Synthesis of Chiral, Monodentate Aminophosphane and Phosphoramidite Ligands Derived from Amino Acid Esters: Application in Rh‐Catalysed Asymmetric Olefin Hydrogenation Reactions

Abstract

Six chiral monodentate ligands combining a 1,3‐dioxa‐2‐phosphacycloheptadinaphthyl moiety [(R)‐ or (S)‐binoP] either with a phenylalanine‐ or with an alanine‐derived fragment were synthesised. The new phosphoramidites are all relatively air stable. Related compounds in which the binoP moiety was replaced by a diphenylphosphanyl group were also prepared for comparison. The X‐ray structures of two phosphoramidite complexes, cis‐PtCl2[(R)‐binoP–NMeR]2 [5, R = (R)‐CH(CH2Ph)(CO2Me); 6, R = (S)‐CH(CH2Ph)(CO2Me)], were determined by single X‐ray analysis. In the solid state, both structures are nearly C2 symmetric, and the nitrogen atoms lies out of the coordination plane. Owing to the particular orientation of the benzyl groups in 6, the environment of the coordination sites occupied in this complex by the chlorine atoms is sterically more crowded than in 5. In the hydrogenation of 2‐(acetylamino)‐3‐(aryl)propenoic methyl esters (aryl = 4‐X‐C6H4, X = H, F, Cl; aryl = 3,4‐Cl2–C6H3), the alanine‐derived phosphoramidites turned out to be ca. twice as active as the corresponding phenylalanine analogues. The highest ee's were observed with the phenylalanine derivatives, for example 92 % in the hydrogenation of 2‐(acetylamino)‐3‐(phenyl)propenoic methyl ester, by using (R)‐binoP–NMe‐(R)‐CH(CH2Ph)(CO2Me). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Efficient, Rhodium‐Catalyzed Hydrogenation of α‐Dehydroamino Acid Esters with Chiral Monodentate Aminophosphanes Bearing Two Binaphthyl Groups

“…This geometry was deduced from the corresponding J P,Pt coupling constant, 5580 Hz (CDCl 3 ), which is in keeping with other cis-phosphoramidite-platinum complexes. [21,[34][35][36] This finding was further confirmed by an X-ray structure determination (Figure 2). The asymmetric unit of 7 consists of a single molecule, which, as in 6, is nearly C 2 -symmetric.…”

“…Interestingly, the P atoms in the two pairs of enantiomers (S aze ,S bin )-4a/(R aze ,R bin )-4b and (R aze ,S bin )-4c/(S aze , R bin )-4d resonated at strikingly different chemical shifts (δ = 146.0 and 143.0 ppm, respectively). In comparison, the chemical shift difference between the corresponding 31 P signals is only 0.4 ppm for the recently reported diastereoisomers C and D. [21] Evidently, the P atoms experience very different environments in each diastereoisomeric pair.…”

“…Following these results, a range of related binaphthyl phosphoramidites (also named as Feringa phosphoramidites) were synthesised in order to explore the scope of these catalysts. [17][18][19][20][21] Most of them turned out to give high ee's in asymmetric catalysis. Interestingly, de Vries et al recently discovered that significant selectivity improvements may be achieved when integrating the nitrogen atom into a cyclic structure, such as in PipPhos and MorfPhos.…”

“…In contrast, the four diastereoisomeric phosphoramidites 4a-d proved to be much more effective ligands. Hydrogenation rates are well known to be higher for phosphoramidites than for aminophosphanes, [21] and it came as no surprise that 4a-d were ca. 16 times more active than 2a and 2b.…”

Synthesis of Dianionic and Trianionic Chiral, Chelating Ligands Based on Amino Acids

“…[5][6][7] For example, chiral amino acids have been incorporated into phosphine and phosphite ligands for asymmetric catalysis. [8,9] However, in many cases the amino acids are not incorporated into the ligand via amide bonds, and most of these ligands contain other functional groups; also, many bear the chiral amino acids remote from the metal.…”