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All authors adopt an applied approach, emphasizing those aspects important for industrial use. With some 2, illustrations and 50 tables, this is a must—have for everyone working in the chemicals and pharmaceutical industries, as well as for graduate students in chemistry. Neem contact met mij op over Events Sprekers Incompany. Welkom terug. Uw account. Agenda Seminars Masterclasses e-learning Sprekers Incompany. Actueel Opinie Interviews Recensies Videos.

Beoordeel zelf slecht matig voldoende goed zeer goed. Gebonden, blz. Auteurs Over dit boek Artikelen en interviews Recensies. Samenvatting Rubriek: Wetenschap en techniek. Lezersrecensies Beoordeel zelf slecht matig voldoende goed zeer goed. Heller, R. Selke, Organometallics , 16, Landis, S. Feldgus, Angew. Feldgus, C. Landis, Organometallics , 20, Rosen, Angew.

Russell Bowers, D. Weitekamp, J. Harthun, R. Kadyrov, R. Selke, J. Bar- gon, Angew. Giernoth, H. Heinrich, N. Adams, R. Deeth, J. Bargon, J. Heinrich, R. Giernoth, J. Muci, K. Campos, D. Imamoto, J. Watanabe, Y. Wada, H. Masuda, H. Yamada, H. Tsuruta, S. Mat- sukawa, K. Yamaguchi, J. Gridnev, N.

Higashi, K. Asakura, T. Gridnev, Y. Yamanoi, N. Higashi, H. Tsuruta, M. Yasutake, T. Maitlis, Acc. Sivak, E. Muetterties, J. Muetterties, Inorg. Acta , 50, 9. Garca, A. Lpez, M. Lahoz, L. Oro, J. Esteruelas, M. Lpez, L. Oro, Organometallics , 10, Robert H. Although iri- dium was still considered potentially to be useful, this was only for the demon- stration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metalligand bonds e.

When low-coordinate iridium fragments in non-coordinating solvents e. The other steps in the catalytic cycle are often very fast for Ir, so if the need for dissociation is avoided, then highly active Ir catalysts can be formed. However, a new consensus has now emerged: rhodium catalysts are often considered to be slower but more selective, whilst iridium catalysts are faster but less selective. Only weakly catalytic itself, Vaskas complex is nevertheless highly relevant to cataly- 31 The Handbook of Homogeneous Hydrogenation.

KGaA, Weinheim ISBN: 2 Iridium sis in providing the classic examples of oxidative addition normally a key step in almost any catalytic cycle. Without alkene binding, hydrogen transfer from the metal to the alkene cannot occur. Once again, the reason was that the adduct [IrH 2 Cl PPh 3 3 ] failed to lose PPh 3 , unlike the Rh analogue, so that the alkenes were unable to bind and undergo reduction [2]. Schrock and Osborn [3] introduced the valuable idea that the reaction should be started with a PR 3 to Rh ratio of 2: 1 in order to avoid the need for ligand dissocia- tion.

In the Rh series, MeOH was easily lost and catalytic alkene reduction was rapid. These proved to be very much less labile and less active than the Rh series [4], and consequently attention was naturally focused on rhodium. At this point, the initial intent of these investigations was to seek stable hy- drides in iridium that were relevant to transient intermediates proposed in the rhodium series.

With this aim in view, attention was focused on a series of complexes [ cod Ir PR 3 2 ]BF 4 , analogous to the Schrock-Osborn Rh catalysts; many of these had been synthesized previously, but had only been tested for catalysis in coordinating solvents and the results had been disappointing. Since solvent dissociation from 3 was needed to generate a site for alkene binding, it seemed appropriate to examine the variation of the solvent, particularly the use of CH 2 Cl 2 ; this was considered 2 Iridium 32 to be non-coordinating because, at the time, it was not known to be capable of binding to metals.

Halocarbon solvents in general had been avoided for Rh cata- lysts, presumably because of the risk of CCl oxidative addition to Rh I. The iridium complexes resisted such pathways, possibly because their resting state is Ir III versus Rh I , and possibly also because of their cationic nature; many neutral Ir I species do add CCl bonds easily. Not only was the catalytic rate very greatly enhanced in CH 2 Cl 2 but, more importantly, the substrate scope was also greatly expanded. At the time, no homogeneous hydrogenation catalysts were known which would reduce tri- and especially tetrasubstituted alkenes effi- ciently; even today, these are very rare.

By using a low PR 3 to M ratio, a non-co- ordinating solvent, and Ir rather than Rh, very high activity was achieved for hindered alkenes [7]. If a PR 3 to M ratio of 2 was so good, then would a ratio of 1 be better? A cata- lyst of this type indeed proved to be the best of the whole series. Even at 0. The deactivation prod- uct is a hydride-bridged polynuclear complex [7], presumably formed by inter- molecular reaction of the catalyst when the depleted substrate is no longer able to compete effectively for binding to the metal. Hydrogenation tends to be fa- vored over deactivation by operating at 08C rather than at room temperature.

