4,4′-(4,5-Dimethyl-1,2-phenyl­ene)bis­(2-methyl­but-3-yn-2-ol): structural variation in vicinal dialkynols

Abstract

The structure of the title compound, C₁₈H₂₂O₂, contains two non-equivalent mol­ecules which differ primarily in the location of the –OH groups on opposite sides or on the same side of the mol­ecular plane. Inversion-symmetric pairs of mol­ecules form inter­molecular O—H⋯O hydrogen-bonded tetra­meric synthons that link non-equivalent mol­ecules into an approximately square double layer parallel to (02). Recently reported fluorinated analogues [Kane, Meyers, Yu, Gerken & Etzkorn (2011 ▶). Eur. J. Org. Chem. pp. 2969–2980] have significantly different structures of varying complexity that incorporate intra­molecular hydrogen bonding and suggest that further study of structure versus substituents in vicinal dialkynols could be fruitful.

Comment  

Dialkynols have been extensively studied as part of a broader survey of gem-alkynol structures (Madhavi, Desiraju et al., 2000a ▶; Nangia, 2010 ▶). Since the gem-alkynol moiety provides two functional groups, viz. –OH and –C CH, there are several competing ways in which groups on neighboring mol­ecules can inter­act which, in turn, lead to a wide variety of synthons and structures (Madhavi, Bilton et al., 2000 ▶; Banerjee et al., 2006 ▶). One recurring structural motif is a tetra­meric synthon of functional groups that are linked by hydrogen bonds in a square arrangement (Madhavi, Desiraju et al., 2000b ▶; Bilton et al., 2001 ▶). The di-gem-alkynols studied often place the alkynol groups on opposite sides of an organic ring, so intra­molecular inter­actions are inhibited. Vicinal dialkynol structures have recently been reported in which the alkynol groups are ortho, but with –C CH groups absent, so the –OH groups are positioned to act as mol­ecular tweezers in which intra­molecular hydrogen bonding is now possible (Kane et al., 2011 ▶). We report here the structure of the title new vicinal dialkynol compound, (I), an inter­mediate obtained in an effort to prepare cyclic diacetyl­enes (Zhou et al., 1994 ▶), which was synthesized by the treatment of 1,2-diiodo-4,5-dimethyl­benzene (Hathaway et al., 2009 ▶) with excess 2-methyl-3-butyn-2-ol in the presence of copper(I) and palladium catalysts (Melissaris & Litt, 1994 ▶).

The two non-equivalent mol­ecules in the asymmetric unit of (I) (the first digit of the atom label denotes assignment to mol­ecule 0 or 1) are each quasi-planar for the core atoms, i.e. those that are, or are bound to atoms that are, sp- or sp ²-hybridized. Bond lengths and angles within each mol­ecule conform to expected values (Ladd & Palmer, 1994 ▶). The mol­ecules have no formal site symmetry, although each approximates mm2 symmetry for the core atoms, with half of the nominally equivalent bond lengths agreeing within 1 s.u. and all within 2 s.u. Nominally equivalent bond angles for the ring atoms agree within 3 s.u. [with one slight exception, for C06—C01—C011 = 120.46 (10)° and C03—C02—C021 = 120.15 (9)°], but the angles in the alkynol groups disagree by more. Similar bond lengths between non-equivalent mol­ecules agree [with one slight exception, for the Cx1—Cx11 and Cx2—Cx21 bonds which are in the range 1.4353 (15)–1.4400 (15) Å] within 3 s.u. for Csp, Csp ² or O atoms, while the Csp ³—Csp ³ bonds agree within 4 s.u. The differences in similar bond angles between non-equivalent mol­ecules are larger and agree within 3–5 s.u. in the ring, with even greater disagreement in the alkynol groups. Bonds in the ring between substituents are elongated compared with the others, with the Cx1—Cx2 bond between alkynol groups being the longest. The substituents are scissored outward, as shown by bond angles at the substituted ring atoms of <120° for aromatic ring atoms only and inter­ior angles of >120° involving the substituent. Angles about the unsubstituted ring atoms of ∼122° indicate a slight scissoring inward of these atoms with the outward scissoring of the substituents. The main geometric difference between the non-equivalent mol­ecules is the –OH group location on opposite sides of the mol­ecular plane (mol­ecule 0) or on the same side (mol­ecule 1), which reduces the approximate point-group symmetry of mol­ecule 0 to 2, and of mol­ecule 1 to m. A displacement ellipsoid plot of the asymmetric unit of (I) is presented in Fig. 1 ▶.

Figure 1.

The asymmetric unit of (I), showing the two non-equivalent mol­ecules with atom labels. Displacement ellipsoids are drawn at the 50% probability level. The dashed line indicates the hydrogen bond between non-equivalent molecules.

