Hydride complexes hydroformylation

Llewellyn, Green, and Cowley isolated the Co-H complex [CoH(CO)3(IMes)] 26, a relatively stable complex under inert conditions [31], The authors examined the hydroformylation activity of 1-octene with Co-hydride complex 26. With 8 atm of syngas (H /CO) at 50°C for 17 h and 1 mol% 26, the conversion to aldehyde products was 47% with a l b of 0.78. However, 83% of the product was the internal aldehyde 2-methyl-octanal, indicating isomerisation competed with hydroformylation and the rate of isomerisation occurred faster than hydroformylation. 

The existence of two different rhodium species co-existing on the silica support can be used as an advantage by controlling their relative amount. Under standard hydroformylation conditions, the cationic species and the neutral hydride complex are both present in significant amounts. Hence hydroformylation and hydrogenation will both proceed under a CO/H2 atmosphere. Indeed a clean one-pot reaction of 1-octene to 1-nonanol was performed, using the supported catalyst for a hydroformylation-hydrogenation cascade reaction. 98 % of the 1-octene was converted in the... 

In 2004 Caporali investigated the hydroformylation of 1-hexene and cyclohexene using HRh(CO)(PPh3)3 [61]. The collected data indicated that the rate-determining step in the hydroformylation cycle depends upon the structure of the olefin. With an alpha-olefin like 1-hexene, the slowest step seems to be the hydrogenolysis of the acyl rhodium complex. In the presence of cyclohexene as a model for an internal olefin, the rate-determining step is the reaction of the olefin with the rhodium hydride complex (intermediate II in Fig. 6). 

However, considerable amounts of 2,3-dihydrofuran 50 and tetrahydro-furan-2-carbaldehyde 53 were present because of an isomerization process. The isomerization takes place simultaneously with the hydroformylation reaction. When the 2,5-dihydrofuran 46 reacts with the rhodium hydride complex, the 3-alkyl intermediate 48 is formed. This can evolve to the 2,3-dihydrofuran 50 via /3-hydride elimination reaction. This new substrate can also give both 2- and 3-alkyl intermediates 52 and 48, respectively. Although the formation of the 3-alkyl intermediate 48 is thermodynamically favored, the acylation occurs faster in the 2-alkyl intermediates 52. Regio-selectivity is therefore dominated by the rate of formation of the acyl complexes. The modification of the phosphorus ligand and the conditions of the reaction make it possible to control the regioselectivity and prepare the 2- or 3-substituted aldehyde as the major product [78]. As far as we know, only two... 

The rhodium complex with bis(diphenylphosphino)phenoxazine was immobilized on silica using the sol-gel technique or by a direct grafting to commercially available silica [127]. Under standard hydroformylation conditions (CO/H2 atmosphere), a neutral hydridic complex (57) and cationic species (58) (Scheme 4.35) coexist on the support and act as a hydroformylation/hydrogenation sequence catalyst, giving selectively 1-nonanol from 1-octene 98% of 1-octene were converted to mainly linear nonanal which was subsequently hydrogenated to 1-nonanol. The... 

The rhodium hydride complex, 18b, was the only rhodium complex observed during the hydroformylation reaction at increased partial hydrogen pressure of 32 bar, as concluded from the absence of carbonyl signals in the IR difference spectra. The spectrum of the rhodium hydride resting state is taken as background at... 

The addition-elimination mechanism, however, is strongly preferred for monohydride systems such as [HCo(CO)4]187 and the Vaska complex193,194 promoting extensive isomerization. Hydroformylation of 2-pentenes in the presence of [HCo(CO)4], for instance, yields mainly the nonbranched aldehyde resulting from double-bond migration.195 Nickel hydride complexes are one of the most active... 

Hybridization, 148-153 energetics of, 225-229 and overlap. 153 and structure, 220-231 Hydrates, 304-306 Hydration enthalpy. 312 Hydride complexes, 641-644 Hydroformylation, 711-712 Hydrogenation of alkenes. 503. 706-708... 

