Resolution, Racemization, and Epimerization in Acyclic (diene)Fe(CO)₂L and [(dienyl)Fe(CO)₂L]X complexes (L = phosphine, phosphite)

A variety of substituted (2,4-pentadienal)-, (methyl 2,4-pentadienoate)-, and (3,5-heptadien-2-one)Fe(CO)₂L complexes have been prepared (L = PPh₃, P(OMe)₃, chiral phosphine); absolute configurations have been assigned by a combination of CD and single-crystal X-ray diffraction studies. The aldehyde and ketone complexes exhibit facile racemization or epimerization in solution (80–100 °C, benzene or toluene); the rates increase with increasing electron donor character of the 5-substituent, while diastereoface selectivity in the chiral phosphine series depends both on the phosphine and on the steric bulk of the 5-substituent. [(dienyl)Fe(CO)₂L]X salts also exhibit epimerization or racemization in solution. VT NMR studies have established axial/basal and s-cis/s-trans ratios in solution. For the aldehyde series, the correlation between the s-cis/s-trans ratio and the diastereoselectivity of nucleophilic attack (MeLi, MeMgI) has been examined. Crystal data: 14a, orthorhombic, space group P2₁2₁2₁:, a = 9.860(4) Å, b = 35.815(9) Å, c = 16.519(4) Å, Z = 4, R₁ = 0.0431 for 669 parameters and 8706 observed reflections; 15b, monoclinic, space group P2₁, a = 8.7353(9) Å, b = 18.055(3) Å, c = 9.9894(11) Å, β = 113.395(8)°, Z = 2, R₁ = 0.0380 for 335 parameters and 4328 observed reflections; 17a, orthorhombic, space group P2₁2₁2₁, a = 10.151(3) Å, b = 14.780(3) Å, c = 17.804(4) Å, Z = 4,R₁ = 0.0481 for 302 parameters and 1583 observed reflections; 23, triclinic, space group P1, a = 12.081(2) Å, b = 13.914(2) Å, c = 16.835(2) Å, α = 93.33(2)°, β = 109.47(2)°, γ = 113.18(2)°, Z = 4, R₁ = 0.0788 for 579 parameters and 9425 observed reflections.