Diastereoselective Synthesis of Arene Ruthenium(II) Complexes Containing Chiral Phosphetane-Based Tethers^,¹

Abstract

Enantiomerically pure cyclic ( R,R )-sulfates have been transformed into novel enantiopure ligands of the general type ( S,S )-2,4-R 2 -1-(3-phenylpropyl)phosphetane ( 7a − c; R = Cy, i -Pr, t -Bu). 7a − c split the arene ruthenium complex dimer [{RuCl 2 (η 6 -C 6 H 5 COOMe)} 2 ] ( 8 ) by forming the mononuclear σ complexes ( S C, S C )-[RuCl 2 (η 6 -C 6 H 5 COOMe){2,4-R 2 -1-(3-phenylpropyl)-η 1 -phosphetane}] ( 9a − c ). An intramolecular arene ligand displacement reaction leads to ( S C, S C )-[RuCl 2 {2,4-R 2 -1-(η 6 -3-phenylpropyl)-η 1 -phosphetane}] ( 10a − c ) with tethered side chains of the arene ligand. Nucleophilic substitution of a chloride ligand by aniline with the assistance of NaPF 6 gives access to the diastereomeric complex salts ( S C, S C )-[RuCl(aniline){2,4-R 2 -1-(η 6 -3-phenylpropyl)-η 1 -phosphetane}] ( 11a − c ). Good diastereoselectivities were obtained with de values between 84 and 88%. The absolute structures of the major diastereomers of 11a − c have been determined by X-ray structure analysis. R Ru, S C, S C configurations were found in all cases. DFT calculations performed on the dechlorinated 16-valence-electron intermediate cation ( S C, S C )-[RuCl{2,4-di- tert- butyl-1-(η 6 -3-phenylpropyl)-η 1 -phosphetane}] + ([ 12 ] + ) are in favor of an attack of aniline from the pro- R Ru side of the complex. Investigation of the relative stabilities of the R Ru and S Ru diastereomers of the complex cation [ 11c ] + revealed an almost isoenergetic situation. The diastereoselectivity of the ligand exchange reaction is therefore believed to be kinetically controlled.