(S P,S P)-(–)-(E)-1,2-Bis(methylphenylphosphinoyl)ethene

The title compound, C16H18O2P2, possesses two stereogenic P atoms and shows a distorted s–cis conformation of each O=P—C=C moiety. This has been suggested on the basis of the stereochemical result of 1,3-dipolar cycloadditions with nitrones and is now confirmed by the present crystal structure analysis. There are two crystallographically independent molecules in the asymmetric unit.

The molecular structure of 1 is displayed in Fig. 1. The absolute configuration of the stereogenic centers has not been determined crystallographically but is evident from that of the starting material (Pietrusiewicz et al., 1984, Pietrusiewicz et al., 1991 as well as from the crystal structure analysis of a cycloaddition product (Vinokurov et al., 2008). The largest substituents of each phosphorus atom are placed in the most distant zigzag positions, and the P1=O1 and P2=O2 dipoles are oriented in opposite directions relative to one another. The deviation from planarity of the O=P-C=C-P=O is reflected by the torsional angles given in Table 1.
As typical for compounds of type R 3 P=O deformations of the tetrahedral environment of the P atoms cause an increase DFT calculations show the observed conformation to be more stable than the corresponding di-s-trans conformation by 16.54 kJ/mol (TURBOMOLE 5.7 Method BP86/SV(P).

S3. Refinement
Note: The asymmetric unit contains two crystallographically independent molecules, one of which is presented here. H atoms were positioned geometrically and refined using a riding model with C-H = 0.93-0.96 Å and with U iso (H) = 1.2 (1.5 for methyl groups) × U eq (C).

Figure 1
Structure of (1) in the crystal with atom labels and 50% probability displacement ellipsoids for non-H atoms.

(S P ,S P )-(-)-(E)-1,2-Bis(methylphenylphosphinoyl)ethene
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 0.37 e Å −3 Δρ min = −0.19 e Å −3 Absolute structure: Flack (1983), 2656 Friedel pairs Absolute structure parameter: 0.01 (9) Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.