The mechanism of propylene oxidation to acrolein over bismuth molybdate, copper oxide, and rhodium catalysts

The oxidation of (E)-propene-1-d₁ to acrolein over bismuth molybdate, copper oxide, and rhodium catalysts was studied to determine if the reaction proceeded with (Z)-(E) randomization of the deuterium stereochemistry. Over Bi₂Mo₂O₉ and Bi₂MoO₆, (E)-acrolein-3-d₁:(Z)-acrolein-3-d₁: acrolein-1-d₁ was 1:1:1 consistent with a σ-allyl intermediate which rapidly converts between two equivalent forms (^*CHDCHCH₂ h CHDCH^*CH₂). With copper oxide, (E)-acrolein-3-d₁: (Z)-acrolein-3-d₁: acrolein-1-d₁, was 1: 1: 1.6. This points to a discrimination isotope effect consistent with a σ-allyl intermediate without interconversion of equivalent forms. Over a Rh/α-Al₂O₃ catalyst, (E)-acrolein-3-d₁: (Z)-acrolein-3-d₁: acrolein-1-d₁ was 1: 0.93: 0.89. This is consistent with an allyl intermediate along with a second minor nonallylic pathway which does not equilibrate the terminal carbon atoms. One or both of these processes occurred with some retention of the (E)-deuterium stereochemistry in the acrolein-3-d₁. Over an unsupported Rh catalyst, (E)-acrolein-3-d₁: (Z)-acrolein-3-d₁: acrolein-1-d₁ was 1: 0.68: 0.85. Only the allylic pathway is evident and the reaction process occurs with incomplete randomization (76 ± 10%) of the (E)-deuterium stereochemistry in the acrolein-3-d₁.