Conformational enanti­omeric disorder in tripivaloylmethane

Tripivaloylmethane [systematic name: 4-(2,2-dimethyl­propano­yl)-2,2,6,6-tetra­methyl­heptane-3,5-dione], C₁₆H₂₈O₃, is a 1,3,3′-triketone with C₃ mol­ecular symmetry, prepared by α-acyl­ation of 2,2,6,6-tetra­methyl­heptane-3,5-dione with 2,2-dimethyl­propanoyl anhydride in the presence of barium metal. The mol­ecules are conformationally chiral and pack so that each mol­ecular site is occupied with equal probability by the two enantiomers. The carbonyl groups of the two superimposed enantiomeric mol­ecules are at an angle of 75.4 (16)°.

S1. Comment

1,3,3'-Triketones can be obtained by acylation, using acyl halides or acid anhydrides, of β-diketones (Lim et al., 2001), their salts (Rogers & Smith, 1955) or their transition metal complexes (Murdoch & Nonhebel, 1962; Collman et al., 1963 or???1962) with acyl halogenides and acid anhydrides. Alkylation and acylation in the α-position of β-diketones, especially with bulkier groups, affects the keto–enol equilibrium in favour of the keto form. Cyclic 1,3,3'-triketones have attracted some interest because of their application as herbicides, since it has been shown that their derivatives inhibit p-hydroxyphenylpyruvate dioxygenase (Lee et al., 1998, and references therein). 1,3,3'-Triketones have also been studied as chelating ligands with group 2 and lanthanide cations (Ismail et al., 1969 or???1968). Although the crystal structures of many α-substituted β-diketones are known, only one crystal structure of an acyclic 1,3,3'-triketone [Cambridge Structural Database (CSD; Allen, 2002) refcode OCIQOG (Lim et al., 2001)] and one transition metal complex with an acyclical 1,3,3'-triketonate ligand (CSD refcode IGAGUS; Carano et al., 2002) have been reported to date.

The title compound, (I) (Fig. 1), is a potential tridentate monoanionic and neutral ligand for coordination to transition metal ions. Compound (I) crystallizes in the trigonal system, space group R3m, with Z = 3. The molecules of (I) are in the 3(a) special position of the space group, with the methine C–H bond along the threefold axis. The molecule is a triketo tautomer with a C2=O1 distance of 1.221 (6) Å, and a C1—C2═O1 angle of 117.6 (4)°. The molecule has C₃ molecular symmetry, and is comformationaly chiral. The space group accommodates equal numbers of M and P enantiomers, so that each molecular site is occupied with equal probability by the two enantiomers. Atoms C1, C3 and C5 and the corresponding H atoms were assigned common sites in both enantiomers. The angle between the C1/C2/O1/C3 planes of the two enantiomers is 75.4 (16)°.

The nature of the disorder raises the question of whether the structure might actually be ordered in space group R3. Refinement of such a model, both with the same unit-cell parameters and with a doubled z, however, led to R values above 1/4, with large and extremely anisotropic U_ij ellipsoids. The shapes of the ellipsoids of atoms C2 and O1, as well as the difference map, indicated that these atoms should be split in two. After splitting the atoms and further refinement, the refinements either became unstable or led to a structure that was disordered, where the carbonyl groups were disordered over two positions with occupancies of 0.5. This model was almost identical to the R3m model, but had a much higher R value (0.13). For these reasons, both the ordered and the disordered R3 model were rejected in favour of the disordered R3m model. The question also arose whether the separate pivaloyl groups rather than the entire molecule are disordered. This was easily shown not to be the case, since the inversion of only one pivaloyl group (reducing the symmetry of the molecule to C1) would create a conformer with an unreasonably short non-bonded contact of 1.156 Å between two carbonyl O atoms.

Equal occupancy by the two enantiomers of the average molecular site may be caused by either spatial or temporal disorder, as well as a combination of both; however, from the available data, it is not possible to distinguish between the two. The disorder may be a result of a random displacement of the two enantiomers, since it is unlikely that, in a crystal that consists of domains containing only one enantiomer, the occupancies of both enantiomers would be identical. The placement of enantiomers may be random in all directions, or it could be ordered in some. There are no significant intermolecular interactions in the [100] and [010] directions. The shortest intermolecular distance is a C–H···Oⁱ contact of 3.478 (9) Å [symmetry code: (i) x, y, 1 + z], involving a methyl C—H bond. Such contacts are of identical length and arrangement for all combinations of enantiomers. These observations support the interpretation that there is no spacial correlation of the site occupancies by the two enantiomers.

