Kinetics of Diosmacyclobutane Exchange Reactions

Abstract

Diosmacyclobutanes Os2(CO)8(olefin) undergo facile exchange reactions with olefins and acetylenes. An associative mechanism is excluded by the observation of saturation kinetics as the concentration of the entering olefin (butyl acrylate) is increased. Multivariate analysis of the rate as a function of [C2H4] (determined from solubility measurements in decane/butyl acrylate mixtures) and [butyl acrylate] suggests a stepwise mechanism in which the departing olefin migrates to a terminal position and then undergoes associative exchange with the entering olefin. The straightforward dissociative exchange of the olefin ligands in Os(CO)4(olefin) shows saturation at much lower concentrations of entering olefin.

Evidence for a Ring-Opening Preequilibrium in the Exchange Reactions of Diosmacyclobutanes

“…The existence of an intermediate is necessary in order to explain the anomalous stability of 7 under conditions where the formation of the ring-opened intermediate is reversible. (As shown in Figure of the previous paper, a [BA] of 2 M is well below thatneat BA!needed to make eq 2 irreversible and its forward step rate-determining.) Any irreversible transformation of the diosmacyclobutanes 1 , 7 , and 8 should reflect the same order of ground-state stabilities observed in the mononuclear reactivity order Os(CO) 4 (ethylene) < Os(CO) 4 (propene) ( 6 ) < Os(CO) 4 ( trans -2-butene) ( 9 ).…”

“…Syntheses of starting materials, purification of solvents and reagents, instrumentation, and the collection and analysis of kinetic data were described in the previous paper …”

“…The resulting solution was diluted to 10 mL ([BA] = 206.8 mM, [ 6 ] = 14.6 mM; [BA]:[ 6 ] = 14.2). The tube was placed in a 65.0 °C bath and the reaction was monitored for 239.2 min; for a the collection and analysis of kinetic data see the preceding paper . A small peak that grew in at 2085 cm -1 suggested the presence of 3 .…”

Organometallic Sandwich Chains Made of Conjugated Polyenes and Metal−Metal Chains

“…The 1 H NMR spectra of 1 ‘ in CD 2 Cl 2 or acetone- d 6 showed only sharp symmetrical DPOT signals at 23 °C, which were significantly broadened at −90 °C . Although we were unable to obtain well-characterizable resonances at temperatures lower than −90 °C, the spectral aspects suggest occurrence of a facile slippage of the DPOT ligands along the Pd 4 array, forming a symmetrical η 2 : η 2 : η 2 : η 2 -complex (Chart ) . It is possible that this complex, which may or may not be coordinated with solvents at the terminals of Pd 4 chain, lies as an equilibrium species with the asymmetrical form 1 ‘ or as an intermediate in the 1 H NMR symmetrization process of 1 ‘.…”

Selective E to Z isomerization of 1,3-Dienes Enabled by A Dinuclear Mechanism

“…We then examined the trans-olefination. Norton et al previously revealed that the reversible trans-olefination of the diosmacyclobutane complex Os 2 (μ-η 1 :η 1 -C 2 H 4 )(CO) 8 proceeds in a stereoretentive manner 36 , 37 . Our group also reported that the trans-olefination of the μ-η 3 :η 3 -1,3,5-triene Pd 2 adduct with a linear 1,3,5-triene proceeds in a stereoretentive manner 25 .…”