4-(4-Methoxyphenyl)-2-methylbut-3-yn-2-ol

The molecular structure of the title compound, C12H14O2, features a nearly coplanar arrangement including the aromatic ring, the C C—C group and the ether O atom. The maximum deviation from the least-squares plane of these ten atoms is 0.0787 (8) Å for the ether O atom. In the crystal, molecules are connected via O—H⋯O hydrogen bonds (involving the hydroxy O atom both as hydrogen-bond donor and acceptor) and weaker (aryl)C—H⋯π(aryl) contacts, leading to the formation of strands running parallel to the b axis. Further stabilization results from weaker (methyl)C—H⋯π(acetylene) interactions between different strands.


Comment
Terminal arylalkynes are of general interest in organic chemistry as they can be used for subsequent coupling reactions leading to diarylalkynes which are important building blocks in materials science (Chinchilla & Nájera, 2007). Compounds of this type can be prepared in a two-step reaction where an aryl halide is reacted with 2-methylbut-3-yn-2-ol (a monoprotected acetylene) in a Sonogashira-Hagihara reaction followed by deprotection of the resulting acetylenic compound using a base-catalysed retro-Favorsky elimination of acetone (Sonogashira, 1998).
Following this strategy, the title compound, which is important as an intermediate for the preparation of the terminal arylalkyne 4-ethinylanisole, was prepared via a Sonogashira-Hagihara coupling reaction starting from 4-bromoanisole and 2-methylbut-3-yn-2-ol. Crystallization from cyclohexane yielded colourless needles suitable for an X-ray crystal structure analysis on which is reported herein.
The title compound, C 12 H 14 O 2 , crystallizes in the orthorhombic space group Pbca. The asymmetric unit consists of one molecule which is illustrated in Fig. 1. The atoms C1-C6, C8-C10 and O1 are arranged nearly coplanar. Only the ether O atom O1 deviates slightly from the least-squares plane involving the mentioned atoms. The deviation between this least-squares plane and O1 is 0.0787 (8) Å, the deviation of all other atoms ranges from 0.0023 (11) to 0.0390 (10) Å.
In the crystal, the hydroxy O atom O2 is involved in an O2-H1···O2 hydrogen bond leading to the formation of one-dimensional strands located parallel to the b axis (Fig. 2). Within these strands further stabilization is reached via a weak C-H···π interaction (Nishio et al., 2009) of the type (aryl)C-H···π(aryl) between C5-H5 and Cg1 (Cg1 corresponds to the centroid of the aromatic ring). Within the packing, these one-dimensional strands are organized antiparallel, which is shown in Fig. 3. Different strands are connected via a weak (methyl)C-H···π(acetylene) contact involving C11-H11B and π2 (π2 is the midpoint of the C≡C bond). This interaction leads to a connection of different strands resulting in the formation of zigzag layers parallel to the ab plane (Fig. 3).

Experimental
The title compound was prepared as follows: To a degassed mixture of 22.44 g (0.12 mol) 4-bromoanisole and 12.20 g (0.145 mol) 2-methylbut-3-yn-2-ol in 100 ml of diethylamine were added 0.27 g (1.2 mmol) palladium(II) acetate, 0.63 g (2.4 mmol) triphenylphosphane and 0.11 g (0.6 mmol) copper(I) iodide. The resulting mixture was refluxed for 15 h under argon, followed by a second addition of the catalyst mixture (same amounts as before) and another 15 h refluxing under argon. The solvent was removed in vacuo and the residue dissolved in water. The aqueous solution was extracted several times with diethyl ether, the combined organic phases dried over Na 2 SO 4 and concentrated in vacuo. Column chromatographic purification [1. Al 2 O 3 , activity 1, eluent: diethyl ether; 2. silica gel, eluent: n-pentane/ethyl acetate (6:1 v/v)] and sup-2 crystallization from cyclohexane yielded the title compound as colourless needles (16.8 g, 74% yield, m.p. 326 K). The analytical data are in agreement with the literature (Mayr & Halberstadt-Kausch, 1982), where the synthesis of the title compound is described using another procedure.

Refinement
H atoms were positioned geometrically and allowed to ride on their respective parent atoms, with C-H = 0.98 Å and U iso (H) = 1.5 U eq (C) for methyl, C-H = 0.95 Å and U iso (H) = 1.2 U eq (C) for aryl and O-H = 0.84 Å and U iso (H) = 1.5 U eq (O) for hydroxy H atoms. Fig. 1. The molecular structure of the title compound, showing the atom numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.    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.