Conformational analysis and electronic interaction study of some ortho-substituted alpha-ethylsulfonylacetophenones; and the oxidative rearrangement study of some ortho-substituted alpha-ethylthioacetophenones / Análise conformacional e estudo das interações eletrônicas em alfa-etilsulfonilacetofenonas-orto-substituídas e o estudo do rearranjo oxidativo de alfa-etiltioacetofenonas-orto-substituídas

Emilene Maria de Andrade

The first part of this thesis reports the conformational study of some -ortho-substituted α-ethylsulfonylacetophenones o-X-PhC(O)CH2SO2Et (X= OMe 1, Me 2, F 3, Cl 4, Br 5 e NO2 6) through the analysis of the carbonyl stretching IR band in solvents of increasing polarity supported by HF/6-31G** ab initio computations. The good match between IR frequencies and intensities of the carbonyl doublet components in carbon tetrachloride and the results of the calculations indicated that the higher frequency component corresponds to the quasi-cis (anti) conformer for derivatives 1 and 3 (conc. 40%) and to the quasi-gauche (syn) conformer for derivative 2 (conc. 30%). For the halogenated derivatives 4 and 5 and for the nitro-derivative 6 the referred component is related to the gauche2 (syn) conformer (conc. 0-20%) and to the gauche2 conformer (conc. 17%), respectively. The lower doublet frequency component is in general related to the most stable gauche conformation., i.e. gauche(anti) for 1 and 3 (conc. 60%) and gauche(syn) (conc. 70%) conformers for 2. For the halogenated derivatives 4 and 5 and the nitro-derivative 6 the referred component is related to the gauche (anti) conformer (conc. 100-80%) and to the gauche1 conformer (conc. 83%), respectively. These trends are discussed in terms of orbital (charge transfer) and electrostatic interactions between pairs of oppositely charged atoms and their variation with the electronic and steric properties of each X substituent throughout the series 1-6.The main interactions which act stabilizing both conformations are: Oδ- SO2...Cδ+CO, Oδ-CO...Sδ+SO2, Xδ-...Cδ+CO, Xδ-...Hδ+CH2, Oδ-CO...Hδ+Ph, Oδ-CO...Hδ+ Et .However the short contacts between the Oδ-SO2...Oδ-CO and Oδ-CO...Xδ atoms for derivatives 1-6 and 4, 5, respectively, whose interatomic distances are close to the sum of their van der Waals radii are responsible for the strong Repulsive Field Effect which occur in the higher frequency conformer leading to its destabilization relative to the more stable lower frequency conformer. The second part of this thesis deals with the mechanistic and synthetic study of the Baeyer-Villiger oxidative rearrangement of some ortho-substituted α-ethylthioacetophenones bearing at the ortho- position the OMe, F, Cl e Me substituents. These reactions were performed with the peracetic and perselenious acids. The obtained products were analysed through IR and 1H NMR spectroscopy. The oxidative rearrangement reactions involving the keto-sulfides, keto-sulfoxides and keto-sulfones with perselenious acid gave the ortho-substituted α-ethylsulfonyl-phenylacetate (ester-sulfone) when the o-substituent is the methoxy-group only. However in the case of the o-F, o-Cl and o-Me the keto-sulfones were the unique products obtained without the formation of any rearranged ester-sulfone. As for the oxidation with peracetic acid of both keto-sulfides and keto-sulfoxides, bearing at the ortho-position the same substituents, the obtained ester-sulfone/keto-sulfone proportions are very close between them. In fact the obtained ratio decreases progressively on going from X = OMe (0.5:0.5) to X = F (0.18:0.82), to X = Cl (0.13:0.87) and to X = Me (0.3:0.7). This trend has been ascribed to a progressive decreasing stabilization of the incipient carbocation formed in the transition state of the Baeyer-Villiger reaction once the mesomeric effect (σR) of the ortho-substituent decreases progressively on going from MeO (-0.42) to F (-0.31) to Cl (-0.18) and to Me (-0.13). In the case of oxidation of the keto-sulfones with peracetic acid the unique ester-sulfone obtained was the o-methoxy derivative whose ester-sulfone/keto-sulfone ratio is 0.4/0.6. In fact in the case of the keto-sulfones bearing F, Me and Cl substituents no oxidative rearrangement products have been detected. This comparative study indicated that the keto-sulfide is promptly oxidized to the keto-sulfoxide which undergoes a Baeyer-Villiger rearrangement, through a cyclic intermediate, giving the ester-sulfoxide, which is further oxidized to an ester-sulfone. For the ortho-methoxy α-ethylsulfonylacetophenone (keto-sulfone) which was the only keto-sulfone where the oxidative rearrangement with both the peracetic and perselenious acids took place, a similar cyclic intermediate has been suggested in order to explain the referred rearrangement.

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