Abstract:an unrelated development, we had an occasion to reinvestigate the e x p e r i m e n t a l conditions for the hydroboration-oxidation of 2-methyl-2-butene with BHBr2.SMe2 and correct the regiochemistry to 99.3:0.7 at the secondary and tertiary carbons, r e s p e~t i v e 1 y . l~ The corrected regiochemistry does n o t call for an alternate m e c h a n i s m at all. Our independent studies on the various dialkylborane dimers20 and BH3.Lewis base complexes7 have shown that t h e y dissociate prior to hydroboratio… Show more
“…Consequently, the slow proton exchange is ascribed to a steric inhibition to mutual approach of two enol molecules, especially in the low dielectric aprotic medium. A similar steric hindrance for hydrogen bonding of 2,2- and 1,2-diarylethenols was previously suggested. 4 Temperature-dependent 1 H NMR spectrum of the OH region of 3a / 3b equilibrium mixture in C 6 D 5 NO 2 . 1 …”
(E)-2-(m-methoxymesityl)-1,2-dimesitylethenol (3a) isomerizes in the absence of a catalyst in solution to a 1.0:0.9 E/Z (3a/3b) equilibrium mixture. In CDCl3 the isomerization is first order in 3a within a run, but the plot of the rate constant k(obs) vs the changing [3a]0 in different runs is a half-parabola, indicating self-catalysis by more than one enol molecule. At 0.09 M enol, the isotope effect k(3a)/k(3a)-OD = 2.1. In the presence of 0.025-0.25 M pyridine-d5, the k(obs) vs [pyridine-d5] plot displays a bell-shaped profile. The change in the shape of the OH signals of the 3a/3b mixture at 295-430 K in C6D5NO2 was followed by DNMR. The four signals of the diastereomeric 3a/3b mixture observed at 330 K coalesce at 350 K with barriers of 18.3 and 18.4 kcal x mol(-1) due to the diastereomerization of the vinyl propellers. The resulting two signals observed at >360 K further coalesce at 425 K with a barrier of 22.9 kcal x mol(-1) due either to oxygen-to-oxygen proton exchange or to E/Z isomerization. The estimated upper limit for the rate of proton exchange of k(ex) < or = (2-4) x 10(3) M(-1) x s(-1) at 425 K between the enol molecules is sufficiently slow to be a rate-controlling step in the isomerization. A process in which several enol molecules catalyze the isomerization is suggested, and several mechanistic routes are analyzed.
“…Consequently, the slow proton exchange is ascribed to a steric inhibition to mutual approach of two enol molecules, especially in the low dielectric aprotic medium. A similar steric hindrance for hydrogen bonding of 2,2- and 1,2-diarylethenols was previously suggested. 4 Temperature-dependent 1 H NMR spectrum of the OH region of 3a / 3b equilibrium mixture in C 6 D 5 NO 2 . 1 …”
(E)-2-(m-methoxymesityl)-1,2-dimesitylethenol (3a) isomerizes in the absence of a catalyst in solution to a 1.0:0.9 E/Z (3a/3b) equilibrium mixture. In CDCl3 the isomerization is first order in 3a within a run, but the plot of the rate constant k(obs) vs the changing [3a]0 in different runs is a half-parabola, indicating self-catalysis by more than one enol molecule. At 0.09 M enol, the isotope effect k(3a)/k(3a)-OD = 2.1. In the presence of 0.025-0.25 M pyridine-d5, the k(obs) vs [pyridine-d5] plot displays a bell-shaped profile. The change in the shape of the OH signals of the 3a/3b mixture at 295-430 K in C6D5NO2 was followed by DNMR. The four signals of the diastereomeric 3a/3b mixture observed at 330 K coalesce at 350 K with barriers of 18.3 and 18.4 kcal x mol(-1) due to the diastereomerization of the vinyl propellers. The resulting two signals observed at >360 K further coalesce at 425 K with a barrier of 22.9 kcal x mol(-1) due either to oxygen-to-oxygen proton exchange or to E/Z isomerization. The estimated upper limit for the rate of proton exchange of k(ex) < or = (2-4) x 10(3) M(-1) x s(-1) at 425 K between the enol molecules is sufficiently slow to be a rate-controlling step in the isomerization. A process in which several enol molecules catalyze the isomerization is suggested, and several mechanistic routes are analyzed.
