Unsaturated substrates hydrogenation

Hydrogenation of unsaturated substrates. Hydrogenation of alkenes and alkynes is one of the most widely useful reactions involving dihydrogen activation. The mechanism of Wilkinson s complex offers a good example of the process (Fig. 2.3) [100]. The cycle shown is just one of several that operate in the real system, depending on the exact conditions. It serves to show a key point, that the activation step is the hrst step in the cycle. As in the Vaska case, the ligands fold back so that the cis-dihydride is formed. An early NMR experiment (Fig. 

Hydroxy radical and sulfate radical anion, though they may sometimes give rise to similar products, show quite different selectivity in their reactions with unsaturated substrates. In particular, the sulfate radical anion has a somewhat lower propensity for hydrogen abstraction than the hydroxyl radical. For example, the sulfate radical anion shows little tendency to abstract hydrogen from mcthacrylic acid.232... 

Because organophosphorus compounds are important in the chemical industry and in biology, many methods have been developed for their synthesis [1]. This chapter reviews the formation of phosphorus-carbon (P-C) bonds by the metal-catalyzed addition of phosphorus-hydrogen (P-H) bonds to unsaturated substrates, such as alkenes, alkynes, aldehydes, and imines. Section 5.2 covers reactions of P(lll) substrates (hydrophosphination), and Section 5.3 describes P(V) chemistry (hydrophosphorylation, hydrophosphinylation, hydrophosphonylation). Scheme 5-1 shows some examples of these catalytic reactions. 

In this review, an attempt is made to exhaustively catalog the rapidly growing subset of metal-catalyzed reductive C-C bond formations comprising the hydrometallative and hydrogenative carbocyclization of 7t-unsaturated substrates, and application of these methods in target-oriented synthesis. Content is organized on the basis of reaction... 

The equilibrium can be shifted by the addition of acid or base, and is also sensitive to the nature of the ligands and solvents. Scheme 1.6 qualitatively accounts for the experimental observations involving three possible pathways by which an unsaturated substrate can be hydrogenated. 

One pervasive mechanistic feature of many of the hydrogenations described in other chapters of this handbook concerns the bonding of the unsaturated substrate to a metal center. As illustrated in generalized form in Eq. (1) for the hydrogenation of a ketone, a key step in the traditional mechanism of hydrogenation is migratory insertion of the bound substrate into a metal hydride bond (M-H). 

The success of stoichiometric ionic hydrogenations is due to achieving a fine balance that favors the intended reactivity rather than any of several possible alternative reactions. The acid must be strong enough to protonate the unsaturated substrate, yet the reaction of the acid and the hydride should avoid producing H2 too quickly under the reaction conditions. The commonly used pair of CF3C02H and HSiEt3 meets all these criteria. 

As documented throughout this handbook, the diversity of reaction patterns of transition-metal complexes leads to a remarkably rich chemistry, with a tremendous mechanistic diversity in the details of how H2 is added to unsaturated substrates. Over forty years ago, Walling and Bollyky reported a catalytic hydrogenation of benzophenone that required no transition metal at all They found that the C=0 bond of benzophenone can be catalytically hydrogenated using KOtBu as a base [88], but harsh conditions (200°C, 100 bar H2) were used (Eq. (49)). Ber-kessel et al. recently examined details of this reaction and provided evidence that it was first order in ketone, first order in hydrogen, and first order in base [89]. 

Finally, the development of modified nanoparticles having better stability and a longer lifetime has involved interesting results in diverse catalytic reactions. Efficient activities are obtained with these transition-metal colloids used as catalysts for the hydrogenation of various unsaturated substrates. Consequently, several recent investigations in total, partial or selective hydrogenation have received significant attention. 

Complex 24 a proved to be a particularly efficient catalyst for the hydrogenation of the cyclic substrate 5, affording 95% ee, which currently is the highest ee-value obtained with any catalyst for this substrate. High enantioselectivities were also obtained with a,/ -unsaturated substrate 6. Catalyst 24 a also gave higher selectivity than the ThrePHOX catalysts 23 in the hydrogenation of substrates 2 and 4. 

Col I(CN)-, 1 is readily formed under mild conditions from Co(CN)2, KCN and H2 [Eqs. (1) and (2)]. It is an active catalyst for the hydrogenation of a variety of unsaturated substrates, and in fact in the first documented examples of two-phase hydrogenations this catalyst was used [48, 49]. The catalysis suffers from several drawbacks such as rapid aging with a loss of activity, and the need to use highly basic aqueous solutions. 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Scheme 6.150 Proposed mechanistic picture for (S)-favored enantioselective Michael addition of O-benzylhydroxylamine to 2,4-dimethyl pyrazole substituted a,P-unsaturated substrates in the presence of hydrogen-bonding thiourea catalyst 139.

Whilst, in principle, kinetic measurements should allow a differentiation between the two possible mechanisms, it must be noted that in catalytic hydrogenation reactions relatively few examples are sufficiently clear cut to allow this differentiation to be made. Thus, for example, it is quite commonly found that the experimentally observed orders of reaction are zero in the unsaturated substrate A and unity in hydrogen. Such results are readily interpreted by the adjacent-site mechanism by assuming A to be much more strongly adsorbed than hydrogen or by the Rideal— Eley type of mechanism. Clearly, kinetic measurements alone are insufficient for the establishment of mechanism. 

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