Catalyst transition metal/phosphine

A variety of Group VIII transition metal phosphine complexes are shown to be active catalysts for hydrogenation of aliphatic nitro compounds. However, chiral phosphines have been found to be noneffective to induce asymmetric induction.110... 

Ferraris et al.108 demonstrated an asymmetric Mannich-type reaction using chiral late-transition metal phosphine complexes as the catalyst. As shown in Scheme 3-59, the enantioselective addition of enol silyl ether to a-imino esters proceeds at —80°C, providing the product with moderate yield but very high enantioselectivity (over 99%). 

Trimethylsilyl cyanide reacts with diphenylcyclopropenone in the presence of Fe2(CO)9 or PPh3 as catalyst to give the aminofuran derivative 73 (40-60%) (Eq. (10)). Other phosphines and transition metal phosphine complexes are effective catalysts. A similar reaction was achieved using cycloheptenocyclopropenone. Desilylprotonation of compound 73 was achieved in hot MeOH containing a trace of p-TsOH, but the primary amine was trapped in situ as a cycloadduct without isolation (87JOC4408). 

Transition metal phosphine complexes provide another important class of hydrogenation catalysts. Wilkinson s complexes, RhClfPPhjlj and RuHClfPPhjlj, well known for their olefin hydrogenation activity, were shown by Fish [77, 97] to be also good precursors for the reduction of polyaromatic substrates under mild reaction conditions (85° C, ca. 20 atm H, ). following a general activity trend consistent with a combination of electronic and steric factors ... 

Space constraints do not allow description of all the imaginative efforts to prepare, characterize, immobilize, and recover water-soluble transitional metal-phosphine complexes as hydrogenation catalysts. Further examples can be found in [11] and in [7[. For the mechanism of asymmetric hydrogenation of alkenes and that of the hydrogenation of aldeyhydes, see Section 6.2.3. 

The number and diversity of transition metal phosphine complexes is vast and a wide range have been used as catalysts for synthetic organic transformations for many years. It therefore comes as little surprise that the preparation of polymer-supported metal phosphine complexes and assessment of their catalytic activity has attracted much attention. Supported phosphine ligands and their metal complexes prepared from 1981 to 2001 can be found in a review published in 2002. Discussed here are examples in the literature from 1996 to the present together with a selected number of those from 1981 to 1996 where particularly notable synthetic methods have been used or where key points should be raised. 

In the quest for additional active catalysts for the Mizoroki-Heck reaction, the advent of N-heterocychc carbene (NHC) ligands was warmly welcomed [32], Transition metal-transition metal-phosphine complexes, and have therefore attracted considerable interest as competitive alternatives in Mizoroki-Heck chemistry, which requires high reaction temperatures. Since the seminal application of NHC ligands in Mizoroki-Heck arylations by Herrmann et al. [33], several research groups have introduced novel palladium catalyst-NHC ligand combinations. These were tested and assessed in standard couplings of simple iodo- or bro-moarenes 60 and activated acceptors such as acrylates 61 or styrene (63) [32], and a selection of impressive examples is summarized in Scheme 7.14. 

Propane, 1-propanol, and heavy ends (the last are made by aldol condensation) are minor by-products of the hydroformylation step. A number of transition-metal carbonyls (qv), eg, Co, Fe, Ni, Rh, and Ir, have been used to cataly2e the oxo reaction, but cobalt and rhodium are the only economically practical choices. In the United States, Texas Eastman, Union Carbide, and Hoechst Celanese make 1-propanol by oxo technology (11). Texas Eastman, which had used conventional cobalt oxo technology with an HCo(CO)4 catalyst, switched to a phosphine-modified Rh catalyst ia 1989 (11) (see Oxo process). In Europe, 1-propanol is made by Hoechst AG and BASE AG (12). 

Transition-metal organometallic catalysts in solution are more effective for hydrogenation than are metals such as platinum. They are used for reactions of carbon monoxide with olefins (hydroformyla-tion) and for some ohgomerizations. They are sometimes immobihzed on polymer supports with phosphine groups. 

Guo et al. [70,71,73] recently attempted to hydrogenate NBR in emulsion form using Ru-PCy complexes. However, successful hydrogenation can only be obtained when the emulsion is dissolved in a ketone solvent (2-butanone). A variety of Ru-phosphine complexes have been studied. Crosslinking of the polymer could not be avoided during the reaction. The use of carboxylic acids or first row transition metal salts as additives minimized the gel formation. The reactions under these conditions require a very high catalyst concentration for a desirable rate of hydrogenation. 

Coordination-catalyzed ethylene oligomerization into n-a-olefins. The synthesis of homologous, even-numbered, linear a-olefins can also be performed by oligomerization of ethylene with the aid of homogeneous transition metal complex catalysts [26]. Such a soluble complex catalyst is formed by reaction of, say, a zero-valent nickel compound with a tertiary phosphine ligand. A typical Ni catalyst for the ethylene oligomerization is manufactured from cyclo-octadienyl nickel(O) and diphenylphosphinoacetic ester ... 

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

Recent studies in our laboratory have demonstrated that formylation of P-H bonds can be achieved without the aid of transition metal catalysts under mild reaction conditions [47]. For example, amide and thioether functionalized primary phosphines, 5 and 9 respectively, upon treatment with 37% formaldehyde produced the corresponding amide/thioether functionaUzed water soluble phosphines 21 and 22 respectively in near quantitative yield (Scheme 10) [47]. 

Indeed, these reactions proceed at 25 °C in ethanol-aqueous media in the absence of transition metal catalysts. The ease with which P-H bonds in primary phosphines can be converted to P-C bonds, as shown in Schemes 9 and 10, demonstrates the importance of primary phosphines in the design and development of novel organophosphorus compounds. In particular, functionalized hydroxymethyl phosphines have become ubiquitous in the development of water-soluble transition metal/organometallic compounds for potential applications in biphasic aqueous-organic catalysis and also in transition metal based pharmaceutical development [53-62]. Extensive investigations on the coordination chemistry of hydroxymethyl phosphines have demonstrated unique stereospe-cific and kinetic propensity of this class of water-soluble phosphines [53-62]. Representative examples outlined in Fig. 4, depict bidentate and multidentate coordination modes and the unique kinetic propensity to stabilize various oxidation states of metal centers, such as Re( V), Rh(III), Pt(II) and Au(I), in aqueous media [53 - 62]. Therefore, the importance of functionalized primary phosphines in the development of multidentate water-soluble phosphines cannot be overemphasized. 

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