Phosphines DUPHOS

Later, in the 1990s a remarkable asymmetric catalyst with the chiral chelating phosphine DuPHOS (3.49) (Figure 3.19) was developed by M. J. Burk at DuPont. This cationic DuPHOS/rhodium complex showed very high ee values of 99% in asymmetric hydrogenation of enamides. As can be seen the choice of chiral phosphine chelating ligand had a dramatic effect on the ee values. 

The numerous chiral phosphine ligands which are available to date [21] can be subclassified into three major categories depending on the location of the chiral center ligands presenting axial chirality (e.g., BINAP 1 and MOP 2), those bearing a chiral carbon-backbone (e.g., DIOP 3, DuPHOS 4), and those bearing the chiral center at the phosphorus atom (e. g., DIPAMP 5, BisP 6), as depicted in Fig. 1. 

Recently, the chiral Pt(0) precatalyst Pt[(R, R)-Me-Duphos](trows-stilbene) (11) has been used to prepare enantiomerically enriched chiral phosphines via hydrophosphination of acrylonitrile, t-butyl acrylate and related substrates. This chemistry is summarized in Scheme 5-13. 

Asymmetric hydrophosphination has been utilized as a route for preparing chiral phosphines. The Pt° complex [(Me-DUPHOS)Pt(t/ tf/ ,s-PhCII ClIPh)] (73) brings about the catalytic P-H addition of bulky secondary phosphines to activated alkenes with modest enantioselectivity. The most promising substrate combinations for further development appear to be bulky alkenes and less bulky phosphines (Scheme 46).195... 

Kovacik, I., Wicht, D.K., Grewal, N.S., Glueck, D.S., Incarvito, C.D., Guzei, I.A., and Rheingold, A.L., Pt(Me-Duphos)-catalyzed asymmetric hydrophosphination of activated olefins enantioselective synthesis of chiral phosphines,... 

In addition to direct DuPhos and BPE analogues, several other ligands containing five-membered phosphacycles have been reported (Fig. 24.6). As early as 1991, non-C2-symme trie phospholane-containing phosphines 37-39 were reported by Brunner and Limmer [7]. These were prepared by base-induced addition of the secondary phospholane to the appropriate diphenylphosphino-substi-tuted olefin. As for the symmetrical 3,4-disubstituted bisphospholanes, enantios-electivities for the Rh-catalyzed reduction of a-acetamidocinnamate were poor. 

In 1993 Burk, Brown, and coworkers confirmed that DuPHOS complexes exhibit the same anti-lock-and-key mechanistic motif as seen for aryl phosphine ligated catalysts [41], In 1998 by Burk and coworkers reported an unexpected and interesting result [34], With substrates having R<, = aryl, selectivity of 99% e.e. for the S product resulted from (S,S)-Me-DuPHOS-Rh hydrogenations, but the R product was obtained with similarly high enantioselectivity when Ra = t-Bu or adamantyl. In other words, the simple change of an aryl substituent to a bulky alkyl completely reverses the sense of enantioselection. 

An important consequence of the often relatively large y( P, P) value is that spectra of bis-phosphine complexes are often second order, e. g., the =CH signals for the two isomers of [Pd(NCCH=CHCN)(Me-Duphos)], 77, see Figure 1.15 [3]. The figure shows that these absorptions appear as complicated multiplets (i.e., the X-part of an ABX spin system). 

Handling, Storage, and Precautions crystalline Me-DuPhos is stable to air oxidation for over 10 days. However, it is generally prudent to store the DuPhos ligands under an inert atmosphere. In benzene solution, the DuPhos ligands are prone to oxidation with ca. 65% conversion to phosphine oxide after 3 weeks. Toxicity data are not available. 

Among the various Rh-phosphine catalysts used to perform enantioselective hydrogenation of lV-(3,4-dihydro-l-naphthyl)ac-etamide, Rh-(R,5,R,S)-Me-PennPhos afforded the desired hydrogenation product in highest optical yield. Thus, the use of Rh-L catalysts, where L = (S,5)-(-e)-DIOP (1), (R)-(-e)-BINAP (2), and (R,R)-(—)-Me-DuPhos (3), afforded lV-(l,2,3,4-tetrahydro-l-naphthyl)acetamide in only 10% ee(S), 24% ee (R), and 1% ee (R, with 57% conversion). ... 

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