Hydrogenation reaction rhodium from

There is more to tire Wilkinson hydrogenation mechanism tlian tire cycle itself a number of species in tire cycle are drained away by reaction to fomi species outside tire cycle. Thus, for example, PPh (Ph is phenyl) drains rhodium from tire cycle and tlius it inliibits tire catalytic reaction (slows it down). However, PPh plays anotlier, essential role—it is part of tire catalytically active species and, as an electron-donor ligand, it affects tire reactivities of tire intemiediates in tire cycle in such a way tliat tliey react rapidly and lead to catalysis. Thus, tliere is a tradeoff tliat implies an optimum ratio of PPh to Rli. 

For the rational design of transition metal catalyzed reactions, as well as for fine-tuning, it is vital to know about the catalytic mechanism in as much detail as possible. Apart from kinetic measurements, the only way to learn about mechanistic details is direct spectroscopic observation of reactive intermediates. In this chapter, we have demonstrated that NMR spectroscopy is an invaluable tool in this respect. In combination with other physicochemical effects (such as parahydrogen induced nuclear polarization) even reactive intermediates, which are present at only very low concentrations, can be observed and fully characterized. Therefore, it might be worthwhile not only to apply standard experiments, but to go and exploit some of the more exotic techniques that are now available and ready to use. The successful story of homogeneous hydrogenation with rhodium catalysts demonstrates impressively that this really might be worth the effort. 

Hydroformylation of phenylacetylene 97 in the presence of -hexylamine 98 catalyzed by (1,5-cyclooctadienyl)-rhodium tetraphenylboronate, [Rh(l,5-COD)] [(7] -C6HsBPh3)], gave the corresponding 2-pyrrolidone 101 (Scheme 16). However, the reaction suffered from competing side-reactions such as hydrogenation of allylamine intermediate 100. ... 

The ionic liquids acting as solvents in these hydrogenation systems do not show any noticeable difference in the turnover rate with the Wilkinson catalyst. It is important to note that at the end of the hydrogenation reaction the product is removed from the two-phase catalytic system by simple decantation and the rhodium catalysts are almost completely retained in the ionic liquid (Steines et al., 2000). 

In the hydrogenation of an alkene using Wilkinson s caialyst. Rh(PPh,)2(RCH=CHj)-D(H)(H) reacts io give Rhreaction rhodium is reduced from +3 to +1 and an alkene is reduced to an alkane. What is oxidized ... 

Two examples of such situations are sketched in Scheme 1.11. Quatemization of tropane occurs mainly from the less hindered pyrrolidine side (equatorial attack at the piperidine ring), even though the main conformer of tropane has an equatorial methyl group. Similarly, l-methyl-2-phenylpyrrolidine yields mainly an anti alkylated product via alkylation of the minor cis conformer when treated with phenacyl bromide [33], In both instances the less stable conformer is more reactive to such an extent that the major product of the reaction results from this minor conformer. A further notable example of a reaction in which the main product results from a minor but more reactive intermediate is the enantioselective hydrogenation of a-acetamidocinnamates with a chiral rhodium-based catalyst [34],... 

Although the hydridorhodacarborane is formally a rhodium (III) derivative, it functions as a facile catalyst in alkenc isomerization, hydrogenation, hydroformylation, and hydrosilylation reactions 80). This catalyst system is extremely stable and may be recovered quantitatively from alkene isomerization and hydrogenation reactions. In addition to these reactions, the hydridorhodacarborane is very effective in the catalysis of deuterium exchange at terminal BH positions 59). These discoveries may soon lead to industrially useful metallocarborane catalysts. 

Rhodium is insoluble in acids, even in aqua regia, although when its alloys are attacked by this latter mixture a portion of the rhodium passes into solution. When fused -with potassium hydrogen sulphate, rhodium dissolves, yielding the sulphate. This reaction is interesting as affording a convenient method of separating the metal from iridium and platinum (see p. 34-1). 

The dihydrido complexes (Table 62) can be obtained by the oxidative addition of molecular hydrogen to rhodium(I) complexes (equation 186).10,119,922""926 The tri(f-butyl)phosphine complexes can be prepared either from the chlororhodium(I) complex,923 or rhodium trichloride.927 The former method seems more reliable since the latter reports the complex as a matt green substance, a color uncharacteristic of tertiary phosphine rhodium(III) complexes. Indeed, Masters and Shaw report that the related tertiary phosphines PBu2R (R = Et, Pr) give green rhodium(II) complexes in this reaction (see Section 48.5.2.1 above).268,269... 

Thus for rhodium, catalyst cost is offset by selectivity. It should be noted that alkene hydrogenation is thermodynamically more favourable than hydroformylation (e.g. by 34 kJ mol-1 for ethene) and so a requirement of the catalyst is that it kinetically diverts the reaction away from simple hydrogenation. 

In recent years the synthesis of chiral and achiral tripodal phosphines and their application in homogeneous catalysis has been studied in more detail [2]. Enantiomerically pure tripodal ligands were synthesized from the corresponding trichloro compounds and chiral, cyclic lithio-phosphanes, e.g. 17, (Scheme 6) [21,22], Using a rhodium(I) complex of ligand 18, an enantiomeric excess of 89 % was obtained in the asymmetric hydrogenation reaction of methyl acetami-docinnamate (19). 

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