The initial studies on the catalyst did not attract the attention of the organic syn- thetic community, partly because the details were not published in an organic chemistry journal, and the substrates used were not real multifunctional organic compounds. On the basis of a suggestion made by Bill Suggs, the catalyst was used for more appropriate substrates, and the results obtained published [8].

This property of the catalyst, which was dis- covered independently by Stork [10], is illustrated in Eq. Any of a variety of directing groups such as ether, ketone or ester is capable of binding to the catalyst before hydrogenation takes place.


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This sets the stereochemistry of as many as two new stereocenters in the reduction. Since Stork is a highly respected member of the organic chemistry community, his intervention was critical in first making the catalyst known, after which time it began to be used more generally. This was suggested by the detection of 5 at low temperature in the reaction of Eq. In commercial practice, however, rate can be more important than e.

This is one of the few enantioselective hydrogenation systems that is in commercial use today. Much later, the pharmaceutical in- dustry developed this aspect of the catalyst for the tritiation of drug candidates, needed for metabolic studies. By introducing the radioactive tritium at the last step, a full organic synthesis involving radioactive intermediates was avoided; this also greatly minimized the production of radioactive organic waste.

As before, pronounced directing effects caused ex- change to occur at well-defined positions on the substrate, notably those imme- diately adjacent to the point on the compound where the catalyst binds. This is usually an O heteroatom, such as in an amide, ester, alcohol or ketone. The hydroboration of terminal and internal alkenes with pinacolborane can be carried out at room temperature in the presence of an iridium I catalyst 3 mol.

The reversal of hydrogenation is also possible, as evidenced by the many iri- dium catalysts for alkane dehydrogenation to alkenes or arenes, though to date this area is of mainly academic interest rather than practical importance [19]. One point of practical importance is the sensitivity of these catalysts to coun- terion and solvent; this is particularly the case in asymmetric hydrogenation, where significant changes in properties have been seen in several cases [20]. This implies that a range of solvents and counterions might usefully be exam- ined in planning trials of the catalyst for a given reduction.

In one case [20a], even the usually satisfactory triflate and tetrafluoroborate counterions almost completely inhibited a cationic iridium-PHOX catalyst. Tetraphenylborate is another undesirable anion as it tends to coordi- nate via an arene ring. In contrast to their sensitivity to anion and solvent, the Ir catalysts are air-stable, unlike typical Rh analogues. The origin of cod as a ligand lies in some of Chatts early studies [21] that were related to the development of the Dewar-Chatt model [21]. The intellectual roots of the concept go back to Langmuir and to Pauling in the s and s, who pro- posed that CO could form multiple bonds with metals such as Ni 0 [22].

Many useful iridium catalysts, such as those mentioned above, are synthetically accessible from [Ir cod Cl] 2 , which is now commercially available. Treatments with PR 3 in a nonpolar solvent gives [Ir cod PR 3 Cl] for the less bulky members of the series, with PEt 3 marking the dividing line between the two types of pathway. Smaller ligands produce neutral bis-phosphine halo-complexes.

In polar solvents e. Typically, reactions are car- ried out at room temperature under N 2 or Ar. A vast number of derivatives of these general types have been prepared by similar routes for catalytic applications, and at this point we can do no more than provide a series of recent references: some have P-donor ligands [24], some have N-heterocyclic carbenes [25], and others have mixed donors [26].

The acetone complex 3 has been characterized crystallographically [27]. These are precursors for the synthesis of a wide variety of unusual derivatives Scheme 2. The first complexes of halocarbons were made by the route of Eq. Agostic species arise from reaction with 8-methylqui- noline Scheme 2.

Instead, benzoquinoline undergoes cyclometalation. Styrene yields a stable g 6 -arene complex Scheme 2. The formation of such stable adducts is highly disadvantageous for rapid catalysis, but not for the exploration of organometallic chemistry. No similar stable complexes have been obtained from the catalyst 4; the faster catalytic rates seen for 4 may correlate with the presence of less stable intermediates in this case [30]. One of the limitations of both 4 and 7 in catalysis is their ready decomposi- tion to inactive cluster hydride complexes in the absence of substrate. If the substrate is a weak ligand e.

A high concentration of substrate favors catalysis 2 Iridium 36 by intercepting unsaturated metal-containing intermediates before they have a chance to cluster [31]. Successive additions of aliquots of catalyst can help in 2. On warming under H 2 to about C, this produces cyclooctane and a trinuclear hydride cluster.

