Iridium Complexes with Phosphine−Phosphite Ligands. Structural Aspects and Application in the Catalytic Asymmetric Hydrogenation of N-Aryl Imines

Abstract

A family of modularly designed phosphine−phosphites (P−OP), possessing a C−C−O backbone, has been synthesized and evaluated in the iridium-catalyzed asymmetric hydrogenation of N-aryl imines. The enantioselectivity of this reaction is highly dependent on the nature of the ligand, and catalysts bridged by an oxyethylene fragment have produced significantly higher enantiomeric excesses (Δee > 20%) than their o-oxyphenylene counterparts. Structural studies by X-ray crystallography and NMR spectroscopy of complexes with the formulation [Ir(COD)(P−OP)]BF4 and Ir(Cl)(CO)(P−OP), complemented by DFT calculations of model compounds of the chlorocarbonyls, have shown important differences between complexes bridged by an aliphatic or an aromatic bridge, regarding the iridacycle conformation and the location of phosphine substituents. Catalyst optimization has afforded enantioselectivities from 72 to 85% ee in the hydrogenation of several N-aryl imines.

Tuning of the Structures of Chiral Phosphane‐Phosphites: Application to the Highly Enantioselective Synthesis of α‐Acyloxy Phosphonates by Catalytic Hydrogenation

“…Keywords: asymmetric catalysis · enantioselectivity · hydrogenation · P ligands · rhodium [ ties around the 80 % ee level in the asymmetric hydrogenation of N-aryl imines. [11] The easily tuneable structures of compounds 1 and 2 make them appropriate tools with which to examine the hydrogenation of challenging substrates: the reduction of enol ester phosphonates for the synthesis of biologically [12] and synthetically [13] important chiral a-hydroxy phosphonates, for instance, is of great interest. Studies covering the scope of this transformation have used important rhodium catalysts bearing DuPHOS, [14a] BisP* or Miniphos ligands, [14b] and these investigations have provided excellent enantioselectivities in hydrogenations of b-alkyl-substituted phosphonates, although catalyst performance is significantly reduced with b-aryl substrates.…”

“…Accordingly, chlorophosphite 3 (Scheme 1) was prepared from the corresponding bisphenol, [18] while to provide the other component a set of phenol-phosphanes 4 was used. In addition to the previously reported 4 a-c, [5,11] additional examples were prepared in order to widen the tuneability of this ligand family. The 1-naphthyl derivative 4 d was thus synthesized by demethylation of the corresponding o-anisyl phosphane, [5] while the tert-butyl phosphane 4 e was prepared by a modification of the procedure described by Schmalz (Scheme 2).…”

“…[11] An interesting feature of this structure is the syn orientation of the bridged phenylene with respect to the exo aryl part (constituted by carbons C7-C12) of the biphenyl fragment. This arrangement, also observed in disubstituted 9 a, is the opposite of the anti orientation observed in complexes derived from 1, which has been attributed to an interaction between the phenylene backbone and a tBu group of the phosphite, additionally causing the rigidity of coordinated 1.…”

“…[5] Although the second element operates in rigid ligands 1, it should be minimized in flexible 2 a. [11] The relatively similar enantioselectivities provided by ligands 1 a and 2 a in the hydrogenation of 12 a (93 and 82 % ee, respectively) and 12 b (85 and 82 % ee, respectively) point to the phosphite as the principal source of stereoinduction.…”

Catalytic Hydrogenation of α‐Iminophosphonates as a Method for the Synthesis of Racemic and Optically Active α‐Aminophosphonates

“…[52,53] The chiral amidophosphite MonoPhos [54] was successfully applied in the Ir(I)-catalyzed hydrogenation of the model 2,3,3-trimethylindolenine. [55] Having in hand a family of ligands of phosphite (L2-6) [56][57][58] and amidophosphite (L7, 8) [59,60] types, we tested them in the hydrogenation of a-iminophosphonate 2b.…”