Synthesis and structure determination of stereoisomeric 2,2’-dibenzoyl-3,3’,4,4’-tetramethyl-1,1’-diphosphaferrocenes

Reaction of 3,3’,4,4’-tetramethyl-1,1’-diphosphaferrocene 1 with an excess of benzoyl chloride and AlCl 3 afforded a ~1:1 mixture of meso - and rac -2,2’-dibenzoyl-3,3’,4,4’-tetramethyl-1,1’-diphosphaferrocenes 2 , which were separated by column chromatography. The less polar diastereomer was transformed into its bis -W(CO) 5 - complex 3 . The single crystal X-ray study revealed that this complex contains the meso - 2 ligand. Therefore, the more polar diastereomer must be rac - 2 . It has been suggested that the splitting pattern of the phospholyl protons signal in the 1 H NMR spectra can be used for stereochemistry assignments.

Herein we report the 2,2'-dibenzoylation of 1 along with the separation of the meso-and racstereoisomers via column chromatography.We determined the structure of one of the stereoisomers after its transformation into crystalline bis-W(CO) 5 -complex by single-crystal Xray diffraction.

Results and Discussion
We found that 1 reacts with benzoyl chloride and AlCl 3 (2,5-2.7 molar equivalents) in dichloromethane at room temperature to afford a ~1:1 mixture of meso-2 [ (R p ,S p )-2] and rac-2 [ (R p ,R p )-2 and [(S p ,S p )-2] in 79% overall yield.The stereoisomers were separated via column chromatography.They are deep red solids giving elemental analyses, IR and 31 P NMR spectra corroborating their structures.Interestingly, their 1 H NMR spectra show substantial difference in the splitting pattern of the phospholyl protons (H-5 and H-5').Whereas the less polar isomer shows expected two-proton doublet (J P-H = 37.1Hz) for these protons, its more polar counterpart displays more complex splitting pattern (Fig. 1).The more complex and apparently higher-order splitting pattern observed for the more polar stereoisomer may suggest its lower symmetry i.e. the rac stereochemistry as was suggested by Mathey et al. for 2,2'-(diphenylphosphino) derivative of 1. 10 However, to provide a direct evidence for the suggested stereochemistry X-ray diffraction study of at least one stereoisomer of 2 was necessary.Unfortunately, attempts to grow crystals of any stereoisomer suitable for such a study failed.Since earlier we often used transformation of a 1,1'-diphosphaferrocene derivative a) b) into its bis-W(CO) 5 complexes, which easily gave X-ray quality crystals, 11 we transformed the less polar diastereomer of 2 into a such derivative via reaction with photochemically generated W(CO) 5 (THF).The single crystal X-ray study revealed the structure 3 (vide infra).
The metallocene ligand in 3 has a meso-configuration, and therefore the less polar and more polar diastereomers of 2 must be meso-2 and rac-2, respectively.
It is worthy noting that diastereomeric 2,2'-diacetyl-3,3',4,4'-tetramethyl-1,1'diphosphaferrocenes also shown a very similar difference in the splitting patterns of phospholyl protons, the less polar displaying a doublet and the more polar a higher-order multiplet similar to that shown in Figure 1b.This suggests that this difference may be used as a tool for stereochemistry determination.

X-Ray diffraction study of 3
Crystals of 3 suitable for X-ray diffraction study were grown from layered dichloromethanehexane.The crystal data and structure refinement are collected in Table 1.Some relevant bond lengths and angles are gathered in Table 2.The molecular structure of 3 is shown in Fig. 2.  The meso-2 ligand in 3 adopts a centrosymmetric antiperiplanar conformation with the largest distance between phosphorus atoms.The W-P bonds length, 2.4719(8) Ǻ, is similar to those observed in similar complexes. 11,12Interestingly, the carbonyl groups in positions 2 and 2' are almost perpendicular to the phospholyl ligands planes (torsional angle C3-C4-C7-O1 is equal to -104(4)˚ and P1-C4-C7-O1 to 81.4(4)˚).It seems therefore that there is no conjugation between these groups and the metallocene moiety.On the other hand, the C7=O1 carbonyl group are almost coplanar with the phenyl rings (torsional angle O1-C7-C8-C13 is equal to -6.6(4)˚ and O1-C7-C8-C9 to 173.1(3)˚), indicating extensive conjugation between these groups.

Experimental Section
General Procedures.All reactions were carried out under argon.All reagents used in this work are commercially available (Aldrich) and were used without further purifications.Compound 1 was prepared according to the literature procedure. 13Dichloromethane was distilled over calcium hydride and THF over sodium-benzophenone before use.Chromatographic separations were carried out using Silica gel 60 (Merck, 230-400 mesh ASTM).The NMR spectra were run on a Varian Gemini 200 BB (200 MHz for 1 H) and IR spectra on a FT-IR Nexus spectrometer.Elemental analyses were performed by Analytical Services of the Center of Molecular and Macromolecular Studies of the Polish Academy of the Sciences (Łódź).

Figure 1 .
Figure 1.Signal of the phospholyl protons in the 1 H NMR spectra of less polar (a) and more polar diastereomer (b) of 2.

Table 1 .
Crystal data and structure refinement for 3