Q2(b). 1,2-Distyrylbenzene from phthalic acid by Wittig reaction

Introduction. The target is 1,2-bis(2-phenylethenyl)benzene. Disconnecting both C=C bonds (Wittig disconnection) gives either phthalaldehyde + 2 PhCH=PPh3 or the bis-ylide of o-xylene + 2 PhCHO. Phthalic acid provides the ortho-disubstituted ring; its two CO₂H groups are first brought to the aldehyde (or CH₂Br) level.

Route: phthalaldehyde and benzylidenetriphenylphosphorane

Scheme: Steps 1–3: the diacid is taken to the dialdehyde.

1. Esterification: phthalic acid + methanol (H₂SO₄) gives dimethyl phthalate. (The acid itself can also be reduced by LiAlH₄, but the ester reacts more cleanly.)
2. Reduction: LiAlH₄ reduces both ester groups to CH₂OH: 1,2-benzenedimethanol.
3. Mild oxidation: PCC (or Swern oxidation) stops at the aldehyde: phthalaldehyde (benzene-1,2-dicarbaldehyde).
4. Ylide: benzyl bromide + PPh₃ (SN2) gives benzyltriphenylphosphonium bromide; BuLi (or NaOEt) removes the benzylic proton: PhCH=PPh₃ (a semi-stabilised ylide).
5. Double Wittig reaction: two equivalents of the ylide add to the two CHO groups. Each addition forms a betaine/oxaphosphetane, which collapses to C=C and Ph₃P=O (strong P=O bond, the driving force).

Scheme: Two Wittig olefinations on one molecule.

Alternative route through the bis-ylide

1,2-Benzenedimethanol + PBr₃ gives α,α'-dibromo-o-xylene; with two PPh₃ it forms the bis(phosphonium) salt, C₆H₄(CH₂PPh₃⁺)₂ 2Br⁻. Two equivalents of base give the bis-ylide, which reacts with 2 PhCHO to give the same product. This route avoids handling the easily oxidised dialdehyde.

Stereochemistry. Benzylic (semi-stabilised) ylides give mixtures of E and Z alkenes. The mixture is isomerised to the more stable (E,E) isomer by a trace of iodine (light) or heating.

Conclusion. Phthalic acid → dimethyl phthalate (MeOH/H⁺) → 1,2-benzenedimethanol (LiAlH₄) → phthalaldehyde (PCC) 2 PhCH=PPh₃ 1,2-distyrylbenzene + 2 Ph₃P=O; E/Z mixtures are equilibrated to E,E with I₂.

Application. Double Wittig reactions on aromatic dialdehydes make stilbene-type optical brighteners and the precursors of helicenes and phenanthrenes (photocyclisation of stilbenes).