Inversion-related mol­ecular pairs are formed, with inter­planar spacings of 3.486 (1) and 3.717 (1) Å for mol­ecules 0 and 1, respectively. The aromatic rings within each pair are offset away from each other, principally along a line parallel to the alkynol arms of O1 or O11, with distances between the ring-atom centroids (measured perpendicular to the mean-plane normal) of 1.394 (2) and 1.547 (2) Å for mol­ecules 0 and 1, respectively. Since the inter­planar spacings are greater than the sum of the van der Waals radii (Bondi, 1964 ▶) and the large ring displacements lead to only minor (30–35%) overlap, it is unlikely that there is a significant attractive inter­action between the mol­ecules of each pair. The long axes of the non-equivalent mol­ecules are almost perpendicular [angle of 86.17 (7)° between the least-squares lines formed by the core atoms], with that of mol­ecule 0 parallel to [201] and that of mol­ecule 1 almost parallel to b. These vectors demarcate axes for an approximately square grid [23.0784 (3) Å along [201] versus 22.7966 (3) Å along b] of inversion-related pairs of mol­ecules linked by hydrogen bonds to form a double layer in (02). Different types of mol­ecule pairs alternate along each repeat axis, with a given mol­ecule forming hydrogen bonds to mol­ecules of the other type on each side. This results in O—H⋯O hydrogen bonds arranged in square tetra­meric synthons between rows of mol­ecules. Deviations from a true square within the synthon are small, e.g. neighboring O⋯O distances are in close agreement and the O⋯O⋯O angles are ∼90°, with the largest due to O—H⋯O angles of 164–171° that place H atoms outside the square. The mean plane of mol­ecule 1 is closer to (02), with both –OH groups directed into the double layer and with the C114 and C125 methyl groups directed outward. The mean-plane normal of mol­ecule 0 forms an angle of 24.86 (2)° to that of mol­ecule 1, with the –OH groups hydrogen bonded to mol­ecules 1 on opposite sides of the double layer. The differences in geometric parameters between the alkynol groups (see above) can thus be ascribed to strains imposed to accommodate hydrogen bonding. The double layer is scored by grooves parallel to [201], generated by the offset of mol­ecules 1, with the tetra­meric O—H⋯O synthons at the floor (or ceiling) of each groove. A diagram of the quasi-square layer is presented in Fig. 2 ▶ and hydrogen-bonding parameters are presented in Table 1 ▶.

Figure 2.

A plot of a portion of the quasi-square layer in (I) (with b vertical and [201] horizontal) formed by hydrogen bonding between inversion-related pairs of mol­ecules. Atoms are represented by circles of arbitrary radii and H atoms have been omitted for clarity, except for those bound to oxygen. Pairs of type 0 and 1 mol­ecules are identified in the diagram and the inset shows the detail of the square tetra­meric hydrogen-bonded synthon. [Symmetry codes: (ii) x − 1, −y + , z − ; (iv) −x + 1, y − , −z + .]

Table 1. Hydrogen-bond geometry (Å, °).

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O01—H01⋯O12i	0.886 (18)	1.901 (17)	2.773 (1)	167.5 (16)
O02—H02⋯O11ii	0.886 (19)	1.876 (19)	2.752 (1)	169.8 (18)
O11—H11⋯O01iii	0.886 (19)	1.892 (19)	2.756 (1)	164.3 (18)
O12—H12⋯O02	0.904 (18)	1.847 (17)	2.744 (1)	170.7 (17)

Symmetry codes: (i) ; (ii) ; (iii) , .

The structure is completed by stacking the double layers so mol­ecule 0 pairs stack along a at the lines of inversion centers passing halfway through the b and c edges, while mol­ecule 1 pairs stack similarly but along the a edges or passing through the center of the bc face. The mean planes within the stacks are tilted relative to a, with the normals forming angles of 60.80 (1) and 36.22 (1)° for mol­ecules 0 and 1, respectively. The stacks of inversion-related mol­ecular pairs are readily observed when the structure is viewed down a, as shown in the packing diagram in Fig. 3 ▶. However, the double layers are less obvious from a cursory examination, since the methyl groups projecting away from the layer are nested into the grooves of neighboring layers so as to obscure the boundaries between layers.

Figure 3.

A unit-cell packing diagram for (I), viewed along a, showing the stacks of inversion–related pairs of mol­ecules. Atoms are drawn as circles of arbitrary radii and H atoms have been omitted for clarity.