“…The study of the effect of solvent on the tautomerization equilibrium constants using the Kamlet and Taft multiparametric method can be used to obtain correlations between the equilibrium constant and the solvatochromic parameters. The use of linear solvation energy relationships to predict solvent effects in chemical equilibria is well documented. ,− In this way, extensive data on related keto/enol equilibria and Kamlet−Taft correlations on chemical shifts are given in the literature. − …”
A significant temperature dependence has been found for the (13)C NMR chemical shifts of pyridoxine in 10%, 20%, 30%, 40%, 50%, and 60% v/v 1,4-dioxane/water mixtures (pH = 7.0). The nuclei most sensitive to the temperature effect were C-3 and C-6 in all of the mixtures. This dependence has been explained on the basis of a thermally induced tautomeric equilibrium shift between the neutral and the dipolar forms of the pyridoxine molecule. The thermodynamic characterization of this tautomeric equilibrium, which interconverts quickly on the NMR time scale, has been achieved by considering the observed average (13)C NMR chemical shifts at different temperatures through fitting the experimental data to a theoretical curve. The fitting accuracy is greatly improved on using linear correlations between the average chemical shifts obtained from different nuclei at the same temperature. The methodology outlined above allows the DeltaH degrees value to be calculated for the tautomeric process and the chemical shifts of the pure extreme forms, i.e., neutral and dipolar, to be deduced. These values have been used to calculate the thermodynamic parameters of the tautomerization equilibrium in each dioxane/water mixture. The effect of solvent on the tautomeric equilibrium and the averaged chemical shift has been explained in terms of a multiparameter equation developed by Kamlet and Taft. The overall solvent effect is the sum of two different effects: the dipolarity and polarizability of the solvent and the ability of the solvent to act as a hydrogen-bond donor toward a solute.
“…The differences between the ∆GЊ acid values in the gas phase and in DMSO solution should reflect the differential solvation of the enol and its anionic species by DMSO. In DMSO the neutral enol is solvated by hydrogen bonding to DMSO as can be deduced by the rough correlation between log K assoc 13 and pK a (DMSO) [K assoc is the association constant of the enols with DMSO]. In addition, the charged species are solvated by the high relative permittivity solvent.…”
Section: Acidity and K Enol Correlationsmentioning
The equilibrium constants for the ionization of nine stable simple enols and of eight ketones were determined by the ion cyclotron resonance (ICR) method in the gas phase. From these pK a values, seven ketone-enol equilibrium constants K Enol were calculated. The most acidic enol in the series Mes 2 C᎐ ᎐ C(OH)R ( 3) is when R = p-CF 3 C 6 H 4 (∆GЊ acid = 323.4 kcal mol Ϫ1 ) and the least acidic one is when R = t-Bu (∆GЊ acid = 334.7 kcal mol Ϫ1 ). There is a good correlation between the ∆GЊ acid values for the ketones and the enols. For 4 α-aryl-substituted enols (R = Ar) and their keto isomers, there is a rough correlation with Hammett's σ values. The ∆GЊ acid 's for the enols where R = Ar correlate linearly with their ∆GЊ acid 's in hexane, but other enols deviate from the relationship. The pK a 's and pK Enol values were calculated by RHF/3-21G* and some values were calculated by B3LYP/3-21ϩG*. The observed ∆GЊ acid 's for the enols give an approximately linear correlation with the calculated ∆EЊ acid values. However, the ∆GЊ eq values for the keto-enol equilibrium in the gas phase or in hexane do not correlate linearly with ∆E eq for all the enols. The calculated Ar-C᎐ ᎐ C dihedral angles in the enols 3 change only slightly to modestly on ionization, presumably due to a relatively rigid geometry caused by steric hindrance. The substituent effects on the acidities and the K Enol values are discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
