Rhodium catalysts diphosphites

Most popular as the substrate for studying asymmetric hydroformylation has been styrene, which might also form linear aldehyde. 

Optically active diols are useful building blocks for the synthesis of chiral diphosphite ligands. Chiral diphosphites based on commercially available optically active 1,2 and 1,4-diols, l,2 5,5-diisopropylidene-D-mannitol, L-a,a,a,a-tetramethyl-l,3-dioxalan-4,5-dimethanol and L-diethyl tartrate, were first used in the asymmetric hydroformylation of styrene [75], 

Sugar backbones have been used many times, and indeed, when a three-carbon bridge is involved good e.e. s may be obtained [76], 

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Asymmetric rhodium catalysts are discussed in section 8.6. The most interesting ligand discovered for asymmetric hydroformylation is undoubtedly BINAPHOS, introduced by Takaya [18], but certain diphosphites also give high enantioselectivities [19,20],... 

Asymmetric hydrogenation of alkenes is efficiently catalysed by rhodium complexes with chiral diphosphite and diphosphoramidite ligands derived from BINOL or diphenylprolinol. Choice of a proper achiral backbone is crucial.341 Highly enantioselective hydrogenation of A-protected indoles was successfully achieved by use of the rhodium catalyst generated in situ from [Rh(nbd)2]SbF6 (nbd = norborna-2,5-diene)... 

Until recently rhodium catalysts gave lower enantioselectivity, but higher chemoselectivity and activity, than the platinum-based catalysts. However, in the past few years rhodium complexes of a few chiral diphosphites and phos-phinophosphito ligands have been reported. These complexes have excellent activities and high chemo-, regio-, and enantioselectivities. 

Less reactive olefins such as 2,2-dialkyl-1-alkenes are hydroformylated at much higher rates than those achieved with TPP-modified rhodium catalysts. Activities of 15 000 mol (aldehyde)/mol (Rh) h have been reported (90 °C, 1-3 MPa) [39]. 1-Alkenes are converted with even higher rates (activity = 160000 mol/mol h). At these high rates the reaction becomes mass-transfer limited. The lack of CO dissolved in the liquid layer leads to formation of unsaturated rhodium species which rapidly isomerize the olefin. The n/i ratio obtained is therefore low (20-30% linear product). The stmcture of hydrido-rhodium diphosphite complexes was investigated in detail by NMR spectroscopy [41]. BASF reported the hydroformylation of methyl 3-pentenecarboxylate with Rh(CO)2(acac) and 3 as a ligand. Methyl 5-formalvalerate was formed with 72 % selectivity [42]. 

Recently van Leeuwen reported the fiigt crystal structure of the diphosphite dicarbonyl rhodium catalyst HRh(CO)2(P P) [258]. Bomer et al. developed a new class of phosphonites which show promising results for the isomerization and subsequent hydroformylation of internal olefins [259]. The number of phosphite ligands based on supramolecular backbones such as calix[4]arenes is growing [260]. They are attractive because of their well defined structure combined with the ability to adopt several discrete conformations. Calix[4]arene diphosphites and calix[6]arene phosphites were first developed by BASF [261]. In... 

Another interesting technique to solve the problem of mass transport in bipha-sic reactions was developed by Fell and Jin, the catalysis with so-called thermo-regulated phase transfer ligands [177-185] (details see Section 4.6.3). In the examples given so far, a smart solvent system controlled the phase behaviour of the reaction mixture. Lemaire et al. have found that also polyether-functionalized chiral mono- or diphosphites are active in thermoregulated conversion [186]. The hy-droformylation of styrene yields conversions of 99%, n/i-ratios of 85/15 and an enantioselectivity of up to 25%. A recycling of the catalyst proved to be possible, however, with a certain leaching of the rhodium catalyst. 

