Hydroformylation, asymmetric catalysts

Polymer supported transition metal catalysts Asymmetric hydroformylation or oxidation reactions d... 

Styrene, a-ethyl-asymmetric hydroformylation catalysts, platinum complexes, 6, 266 asymmetric hydrogenation catalysts, rhodium complexes, 6, 250 Styrene, a-methyl-asymmetric carbonylation catalysis by palladium complexes, 6, 293 carbonylation... 

Some chiral mono-, acyl- and di-thioureas have been used as ligand for the Rh-catalysed asymmetric hydroformylation of styrene. Although thiourea ligands form inactive systems with [Rh(COD)Cl]2 as the catalyst precursor, in standard conditions (40 °C, 40 bar CO -l- H2 1/1), the cationic Rh complex [Rh(COD)2]Bp4 combined with monothioureas as the ligand showed moderate to good activity (Scheme 29) [114]. 

In 1999, Casado et al. developed heterotetranuclear complexes (TiRh3) depicted in Scheme 10.3 with bridging sullido ligands combined with P-donor ligands. These complexes were further tested as catalysts for the asymmetric hydroformylation reaction of styrene. In this process, [CpTi((/i3-S)3 Rh(tfbb 3] was efficiently active under mild conditions (10 bar, CO/H2 = 1 atm, 353 K). In order to explore the effect of the added phosphorus ligand and the possibilities of this system for the asymmetric hydroformylation of styrene, achiral diphosphines such as dppe (l,2-bis(diphenylphosphine)ethane) and... 

Platinum complexes with chiral phosphorus ligands have been extensively used in asymmetric hydroformylation. In most cases, styrene has been used as the substrate to evaluate the efficiency of the catalyst systems. In addition, styrere was of interest as a model intermediate in the synthesis of arylpropionic acids, a family of anti-inflammatory drugs.308,309 Until 1993 the best enantio-selectivities in asymmetric hydroformylation were provided by platinum complexes, although the activities and regioselectivities were, in many cases, far from the obtained for rhodium catalysts. A report on asymmetric carbonylation was published in 1993.310 Two reviews dedicated to asymmetric hydroformylation, which appeared in 1995, include the most important studies and results on platinum-catalogued asymmetric hydroformylation.80,81 A report appeared in 1999 about hydrocarbonylation of carbon-carbon double bonds catalyzed by Ptn complexes, including a proposal for a mechanism for this process.311... 

Platinum(II) complexes with diphosphines based on DIOP (85),315-321 CHIRAPHOS (86),316,320 and bdpp (87)322-325 backbones have been prepared to be used, in the presence of SnCl2, as catalyst precursors in asymmetric hydroformylation of styrene and other alkenes. 

MeOBIPHEP is the atropisomeric diphosphine 2,2,-bis(diphenylphosphino)-6,6 -dimethoxy-l,-l -biphenyl (100), has been synthesized. In the presence of SnCl2, this species is an efficient catalyst for the asymmetric hydroformylation of styrene. Asymmetric inductions are higher than those attainable using the system [PtCl2 (i )-(+)-BINAP ]/SnCl2, where BINAP is 2,2 -bis(di-phenylphosphino)-l,l,-binaphthyl. The influence of CO and H2 partial pressures on the catalytic activity of the (99)/SnCl2 system has also been studied.328 Complexes [PtMeCl(P-P)][(101), P-P = (5)-6,6,-(dimethoxybiphenyl)-2,2,-diylbis(diphenylphosphine) ((5)-MOBIPH) (102),... 

Chiral bis-(binaphthophosphole) (bis(BNP)) ligands have been used in the asymmetric hydroformylation of styrene. In solution, the free diphospholes display fluxional behavior. Consistent with their structure, the reaction of the bis(BNP) compounds with platinum(II) derivatives gives either cis chelate mononuclear complexes or trans phosphorus-bridged polynuclear derivatives. Coordination to platinum enhances the conformational stability of bis(BNP)s and diastereomeric complexes can be detected in solution. In the presence of SnCl2, the platinum complexes give rise to catalysts that exhibit remarkable activity in the hydroformylation of styrene. Under optimum conditions, reaction takes place with high branched selectivity (80-85%) and moderate enantio-selectivity (up to 45% ee). 

In 1992, an important breakthrough appeared in the patent literature when Babin and Whiteker at Union Carbide reported the asymmetric hydroformylation of various alkenes with ees up to 90%, using bulky diphosphites derived from homochiral (2i ,4R)-pentane-2, 4-diol, UC-PP (1 19).359 360 van Leeuwen et al. studied these systems extensively. The influence of the bridge length, of the bulky substituents and the cooperativity of chiral centers on the performance of the catalyst has been reported.217 218 221 361-363... 

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... 

Bakos et al. reported a series of diastereomeric diphosphites that were used in the Pt- and Rh-catalyzed asymmetric hydroformylation of styrene. Systematic variation in chirality at both the chelate backbone and the terminal groups revealed a remarkable effect on the enantioselect-ivity of the catalysts. These systems have been described in Section 9.3.3.5. 

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... 

The monosulfonated PPh derivative, Ph2P(m-C6H4S03K) (DPM) and its rhodium complex, HRh(CO)(DPM)3 have been synthesized and characterized by IR and NMR spectroscopic techniques. The data showed that the structure was similar to [HRh(CO)(PPh3)3]. The catalytic activity and selectivity of [HRh(CO)(DPM)3] in styrene hydroformylation were studied in biphasic catalytic systems.420 421 Rh1 complexes [Rh(acac)(CO)(PR3)] with tpa (131), cyep (132), (126), ompp (133), pmpp (134), tmpp (135), PPh2(pyl), PPh(pyl)2, and P(pyl)3 were characterized with NMR and IR spectra. Complexes with (131), (132), and (126) were catalysts for hydrogenation of C—C and C—O bonds, isomerization of alkenes, and hydroformylation of alkenes.422 Asymmetric hydroformylation of styrene was performed using as catalyst precursor [Rh(//-0 Me)(COD)]2 associated with sodium salts of m-sulfonated diarylphosphines.423... 

TABLE 3.1. Asymmetric hydroformylation of styrene using polystyrene supported rhodium catalysts based... 

The first highly enantioselective asymmetric hydroformylation was the asymmetric hydroformylation of styrene.120 In 1991, Stille et al.121 reported the achievement of up to 96% ee using a chiral bisphosphine complex of PtCl2 as the catalyst in combination with SnCl2. However, the Pt(II)-catalyzed hydroformylation of arylethenes and some functionalized olefins has several disadvantages, such as low reaction rates, a tendency for the substrates to undergo hydrogenation, and poor branched-to-linear ratio. 

A big problem in asymmetric hydroformylation is that the chiral aldehyde products may be unstable and may undergo racemization during the reaction. This problem is even more serious for the Pt catalyst systems, which are usually plagued by slow reaction rates. Stille et al.121 tackled this problem by using triethyl orthoformate to trap the aldehyde products as their diethyl acetals and consequently increased the product ee values significantly. 

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... 

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