Related literature

For related literature, see: Allen (2002); Carano et al. (2002); Collman et al. (1963); Ismail et al. (1969); Lee et al. (1998); Lim et al. (2001); Murdoch & Nonhebel (1962); Rogers & Smith (1955).

S2. Experimental

Compound (I) was prepared by a method similar to that described by Lim et al. (2001). 2,2,6,6-Tetramethylheptane-3,5-dione (1.0 mmol) was dissolved in cyclohexane (30 ml); powdered barium metal (0.5 mmol) was added, and the mixture was heated under reflux for 45 min. 2,2-Dimethylpropanoyl anhydride (2.0 mmol) was added to the resulting solution, which was then heated under reflux for a further for 6 h. The resulting precipitate of barium pivaloate was removed by filtration, and the solution was then left to cool to ambient temperature, at which point white needles of (I) (0.3 mmol) crystallized from the mother liquor. NMR (CDCl₃): δ(H) 5.92 (s, 1H, –CH), 1.18 (s, 27H, –CH₃); δ(C) 27.5 (C4, C5, C6), 45.2 (C3), 64.6 (C1), 207.1 (C2). The compound was crystallized as elongated colourless prisms by slow isothermal evaporation of a solution in dimethyl sulfoxide.

S3. Refinement

Crystals of compound (I) are trigonal. Initially, space group R3 was selected, but during the refinement it was found that the symmetry is higher as a result of the disorder and the space group was assigned as R3m, which was confirmed by the successful structure analysis. Since the space group is achiral, the Friedel equivalents were averaged. Methyl H atoms were placed in calculated positions and treated as riding, with C–H = 0.96 Å and U_iso(H) = 1.5U_eq(C). The methine H atom was located in a difference map and refined isotropically with x and y coordinates restrained to 0.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2003); cell refinement: CrysAlis RED (Oxford Diffraction, 2003); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999) and PARST97 (Nardelli, 1995).

Figures

	Fig. 1. A view of the M enantiomer of (I), with the atom labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. H atoms have been omitted for clarity.
	Fig. 2. The contents of a single molecular site in (I), showing the relative orientations of the two enantiomers. The bonds of the M enantiomer are shown as full bonds, those of the P enantiomer as open bonds and the bonds common to both enantiomers as open bonds filled with one line. Displacement ellipsoids are drawn at the 10% probability level and H atoms have been omitted for clarity.

4-(2,2-dimethylpropanoyl)-2,2,6,6-tetramethylheptane-3,5-dione

Crystal data

C16H28O3	Dx = 1.08 Mg m^−3
Mr = 268.38	Mo Kα radiation, λ = 0.71073 Å
Trigonal, R3m	Cell parameters from 1072 reflections
Hall symbol: R 3 -2"	θ = 4.6–32.0°
a = 15.711 (7) Å	µ = 0.07 mm^−1
c = 5.792 (2) Å	T = 298 K
V = 1238.1 (9) Å^3	Prism, colourless
Z = 3	0.65 × 0.29 × 0.22 mm
F(000) = 444

Data collection

Oxford Diffraction Xcalibur CCD diffractometer	433 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.035
Graphite monochromator	θmax = 31.9°, θmin = 4.5°
ω scan	h = −23→22
3386 measured reflections	k = −21→23
518 independent reflections	l = −8→8

Refinement

Refinement on F^2	2 restraints
Least-squares matrix: full	H atoms treated by a mixture of independent and constrained refinement
R[F^2 > 2σ(F^2)] = 0.075	w = 1/[σ^2(Fo^2) + (0.1632P)^2 + 0.058P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.234	(Δ/σ)max < 0.001
S = 1.07	Δρmax = 0.12 e Å^−3
518 reflections	Δρmin = −0.13 e Å^−3
50 parameters

