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

Pt(eta(5)-C5Me5)(PR3)I (PR3 = PPh3, PPhCy2, (PPhMeBu)-Bu-t) reacts with O-3(2) to form the metal-bound endoperoxide Pt(eta(1),eta(2)-C5Me5O2)(PR3)I; analogous reactions of Pt(eta(5)-C5Me5)(PPh3)I with a variety of other dienophiles to form Pt(eta(1),eta(2)-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 eta(5)-C5Me5 ring.