Synthesis and crystallographic studies of 2-(diphenylphosphinothioyl)-2-(3-oxobut-1-en-yl)ferrocene

The title molecule is built up from a ferrocene unit disubstituted by an S-protected diphenylphosphine group and by a methylvinylketone chain. In the crystal, weak C—H⋯O and C—H⋯S interactions build a two-dimensional network.


Chemical context
Over the last few years, our team has developed several bidentate phosphine-containing planar chiral ferrocene ligands and tested them in various asymmetric catalytic reactions (Manoury & Poli, 2011). In particular, some P,O ligands were synthesized from 2-(diphenylphosphinothioyl)ferrocenecarboxaldehyde (Mateus et al., 2006). This compound can be easily obtained as a racemic mixture or as each pure enantiomer and bears a versatile aldehyde function, which can be used to obtain more complex molecules. In this context, we were delighted to report a new and efficient aldol/elimination reaction of the aldehyde group to yield the corresponding eneone under mild conditions (see Scheme) using a weak base (pKa of 2-picolyl amine is 8.60; Miletti et al., 2010). Similar compounds have been synthesized but using the Wittig reaction, which requires the synthesis of a specific phosphonium reagent and the use of a strong base, such as nbutyllithium (Ye et al., 2017;Schaarschmidt et al., 2014;Š tě pnička et al. 2008) or sodium hydride (Stepnicka et al., 2008). Indeed, the aldol/elimination sequence has been used to functionalize ferrocenecarboxaldehyde, which is a much less crowded analog of 2-(diphenylphosphinothioyl)ferrocenecarboxaldehyde but with a much stronger base such as NaOH, KOH or tBuOK (see, for instance, Achelle

Structural commentary
The molecule is built up from a ferrocene unit disubstituted by an S-protected diphenylphosphine group and by a methylvinylketone chain (Fig. 1). As is usually observed for thiophenylphosphine ferrocenyl derivatives, the P atom is roughly in the plane of the Cp ring, deviating from the mean plane by À0.034 (5) Å , whereas the S atom is offset from this plane by 1.159 (6) Å . The two Cp rings have a staggered conformation with a twist angle of ca 37.1 . The O atom is trans to the ferrocene unit with respect to the C C double bond. The torsion angle of the C2-C21-C22-C23 chain is 172.4 (4) and the plane containing the double bond is twisted with respect to the Cp ring by 22.8 (2) . This molecule has a planar chirality related to the occurrence of two different substituents on the Cp ring; however, as the space group is centrosymmetric, the two enantiomers R/S are present in equal numbers within the crystal. Two intramolecular C-HÁ Á ÁS interactions occur (Table 1).

Supramolecular features
The packing of the structure is stabilized by weak C-HÁ Á ÁO and C-HÁ Á ÁS interactions (Table 1). The C-HÁ Á ÁO interaction results in the formation of a pseudo-dimer through an R 2 2 (8) graph-set motif (Etter et al., 1990;Bernstein et al., 1995) (Fig. 2). The C-HÁ Á ÁS intercations build up a chain parallel to the b axis and these chains are further associated by the C-HÁ Á ÁO interactions of the pseudo-dimer, building a ribbon parallel to the (011) plane (Fig. 3).

Database survey
A search of the Cambridge Structural Database (CSD version 5.42, update 2020.3;Groom et al., 2016) does not reveal any structures with ferrocenyl disubstituted by a thiodiphenylphosphine and a vinyl; however, a search using a fragment containing a ferrocenyl disubsituted by an unprotected phosphine and a vinyl substituent (Fig. 4) reveals 15 hits of which seven can be compared with the title compound, having only different substituents R 1 and R 2 (Fig. 4). A comparison of C-C and C-P distances and dihedral angles between the Cp ring Symmetry codes: (i) Àx þ 1; Ày þ 2; Àz þ 1; (ii) x À 1; y; z.

Figure 1
Molecular structure of the title compound with the atom-labeling scheme. Ellipsoids are drawn at the 50% probability level and the H atoms are represented as small circle of arbitrary radii.   The model used for the CCDC search. and vinyl mean plane are shown in Fig. 5. Clearly the substituent on the phosphine has some influence on the C-P bond lengths, which range from 1.795 (3) Å for the title compound to 1.827 Å for the [ 5 -1-dicyclohexylphosphino-2-(2-phenylethenyl)cyclopentadienyl]( 5 -cyclopentadienyl)iron compound (Schaarschmidt et al., 2014) in which the phosphine bears two cyclohexyl substituents that are rather bulky. The occurrence of the S atom attached to the phosphine in the title compound may explain why the shortest value observed for the title compound. There is no significant difference in the C-C bonds within the vinyl moiety, showing that these values are not affected by the substituent, whereas the discrepancy observed for the dihedral angles between the vinyl unit and the Cp rings (6.4 to 22.8 ) is related to the nature of the R 1 and R 2 substituents on the vinyl unit. The largest value of 22.8 , observed for the title compound, is related to the weak C21-H21Á Á ÁS1 interaction.

Synthesis and crystallization
To a solution of 2-(diphenylphosphinothioyl)ferrocenecarboxaldehyde (220 mg, 0.51 mmol) in acetone (40 mL) was added 2-picolylamine (0.2 mL, 1.53 mmol). The reaction mixture was refluxed for 24-36h with TLC monitoring of the consumption of aldehyde. After complete consumption, the reaction mixture was evaporated in vacuo and extracted with dichloromethane and washed with three portions of water. The combined organic layers were dried over Na 2 SO 4 , filtered and evaporated to dryness. The crude material was purified by silica gel column chromatography with a hexane-ether mixture (1/1, v/v) to obtain the product as a red solid (0.13 g, 55%). Monocrystals suitable for X-ray diffraction analysis were obtained by slow diffusion of pentane into a dichloromethane solution of 4-(2-thiodiphenylphosphinoferrocenyl)but-3-ene-one.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms attached to C atoms were fixed geometrically and treated as riding with C-H =

2-(Diphenylphosphinothioyl)-2-(3-oxobut-1-en-yl)ferrocene
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.43 e Å −3 Δρ min = −0.50 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.