Effect of Ligand on Ring Contraction of Six-Membered Nickel-Containing Cyclic Esters, , to Their Five-Membered-Ring Isomers, . Kinetic and Thermodynamic Control of Asyammetric Induction by Chiral Diphosphines in the Ring Contraction

Article abstract of DOI:10.1021/ja00238a017

Ring contraction of (1) to (2) is accelerated by coordination of bulky ligands: coordination of a bulky diphosphine like 1,2-bis(diphenylphosphino)ethane (dpe) causes an extensive ring contraction to afford (dpe).Use of a chiral diphosphine affords a mixture of unequal amounts of diastereomers (chiral diphosphine)-(R) ((R)-2) and (chiral diphosphine)-(S)- ((S)-2).When (S,S)-chiraphos is used, (R)-2 is kinetically favored, but it isomerizes to thermodynamically favored (S)-2 obeying the first-order kinetics.The equilibrated reaction mixture after the isomerization contains ((S,S)-chiraphos) in 54percent diastereomer excess at 24 deg C, and the kinetic and thermodynamic parameters for the R to S isomerization are as follows: ΔH = 93 +/- 2 kJ/mol, ΔS = -8 +/- 6 J/(K mol), ΔG = 95 kJ/mol, ΔH = 13 +/- 2 kJ/mol, ΔS = 54 +/- 6J (K mol), and ΔG = -3.0 kJ/mol at 24 deg C.Use of (R,R)-dipamp gives a result opposite to that of (S,S)-chiraphos concerning the kinetically and thermodynamically favored species.Data obtained by use of (R)-prophos, trans-cypenphos, and trans-renorphos are also given.The kinetic and thermodynamic control of the asymmetry can be explained by considering the effects of arrangement of two phenyl groups bonded to each phosphorous atom of the diphosphine ligands on the metallacycle entity.