Preliminary findings suggest non-acute ischaemic stroke patients can improve their cardiovascular fitness and reduce their CRS with a cardiac rehabilitation programme. The intervention was associated with improvement in self-reported depression.
Equilibrium constants for keto-enol tautomerisation and migration of hydrogen from carbon to nitrogen to form enamine or zwitterion tautomers have been measured for COPh) in aqueous solution at 25 "C. Relative tautomeric stabilities fall in the order ketoimine > enamine > enol and (for the 3-isomer) enol > zwitterion. Values of pKT, (-log KT) where KT = [enamine (or zwitterion)]/[imine] or [enol]/[ketone], are 1.05, 5.87 and 2.42 for the enamine or zwitterion tautomerism and 2.0, 4.86 and 4.4 for keto-enol tautomerism of the 2-, 3-and 4-isomers respectively. For the enamines KT was determined kinetically by quenching the enolate anion at a pH below its pKa and monitoring its relaxation to the ketoimine spectrophotometrically: combining rate constants for this process and the reverse reaction measured by halogen trapping of the enol or enamine gave KT. Values are compared with results * This pK, was earlier estimated4 as 8.4 instead of 8.1 and the derived pK, for the keto-enol tautomerisation as 4.2 instead of 4.4. The present values are preferred.
A comparison of the effects of phenyl, pyridyl, and pyridinio substituents upon keto-enol tautomerisation and enol ionisation equilibria of acetophenone shows that polar effects upon bond hybridisation and resonance interaction with 'neutral' double bonds are important influences upon enol stability.In the past five years the availablity of measurements of keto-enol tautomerisation constants of a number of simple aldehydes and ketones, including acetaldehyde, acetone, and acetophenone ,1--4 has made possible an examination of substituent effects upon these equilibria. 135.6 Steric effects have been discussed in detail by Rappoport,s,6 and the purpose of this communication is to consider electronic effects. These are interesting because keto-enol equilibria involve only neutral molecules whereas electronic effects normally reflect resonance or inductive stabilisation of ionic charges.Enols can be substituted at aor P-carbon atoms [see (2)]. At either position the main effect of the substituent is upon the double bond of the enol. At the a-position however the effect is complicated by a compensating interaction with the carbonyl group of the keto tautomer,176 and for aryl and alkyl groups this leads to opposite effects of substituents upon the equilibrium at the the two positions. Thus the enol content of acetaldehyde (pKE = 6.23 in aqueous solution)4 is greater than that of acetophenone (pKE = 7.96) but much less than that of diphenylacetaldehyde7 (pKE = 0.98) or 9-formyl-fluorene8 (4) (pKE = -1.7).
Reactivity and selectivity are discussed in terms of quadratic rate—equilibrium relationships for reaction series sharing a common intrinsic barrier, notably Marcus' equation. R.D. Levine's emphasis on such reactions as forming a “Brønsted Series” is suggested as a basis for defining “ideal” reactivity behavior. Ideal relationships between selectivity and reactivity and selectivity and equilibrium are derived based on reaction schemes drawing a formal distinction between reagent and substrate. In a simple Brønsted relationship, the reagent varies and the substrate is held constant. In an extended relationship, in which both substrate and reagent vary, the Brønsted series becomes a selectivity “network” or matrix characterized by direct and cross quadratic coefficients reflecting curvature of a free energy relationship and the dependence of selectivity upon rate or equilibrium respectively. In the ideal case, the quadratic coefficients are equal but, in practice, they generally differ. Inspection of the relevant reaction schemes shows that in evaluating the coefficients, there is greater cancellation of experimental and chemical scatter arising from non‐systematic variations in reagent structure for the cross than the direct coefficient; this phenomenon accounts in part for the wider observation of reactivity—selectivity than non‐linear free energy relationships. New measurements are reported for reactions of carboxylic acids with enolate and imine substrates. The results are combined with literature data and plotted as reactivity—selectivity and extended Brønsted relationships. The correlations are satisfactorily considered in the context of departures of real from ideal behaviour.
Measurement of equilibrium constants for keto-enol tautomerism (KT) and ionisation (K,) of 1phenacylpyridinium and 1 -phenacyl(4-dimethylamino) pyridinium ions gives pK, (-log KT) = 6.1 0 and 5.55 and pK, = 10.90 and 13.2 respectively. The enol content and acidity of the 1phenacylpyridinium ion is lower than that of its 2-, 3and 4-isomers, and the possibility that this reflects impaired -M resonance by a 1 -pyridinium substituent is discussed. Notional (proton) activating factors reflecting the influence of the positive charge of the 1 -pyridinium substituent upon equilibrium ionisation and rates of deprotonation by lutidine and hydroxide bases are estimated from free energy correlations as lo3, 17 and 5 x lo3 respectively. These compare with a (methyl) activating factor of 1 O9 derived from equilibrium ionisations of 4-chlorobenzaldehyde oxime and nitrone and a (notional) value of lo8 for pyridine-N-oxide. The implications of these values for the activating effect of N-protonation of an azomethine group in models for pyridoxalcatalysed azomethine rearrangements are discussed.
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