Highly functionalized alkenes produced from base-free organocatalytic Wittig reactions: (E)-3-benzylidenepyrrolidine-2,5-dione, (E)-3-benzylidene-1-methylpyrrolidine-2,5-dione and (E)-3-benzylidene-1-tert-butylpyrrolidine-2,5-dione

<jats:p>The Wittig reaction is a fundamental transformation for the preparation of alkenes from carbonyl compounds and phosphonium ylides. The ylides are prepared prior to the olefination step from the respective phosphonium salts by deprotonation utilizing strong bases. A first free-base catalytic Wittig reaction for the preparation of highly functionalized alkenes was based on tributylphosphane as the catalyst. Subsequently we developed a system employing a phospholene oxide as a pre-catalyst and trimethoxysilane as reducing agent which operates under milder conditions. The title compounds, (<jats:italic>E</jats:italic>)-3-benzylidenepyrrolidine-2,5-dione, C<jats:sub>11</jats:sub>H<jats:sub>9</jats:sub>NO<jats:sub>2</jats:sub>, (I), the methylpyrrolidine derivative, C<jats:sub>12</jats:sub>H<jats:sub>11</jats:sub>NO<jats:sub>2</jats:sub>, (II), and the<jats:italic>tert-</jats:italic>butylpyrrolidine derivative, C<jats:sub>15</jats:sub>H<jats:sub>17</jats:sub>NO<jats:sub>2</jats:sub>, (III), have been synthesized by base-free catalytic Wittig reactions. In the crystal of (I), molecules are linked into centrosymmetric dimers<jats:italic>via</jats:italic>pairs of N—H...O hydrogen bonds. Furthermore, in the crystal structure of (III), there are two molecules in the asymmetric unit, whereas in (I) and (II), only one molecule is present.</jats:p>