2,2',4,4',6,6'-Hexa­methyl-4,4'-bi­[4H-pyran­yl]

The title mol­ecule, C₁₆H₂₂O₂, reveals C_i point symmetry in the crystal structure. The structure was disordered. The pyran ring is not planar; the O atom lies significantly out of the least-squares plane (ten times the r.m.s. deviation of all six atoms).

S1. Comment

Bipyranyls and pyrylium salts are used for photochemical redox reactions. Bipyranyls may be applied as 2 e donors in photoreductions, e.g. such as

2acrH⁺ + (tmp)₂ + hν → 2tmp⁺ + (acrH)₂,

where acrH⁺ is acridinium and tmp⁺ is 2,4,6-trimethylpyrylium. For photo-oxidations with pyrylium salts as e acceptors, attention has to be paid to the equilibrium of bipyranyl and the pyranyl radical.

tmp⁺ + two-dimensional + hν → two-dimensional^+. + 2tmp^. ← K → (tmp)₂;

where D = e donor.

The title molecule, C₁₆H₂₂O₂, reveals C_i point symmetry in the crystal structure. The structure was disordered in such a way that each of three C atoms of the pyran ring occupies statisticially two sites with a ratio of 89/11. The sites of the other three ring atoms are not split. This means that the two ring positions are arranged like a roof with an interfacial angle of about 24°. A second crystal studied showed the same effect with a similar occupation ratio of 81/19 and a corresponding interfacial angle of about 27°. The pyran ring is not planar; the O atom lies significantly out of the least-squares plane (10 × the r.m.s. deviation of all six atoms).

S2. Experimental

The title compound was synthesized according to Balaban et al. (1964) and crystallized from ethanol.

S3. Refinement

The structure is disordered so that split atomic positions of C3, C4 and C5 had to be introduced for the least squares refinement. Without such a `split model', all parameters of quality for a structure determination become worse as follows: R₁(gt) increases from 0.0438 to 0.0751; wR₂(all) increases from 0.1185 to 0.2182; GoF changes from 1.039 to 1.066; the long axes of the thermal ellipsoids of C3, C4 and C5 increase by a factor of about 1.35; and last but not least the three highest peaks in the final difmap become significantly larger (1.02, 0.82 and 0.77 e Å⁻³) than its r.m.s. (0.07 e Å⁻³); the peaks lie nearest to C4 (1.09 Å), C5 (0.82 Å) and C3 (0.79 Å), respectively.

Computing details

Data collection: IPDS2.87 (Stoe & Cie, 1997); cell refinement: IPDS2.87; data reduction: IPDS2.87; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); software used to prepare material for publication: SHELXL97.

2,4,6,2',4',6'-Hexamethyl-4H,4'H[4,4']bipyranyl

Crystal data

C16H22O2	Dx = 1.132 Mg m^−3
Mr = 246.34	Melting point: 385(1) K
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 7.842 (2) Å	Cell parameters from 5000 reflections
b = 11.458 (3) Å	θ = 2.0–24.0°
c = 8.824 (2) Å	µ = 0.07 mm^−1
β = 114.26 (3)°	T = 180 K
V = 722.9 (3) Å^3	Plate, colourless
Z = 2	0.52 × 0.40 × 0.22 mm
F(000) = 268

Data collection

Stoe IPDS diffractometer	1393 reflections with I > 2σ(I)
Radiation source: fine-focus sealed X-ray tube	Rint = 0.033
Planar graphite monochromator	θmax = 28.1°, θmin = 3.4°
Detector resolution: 6.667 pixels mm^-1	h = −10→10
ϕ–rotation, ϕ–incr. = 1.5°, 160 exposures scans	k = −15→15
5901 measured reflections	l = −11→11
1716 independent reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.044	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.119	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ^2(Fo^2) + (0.065P)^2 + 0.1273P] where P = (Fo^2 + 2Fc^2)/3
1716 reflections	(Δ/σ)max = 0.003
121 parameters	Δρmax = 0.22 e Å^−3
0 restraints	Δρmin = −0.16 e Å^−3

Special details

Experimental. Recrystallized from methanol. During data collection the crystal was in cold N₂ gas of the Cryostream Cooler (Oxford Cryosystems, 1992) mounted on a ϕ-axis diffractometer supplied with an area detector.

