Influence of solvent and cation on the properties of oxygen-containing organic anions. Part 4. Mechanism and reactivity of tetraaryloxirane cleavage with alkali metals

Abstract

Six tetraaryloxiranes 1a–f(Scheme 4) were reduced (Schemes 1–3) with alkali metals (M = Li, Na, K, Cs) in eight polar aprotic solvents under an inert atmosphere. The organometallic solutions thus obtained were hydrolysed and the reaction products analysed. Similar experiments were carried out where the same solutions were quenched with D₂O or Mel. In some cases the same solutions were studied by NMR and ESR spectroscopy before quenching. A stepwise reduction mechanism was established where the transfer of a first electron produces CO-bond scission in the oxirane ring, yielding a short-lived radical anion 4 or 5(Scheme 1), i.e. a tetraaryl-β-oxidoethyl radical. This intermediate can either eliminate oxygen as metal oxide (MO) to produce a tetraarylethylene 24(Scheme 2) or be further reduced to a dianion 8 or 9(Scheme 1). This anion yields, upon hydrolysis, low yields, if any, of the corresponding tetraphenylethanol 15 or 16(Z = H). The larger proportion of the dianion, after the first protonation step, yielding anion 11 or 12, undergoes CC-bond scission which leads eventually to the corresponding ketone and diarylmethane 19+ 20 or 21+23(Z = H)(Scheme 2). Other possible pathways were excluded through experiments where other possible intermediates were generated. These led to different end products. A triparametric linear correlation as a function of solvent parameters E_T^N and DN, as well as the cationic radius, was established for the influence of the nature of the solvent and counter-ion on the ratio between the rates of formation of products stemming from metal oxide (MO) elimination by the ring-opened radical anion 4 or 5(Schemes 1 and 2) and rates of formation of products stemming from further reduction of the same radical anion to the dianion 8 or 9, thus confirming the mechanism established.