[1,5]-Sigmatropic hydrogen shift template

A hydrogen moves suprafacially from one end of a five-carbon π system to the other: degenerate in (Z)-penta-1,3-diene (shown by D labelling), and in cyclopentadienes it walks substituents round the ring (5-methyl → 1-methyl → 2-methyl).

Mechanism step by step

A suprafacial [1,5]-H shift moves substituents round a cyclopentadiene ring at room temperature

1. Open chain: (Z)-penta-1,3-diene – In the s-cis diene the C5–H σ bond and two π bonds move round a six-membered cycle: H goes from C5 to C1 on the same face
2. In a ring – A hydrogen of the CH₂ moves to the neighbouring ring carbon: the methyl group ends up on C2

Key points

• Six electrons (σ C–H + two π bonds): the thermal [1,5]-H shift is suprafacial and allowed (Woodward–Hoffmann, 4n + 2).
• FMO picture: the H 1s orbital bridges the termini of the pentadienyl radical SOMO (ψ3), which have the same phase on one face.
• In a five-membered ring the “1,5” shift looks like a 1,2 shift: 5-substituted cyclopentadienes must be kept cold (below about −20 °C) or they isomerise.
• Thermal [1,3]-H shifts would need an antarafacial path (impossible geometrically); [1,7]-H shifts go antarafacially (previtamin D₃ → vitamin D₃).

Drawn with 42 atoms, 10 arrows, 4 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 35 Textbook
• Fleming, Molecular Orbitals and Organic Chemical Reactions, Wiley 2010 Textbook