The few known vicinal dialkynols exhibit a variety in their structures that is sensitive to mol­ecular geometry and com­position, as is already well known among the much larger gem-alkynol family. The structure of (I) is, perhaps, the most conventional, since it contains the recurring tetra­meric O—H⋯O synthon, yet at the same time shows no intra­molecular inter­actions that might be expected for the tweezer-like ortho arrangement of the alkynol groups. In contrast, the difluoro analog [Cambridge Structural Database (Version 5.32; Allen, 2002 ▶) refcode OQAZUC (Kane et al., 2011 ▶)], in which F replaces the ring CH₃ groups, has quite a complex structure (Z′ = 9) in which intra­molecular O—H⋯O inter­actions are also found. This striking difference in complexity between the dimethyl and difluoro analogs raises the question of what other structures might be observed by replacing substituents in these positions. Meanwhile, the tetra­fluoro analog (OQEBOC; Kane et al., 2011 ▶), in which F replaces the ring CH₃ groups and H atoms, has a simpler structure (Z′ = 1) and only localized intra- and inter­molecular O—H⋯O hydrogen bonding between two mol­ecules in the tetra­meric synthon, although with a more distorted square than in (I). In contrast, in OQEBIW (Z′ = 1; Kane et al., 2011 ▶), the analog to OQEBOC with alkynol CH₃ groups replaced by H, a strictly inter­molecular O—H⋯O hydrogen-bonding network of zigzag chains is found. Thus, a two-dimensional hydrogen-bonding network with no intra­molecular hydrogen bonding is found in (I), in spite of the presence of alkynol CH₃ groups, the presence of which allows for intra­molecular hydrogen bonding in OQAZUC and OQEBIW and localized inter­actions only in OQEBIW. [Note that the carb­oxy­lic acid analog of (I) contains both intra- and inter­molecular bonding to form zigzag O—H⋯O hydrogen-bonded chains (Saravanakumar et al., 2009 ▶).] Two other vicinal dialkynols, ­octa-2,4,6-triyne-1,8-diol (LILHUJ, Z′ = 0.5; Enkelmann, 1994 ▶) and 2,9-dimethyl­deca-3,5,7-triyne-2,9-diol (LILJAR, Z′ = 1; Enkelmann, 1994 ▶), contain a –C  C– core that dictates a linear geometry, so the –OH groups are at opposite ends (and intra­molecular hydrogen bonding is impossible) and O—H⋯O hydrogen-bonded networks are present as zigzag single or double layers, respectively. This variation of structure within the vicinal dialkynols (in particular ortho-dialkynols), where small variations in ring or alkynol substituents lead to large changes in structure from competition between intra- and inter­molecular hydrogen bonding, seems a fruitful area for further investigation.

Experimental  

Nitro­gen (N₂) was bubbled through triethyl­amine (75 ml) for 15 min to remove dissolved oxygen, with the N₂ atmosphere maintained throughout the subsequent reaction. 1,2-Diiodo-4,5-dimethyl­benzene (5.0 g, 0.014 mol), 2-methyl-3-butyn-2-ol (5.88 g, 0.070 mol), tri­phenyl­phosphane (0.10 g) and copper(I) iodide (0.03 g) were added, and the mixture was stirred for 10 min. Dichloridobis(triphenyl­phosphane)palladium (0.030 g) was added, and the reaction was heated to ∼330 K overnight. The resulting green solution was diluted with diethyl ether and filtered. The filtrate was evaporated to yield a green–brown solid that was then redissolved in hot toluene on a steam bath. Crystals of (I) formed as a sand-colored solid upon slow cooling of this solution, first to ambient temperature and then to 273 K in an ice bath (yield: 2.67 g, 70.6%; m.p. 414 K). Single crystals of this sample appeared colorless under microscopic examination and were suitable for X-ray diffraction.

Crystal data  

• C₁₈H₂₂O₂

• M _r = 270.37

• Monoclinic,

• a = 8.4720 (1) Å

• b = 22.7966 (3) Å

• c = 16.8592 (2) Å

• β = 93.886 (1)°

• V = 3248.58 (7) ų

• Z = 8

• Mo Kα radiation

• μ = 0.07 mm⁻¹

• T = 100 K

• 0.36 × 0.32 × 0.28 mm

Data collection  

• Nonius KappaCCD area-detector diffractometer

• 18231 measured reflections

• 9466 independent reflections

• 7185 reflections with I > 2σ(I)

• R _int = 0.027

Refinement  

• R[F ² > 2σ(F ²)] = 0.046

• wR(F ²) = 0.122

• S = 1.01

• 9466 reflections

• 389 parameters

• H atoms treated by a mixture of independent and constrained refinement

• Δρ_max = 0.33 e Å⁻³

• Δρ_min = −0.24 e Å⁻³

Solution and refinement of the structure were straightforward; all non-H atoms were found in the initial electron-density map and their anisotropic displacement parameters were refined. All H atoms were visible in subsequent electron-density difference maps, and the positions and isotropic displacement parameters of those bound to O atoms were refined freely. Other H-atom positions were calculated to give an idealized geometry, with C—H = 0.96 or 0.93 Å and U _iso(H) = 1.5 or 1.2U _eq(C) for CH₃ or aromatic H atoms, respectively. The CH₃ torsion angle was refined to match the electron density. 11 low-angle reflections obscured by the beam stop (as indicated by F _o ² << F _c ²) were omitted from the refinement.

Data collection: COLLECT (Nonius, 1998 ▶); cell refinement: SCALEPACK (Otwinowski & Minor, 1997 ▶); data reduction: DENZO (Otwinowski & Minor, 1997 ▶) and SCALEPACK; program(s) used to solve structure: SIR92 (Altomare et al., 1993 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997 ▶) and ORTEPIII (Burnett & Johnson, 1996 ▶); software used to prepare material for publication: WinGX (Farrugia, 1999 ▶) and PARST (Nardelli, 1995 ▶).