Activation of metal-bound η5-C5Me5 groups to Diels–Alder addition of 3O2 and other dienophiles

Pt(eta5-C5Me5)(PR3)I (PR3=PPh3, PPhCy2, PPhMetBu) reacts with 3O2 to form the metal-bound endoperoxide Pt(eta1,eta2-C5Me5O2)(PR3)I; analogous reactions of Pt(eta5-C5Me5)(PPh3)I with a variety of other dienophiles to form Pt(eta1,eta2-C5Me5CH2CHR)(PPh3)I (R=C(O)H, CO2Me, C(O)Et) in which the metal is anti to the incoming dienophile are indicative of direct Diels-Alder addition to the eta5-C5Me5 ring.