labeled carboxylic acid in the hydration of the unlabeled ketene as demonstrated in Scheme 6.…”

A New and Efficient Strategy for the Total Synthesis of Polycyclic Diterpenoids: The Preparation of Gibberellins (±)-GA103 and (±)-GA73

“…During studies of the controlling factors in this reaction, it was observed that prolonged irradiation of 11 for extended periods afforded 10 , but the rate was such that it could account for only a small amount of this product. Although fission of the C(8)−C(15) bond has been initiated by enolization of the ester function in similar substrates, solvation of the assumed intermediate ketene under the reaction conditions (pH 7−10) would be expected to involve C-protonation, i.e. , the enol would be bypassed.…”