Benzyl(methyl)phosphinic acid

The title compound, C8H11O2P, is a phosphinic compound with a tetracoordinate pentavalent P atom. The phosphinic function plays a predominant role in the cohesion of the crystal structure, both by forming chains along the b axis via strong intermolecular O—H⋯O hydrogen bonds and by cross-linking these chains perpendicularly via weak intermolecular C—H⋯O hydrogen bonds, generating a two-dimensional network parallel to (001).


Comment
The title compound, C 8 H 11 O 2 P, belongs to the phosphinic acid family (R'P(O)OHR''). These compounds are important substrates in the study of biochemical processes, and those comprising tetracoordinate pentavalent phosphorus are widely used as biologically active compounds. Mimics of amino acids in which the carboxylic function is replaced by phosphorus analogues have attracted particular interest. Among these phosphorus functions, phosphinic acid moiety is an excellent mimic of the tetrahedral transition state of amid bond hydrolysis and is more stable than phosphonic or phosphonamidic isosters.
The development of methods for the preparation of phosphinic acids is so important and currently attracting growing interest (Montchamp, 2005;Dingwall et al., 1989). The most commonly employed methods to prepare phosphinic acids suffer from several limitations: large excess of reagents, difficulties to avoid formation of symmetrically disubstituted phosphinic acids, handling difficulties of some starting materials. A new synthesis of unsymmetrical phosphinic acids R'P(O)OHR'' was performed. The first P-C bond formation was achieved using a base-promoted H-phosphinate alkylation from a protected H-phosphinate, easier and safer to handle. A one pot methodology was developed for the second P-C bond formation involving sila-Arbuzov reaction (Fougère et al., 2009).
An ORTEP plot of the molecule is given in Fig. 1. Geometric parameters are in the usual ranges, e.g.; typical P = O, P-O and P-C bonds as it was found earlier in phosphonic acid crystal structures (Langley et al., 1996;Frantz et al., 2003;Meyer et al., 2003;Cai et al., 2003 ).
In the crystal packing, one molecule is linked to two adjacent symmetric molecules via strong intermolecular O-H···O==P hydrogen bonds (Table 1). These hydrogen bonds between phosphinic groups built an infinite intermolecular hydrogen-bond network along the b direction (Fig. 2), forming chains of molecules. These chains are perpendicularly cross-linked via weak hydrogen bonds between C-H from the aromatic ring and O from the phosphinic group (Table 1, Fig 2), that give rise to a bidimensionnal organization parallel to the (001) plane. The packing of the structure can also be described as a bidimensionnal organization piled up to the third direction with hydrophobic functions face to face.
The triethylamine (2 equiv) was added, followed 5 minutes later by the bromide derivatives (1 equiv). The mixture was cooled to 0°C and absolute ethanol was added to quench the reaction. After 30 min., the solvent was removed and the residue was taken up in distilled water and extracted with ethyl acetate. The organic layer was dried under MgSO 4 ; filtrated and evaporated under reduced pressure to give the crude product. This product was taken up in water (20 ml) and washed with supplementary materials sup-2 ether (3 x 20 ml), followed by a reversed phase column chromatography (water/methanol 1:1) to give a white solid with high yield (76%). Single crystals suitable for X-ray structure analysis could be obtained by slow evaporation of a concentrated water/methanol (1/1) solution at room temperature.

Refinement
All Hydrogen atoms attached to C atoms were fixed geometrically and treated as riding with C-H = 0.93 Å (aromatic), 0.96 Å (methylene) or 0.97 Å (secondary CH2 group) with U iso (H) = 1.2 U eq (C) (aromatic) or 1.5 U eq (C) for others. H atom of the hydroxyl was located in difference Fourier syntheses and was treated in the last stage of refinement as riding on it parent O atom with O-H = 0.82 Å and U iso (H) = 1.2 U eq (O).  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 Rfactors(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.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )