Generation of Cyclopentadiene for Diels–Alder Reactions by Visible‐Light Irradiation of Iron Sandwich Complexes.

Cyclopentadienes readily undergo fast and selective Diels–Alder reactions which can be used for conjugation of biomolecules and polymers. Herein we present a new method for the photochemical generation of cyclopentadienes by the visible light irradiation of the iron complexes [(C5H4R)Fe(arene)]+. This reaction proceeds at room temperature, in water, and in the presence of potentially interfering amino acids. Free cyclopentadiene is trapped by dienophiles to give the corresponding Diels–Alder adducts in 72–95 % yield. [ABSTRACT FROM AUTHOR]