As indicated in the introduction, bis-l,3-diphenylphosphino-propane (dppp) and bis-l,2-diphenylphosphinoethane (dppe) were reacted with tris(triphenylphosphine)rhodium(II) carbonyl hydride in toluene-deuterobenzene solution to derive cis-chelate complex hydroformylation catalysts. These complexes were expectedly non-selective terminal hydroformylation catalysts for 1-butene hydroformylation (see Table I) because of their cis-stereochemistry. They were also somewhat less active due to their specific structural features. The structure of these complexes in solution was studied in detail by P-31 NMR spectroscopy. 

Since the 1-olefin concentration-dependent hydroformylation in the presence of the above catalyst system has a slightly higher activation energy of about 22 kcal mol-1, it is proposed that the ratedetermining step of selective terminal 1-olefin hydroformylation may involve a transition state leading to the formation of a 1-alkyl bis-(trans-phosphine)rhodium carbonyl hydride complex rather than the dissociation of the trisphosphine complex. 

As a second example, we generated possible intermediates and synthetic sequences for the hydroformylation of olefins [Eq. (8)] with the hydride complex HRh(C02)L2 (L = PPh3). 

The catalyst for hydroformylation is a rhodium(I) hydride species, which is clearly distinct from the species that are active for hydrogenation. The hydrogenation catalysts are cationic Rh(I)+ or neutral Rh(I)Cl species. Carbonylation of alcohols also requires an ionic Rh(I) species, e.g. [Rl CO y-- Often rhodium(I) salts are used as the precursor for hydroformylation catalysts. Under the reaction conditions (H2, CO, ligands, temperature >50°C) these salts are converted to a rhodium hydride complex, although there are several papers that seem to invoke cationic rhodium species as the catalysts. Chlorides have a particularly deleterious effect on the activity (i.e. they are not converted into hydrides under mild conditions) and it has been reported that the addition of bases such as amines has a strong promoting effect on such systems ... 

Asymmetric hydroformylation of styrene has been studied with a range of bidentate phosphines, van Leeuwen reported phosphorus-chiral ferrocenyl-based diphosphines, the hydride complexes of which showed varying ee-ea ratios depending on ligand basicity, as determined by HP-IR and HP-NMR. This contrasted to the clear ea preference shown by complexes of the achiral l,l -bis(diphenylphosphino)ferrocene (dppf) ligand. Claver and van Leeuwen " " " investigated BDPP [(2A,4A)-bis(diphenylphosphino)pentane] and CHIRAPHOS [(2R,3R)-bis (diphenylphosphino)butane] for the same transformation. High pressure spectroscopic data for the catalytic species... 

More complex branching occurs in mechanisms where selectivities arise. Therefore, consider as an example the overall transformation of a symmetric alkene cyclopentene in the presence of CO and H2 to both cyclopentane carboxaldehyde and cyclopentane, facilitated by the addition of a Group 9 (or CAS systems VIIIB) metal hydride complex HML capable of performing both hydroformylation [27,28] and hydrogenation [29]. Upon addition of the metal hydride to the system, one would expect the steps HML —> HML i —> H(ji-cyclopentene)ML i CsH9ML i where the bold intermediates should be common to both unicycUc catalytic sequence of steps. At this point there is a branching in the network of intermediates, and one of either two paths is followed ... 

Discovered more than 70 years ago, hydroformylation is nowadays one of the most important reactions in the chemical industry because aldehydes can be transformed to many other products. In the enantioselective version, rhodium/ diphosphorus ligand complexes are the most important catalytic precursors, although cobalt and platinum complexes have also been widely used. For these systems, the active species are pentacoordinated trigonal-bipyramidal rhodium hydride complexes, [HRh(P-P)(CO)2]. In those complexes, the coordination mode of the bidentate ligand (equatorial-equatorial or equatorial-apical) is an important parameter to explain the outcome of the process. The most common substrates of enantioselective hydroformylation are styrenes followed by vinyl acetate and allyl cyanide. With these substrates, mixtures of the branched (b, chiral) and linear (1, not chiral) aldehydes are usually obtained. In addition, some hydrogenation of the double bond is often observed. Therefore, chemo- and regioselectivity are prerequisites to enan-tioselectivity and all of them must be controlled. An additional eomplieation is that chiral aldehydes are prone to racemise in the presenee of rhodium spe-... 

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