If excess cod is present during the warming procedure, a new alkene complex 9 is formed. This is much more stable than species 8 and survives to room temperature. As before, catalyst 4 does not give rise to stable intermediates of similar structure, although they are as- sumed to be present [32]. These transfer coordinated H 2 to olefin on warming to C, and so can be considered as probable intermediates in hydrogenation. This loses H 2 at 08C when the H 2 is removed, to form the dinuclear hydride of Eq. Monoolefins containing coordinating groups often chelate, as in 5. An ex- 2 Iridium 38 Scheme 2.

Coordinating anions react with the catalyst, again with deactivation of the catalyst, so any halide counterions should be replaced by BF 4 or PF 6. Carboxylate salts also react with the system to give inactive [IrH 2 O 2 CR PPh 3 2 ], so carboxylates should be reduced in the protonated form or as the ester. Amides bind via the carbonyl oxygen, albeit reversibly, so they can affect the rate of reaction and the stereochemistry of the product via direct- ing effects, but are otherwise well tolerated. Esters and alcohols bind less strongly and have little effect on the rate, but still show directing effects.

The Ir catalyst has been used for a wide variety of transformations in the organic syn- thesis of complex molecules. When attention is paid to the points mentioned above, the results have often proved very satisfactory. Among these is one that in- volves the enantiomeric reduction of imines by catalyst 6: Syngentas process for S -metolachlor [11]. The latter is now the largest scale industrial enantio- meric catalytic process, with annual sales of the product, Dual-Magnum. Imines tend to be difficult substrates because of the possi- bility of unproductive ligand binding via the imine lone pair.

For reasons that are still not entirely clear, the Ir catalysts are less seriously affected by such binding as are the Rh analogues. It is possible that the high trans-effect of the hydrides in the Ir III resting state labilizes the substrate binding sites, located trans to the hydrides. Enhanced back-bonding by the third row metal may also enhance the relative stability of the g 2 -bound form of the imine that leads to in- sertion and productive catalysis.

A ketimine is normally required for the reduction product to contain an asymmetric carbon a to nitrogen, as in the case of metolachlor Eq. Finally, the presence of an acid of a non-coordinating anion helps to protonate the nitrogen lone pair and disfavor g 1 binding to the metal via this lone pair. The iodide additive leads to the formation of iodoiridium species that are beneficial for precatalyst 6.

Rates of up to 1. This is said to be one of the fastest homogeneous catalysts of any type known. A more appropriate figure of merit FOM [11] might be obtained by multiplying the ee by the rate; hence, an FOM value for the metolachlor catalyst system is 1. This implies that efficient stirring is desirable for the most effective use of the catalyst.

The rate is first order in catalyst and H 2 , but zero order in substrate. Taken to- gether with the density functional theory DFT calculations, this is consistent with the mechanism of Scheme 2. This explains the insensitivity of the iridium system to air and to oxidizing sol- vents, since Ir III and Ir V tend to be more stable than Ir I both to air and to oxidants in general. It also explains the markedly different catalytic selectivities of what are entirely analogous Rh I and Ir I catalyst precursors. Related iridium species are effective alkane dehydrogenation catalysts, for which a simi- lar reverse-hydrogenation mechanism could readily apply.

Oxidative addition regenerates the Ir III species [34]. Styrene is formed rapidly, whilst subsequent reduc- tion to ethyl benzene is much slower.

Citações duplicadas

Stopping the reaction after the appropri- ate time led to essentially complete selectivity for styrene formation [35]. Sur- prisingly, the cod remains coordinated to Ir throughout the catalytic cycle, in contrast to every other case, where cod is proposed to be hydrogenated or the cyclooctane hydrogenation product is detected. In view of the case with which 6-alkynes rearrange to vinylidenes, such a pathway might easily be involved in 1-alkyne hydrogenation. The appropriate isotope labeling experiments seem to be carried out only rarely.

A detailed combined experimental computational mechanistic study, per- formed for isotope exchange in 2-dimethylamino pyridine, showed how the presence of hydrides in the Ir III intermediates helps to flatten the potential energy surface, accounting for the extremely high rates of exchange. In this case, carbene intermediates were also involved as a result of double CH activa- tion. These catalysts are normally stable to air as solids, but are somewhat air-sensi- tive in solution.

An inert atmosphere N 2 or Ar is typically used for the storage of solids and to protect solutions, as the catalysts deactivate in the absence of substrate. The order of addition must be: substrate first, followed by H 2. Weakly coordinating solvents are required for optimum activity. Dichloromethane is typ- ical, but tetrahydrofuran THF has also been used. The presence of water is tolerated. Basic substrates should be neutralized by the addition of HOAc or HBF 4 in an amount equivalent to the number basic groups to be neutralized, though an excess does not seem to be detrimental.

