Mechanistic and synthetic aspects of stereoselective reactions of lithium derivatives of chiral phosphine oxides: X-ray crystal structure of (1R*,2S*,1′S*,2′R*)-1-(1′-diphenylphosphinoyl-2′-phenylpropyl)-2-phenylsulfanylcyclohexan-1-ol

Abstract

Lithiation of four different chiral phosphine oxides and reaction with seven different electrophiles has been carried out in order to discover what factors govern the sense and degree of asymmetric induction imparted by the resident chiral centre. Provided the groups at the chiral centre are significantly different in steric size (e.g. Ph and Me) and ketones, esters or Me₃SiCl are used as the electrophile, surprisingly high levels of syn-diastereoselectivity are observed. Reactions with benzaldehyde or methyl iodide proved to be rather unselective. The stereochemistry of the reaction of lithiated 1-diphenylphosphinoyl-2-phenylpropane 9 with cyclohexanone was elucidated by conversion into (1R*,2S*,1′S*,2′R*)-1-(1′-diphenylphosphinoyl-2′-phenylpropyl)-2-phenylsulfanylcyclohexan-1-ol syn,syn,anti-39 and subsequent X-ray crystal structure analysis. The stereochemistry of the remainder of the compounds were determined by analogy and by using a reliable ¹³C NMR spectroscopy ³J_PC coupling constant correlation. A detailed (and potentially general) mechanistic interpretation of the results which combines knowledge of the structure and configurational stability of lithiated phosphine oxides is discussed. In particular, it is suggested that a dynamic kinetic diastereoselection of rapidly equilibrating diastereomeric lithiated phosphine oxides explains the observed high levels of stereocontrol; a tentative transition state model is proposed to explain the syn-selectivity. Finally, the use of one of the addition products in the concise and stereoselective synthesis of an alkene with 1,4-disposed chiral centres is described.