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
C5	0.2751 (5)	0.154 (3)	0.8404 (13)	0.142 (14)	0.5
H5A	0.264	0.202	0.7673	0.213*	0.5
H5B	0.2737	0.1086	0.7263	0.213*	0.5
H5C	0.3381	0.1856	0.915	0.213*	0.5
C4	0.2015 (5)	0.1767 (4)	1.1820 (14)	0.137 (2)
H6A	0.1385	0.1544	1.2526	0.205*
H6B	0.2202	0.2359	1.0968	0.205*
H6C	0.2495	0.1895	1.2996	0.205*
C1	0	0	0.9920 (7)	0.0443 (9)
C3	0.1954 (2)	0.09771 (11)	1.0186 (7)	0.0651 (9)
C2	0.1002 (3)	0.0802 (3)	0.9009 (6)	0.0507 (9)	0.5
O1	0.1007 (3)	0.1278 (4)	0.7338 (7)	0.0811 (14)	0.5
H1	0	0	1.160 (4)	0.066 (19)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C5	0.058 (3)	0.18 (2)	0.096 (4)	−0.007 (10)	0.015 (3)	0.001 (9)
C4	0.162 (6)	0.112 (4)	0.135 (4)	0.068 (4)	−0.012 (4)	−0.032 (3)
C1	0.0465 (12)	0.0465 (12)	0.0401 (16)	0.0232 (6)	0	0
C3	0.0483 (15)	0.0738 (16)	0.0647 (16)	0.0241 (8)	−0.0025 (13)	−0.0013 (6)
C2	0.0490 (18)	0.0504 (16)	0.0494 (18)	0.0224 (14)	0.0041 (14)	0.0019 (13)
O1	0.071 (2)	0.086 (3)	0.076 (2)	0.032 (2)	0.0107 (17)	0.0382 (19)

Geometric parameters (Å, º)

C5—C3	1.518 (11)	C4—H6C	0.96
C5—H5A	0.96	C1—C2	1.537 (4)
C5—H5B	0.96	C1—H1	0.97 (2)
C5—H5C	0.96	C3—C4^i	1.525 (7)
C4—C3	1.526 (7)	C3—C5^i	1.518 (11)
C4—H6A	0.96	C3—C2	1.538 (5)
C4—H6B	0.96	C2—O1	1.221 (6)
C3—C5—H5A	109.5	C4^i—C3—C5^i	104.7 (18)
C3—C5—H5B	109.5	C4—C3—C2	94.6 (3)
H5A—C5—H5B	109.5	C4^i—C3—C2	124.1 (3)
C3—C5—H5C	109.5	C5—C3—C2	104.4 (9)
H5A—C5—H5C	109.5	C5^i—C3—C2	110.6 (6)
H5B—C5—H5C	109.5	C4—C3—C2^i	124.1 (3)
C3—C4—H6A	109.5	C4^i—C3—C2^i	94.6 (3)
C3—C4—H6B	109.5	C5—C3—C2^i	110.6 (6)
H6A—C4—H6B	109.5	C5^i—C3—C2^i	104.4 (9)
C3—C4—H6C	109.5	C2^i—C2—O1	127.6 (3)
H6A—C4—H6C	109.5	C2^i—C2—C1	72.08 (16)
H6B—C4—H6C	109.5	O1—C2—C1	117.6 (4)
C2^ii—C1—C2	108.87 (19)	C2^i—C2—C3	72.10 (16)
C2^ii—C1—H1	110.07 (19)	O1—C2—C3	122.3 (4)
C2^iii—C1—H1	110.07 (19)	C1—C2—C3	120.0 (3)
C2^i—C1—H1	110.07 (18)	C2^i—C2—C2^iv	120.000 (3)
C2—C1—H1	110.07 (19)	O1—C2—C2^iv	72.5 (3)
C2^iv—C1—H1	110.07 (18)	C3—C2—C2^iv	152.00 (19)
C2^v—C1—H1	110.07 (18)	C2^i—C2—O1^iv	133.6 (2)
C4—C3—C4^i	103.0 (6)	C1—C2—O1^iv	85.7 (2)
C4—C3—C5	104.7 (18)	C3—C2—O1^iv	150.8 (3)
C4^i—C3—C5	120.9 (17)	O1^iv—O1—C2	107.5 (3)
C4—C3—C5^i	120.9 (17)	C2—O1—C2^iv	70.1 (3)

Symmetry codes: (i) x, x−y, z; (ii) −x+y, −x, z; (iii) −y, −x, z; (iv) −x+y, y, z; (v) −y, x−y, z.