Binaphos system

Takasago group and Nozaki reported the synthesis of the 1-methylcarbapenem intermediate 78 by hydroformylation of the 4-vinyl / -lactam, (3[Pg.460]

Hydroformylation. The asymmetric version of hydroformylation has been studied using the Rh(acac)(CO)2-(/ ,5)-BINAPHOS system. Substituent effects on the olefins on the regioselectivity of the reaction were evaluated. 

Mechanistic studies have been made of the asymmetric hydrofomylation of the BINAPHOS system [4, 44], Deuterioformylation experiments (Chapter 2) have been used to study the aUcene insertion step [45]. Studies using [RhH(CO)2(R,S-BINAPHOS)l indicate that the alkene-insertion is irreversible for styrene at CO/H2 total pressures between 20 and 100 bar, but the insertion is partially irreversible at a total pressure of 1 bar [44]. 

The models which explain the enantiofacial selection in the Rh-BINAPHOS systems assume that the aUcene insertion in the Rh-H bond is irreversible. The old model proposed by Consiglio and Pino [46] has been used by Takaya [ 14]. The model involves partitioning the space around the metal center into four quadrants and for each of these the steric encumbrance is estimated. This simple model differentiates between the enantiofaces of the alkene and it has often been successfully applied. For RhH(CO)2(R,S-BINAPHOS) the aldehyde enantiomer expected from this model was indeed obtained [15]. 

The BINAPHOS system has also been modified by using the perfiuoroalkyl-substituted derivative of (R,S)-B1NAPH0S (see Chapter 10) in the rhodium catalyzed hydroformylation of vinyl arenes in liquid or supercritical carbon dioxide as solvent This system leads to similar catalytic activity and the same level of enantiocontrol as the rhodium-(R,S)-BINAPHOS system in organic solvents [48]. 

Rhodium-Catalyzed Asymmetric Hydroformylation 77 Table 3.5 Several substrates efficiently hydroformylated using Rh-BINAPHOS system. 

A new structural feature is obtained by replacement of the phosphite donor within BINAPHOS by a phosphoramidite system. Improved enan-tioselectivities were noted, albeit the problem of regioselectivity persists (Scheme 20) [69]. 

The hydroformylation reactions were carried out in a number of different solvent systems, Table 2, at 50-60°C and 40 bar CO/H2 (1 1) using the (S,R)-1 m-C6F13(CH2)3]2-BINAPHOS ligand, Figure 6.11. After 18 hours in the PFMC/toluene system, 100% conversion was achieved with 100% selectivity to aldehydes of which 92% were... 

Rhodium (I) complexes of chiral phosphines have been the archetypical catalysts for the hydrocarbonylation of 1-alkenes, with platinum complexes such as (61) making an impact also in the early 1990s[1461. More recently, rhodium(I)-chiral bisphosphites and phosphine phosphinites have been investigated. Quite remarkable results have been obtained with Rh(I)-BINAPHOS (62), with excellent ee s being obtained for aldehydes derived for a wide variety of substrates1 471. For example, hydroformylation of styrene gave a high yield of (R)-2-phenylpropanal (94% ee). The same catalyst system promoted the conversion of Z-but-2-ene into (5)-2-methylbutanal (82% ee). 

The results obtained for ligands 48 and 50, which contain only one fixed stereocentre, are interesting and very informative about the system. Ligand 48-(R,—), in which only the binaphthyl bridge has a predetermined absolute configuration R, leads to an e.e. of 83% (R-aldehyde), which is quite close to the value of 94% for (R,S)-BINAPHOS. This suggests that in the formation of the complex the binaphthyl bridge controls the conformation of the bisphenol... 

The C02-philic perfluoroalkyl-substituted (R,S)-3-H F -BINAPHOS ligand [34] was successfully applied to enantioselective hydrogenation in the inverted SCCO2/H2O system. The complex [Rh(cod)2]BARF was chosen as metal source and the active catalyst was formed in situ. Using the same procedure as above, similar activities and more than 98% ee were obtained consistently over five subsequent cycles in the hydrogenation of methyl 2-acetamido acrylate. The results demonstrate the potential of the inverted SCCO2/H2O system for asymmetric synthesis of chiral biologically active products. 

Effects of GO and H2 partial pressures on the reaction rate and selectivity of asymmetric hydroformylation of 1-hexene and styrene are examined using (7 ,A)-BINAPHOS-Rh catalyst system. For both substrates, high GO partial pressure tends to retard the reaction the partial pressure of H2 hardly affects the reaction rate (Phz -5 MPa). In most cases, the regio- and enantioselectivities are independent of H2 and GO pressure. Deuterioformylation experiments clearly demonstrate the irreversibility of the olefin-insertion step at total pressures of 2-10MPa (D2/G0=I/I). This fact proves that the regio- and enantioselectivity of the present hydroformylation should be controlled by the olefin-insertion step. Herrmann reported the theoretical calculation of the olefin coordination step, explaining selectivity obtained with (i ,A)-BINAPHOS/Rh system for the hydroformylation of styrene. 

As a unique medium for asymmetric hydroformylation, supercritical carbon dioxide has recently been examined, which can be carried out in an extremely low catalyst concentration. The reactions of styrene (16a) and pentafluorostyrene (16e) catalyzed by Rh-BINAPHOS appear to give mixed results that are highly dependent on the reaction conditions [77,78], Enantioselectivity up to 92-95% ee for 16a or 85 % ee for 16e has been observed [78]. A biphasic reaction system has also been examined for the reaction using Rh(acac)(CO)2 with a sulfonated diphosphine ligand BINAS [79], The reaction proceeds smoothly at 40°C and 100 atm in high conversion with excellent branched aldehyde selectivity (95%), but enantioselectivity is very low (18% ee). The use of these newer reaction conditions is still in the very early stage and further development is expected in the next decade. 

Rh-BINAPHOS catalyst system mechanisms, 11, 444 Asymmetric hydrogenation in C-H bond formation... 

ESPHOS (178), a chiral bis(diazaphospholidine) ligand developed by the Wills group, has been found to be active in the asymmetric hydroformylation of vinyl acetate.231 (This ligand system is being commercialized by Stylacats.) Enantioselectivities of 87-89% ee and regioselectivities of 95/5 (b/1) have been obtained at 120 psig pressure of syn gas.232 Unfortunately, the substrate scope is not as broad as BINAPHOS. 

The catalytic system based on the precatalyst [Rh(acac)(R,S BINAPHOS)] shows much higher activity for asymmetric hydroformylation of styrene than that based on RhH(CO)L3 (L=PPh3) plus R,S BINAPHOS. Why ... 

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