Photocatalytic hydrogenation of dienes with chromium carbonyls

The photochemical hydrogenation of norbornadiene (NBD) in the presence of Cr(CO)6 or Cr(CO)4NBD at normal pressure yields nortricyclene (NTC) and norbornene (NBN) in a ratio of 3/1. With increasing hydrogen pressure the NTC/NBN ratio changes to 0.8/1 at 100 bar H2, due to a faster formation of norbornene. The formation of NBN is more strongly inhibited than that of NTC by 50 bar of CO. Other conjugated dienes such as cyclohexadiene, isoprene, or 1,3-pentadiene give exclusively 1.4-hydrogenation products. The results are rationalized by a mechanism in which the 1.4-addition and the NTC formation are initiated by a CO dissociation, whereas the norbornene is formed via initial cleavage of a Crdiene bond in the Cr(CO)4diene catalyst.