Ketones kinetics

In Table IV are listed characteristic visible absorptions observed for the heavy ketones kinetically stabilized by Tbt group. 

Enolization and ketonization kinetics and equilibrium constants have been reported for phenylacetylpyridines (85a), and their enol tautomers (85b), together with estimates of the stability of a third type of tautomer, the zwitterion (85c). The latter provides a nitrogen protonation route for the keto-enol tautomerization. The two alternative acid-catalysed routes for enolization, i.e. O- versus Af-protonation, are assessed in terms of pK differences, and of equilibrium proton-activating factors which measure the C-H acidifying effects of the binding of a proton catalyst at oxygen or at nitrogen. 

Kinetic Resolution of a Racemic Ketone. Kinetic resolution is a process by which one of the enantiomeric constituents of a racemate is more readily transformed into a product than the other (63b). For example, in the presence of an (/ )-BINAP-Ru catalyst, the S enantiomer of the a-hydroxy ketone is hydrogenated 64 times faster than R enantiomer, and, after 50.5% conversion, both the syn 1,2-diol and unreacted R hydroxy ketone are obtained in high ee (Scheme 61). 

In 1978, we observed that flash photolysis of butyrophenone produced acetophenone enol as a transient intermediate, which allowed us to determine the acidity constant KE of the enol from the pH-rate profile (section pH Rate Profiles ) of its decay in aqueous base.4 That work was a sideline of studies aimed at the characterization of biradical intermediates in Norrish Type II reactions and we had no intentions to pursue it any further. Enter Jerry Kresge, who had previously determined the ketonization kinetics of several enols using fast thermal methods for their generation. He immediately realized the potential of the photochemical approach to study keto enol equilibria and quickly convinced us that this technique should be further exploited. We were more than happy to follow suit and to cooperate with this distinguished, inspiring, and enthusing chemist and his cherished wife Yvonne Chiang, who sadly passed away in 2008. Over the years, this collaboration developed into an intimate friendship of our families. This chapter is an account of what has been achieved. Several reviews in this area appeared in the years up to 1998.5 10... 

UV-vis Spectra. In Table 6 are listed characteristic visible absorptions observed for the heavy ketones kinetically stabilized by a Tbt group, ft is known that the absorption maxima attributable to the n-jr electron transitions of a series of R2C=Ch (Ch = O, S, Se, Te) undergo a systematic red-shift on going down the periodic table. A similar tendency is observed for the Si, Ge, and Sn series of Tbt(R)Si=Ch (Ch = S, Se, Tbt(R)Ge=Ch... 

Imidazole and benzimidazole esters are quite reactive towards lithium enolates of ketones. Kinetic studies show that the reaction involves primarily monomers of the enolates <19990L145>. This is in contrast to reactions of aryl esters where both monomers and aggregates of the enolates participate in the reaction. Although (thio)carbonylimidazole esters are normally formed between alcohols and Kl -carbonyldiimidazole or l,l -(thiocarbonyl)diimidazole 1039, transfer of the imidazole group has been reported in reactions with activated alcohols to give, for example, 1040 (Scheme 250) <1997JOC7319>. 

Ashby, E. C., Boone, J. R., Mechanism of Lithium Aluminum Hydride Reduction of Ketones. Kinetics of Reduction of Mesityl Phenyl Ketone, J. Am. Chem. Soc. 1976, 98, 5524 5531. 

Little precise mechanistic studies have been undertaken with these inhibitors with the exception of the time-dependent inhibition of SAH hydrolase by 176. Stoichiometric loss of fluoride was observed by 19F-NMR during the inactivation process. However, there is only circumstantial evidence to support the addition-elimination mechanism proposed in Figure 1 all attempts to isolate an enzyme fragment covalently bound to an inhibitor have so far been unsuccessful. If the rate-determining step in the enzyme inhibition process is an attack of an enzyme nucleophilic residue on a 0-fluoro-a, 0-unsaturated imine or ketone, kinetic analysis of addition-elimination reactions to similar systems indicate that the ( )-isomer is the more active isomer (73) this could explain in part the isomeric preference seen with MAO and SAH hydrolase inhibitors. 

Thus four mechanistic pathways have been described, all in basic disagreement with each other. It is very possible that the often expressed warning (76) that the structure of the Grignard reagent may depend on solvent, R, and X may well carry over into its mechanistics with R2 CO, and that in fact no unique mechanism exists. Other ketone kinetic data are available, but do not seem pertinent at this point, because of their limited or tentative nature, or because they do not reveal information that could be of value in distinguishing mechanistic pathways. 

The Pd(Quinox)Cl2, (Quinox = 2-(2-quinolyl)-4,5-dihydrooxazole) catalysed Wacker-type oxidation of olefins bearing homoallylic alcohols by TBHP led to the corresponding /3-hydroxy ketones in good yields. Since the oxidation was catalyst controlled, it was significantly faster than the substrate-controlled Tsuji-Wacker oxidation. The bis- and fra-homoallylic alcohols were oxidized to cyclic peroxyketals, presumably via nucleophilic attack of the methyl ketone. Kinetics of the Wacker-type oxidation of olefins by TBHP in the presence of Quinox (ligand), and (54) as the catalyst reveal first-order dependence on ligand and olefin, and rate saturation in TBHP, as expected of the proposed mechanism (Scheme 9)... 

Ketones Examples in which unprotected ketones kinetically survive McMurry reactions are very rare. Coupling at the enone site of androsta-l,4-diene-3,17-dione... 

Jimenez, E., B. Ballesteros, E. Martinez, and J. Albaladejo (2005a), Tropospheric reaction of OH with selected linear ketones Kinetic studies between 228 and 405 K, Ertviroru ScL Tech.,... 

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