Abstract
The use of solid–state molecular organometallic chemistry (SMOM–chem) to promote the efficient double bond isomerization of 1-butene to 2-butenes under flow–reactor conditions is reported. Single crystalline catalysts based upon the σ-alkane complexes [Rh(R2PCH2CH2PR2)(η2η2-NBA)][BArF4] (R = Cy, tBu; NBA = norbornane; ArF = 3,5-(CF3)2C6H3) are prepared by hydrogenation of a norbornadiene precursor. For the tBu-substituted system this results in the loss of long-range order, which can be re-established by addition of 1-butene to the material to form a mixture of [Rh(tBu2PCH2CH2PtBu2)(cis-2-butene)][BArF4] and [Rh(tBu2PCH2CH2PtBu2)(1-butene)][BArF4], in an order/disorder/order phase change. Deployment under flow-reactor conditions results in very different on-stream stabilities. With R = Cy rapid deactivation (3 h) to the butadiene complex occurs, [Rh(Cy2PCH2CH2PCy2)(butadiene)][BArF4], which can be reactivated by simple addition of H2. While the equivalent butadiene complex does not form with R = tBu at 298 K and on-stream conversion is retained up to 90 h, deactivation is suggested to occur via loss of crystallinity of the SMOM catalyst. Both systems operate under the industrially relevant conditions of an isobutene co-feed. cis:trans selectivites for 2-butene are biased in favor of cis for the tBu system and are more leveled for Cy.
Highlights
The Shell higher olefins process relies on an isomerization/metathesis step to maximize the production of desired C12−20 α-olefin fractions,[3] while “on-purpose” olefin conversion technologies allow for propene to be generated from ethene/butenes, via isomerization from 1-butene to 2butenes followed by metathesis with ethene, Scheme 1.4,5 Given the world-wide demand for propene,[6] the energyefficient isomerization of butenes is important in an industrial context
We have recently reported the development of solid-state molecular organometallic (SMOM) chemistry in which precisely defined, molecular, organometallic systems undergo reactivity in single-crystal to single-crystal (SC−SC) transformations in the absence of solvent.[22]
We show here that well-defined single-crystal solid-state molecular organometallic (SMOM) systems can be deployed under flow-reactor conditions for the industrially relevant isomerization of 1-butene under the attractive conditions of room temperature and low pressures
Summary
The double bond isomerization of simple alkenes is a truly atom efficient process that is used in industry and fine chemical synthesis to add value to chemical feedstock streams.[1,2] For example, the Shell higher olefins process relies on an isomerization/metathesis step to maximize the production of desired C12−20 α-olefin fractions,[3] while “on-purpose” olefin conversion technologies allow for propene to be generated from ethene/butenes, via isomerization from 1-butene to 2butenes followed by metathesis with ethene, Scheme 1.4,5 Given the world-wide demand for propene,[6] the energyefficient isomerization of butenes is important in an industrial context. In CD2Cl2 solvent rapid decomposition to butadiene (R = Cy, 5)[30] and chloride bridged (12, R = tBu) occurs This shows a role for the fluoroarene as a chaperone ligand in homogeneous catalysis by stabilizing the reactive metal center but being labile enough to reveal reactivity.[58] These experiments demonstrate a significant difference between solution and molecular solidstate reactivity, in terms of speciation, comparative stabilities of the two catalysts and the ratio of 2-butene isomers formed. They indicate that the deactivation of single-crystalline complex 8 under flow is likely associated with a structural change that quenches activity of the metal sites in the solidstate
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