Optically-active tris(O,O′-dialkyl dithiophosphato-S,S′)chromium(III) complexes. Isolation of diastereoisomers and isomerization reactions

Abstract Two optically-active O , O ′-dialkyl dithiophosphate ligands, (Bor) 2 PS 2  − and (Men) 2 PS 2  − , were prepared from P 2 S 5 and the monoterpene alcohols, d -borneol and l -menthol. The chromium(III) complexes, [Cr{(Bor) 2 PS 2 } 3 ] and [Cr{(Men) 2 PS 2 } 3 ], were prepared by reaction with Cr(NO 3 ) 3 ·9H 2 O in acetone. The diastereoisomers (Δ and Λ) were separated by fractional crystallization of the complexes from toluene ([Cr{(Bor) 2 PS 2 } 3 ]) or acetone ([Cr{(Men) 2 PS 2 } 3 ]). The (−) 680  CD -Δ-[Cr{(Bor) 2 PS 2 } 3 ] isomer was obtained from the less soluble part. This isomer shows extremely strong CD, Δ e (680 nm, toluene)=−15.0 M −1 cm −1 . The Δ-[Cr{(Men) 2 PS 2 } 3 ] isomer could not be obtained in the optically-pure state, since the complex inverts appreciably during the recrystallization procedure. Activation parameters for isomerization (epimerization), Δ→Λ and Λ→Δ were determined for both complexes, [Cr{(Bor) 2 PS 2 } 3 ] and [Cr{(Men) 2 PS 2 } 3 ]. [Cr{(Men) 2 PS 2 } 3 ] isomerizes much faster than [Cr{(Bor) 2 PS 2 } 3 ].