Bulky diphosphite

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... 

The first report to use diphosphite ligands in the asymmetric hydroformylation of vinyl arenes revealed no asymmetric induction [46]. An important breakthrough came in 1992 when Babin and Whiteker at Union Carbide patented the asymmetric hydroformylation of various alkenes with ee s up to 90%, using bulky diphosphites 2a-c derived from homochiral (2R, 4R)-pentane-2,4-diol (Scheme 4) [17]. Their early results showed that (a) bulky substituents are required at the ortho positions of the biphenyl moieties for good regio- and enantio-selectivity and (b) methoxy substituents in the para positions of the biphenyl moieties always produced better enantio-selectivities than those observed for the corresponding ferf-butyl-substituted analogues. 

Table 8.6. Hydroformylation using rhodium bulky diphosphite catalysts3...

Bulky diphosphites not only express a high selectivity toward 1-alkenes [255] but also for less reactive internal [256] and functionalized alkenes. Recently DSM and Du Pont reported on a ligand (24) which has a high regio- and chemoselectivity for the hydroformylation of methyl 3-pentenoate [253]. The synthesis of monophenols containing bulky substituents (25) is described in patents from Mitsubishi [257]. High yields with 1-alkenes and l/b ratios up to 20 are reported. 

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]. 

Diphosphites came into focus after the discovery of Bryant and CO workers at Union Carbide Corporation that certain bulky diphosphites lead to high selectivities in the rhodium catalyzed hydrofonnylation of terminal and internal alkenes [34] (see Figure 13). A plethora of diphosphites has been tested and recorded in many patents authored by coworkers of several companies [35-37], indicating their importance for the near future in this field. The patents included in the references of this chapter serve only as examples. 

Among the rhodium complexes, the catalyst containing the nonsymmetrical atropisomeric phosphine-phosphite chelating ligand (1) gave the best enantioselec-tivity (up to 92% ee) in the hydroformylation of the ibuprofen precursor 4-isobutyl-styrene [2] whereas the use of the bulky diphosphite ligand 2 resulted in a remarkable regioselectivity with a branched/normal ratio of 98.5/1.5 and somewhat lower optical yield (82% ee) [3]. 

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