Trifluoroacetophenone reduction rates (by a dihydronicotinamide)
and hydration equilibria are identically affected by ring substituents.
High yields of alcohol are obtained when electron donating substituents are absent from the ring.
We are interested in the effect of substituents on organic oxidation reactions since the often vexing question of hydride transfer vs. hydrogen-atom transfer 1.2 can sometimes be addressed through such studies.
We accordingly have measured the rates of reduction of a series of m_ and E-substituted trifluoroacetophenones 3 by N-carbamoylmethyl-3-carbamoyl-1, 4-dihydropyridine t, an NADH analogue with a relatively high degree of stability with respect to decomposition in aqueous buffer. 4 (Most trifluoromethyl ketones are highly hydrated in aqueous media' and their observed reduction rates must be corrected for the extent of hydration if meaningful results are to be obtained.) H H CONH2 2 +H+ 4 2DrCF3+ +y;r* H CH2CONH2 2 2 1 2 ?I 'L The reaction was followed at 43.4"C in a 21.8% (v/v) sulfolane-water solution containing 0.40 M sodium perchlorate, 0.10 M triethanolamine, and 0.10 M triethanolamine hydrochloride buffer (glass electrode 'pH' 8.3) by following the disappearance of the 348 nm band of 2. The latter's concentration was 10 -4 M and the ketones were always present in excess (6-ZOO times).
Accompanying each run was a determination of the rate of decomposition of z under the same conditions, except for the absence of ketone.
Each observed first-order rate constant was corrected for this decomposition by simply subtracting the first-order decomposition rate.
A further correction was made for the degree of hydration of the ketone using equilibrium constants that are either known or that can be calculated.
Sb,c
The second-order rate constant k, which represents the rate of reaction of 1 and I, was then obtained.
(The relaxation rate for the dehydration equilibria was much faster tcan the reduction in all the cases studied.