How Do Positions of Phosphito Units on a Calix[4]Arene Platform Affect the Enantioselectivity of a Catalytic Reaction?

Abstract

Three chiral diphosphites, (S,S)-5,17-bis(1,1′-binaphthyl-2,2′-dioxyphosphanyloxy)-25,26,27,28-tetrapropyloxycalix[4]arene (1), (S,S)-5,11,17,23-tetra-tert-butyl-25,27-dipropoxy-26,28-bis(1,1′-binaphthyl-2,2′-dioxyphosphanyloxy)calix[4]arene (2) and (S,S)-5,11,17,23-tetra-tert-butyl-25,26-dipropoxy-27,28-bis(1,1′-binaphthyl-2,2′-dioxyphosphanyloxy)calix[4]arene (3), based on conical calix[4]arene were investigated in the rhodium-catalyzed asymmetric hydrogenation of α-dehydroamino esters. High conversions were observed after 24 h under 5 bar of hydrogen whatever the employed diphosphite, and the chiral induction increases in the order 1 < 3 < 2. This may be due to the presence of the calix[4]arene moiety, which by its presence modifies the second coordination sphere of the catalytic center. The larger steric hindrance around the rhodium atom leads to the higher enantiomeric excess.

Optically Pure Calixarenyl Phosphine via Stereospecific Alkylation on Evans’ Oxazolidinone Moiety

“…These differences could be explained by steric factors generated by the second coordination sphere of the ligand, i.e., by the calix[4]arene skeleton. In accordance with molecular models, the more sterically constrained active species (encapsulation of the catalytic center inside a molecular pocket), obtained when the diphosphite B was employed, led to the more efficient chirality transfer from the ligand to the substrate [ 18 ].…”

“…strained active species (encapsulation of the catalytic center inside a molecular pocket), obtained when the diphosphite B was employed, led to the more efficient chirality transfer from the ligand to the substrate [18]. Regarding the carbon-carbon bond formation, the most popular reaction is the palladium-catalyzed asymmetric allylic alkylation of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate, released generally with the N,O-bis(trimethylsilyl)acetamide and acetate salt as bases (Equation (3) in Figure 2).…”