Multiple Tandem Catalysis:  Facile Cycling between Hydrogenation and Metathesis Chemistry

Abstract

Tandem ROMP-hydrogenation via a single Ru precursor permits catalytic reduction of ROMP polymers under exceptionally mild, homogeneous conditions (1 atm of H 2 , 60°C). Repeated catalyst cycling between metathesis and hydrogenation chemistry is effected in a one-pot procedure.Interest in rapid-throughput chemical synthesis has spurred development of technologies in which one catalyst supports several functions. 1 Particularly attractive in this context is the ability to simply, controllably, and reversibly "turn on" different modes of catalysis. Here we describe facile cycling of a single catalyst precursor between metathesis and hydrogenation activity (eq 1). Use of methanol in the hydrogenation step permits complete polymer hydrogenation under extraordinarily mild conditions, at 1 atm of H 2 . This approach is equally relevant to reduction of unsaturated molecular species derived from ring-closing metathesis (RCM) or cross-metathesis.Industrial demand continues to increase for lightweight, moldable optical-grade materials. Ring-opening metathesis polymerization (ROMP) of cycloolefins, followed by hydrogenation, enables synthesis of highmolecular-weight, narrow-polydispersity polyolefins with desirable optical characteristics. 2 Hydrogenation expands the range of applications open to ROMP materials by eliminating the inherent susceptibility of olefinic linkages to oxidative and thermal degradation. 3,4 Hydrogenation of all unsaturated polymers typically requires forcing conditions, as local steric effects are exacerbated by diffusion constraints associated with the random coil. 3 Steric problems are particularly acute in ROMP polymers, in which the olefinic groups are part of the polymer backbone; 5 in ROMP via 1, they are also of predominantly trans geometry. 6 Nickel and rhodium -Olefins in polybutadiene are reduced faster than 1,4-olefins and cis-1,4 units faster than trans-1,4 units. 3a (6) Schwab, P.; Grubbs, R. H.; Ziller, J. W.

Alkene Metathesis Catalysis: A Key for Transformations of Unsaturated Plant Oils and Renewable Derivatives

“…A tandem metathesis/hydrogenation process in the presence of ruthenium alkylidene catalyst has already been used to prepare saturated polymers [66][67][68]. Thus, we have explored whether ruthenium-benzylidene and ruthenium-indenylidene olefin metathesis catalysts could also perform hydrogenation of fatty alkyl nitriles and could be more efficient than the residual ruthenium arising from cross-metathesis.…”

Improved Syntheses of Versatile Ruthenium Hydridocarbonyl Catalysts Containing Electron‐Rich Ancillary Ligands

“…[4,5] Complex 2, used either directly or generated in situ, is among the most efficient catalysts in current use for the reduction of C=C bonds in nitrile-butadiene polymers [5][6][7][8] and unsaturated polymers derived from ringopening metathesis polymerization (ROMP). [9,10] Its recent applications to molecular transformations span a wide range of applications. Reactions of alkenes range from hydrogenation [11,12] and isomerization, [11,13] to diverse addition and coupling reactions.…”

Exploration of the versatility of ring opening metathesis polymerization: an approach for gaining access to low density polymeric aerogels

“…The alcohol functionalized norbornene monomers can be used to form derivatives having various functional groups, as previously reported. 17,18 Norbornene-based monomers were selected for their mild polymerization conditions and ability to form statistical copolymers with dicyclopentadiene rings. In this study, we prepared alkyl norbornene monomers (4-7) by protecting a pendant alcohol group with various alkyl halides including methyl iodide, hexyl bromide, dodecyl bromide, and benzyl bromide, respectively (Scheme 1b).…”