Electronic Differences between Coordinating Functionalities of Chiral Phosphine-Phosphites and Effects in Catalytic Enantioselective Hydrogenation
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- 作者：André ; s Suá ; rez ; Miguel A. Mé ; ndez-Rojas ; and Antonio Pizzano
- 刊名：Organometallics
- 出版年：2002
- 出版时间：October 28, 2002
- 年：2002
- 卷：21
- 期：22
- 页码：4611 - 4621
- 全文大小：253K
- 年卷期：v.21,no.22(October 28, 2002)
- ISSN：1520-6041
文摘
A convenient synthesis of new chiral phosphine-phosphites (P-OP) has been described.The versatility of the synthetic protocol developed has allowed the preparation of ligandswith different phosphine fragments and the choice of the stereogenic element location.Analyses of the values of 1JPSe of the corresponding diselenides are in accord with the expectedlower 
-donor ability of the phosphite fragment, with respect to the phosphine group, andwith an increase of phosphine basicity after substitution of phenyl substituents by methylgroups. Inspection of 
(CO) values on a series of complexes RhCl(CO)(P-OP) demonstrateda variable 
-aceptor ability of the phosphite group, compensating for the change of basicityof the phosphine functionality, as well as having a rather reduced electron density at themetal center compared with diphosphine analogues. The distinct nature of the phosphorusfunctionalities has also been evidenced in rhodium-catalyzed enantioselective hydrogenationof methyl Z-
-acetamido-cinnamate (MAC). Thus, the coordination mode of the substrate isgoverned by the chiral ligand, directing the olefinic bond to a cis position with respect to thephosphite group, as demonstrated by NMR studies performed with [Rh(P-OP)(MAC)]+complexes. In consequence, the phosphite group has a greater impact on the enantioselectivityof the product. However, the optical purity of the process also depends on the nature of thephosphine group, and hence, an appropriate election of both phosphorus functionalities isrequired for the attainment of excellent enantioselectivities (99% ee).
-donor ability of the phosphite fragment, with respect to the phosphine group, andwith an increase of phosphine basicity after substitution of phenyl substituents by methylgroups. Inspection of (CO) values on a series of complexes RhCl(CO)(P-OP) demonstrateda variable -aceptor ability of the phosphite group, compensating for the change of basicityof the phosphine functionality, as well as having a rather reduced electron density at themetal center compared with diphosphine analogues. The distinct nature of the phosphorusfunctionalities has also been evidenced in rhodium-catalyzed enantioselective hydrogenationof methyl Z--acetamido-cinnamate (MAC). Thus, the coordination mode of the substrate isgoverned by the chiral ligand, directing the olefinic bond to a cis position with respect to thephosphite group, as demonstrated by NMR studies performed with [Rh(P-OP)(MAC)]+complexes. In consequence, the phosphite group has a greater impact on the enantioselectivityof the product. However, the optical purity of the process also depends on the nature of thephosphine group, and hence, an appropriate election of both phosphorus functionalities isrequired for the attainment of excellent enantioselectivities (99% ee).