A catalyst loading of 0. Coordinating anions such as ha- lides are to be avoided in the substrate, but the presence of some iodide has proved beneficial in one case. In the relatively low-polarity solvents used, the complexes form tight ion pairs. Hydrogen is usually supplied at 1 atm pressure, although commercial applications use pressures up to 80 atm.

Rates may also slow at low H 2 pres- sures, but the reaction still occurs. Reaction temperatures from 08C to C have been used successfully. Nitriles can bind to the metal, and the N lone pair is not effectively masked by acid addition so lower rates can be en- countered if this group is present. Alkynes, alkenes, and imines are the best- studied substrates for which reduction is efficient. The isolation of product is usually possible after evaporation of the solvent and extraction with hexane, ether, or toluene.

Supported versions, for example on polystyrene grafted with PPh 2 groups, have proved unsatisfactory because the rate of deactivation is greatly enhanced under these conditions [37]. Asym- metric versions exist, but the ee-values tend to be lower than in the Rh series [38]. With acid to neutralize the basic N lone pair, imine reduction is fast. Should it be necessary to remove the catalyst from solutions in order to isolate a strictly metal-free product, a resin containing a thiol group should prove satis- factory.

A thiol group in the substrate deactivates the catalyst, however. Department of Energy and Johnson Matthey for the support of these studies, and also those coworkers mentioned in the references. Vaska, D. Crabtree, H. Felkin, G. Morris, J. Felkin, T. Fillebeen- Khan, G.

Crabtree, Acc. Suggs, S. Cox, R. Crabtree, J.

Johannes G. de Vries - Citações do Google Acadêmico

Quirk, Tetrahedron Lett. Crabtree, M. Davis, Organometal- lics , 2, ; J. Stork, D. Kahne, J. Buser, K. Coers, R. Hanreich, H. Jalett, E. Jelsch, B. Pugin, H. Schneider, F.

The handbook of homogeneous hydrogenation, Volume 1

Spindler, A. Wegmann, Chimia , 53, Togni, C. Breutel, A. Schnyder, F. Landert, A. Tijani, J. Crabtree, E. Holt, M. Lavin, S. Morehouse, Inorg. Valsborg, L. Sorensen, C. Foged, J. Shu, D. Saunders, S. Levinson, S. Landvatter, A. Mahoney, S. Senderoff, J. Mack, J. Heys, J. Chen, Y.

Li, Z. Dong, B. Li, J. Gao, Tetrahedron Lett. Sablong, J. Osborn, Tetrahedron Lett. Tani, J. Onouchi, T. Yamagata, Y. Ka- taoka, Chem. Yamamoto, R. Fujikawa, T. Umemoto, N. Miyaura, Tetrahedron , 60, Crabtree, C. Parnell, Organo- metallics , 4, ; b C. Jen- sen, Chem. Braunstein, Y. Chauvin, J. Nah- ring, A. DeCian, J. Fischer, A. Tiripic- chio, F. Ugozzoli, Organometallics , 15, ; d F. Liu, A. Gold- man, Chem. Tellers, R. Bergman, Organo- metallics , 20, Smidt, N. Zimmermann, M. Studer, A. Pfaltz, Chem. Kinoshita, K.

Marx, K. Ta- naka, K. Tsubaki, T. Kawabata, N. Yoshi- kai, E. Nakamura, K.

Wilkinson's catalyst

Fuji, J. Chatt, R. Wilkins, Nature , , ; J. Chatt, L. Venanzi, J. Crabtree, G. Organo- met. Focken, G. Raabe, C. Bolm, Tetrahe- dron: Asymmetry , 15, Bolm, T. Raabe, Tetra- hedron: Asymmetry , 14, ; b A. Hillier, H. Lee, E. Stevens, S. Nolan, Organometallics , 20, Pamies, M. Dieguez, G.

PROFESSIONAL CONSULTANCY

Net, A. Ruiz, C. Claver, Organometallics , 19, ; b X.


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    Khan, J. Richards, J. Cho- dosh, R. Chodosh, R. Crab- tree, H. Felkin, S. Morehouse, G. Mor- ris, Inorg. Brandt, E. Hedberg, P. Anders- son, Chem. Pfaltz, J. Blankenstein, R. Hilgraf, E. Hormann, S. McIntyre, F. In 45 clearly structured chapters, the book includes all hydrogenation reactions catalyzed by soluble transition metal-based catalysts. All authors adopt an applied approach, emphasizing those aspects important for industrial use.

    With some 2, illustrations and 50 tables, this is a must-have for everyone working in the chemicals and pharmaceutical industries, as well as for graduate students in chemistry. About The Author. Vermeer and Professor H. Bos, on the topic of "transition-metal mediated synthesis of chiral allenes". Subsequently he moved to the University of Amsterdam, where he has developed his int Select Parent Grandparent Teacher Kid at heart. Age of the child I gave this to:. Hours of Play:. Tell Us Where You Are:.