Activated alkenes, hydrogenation

The cyclopentadienyl group is another interesting ligand for immobilization. Its titanium complexes can be transformed by reduction with butyl lithium into highly active alkene hydrogenation catalysts having a TOF of about 7000 h 1 at 60 °C [85]. Similar metallocene catalysts have also been extensively studied on polymer supports, as shown in the following section. 

The rates of activated alkene hydrogenations seen in the a-bove studies are comparable to those seen earlier by Wilkinson when RhCl(PPh3) was reportedly formed in situ from PPI13 and [RhCl(C0D)]2 (21) supporting our conclusion that removal of phosphine was responsible for the activations seen in our procedures. However alternative explanations involving possible formation of a hydridorhodium catalyst (e.g. eq 4 and 5 must also be considered. The hydridorhodium catalysts formed in these equa-... 

Adams catalyst, platinum oxide, Pt02 H20. Produced by fusion of H2PtCl6 with sodium nitrate at 500-550 C and leaching of the cooled melt with water. Stable in air, activated by hydrogen. Used as a hydrogenation catalyst for converting alkenes to alkanes at low pressure and temperature. Often used on Si02... 

A well-understood catalytic cycle is tliat of the Wilkinson alkene hydrogenation (figure C2.7.2) [2]. Like most catalytic cycles, tliat shown in figure C2.7.2 is complex, involving intennediate species in tire cycle (inside tire dashed line) and otlier species outside tire cycle and in dead-end patlis. Knowledge of all but a small number of catalytic cycles is only fragmentary because of tire complexity and because, if tire catalyst is active, tire cycle turns over rapidly and tire concentrations of tire intennediates are minute thus, tliese intennediates are often not even... 

Systems usually fluonnated by electropositive fluorine reagents include acti-vated alkenes (enol ethers, enol acetates, silyl enol ethers, and enamines), activated aromatic systems, certain slightly activated carbon-hydrogen bonds, and selected organometallics. 

RhCl(PPh3)3 is a very active homogenous hydrogenation catalyst, because of its readiness to engage in oxidative addition reactions with molecules like H2, forming Rh—H bonds of moderate strength that can subsequently be broken to allow hydride transfer to the alkene substrate. A further factor is the lability of the bulky triphenylphosphines that creates coordinative unsaturation necessary to bind the substrate molecules [44]. 

Free-radical addition of an aryl group and a hydrogen has been achieved by treatment of activated alkenes with a diazonium salt and TiCls. ... 

In addition to the use of hydrogen directly, hydrogen generated from CO and water (water-gas shift reaction) is also very effective in hydrogenating activated alkenes under basic conditions (Eq. 10.3).5... 

Calvin reports the first homogeneous hydrogenation of an organic molecule [18,19] Halpern reports the first homogeneous hydrogenation of an activated alkene [20,21]... 

The mechanism of alkene hydrogenation catalyzed by the neutral rhodium complex RhCl(PPh3)3 (Wilkinson s catalyst) has been characterized in detail by Halpern [36-38]. The hydrogen oxidative addition step involves initial dissociation of PPI13, which enhances the rate of hydrogen activation by a factor... 

Ru3(CO)12(117)3] and [H4Ru4(CO)11(117)] as catalyst precursors in the hydrogenation of non-activated alkenes under biphasic conditions. Each cluster displays activity under moderate conditions, ca. 60 atm. H2 at 60 °C with catalytic turnovers up to ca. 500. The trinuclear clusters undergo transformations during reaction but can be used repeatedly without loss of activity.325... 

Hydrogenation catalysts from non-platinum group have been also reported. For example, some organolanthanides of formula [ (r/5-C5Me5)2MH)2] are active catalyst for alkene hydrogenation.384 It has been proposed on the basis of kinetic studies that first the dimer dissociates according to (Equation (18)),... 

Besides the known activity of HRuX(PPh3)3 complexes where X is a carboxylate (/, p. 85), other complexes with X an a-hydroxycarboxylate and related bridged dimers [HRu(PPh3)3]2X have been found to effect alkene hydrogenation (124). 

Much emphasis has been placed in recent times on easily recoverable liquid bi-phasic catalysts, including metal clusters in nonconventional solvents. For instance, aqueous solutions of the complexes [Ru3(CO)12.x(TPPTS)x] (x=l, 2, 3 TPPTS = triphenylphosphine-trisulfonate, P(m-C6H4S03Na)3) catalyze the hydrogenation of simple alkenes (1-octene, cyclohexene, styrene) at 60°C and 60 bar H2 at TOF up to 500 h 1 [24], while [Ru i(CO)C (TPPMS) >,] (TPPMS = triphenylphos-phine-monosulfonate, PPh2(m-C6H4S03Na) is an efficient catalyst precursor for the aqueous hydrogenation of the C=C bond of acrylic acid (TOF 780 h 1 at 40 °C and 3 bar H2) and other activated alkenes [25]. The same catalysts proved to be poorly active in room temperature ionic liquids such as [bmim][BF4] (bmim= Tbutyl-3-methylimidazolium). No details about the active species involved are known at this point. 

Alkene hydrogenation is a common field of catalytic application for metal nanoparticles. Various approaches have been utilized to obtain stable and active nanocatalysts in hydrogenation reactions. The main approaches are described in the following sections, and are classified according to the stabilizing mode retained for the nanoparticles. 

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