Rhodium catalysts modified with diphosphites affect the isomerization of higher a-olefins, especially at low partial pressure of carbon monoxide or higher temperature. Isomerization of the alkene occurs by /3-H elimination of the branched alkyl rhodium intermediate. This results in the formation of 2-alkenes, which are less reactive in hydroformylation than 1-alkenes. In the case of propene, however, this /3-H elimination reaction converts the branched alkyl rhodium intermediate back to propene, and therefore very high n iso ratios can... 

Table 2. Hydroformylation using rhodium bulky diphosphite catalysts ...

For lower alkenes such as 2-butenes UCC has achieved high contents of linear products (see Table 2). Bryant reported 74% selectivity for the formation of linear pentanal by hydrofonnylation of 2-butene using the rhodium bulky diphosphite catalyst [22, 23]. 

Under these conditions, the bisdiazaphospholane ligand with electron-withdrawing groups induced the highest conversion, which corresponds to the well-accepted experience in hydroformylation that electron-poor phosphines produce particularly active rhodium catalysts. Interestingly, the relevant catalyst is even more active than those derived from the diphosphites Chiraphite and Kelliphite. 

Chiral diphosphites based on (2R,3R)-butane-2,3-diol, (2R,4R)-pentane-2,4-diol, (25, 5S)-hexane-2,5-diol, (lS -diphenylpropane-hS-diol, and tV-benzyltartarimide as chiral bridges have been used in the Rh-catalyzed asymmetric hydroformylation of styrene. Enantioselectivities up to 76%, at 50% conversion, have been obtained with stable hydridorhodium diphosphite catalysts. The solution structures of [RhH(L)(CO)2] complexes have been studied NMR and IR spectroscopic data revealed fluxional behavior. Depending on the structure of the bridge, the diphosphite adopts equatorial-equatorial or equatorial-axial coordination to the rhodium. The structure and the stability of the catalysts play a role in the asymmetric induction.218... 

A chiral diphosphite based on binaphthol, coordinated with rhodium (I) forming a nine-member ed ring, led to an efficient hydroformylation of vinylarenes, although moderate ees were obtained (up to 46%) at mild pressure and temperature reaction conditions.364 Chiral diphosphites and phosphinite-phosphites derived from spiro[4.4]nonane-l,6-diol were synthesized. Using these catalysts in the asymmetric hydroformylation of styrene, high regioselectivity (97%) and... 

However, platinum catalysts have several disadvantages they have low reaction rates, they hydrogenate the substrate and their regioselectivity to the branched aldehyde is low. The selectivity of Pt-diphosphite/SnCl2 systems is also low. When the appropriate diphosphite is used, ee s can be as high as 90% [13]. In the early 90s, several reports were published which described the state of the art in hydroformylation with both rhodium and platinum systems [14-16]. 

After the discovery of the high ee provided by rhodium/diphosphite and rhodium/phosphine-phosphite complexes, with total conversion in aldehydes and high regioselectivities, rhodium systems became the catalysts of choice for asymmetric hydroformylation. Important breakthroughs in this area have been the use of rhodium systems with chiral diphosphites derived from... 

The combination of rhodium dicarbonyl acetylacetonate complex (Rh(acac)(CO)2) and a diphosphite ligand, (2,2 -bis[(biphenyl-2,2 -dioxy)phosphinoxy]-3,3 -di-/i t/-butyl-5,5 -dimethoxy-l,T-biphenyl (BIPHEPHOS), is an excellent catalyst system for the linear-selective hydroformylation of a wide range of alkenes. This catalyst system has been successfully applied to the cyclohydrocarbonylation reactions of alkenamides and alkenylamines, which are employed as key steps for the syntheses of piperidine,indolizidine, and pyrrolizidine alkaloids. ... 

A variety of enantiopure or enantiomerically enriched phosphines, diphosphines, phosphites, diphosphites, phosphinephosphites, thiols, dithiols, P,N-ligands, and P,S-ligands have been developed as chiral modifiers of rhodium and platinum catalysts [1-7], Representative chiral ligands discussed in this chapter are shown in Figure 7.1. 

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