BINAPHOS

Second generation BINAPHOS-type ligands have been developed recently. Placing 3-methoxy substituents on the aryl phosphine unit furnishes a catalyst which allows for an enantioselective hydroformylation of vinylfurans (Scheme 19) [68]. 

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]. 

Figure 6.11. (S,R) [CsFutCFCbL-BINAPHOS and related ligands used in the rhodium catalysed asymmetric...

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... 

Scheme 8.3. Hydroformylation of styrene with the C02-philic in situ catalyst 3-H2F6-BINAPHOS/[Rh(acac)(CO)2] (2 1) using a batch-wise CESS procedure...

Zhang reported two new (S)-BINOL based ligands phosphine-phosphite (S,R)-o-BINAPHOS 163 and phosphine-phosphinite (S)-o-BIPNITE 164 [128]. Applications of these ligands in the Rh-catalyzed hydrogenation of methyl N-2-acetamido-cinnamate and methyl N-2-acetamidoacrylate induced very high enantioselectiv-ities (>99% ee), and with a wide range of substrates. 

Scheme 39.4 Enantioselective hydrogenation in scC02 using a C02-philic derivative of BINAPHOS.

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). 

BINAPHOS (R)-2-(diphenylphosphino)-l,l -binaphthalene-2 -yl (S)-l,l -binaphthalene-2,2 -diyl phosphite)... 

Takaya and co-workers in 1993 were the first to report on asymmetric hydroformylation using phosphite-phosphine ligands [59]. In an attempt to combine the effectiveness of the BINOL chemistry for asymmetric catalysis and the effectiveness of the phosphite moiety for asymmetric hydroformylation, they developed the (.R,S)-BINAPHOS ligand 3, which turned out to be very efficient (Fig. 6). 

The characterization of the rhodium complexes formed under hydroformylation conditions by NMR techniques and in situ IR spectroscopy showed that there is a relationship between the structure of the [HRh(CO)2 (BINAPHOS)] species and their enantiodiscriminating performance. Thus, (R,S)- and (S,R)-BINAPHOS ligands show high equatorial-axial (ea) coordination preference with the phosphite moiety in the axial position. Meanwhile, the characterization of the (R,R)- and (S,S)-BINAPHOS ligands suggests that there is either a structural deviation of the monohydride complexes from an ideal TBP structure or an equilibrium between isomers [20,34],... 

Highly crosslinked polymer-supported-BINAPHOS ligands were effective for the hydroformylation of styrene and other functionalized olefins (ee s up to 89%). The catalyst could be recovered and reused at low stirring conditions [61,62]. 

Perfluoroalkyl-substituted BINAPHOS ligand 29c was also developed for the asymmetric hydroformylation of vinyl arenes in SCCO2. With this ligand regio-and enantio-selectivity (ee s up to 93.6%) were high without the need for hazardous organic solvents [63,64],... 

Inspired by the excellent results using the BINAPHOS ligands, new phosphine-phosphite ligands with different backbones have been developed in recent years. Unfortunately, their Rh-catalyzed hydroformylation provided low-to-moderate enantioselectivity (ee s from 20 to 62%) [35,36,65]. 

This equatorial axial phosphorus exchange shows that in the most stable diastereoisomer, the phosphine is in the axial position while the phosphite, the best 7r-acceptor is located in the equatorial position. This behavior contrasts with the coordination mode found for the BINAPHOS in which the phosphine occupies the equatorial position and the phosphite the axial position. 

The phosphine-phosphite BINAPHOS ligand was first used in the Rh-catalyzed asymmetric hydroformylation of heterocyclic olefins such as 2,5-dihydrofuran, 3-pyrroline derivatives, and 4,7-dihydro-1,3-dioxepin derivatives. It provided the optically active aldehydes as single products with enantioselectivity between 64-76% ee. In the hydroformylation of 2,5-di-... 

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