¹⁷O NMR CHEMICAL SHIFTS OF 1,1,1-TRIFLURO-

4-DIMETHYLSULFOXIMIDE-3-ALKEN-2-ONES

Helio G. Bonacorso (PQ), Marcos A. P. Martins* (PQ), Nilo Zanatta (PQ),

Sandra R.T. Bittencourt (PG)

Departamento de Química, Universidade Federal de Santa Maria

97.105-900 Santa Maria, RS, Brazil

Key words: ¹⁷O NMR, substituent effects, ketones, sulfoxides

It is well documented that ¹⁷O chemical shifts are very sensitive to different intramolecular interactions and the influence of small changes in molecular structure on the ¹⁷O chemical shift of several functional groups have been extensively reported¹. The substitution of a hydrogen by a methyl group at various positions in organic compounds containing oxygen atom induces ¹⁷O chemical shift perturbations² which have been interpreted in terms of the substituent chemical shifts (SCS), a concept³ widely used in ¹³C NMR. Despite the abundance of ¹⁷O NMR data for aliphatic compounds^1,2,4 in the literature, there is little work devoted to the systematic determination of substituent oxygen chemical shifts in organic compounds⁵. The use of empirical relationships in NMR are extremely useful for the structural determination of unknown compounds as well as their conformational analysis. The aim of this work is to study the substituent chemical shift (SCS) of ¹⁷O chemical shifts in a series of 1,1,1-trifluro-4-dimethylsulfoximide-3-alken-2-ones.

The ¹⁷O chemical shifts values of O2 and O6 in a series of 1,1,1-trifluro-4-dimethylsulfoximide-3-alken-2-ones 1 (Scheme), can be described using the additivity relationship given in Eq. 1, where C and B are constants, Ai_nm is the additive parameter of substituent in position i, and nm are the numbers that indicate the position i with respect to oxygen atom.

1

Scheme

d¹⁷O = C + B • å Ai_nm [1]

d_O2 = 446.2 + (d₄₂ + g₃₂) [2]

d_O6 = 126.0 + (d₄₆ + e₃₆) [3]

For determination of the additive parameters, Ai_nm, the experimental data of non-substituted compound 1 was used. Considering that the effect of the substituent R¹ and R² on the ¹⁷O chemical shift of O2 and O6 are additive and independent, we can write the Eqs. 2 and 3, where d₄₂, d₄₆, g₃₂, e₃₆ are the additive parameters of the substituents R¹ and R². The effect of R¹ on O2 (d₄₂) show a deshielding of the oxygen atom (+16 ppm for R¹ = Me, Et, n-Pr; +5.5 ppm for iso-Pr, tert-Bu; and +19 ppm for Aryl). Also the effect of R¹ on O6 (d₄₆) show a deshielding of the oxygen atom (+18.5 ppm for R¹ = Me, Et, n-Pr; +12.5 ppm for iso-Pr, tert-Bu; and +15 ppm for Aryl). The effect of R² on O2 (g₃₂) show a deshielding of the oxygen atom (+44 ppm for R² = Me), and on O6 (e₃₆) show a deshielding of the oxygen atom (+4.5 ppm for R² = Me). The Ai_nm observed for a series of compounds 1 showed the same trend that was observed for a series of 1,1,1-trichloro-4-alkoxy-3-alken-2-ones previously studied in our laboratory⁵.

The ¹⁷O NMR spectra were recorded on a Bruker DPX 400 at 54.25 MHz, 323 ±1 K, at natural abundance, in acetonitrile-d₃. The spectra were recorded with a RIDE (RIng Down Eliminate) sequence⁶ for suppression of acoustic ringing. The general reproducibility of chemical shift data is estimated to be better than ±1.0 ppm (±0.2 within the same series). The half-height widths were in the range 200-700 Hz. All spectra were acquired in a 10mm, the concentration of the compounds employed in these experiments was 2.0 M, and the signals were referenced to external H₂O (in a capillary coaxial tube), d(H₂O) = 0.0 ppm.

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(CNPq, PADCT II and III, CAPES, FAPERGS)