Abstract
Optical resolution of several dialkyl-arylphosphine oxides was elaborated using the Ca2+ salt of (−)-O,O’-dibenzoyl-(2R,3R)-tartaric acid as the resolving agent. The conditions of crystallization and purification of the enantiomerically enriched phosphine oxides were optimized. Ethyl-phenyl-propylphosphine oxide and butyl-methyl-phenylphosphine oxide were prepared with an enantiomeric excess higher than 93%, whereas, three other dialkyl-arylphosphine oxides were obtained with an enantiomeric excess of 37–85%. It was also found that the sterically demanding alkyl chains hinder the formation of stable diastereomeric complexes, which consequently led to less efficient resolution procedures.
Highlights
Chiral compounds form an important class in organophosphorus chemistry
As coordination of the chiral tartaric acid scaffold and phosphine oxide to the cation is one of the most important interactions in the diastereomers formed, (−)-O,O0 -dibenzoyl-(2R,3R)-tartaric acid was converted to its calcium, magnesium, copper, nickel- or cobalt-salts to test which derivative is the most suitable for enantiomeric separation of ethyl-phenyl-propylphosphine oxide (3)
This study demonstrated that the coordination mediated optical resolution method originally developed for cyclic phosphine oxides has a more general scope, and this technique can be used for acyclic phosphine oxides
Summary
Chiral compounds form an important class in organophosphorus chemistry. These species may contain an asymmetric element either on the phosphorus atom or at other parts of the molecular scaffold [1,2,3]. Chiral organophosphourus compounds were traditionally used as ligands in transition metal-catalyzed transformations [4,5]. In the past two decades, application of phosphines [6,7,8], phosphine oxides [9,10] or phosphonium salts [11,12] as enantioselective organocatalysts became an emerging area. Many enantioselective (organo)catalytic transformations take place in the close proximity of the phosphorus atom, which signifies the application of the corresponding P-stereogenic species [13]
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