CRYSTALLOGRAPHIC DETERMINATION OF ABSOLUTE-CONFIGURATION AT IRON OF A SERIES OF CHIRAL IRON ALKYLS - EMPIRICAL CIRCULAR-DICHROISM SPECTROSCOPIC CORRELATIONS

The absolute configurations of two optically active iron complexes, (+)578-CpFe(CO)(PPh3)(CH2O-menthyl) (1) and (+)578-CpFe(CO)(PPh3)(CH2COO-menthyl) (2), have been determined by single-crystal X-ray diffraction methods and have been correlated with their circular dichroism spectra. The configuration of 1 is S and that of 2 is R (based on the sequencing rules proposed recently by Stanley and Baird). Anomalous dispersion effects were used to elucidate the absolute configuration of 1, while that of 2 was determined using the (—)-menthyl group as a reference. From these results, the absolute configurations of a large number of complexes of the type CpFe(CO)(PPh3)(X) can be deduced and compared to their CD spectra [X=Br, I, Me, Et, n-Pr, i-Bu, CH2-c-C3H5, CH2Cl, CH2Br, CH21, CH2Ph, C(O)Me, S(O)2R, CH2S(O)20(menthyl), (η2-C2H4)+BF4-]. It is concluded that the maxima found at the 300-350- and 350-450-nm regions of the CD spectra can serve as a reliable indicator of the absolute configuration at iron, p ovided that the X groups are not too dissimilar. Crystal details: 1-S crystallizes in space group P2l (monoclinic) with a=10.882 (3) Å, b=11.054 (4)Å, c=13.664 (4)Å,β=102.66 (2)°, V=1603.7 Å3, Z= 2; 2-R crystallizes in space group P1 (triclinic) with a=7.660 (1) Å, b=13.806 (1) Å, c=15.948 (1) Å, α=108.23 (1)°. β = 88.62 (1)°, γ=95.06 (1)°, V=1595.5 A3, Z=2. Final agreement factors are for 1-5, R = 6.2% (3655 reflections); for 2-R, R = 8.4% (3665 reflections). © 1978, American Chemical Society. All rights reserved.