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 F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² 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 (Ų)

	x	y	z	Uiso*/Ueq	Occ. (<1)
C2	0.73063 (16)	0.83938 (10)	0.06204 (13)	0.0281 (3)
C3	0.8723 (3)	0.85917 (15)	0.0177 (2)	0.0266 (4)	0.891 (4)
H3	0.8907	0.8051	−0.0559	0.029*	0.891 (4)
C3S	0.803 (2)	0.8875 (12)	−0.0326 (19)	0.026 (3)	0.109 (4)
H3S	0.7959	0.8418	−0.1247	0.028*	0.109 (4)
C4	1.00485 (16)	0.96160 (10)	0.07662 (14)	0.0234 (4)	0.891 (4)
C4S	0.8967 (14)	1.0075 (7)	−0.0162 (11)	0.023 (3)	0.109 (4)
C5	0.9479 (3)	1.03271 (15)	0.1930 (2)	0.0269 (3)	0.891 (4)
H5	1.0195	1.1004	0.2418	0.030*	0.891 (4)
C5S	0.879 (2)	1.0618 (11)	0.1365 (17)	0.022 (3)	0.109 (4)
H5S	0.9228	1.1394	0.1657	0.024*	0.109 (4)
C6	0.80534 (16)	1.00757 (10)	0.23210 (13)	0.0283 (3)
C7	0.59953 (19)	0.73796 (11)	0.01132 (17)	0.0387 (3)
H7A	0.6207	0.6892	0.1086	0.043*
H7B	0.6216	0.6918	−0.0724	0.043*
H7C	0.4703	0.7664	−0.0356	0.043*
C8	1.20699 (16)	0.91696 (11)	0.17409 (15)	0.0319 (3)
H8A	1.294 (2)	0.9824 (13)	0.2193 (17)	0.035*
H8B	1.251 (2)	0.8719 (13)	0.1089 (17)	0.035*
H8C	1.209 (2)	0.8641 (12)	0.2617 (18)	0.035*
C9	0.7508 (2)	1.07188 (12)	0.35281 (16)	0.0406 (3)
H9A	0.6195	1.0957	0.2976	0.045*
H9B	0.8299	1.1412	0.3930	0.045*
H9C	0.7673	1.0209	0.4470	0.045*
O1	0.69443 (12)	0.90995 (8)	0.17293 (11)	0.0371 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C2	0.0295 (6)	0.0277 (5)	0.0284 (5)	−0.0042 (4)	0.0132 (4)	−0.0009 (4)
C3	0.0273 (9)	0.0237 (8)	0.0317 (8)	−0.0012 (6)	0.0150 (7)	−0.0020 (6)
C3S	0.028 (7)	0.023 (6)	0.028 (7)	−0.006 (5)	0.014 (6)	−0.014 (5)
C4	0.0198 (6)	0.0248 (6)	0.0269 (6)	−0.0009 (4)	0.0108 (5)	−0.0002 (5)
C4S	0.039 (6)	0.015 (4)	0.017 (4)	0.000 (4)	0.013 (4)	0.002 (3)
C5	0.0275 (8)	0.0262 (7)	0.0272 (8)	−0.0034 (6)	0.0113 (6)	−0.0030 (6)
C5S	0.029 (7)	0.018 (5)	0.022 (6)	−0.005 (4)	0.013 (5)	−0.009 (4)
C6	0.0310 (6)	0.0288 (5)	0.0279 (5)	−0.0006 (4)	0.0149 (4)	−0.0013 (4)
C7	0.0374 (7)	0.0350 (6)	0.0458 (7)	−0.0121 (5)	0.0193 (5)	−0.0038 (5)
C8	0.0247 (6)	0.0351 (6)	0.0340 (6)	0.0055 (5)	0.0103 (5)	0.0054 (5)
C9	0.0528 (8)	0.0392 (7)	0.0412 (7)	0.0004 (6)	0.0307 (6)	−0.0045 (5)
O1	0.0385 (5)	0.0388 (5)	0.0451 (5)	−0.0113 (4)	0.0285 (4)	−0.0098 (4)

