Norbornadiene hydrogenation

The most efficient cofactor found for inducing rate accelerations in norbornadiene hydrogenations was simply KOH at a KOH/Rh molar ratio of 160. The formation of a catalytically active species other than RhCl(PPh3)(norbornadiene) is highly likely. Initially we did note an obvious color change on introduction of norbornadiene into a solution of RhCl(PPh3)3 the major species formed initially was probably RhCl(PPh3)(norbornadiene). 

Scheme II Proposed mechanisms for photocatalysis of norbornadiene hydrogenation by the group VI carbonyls M(CO)6...

The first successful hydrogenation reactions in ionic liquids were studied by the groups of de Souza [45] and Chauvin [46] in 1995. De Souza et al. investigated the Rh-catalyzed hydrogenation of cyclohexene in l-n-butyl-3-methylimidazolium ([BMIM]) tetrafluoroborate. Chauvin et al. dissolved the cationic Osborn complex [Rh(nbd)(PPh3)2][PFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions (e.g., [PFg] , [BFJ , and [SbF ] ) and used the obtained ionic catalyst solutions for the biphasic hydrogenation of 1-pentene as seen in Scheme 5.2-7. 

When irradiated in the presence of norbornadiene and high-pressure synthesis gas, rhodium chloride is converted to a catalyst which is active for a variety of reactions. /2A/. The salt is probably converted photochemically to the rhodium norbornadiene complex 9. This dimer may undergo a consecutive photoreaction to give the monomeric hydrido complex 10, which is the actual catalyst for polymerisation, hydrogenation, and hydroformylation reactions. 

The hydride route involves the initial reaction with hydrogen followed by coordination of the substrate the well-known Wilkinson catalyst [RhCl(PPh3)3] is a representative example. A second possible route is the alkene (or unsaturated) route which involves an initial coordination of the substrate followed by reaction with hydrogen. The cationic catalyst derived from [Rh(NBD)(DIPHOS)]+ (NBD = 2,5-norbornadiene DIPHOS = l,2-bis(diphenyl)phosphinoethane) is a well-known example. The above-mentioned rhodium catalysts will be discussed, in the detail, in the following sections. 

The characteristics of the hydrogenation of norbornadiene, substituted butadienes and conjugated and cyclic dienes are all very similar. In the case of conjugated dienes, there appears to be hardly any isomerization activity, while in the case of 1,4-dienes an isomerization step to form the corresponding 1,3-diene is assumed prior to hydrogenation. The catalyst behavior changes after the diene has been completely converted to the monoene, whereupon the rhodium catalyst resumes its normaF monoene hydrogenation behavior. 

Beside SILP experiments with silica as support material, reports have also been made on the use of membranes coated with ionic liquid catalyst solution for the hydrogenation reaction of propene and ethene. The membranes were obtained by supporting various ionic liquids, each containing 16 to 23 mmol Rh(I) complex Rh(nbd)(PPh3)2 (nbd=norbornadiene), in the pores of poly(vinylidene fluoride) filter membranes [118]. 

SILC was also used without covalently anchoring the ionic liquid fragment to the silica support. In this case, [bmim][PF6] was simply added to silica in acetone together with the catalyst. [Rh(norbornadiene)(PPh3)2]PF6 and the solvent evaporated to yield the supported catalyst-philic phase. Catalyst evaluation on the hydrogenation of model olefins showed enhanced activity in comparison to homogeneous and biphasic reaction systems, in analogy to Davis s observations. Also... 

Olah and co-workers reported the synthesis of 2,2,5,5-tetranitronorbornane (127) from 2,5-norbornadiene (122). In this synthesis formylation of (122) with formic acid yields the diformate ester (123), which on treatment with chrominm trioxide in acetone yields 2,5-norbomadione (124). Formation of the dioxime (125) from 2,5-norbornadione (124) is followed by direct oxidation to 2,5-dinitronorbomane (126) with peroxytriflnoroacetic acid generated in situ from the reaction of 90 % hydrogen peroxide with TFAA. Oxidative nitration of 2,5-dinitronorbornane (126) with sodium nitrite and potassium ferricyanide in alkaline solution generates 2,2,5,5-tetranitronorbornane (127) in excellent yield. 

The toroidal pressure probe, introduced in 1989 by Rathke and coworkers [249, 250], has been modified by Woelk and coworkers [249, 251], who have used a toroid cavity NMR autoclave for high pressure PHIP NMR experiments. Figure 1.43 shows the PHIP spectrum of the [Rh(norbornadiene)(PPh3)2]PF6-catalyzed hydrogenation of 1,4-diphenylbutadiyne with 40 bar of 50% emiched para-H2 [252]. The spectrum from the same reaction at ambient H2 pressure is shown in the inset [253]. The two absorption/emission PHIP patterns in both spectra indicate that para-H2 is transferred pairwise during the catalytic cycle. 

Figure 1.43 HP PHIP NMR spectrum of the [Rh(norbornadiene)(PPh3)2](PFd-catalyzed hydrogenation of 1,4-diphenylbutadiyne with 40 bar of 50% enriched P-H2, plus the spectrum from the same reaction at ambient H2 pressure (in the inset).

The rhodium complexes were prepared in situ from the rhodium precursor [Rh(nbd)2](C104) (nbd = 2,5-norbornadiene) and applied in the hydrogenation experiments under an initial hydrogen pressure of 5 bar at 35°C. The dendrimer structure had almost no effect on the activity of the catalyst in the batch-wise rhodium-catalyzed hydrogenation of dimethyl itaconate (Scheme 4). 

Scheme 5.2-7 Biphasic hydrogenation of 1 -pentene with the cationic Osborn complex [RhfnbdjfPPhjjjjjPFg] (nbd = norbornadiene) in ionic liquids with weakly coordinating anions.

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