Solid‐State NMR Study of the Tautomerism of Acetylacetone Included in a Host Matrix

Abstract

The tautomerism of the enol form of acetylacetone (=pentane‐2,4‐dione; 1) inside a host cavity has been studied by means of solid‐state 13C‐NMR spectroscopy (SSNMR) using the variable‐temperature CPMAS technique. It appears that the enol form, 4‐hydroxypent‐3‐en‐2‐one (1a), exists in an equilibrium with an identical tautomer (1c) trough OH ⋅⋅⋅O proton transfer. The experimental results (energy barrier and chemical shifts) were rationalized by means of MP2 and GIAO calculations.

Structural and electrochemical studies of Co(II), Ni(II), Cu(II) and Cd(II) complexes with a new symmetrical N2O2 Schiff base: Crystal structure of the ligand 1,2-di[4-(2-imino 4-oxo pentane)phenyl]ethane

“…Though the ligand crystallises in keto-imine tautomer I, the solution NMR spectra of both compounds show signals corresponding to keto-amine tautomer III: the singlet at 12.3 ppm corresponds to the NAH proton [16] (the hydroxyl proton of the enol-imino tautomer leads to a singlet in the 8-9 ppm range [23]). This signal could be also observed in the spectrum of the cadmium complex with the same chemical shifts as it has been observed in similar compounds [24,25]. The signal of the aromatic protons, of the ethylene moiety and of the methyl groups are not modified by coordination.…”

Curcumin as the OO Bidentate Ligand in “2 + 1” Complexes with the [M(CO)3]+ (M = Re, 99mTc) Tricarbonyl Core for Radiodiagnostic Applications

“…This is in agreement with a number of studies on complexes of the enolate form of β-diketones with a variety of divalent and trivalent ions that report small or nonexistent chemical shift changes upon coordination. In the 13 C spectra of complex 2 (and the related fac- Re(CO) 3 (acac)(isc) and fac- Re(CO) 3 (acac)(pyr) complexes), it is interesting to note that while the C-1 and C-3(C-3′) carbons shift downfield by approximately 1.5 and 2.5 ppm, respectively, compared to plain acetylacetone, the C-2(C-2′) carbonyl carbons are shifted upfield by approximately 2.5 ppm, apparently reflecting an increase in π-electron density at the carbonyl carbons in this type of complexes.…”

“…This is in agreement with a number of studies on complexes of the enolate form of β-diketones 21 with a variety of divalent and trivalent ions that report small or nonexistent chemical shift changes upon coordination. In the 13 C spectra of complex 2 (and the related fac-Re(CO) 3 (acac)(isc) 1 and fac-Re(CO) 3 (acac)(pyr) 2 complexes), it is interesting to note that while the C-1 and C-3(C-3 0 ) carbons shift downfield by approximately 1.5 and 2.5 ppm, respectively, compared to plain acetylacetone, 22 the C-2(C-2 0 ) carbonyl carbons are shifted upfield by approximately 2.5 ppm, apparently reflecting an increase in π-electron density at the carbonyl carbons in this type of complexes. For the imidazole coligand, upon coordination downfield shifts of approximately 0.2 and 3 ppm are noted for H-1 00 and C-1 00 , respectively, relative to their values in free imidazole in the same solvent (our data, Figure S1, Supporting Information), while the previously equivalent H-2 00 , H-3 00 , C-2 00 , and C-3 00 are magnetically differentiated.…”

A theoretical study of dynamic processes observed in trimethylsilyl-1H-pyrazoles: prototropy and silylotropy

“…Although prototropic tautomerism is important in general organic chemistry, for instance in β-diketones [3], most results come from heterocycles [2], particularly from azoles that have a N-substituted nitrogen atom and a N-unsubstituted one. In pyrazoles (but also in triazoles and tetrazoles), these nitrogen atoms occupy contiguous positions that facilitate the transfer of the migrating atom.…”