Geometric parameters (Å, º)

C2—C3S	1.306 (14)	C4S—C4S^i	1.53 (2)
C2—C3	1.340 (2)	C4S—C5S	1.542 (13)
C2—O1	1.3856 (14)	C4S—C8^i	1.556 (9)
C2—C7	1.4931 (16)	C5—C6	1.3303 (19)
C3—C4	1.511 (2)	C5S—C6	1.353 (12)
C3S—C4S	1.539 (15)	C6—O1	1.3810 (14)
C4—C5	1.5147 (18)	C6—C9	1.4948 (15)
C4—C8	1.5472 (16)	C8—C4S^i	1.556 (9)
C4—C4^i	1.589 (2)
C3S—C2—C3	27.7 (7)	C4S^i—C4S—C5S	110.5 (10)
C3S—C2—O1	118.2 (5)	C3S—C4S—C5S	103.3 (8)
C3—C2—O1	122.85 (11)	C4S^i—C4S—C8^i	110.7 (8)
C3S—C2—C7	124.2 (5)	C3S—C4S—C8^i	111.0 (9)
C3—C2—C7	126.54 (12)	C5S—C4S—C8^i	110.9 (8)
O1—C2—C7	110.50 (10)	C6—C5—C4	124.93 (13)
C2—C3—C4	124.49 (14)	C6—C5S—C4S	125.1 (8)
C2—C3S—C4S	129.3 (9)	C5—C6—C5S	28.3 (6)
C3—C4—C5	107.37 (11)	C5—C6—O1	122.91 (11)
C3—C4—C8	109.71 (12)	C5S—C6—O1	120.0 (4)
C5—C4—C8	108.71 (11)	C5—C6—C9	126.51 (12)
C3—C4—C4^i	110.59 (12)	C5S—C6—C9	122.5 (5)
C5—C4—C4^i	110.77 (13)	O1—C6—C9	110.40 (10)
C8—C4—C4^i	109.64 (11)	C4—C8—C4S^i	43.9 (4)
C4S^i—C4S—C3S	110.3 (10)	C6—O1—C2	117.37 (9)
C3S—C2—C3—C4	86.6 (12)	C8^i—C4S—C5S—C6	122.4 (13)
O1—C2—C3—C4	−2.3 (2)	C4—C5—C6—C5S	−91.3 (10)
C7—C2—C3—C4	−178.11 (13)	C4—C5—C6—O1	1.9 (3)
C3—C2—C3S—C4S	−92.3 (19)	C4—C5—C6—C9	176.63 (13)
O1—C2—C3S—C4S	15 (2)	C4S—C5S—C6—C5	86.8 (16)
C7—C2—C3S—C4S	163.0 (11)	C4S—C5S—C6—O1	−17.7 (17)
C2—C3—C4—C5	0.8 (2)	C4S—C5S—C6—C9	−165.5 (9)
C2—C3—C4—C8	118.79 (18)	C3—C4—C8—C4S^i	120.8 (5)
C2—C3—C4—C4^i	−120.16 (18)	C5—C4—C8—C4S^i	−122.0 (5)
C2—C3S—C4S—C4S^i	115.7 (16)	C4^i—C4—C8—C4S^i	−0.8 (5)
C2—C3S—C4S—C5S	−2 (2)	C5—C6—O1—C2	−3.08 (19)
C2—C3S—C4S—C8^i	−121.3 (15)	C5S—C6—O1—C2	30.1 (8)
C3—C4—C5—C6	−0.6 (2)	C9—C6—O1—C2	−178.60 (10)
C8—C4—C5—C6	−119.22 (18)	C3S—C2—O1—C6	−28.5 (9)
C4^i—C4—C5—C6	120.25 (19)	C3—C2—O1—C6	3.30 (19)
C4S^i—C4S—C5S—C6	−114.6 (14)	C7—C2—O1—C6	179.71 (10)
C3S—C4S—C5S—C6	3.4 (19)

Symmetry code: (i) −x+2, −y+2, −z.