3,5-Bis(benzyloxy)benzoic acid

In the title compound, C21H18O4, the outer benzyl rings are disordered over two resolved positions in a 0.50 ratio. The O—CH2 groups form dihedral angles of 4.1 (2) and 10.9 (4)° with the central benzene ring, adopting a syn–anti conformation with respect to this ring. In the crystal, the molecules are linked by O—H⋯O hydrogen bonds and weak C—H⋯O interactions, forming chains along [010].


Ellena Comment
Dendrimer chemistry provides new opportunities of research in design of supramolecular architectures (Fréchet, 2002).
3,5-Bis-benzyloxy-benzoic acid (I) was used for the synthesis of luminescent lanthanide coordination complexes that display unique line-like emission bands (Sivakumar et al., 2010;Remya et al., 2008). Lanthanide benzoates and their derivatives have potential applications in a wide variety of fields because their novel luminescent and magnetic properties (Busskamp et al., 2007). The title compound was also used in the synthesis of monodispersed dendritic polyesters with removable chain ends using a convergent growth process (Hawker & Fréchet, 1992). Other related compounds were crystallized and studied by X-ray diffraction (Gainsford et al., 2009;Zhu et al., 2009) and their parameters can be used to compare with the parameters of the title system. A perspective view of the molecule of (I), showing the atomic numbering scheme, is given in Fig. 1. The title compound crystallizes in the triclinic system with a P-1 space group. The outer benzyl rings are disordered over two resolved positions in a 0.50 ratio. The molecules are bonded by intermolecular O-H···O hydrogen bonds of moderate character (Desiraju & Steiner, 1999). Indeed, carbonylic O2 and O1 are linked with an O···O distance of 2.633 (2) Å. The propagation of these interactions generate centrosymmetric rings with graphs-set notation R 2 2 (8) (Etter, 1990). Other weak C-H···O intermolecular interactions (Nardelli, 1995) contribute to stabilization of the molecules along b (Fig. 2). Other classical hydrogen bond interactions are not exhibited in the crystal packing. In the title structure, the O-CH 2 groups adopt a syn-anti conformation with respect to the central phenyl ring, similar to the behavior presented in the 1,3-Dibenzyloxy-5-(bromomethyl)-benzene system (Zhu et al., 2009), while in other similar structures, the O-CH 2 groups adopt a syn-syn conformation (Xiao et al., 2007). The O-CH 2 groups, C4-O3-C8-C9 and C6-O4-C15-C16 are essentially planar (r.m.s. deviation of non-hydrogen atoms= 0.0355 Å and 0.0217 Å respectivelly) and form dihedral angles of 4.1 (2)° and 10.9 (4)° with the central phenyl ring.

Experimental
Methyl 3,5-dihydroxybenzoate (2.0g, 12 mmol) was dissolved in 50 ml of acetonitrile and refluxed with potassium carbonate (8.0 g, 58 mmol) for 30 min. The resulting reaction mixture was refluxed at 68° C for 48 h following the addition of benzyl bromide (4.0 g, 24 mmol). The acetonitrile was evaporated off, and the residual mixture was poured into ice cold water. Methyl 3,5-bis-(benzyloxy)benzoate was obtained as a white precipitate. (2.0 g, 5.74 mmol), were taken from the precipitate, which was dissolved in 50 ml of ethanol. To this solution was added (1 g, 17.77 mmol) of KOH and it was placed under reflux. The reaction was followed by TLC until the presence of KOH was not longer observed. The reaction mixture was poured into ice cold water, acidified with dilute HCl, and the resulting precipitate was filtered, washed, dried, and recrystallized from ethanol. Yield, 1.67 g (89%

Refinement
All H-atoms were positioned geometrically using riding model with [C-H= 0.93 Å for aromatic, C-H= 0.82 Å for hydroxyl and C-H= 0.97 Å for methylene H atoms. U iso (H)= 1.2U eq (C) for aryl and methylene H atoms and 1.5U eq (O) for hydroxyl H-atom]. During the structure determination disordered sites around the two benzyl groups were found.
Trial refinements were used with the split-atom approach for these extra sites with a constrained 50% occupancy each.    Part of the crystal structure of (I), showing the formation of chains running along [010]. Symmetry code: (i) -x,-y + 1,-z + 1; (ii) -x + 2,-y,-z + 1  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.