EVIDENCE FOR A RING-OPENING PREEQUILIBRIUM IN THE EXCHANGE-REACTIONS OF DIOSMACYCLOBUTANES

Variable-temperature C-13 NMR does not show any evidence for intramole cular ethylene rotation in 1-C-13. The rates of alkene dissociation fo r the propene (8) and trans-2-butene (7) adducts of Os-2(CO)(8) have b een measured in hydrocarbon solution and compared with the rates of al kene dissociation from the corresponding Os(CO)(4)(alkene) adducts 6 a nd 9. The kinetic labilities of propene and trans-2-butene are reverse d in the Os-2(CO)(8)(alkene) and Os(CO)(4)(alkene) systems; propene is replaced 2.5 times faster than trans-2-butene in the Os-2(CO)(8)(alke ne) system, while trans-2-butene is replaced 55.9 times faster than pr opene in the Os(CO)(4)(alkene) system. We have used molecular mechanic s to explore the reasons for this unusual reactivity pattern and have found that these results may be easily reconciled with a ring-opening mechanism for alkene replacement in the Os-2(CO)(8)(alkene) system. We have confirmed that alkene exchange with Os(CO)4(alkene) is dissociat ive, in agreement with precedent. The secondary deuterium kinetic isot ope effect (KIE) has been measured for the replacement of C2H4 and C2D 4 in Os-2(CO)(8)(mu-eta(1),eta(1)-C2H4) (1) and Os-2(CO)(8)(mu-eta(1), eta(1)-C2D4) (1-d(4)); it is 1.30(1) at 39 degrees C. The measured KIE is consistent with a ring-opening associative mechanism for alkene ex change (mechanism II in the previous paper).