Tautomerism and metal complexation of 2-acylmethyl-2-oxazolines: a combined synthetic, spectroscopic, crystallographic and theoretical treatment

Abstract

A synthetic, structural and theoretical investigation into the solid-state, solution and gas phase structure(s) of six 2-acylmethyl-4,4-dimethyl-2-oxazolines is reported. Four of these materials, viz.α-[(4,5-dihydro-4,4-dimethyl-2-oxazolyl)methylene]benzenemethanol (3a), α-[(4,5-dihydro-4,4-dimethyl-2-oxazolyl)methylene]-(4-nitrobenzene)methanol (3b), 1-(4,5-dihydro-4,4-dimethyl-2-oxazolyl)-3,3-dimethyl-1-buten-2-ol (3d) and (E)-1-phenyl-2-((3aR)-3,3a,8,8a-tetrahydro-2H-indeno[1,2-d]oxazol-2-ylidene)ethanone (3f) have been characterised in the solid-state by single crystal X-ray diffraction studies. These data represent the first solid-state structural studies of this class of compounds and details the first synthesis and full characterisation of chiral derivative 3f. All four of these materials are shown to exist in the solid phase in the enamine tautomeric form (e.g., 3a is best described as 2-[4,4-dimethyl-2-oxazolidinylidene]-1-phenylethanone) and it is suggested (NMR, IR) that this isomeric form is likely also retained in solution (e.g., CDCl3) as the more stable isomer. An investigation of the relative gas phase stabilities of the three possible (i.e., the (Z)-enol, keto and enamine) isomers of all five compounds by DFT at the B3LYP/6-311G(d) level of theory confirms the latter as the most stable form. The energy differences between the enamine and keto tautomers have been calculated to be the lowest for derivative 3d. These results are compared and contrasted with the previously reported NMR studies of such compounds which have identified the keto form as being a minor (albeit solution) tautomer. Equilibrium solution tautomer distributions for 3d are found to be solvent dependent. The protonated form of 3a, isolated as the HSO4(-) salt (i.e.4a), has been further characterised in the solid state by single crystal X-ray diffraction. These data represent the first example of a protonated oxazoline to be structurally elucidated and confirms that upon protonation, the keto (oxazoline) tautomer is the energetically favoured form in the solid-state. This observation is further supported by DFT studies for the gas phase protonated forms of such materials. Further DFT (B3LYP/6-311G(d)) calculations employing the SM8 or SMD solvation models were then applied to address the observed solution isomeric distribution for 3d; these results corroborate the gas phase theoretical treatment and also yield values that predict the higher solution stability of the enamine form as observed, although they fail to account for the existence of the keto form as a minor solution state tautomer. To access the availability of an enol-form, via hypothetical de-protonation to the enolate, compound 3a was treated with hydrated Cu(NO3)2 in EtOH solution. The resulting isolated green-coloured product (5), the first metal derivative of this entire class of ligands, is best described (IR, X-ray diffraction) as a coordinated enolate complex, i.e., Cu(3a-H)2. Complex 5 crystallizes in the P21/c space group with four molecules in the unit cell. T...

Synthesis and Antimicrobial Activity of 2-Trifluoroacetonylbenzoxazole Ligands and Their Metal Complexes

“…33,34) In the Cu(II) complex 2f, the Cu(II) atom was coordinated by two N atoms and two O atoms from two 1a, forming a strictly square planar geometry (dihedral angle between two N-Cu-O planes: 0°). [35][36][37] In the Pd(II) complex 2g, the geometry at Pd(II) center was coordinated as nearly perfect square planar which are similar to that of the Cu(II) atom in 2f. 38) The crystal structure of Ag(I) complex 2h revealed to consist a 1 : 1 metal-ligand molar ratio and the Ag(I) ion exhibits a distorted trigonal pyramidal geometry by two 1a and the another Ag(I) atom, forming a 4-membered parallelogram Ag 2 O 2 ring.…”

“…1 [29][30][31][32][33][34][35][36][37][38][39] In the Zn(II) complex 2b, the geometry at the Zn(II) center can be best described as a distorted tetrahedron in which two benzoxazole-2-chlorodifluoroacetonate (1b) ligands are placed in a similar disposition about the zinc center. [29][30][31] One coordination plane (N-O-Zn) in 2b is approximately perpendicular to the other with a dihedral angle of 89.2°.…”

Divalent cobalt and copper coordination complexes of κ2-N, O-derivatives of (Z)-1-R-2-(2′-oxazolin-2′-yl)-eth-1-en-1-ates: Structure and reactivity patterns

“…[28][29][30][31] <Insert Figure 1> These organics have been investigated in some detail by Yasuo Tohda since 1984 and indeed random related examples of this general structural class have been studied since the 1970s. [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] The compounds themselves exist in the solid-state in the enamine form (Figure 1: centre) and in some cases equilibrate in solution to include the keto isomer (Figure 1: right). [28-31, 32, 37, 43] This aspect depends on both the solvent and the nature of the R / R' groups.…”

“…[28][29][30][31][32][33][34][35][36][37][38][39][40] Protonation at the N-atom leads to complete conversion to the keto tautomer. [43] The presence of base typically leads to H + loss and the formation of a reactive enolate. [28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] Despite the fact that these materials have been studied for many years, there are only a few reports of metal chelation by these reagents and their analogues.…”

“…[28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] Despite the fact that these materials have been studied for many years, there are only a few reports of metal chelation by these reagents and their analogues. [41][42][43] Sometime ago, we briefly reported on such a Cu(II) example of formula bis-([Z]-1-phenyl-2-{4',4'-dimethyl-2'oxazolin-2'yl}-eth-1-en-1-ate)copper(II), i.e., complex 3aa: Figure 2).…”