Supporting information
Crystallographic Information File (CIF) https://doi.org/10.1107/S010827010001235X/gd1104sup1.cif | |
Structure factor file (CIF format) https://doi.org/10.1107/S010827010001235X/gd1104IIIsup2.hkl |
CCDC reference: 156166
Preparation of sp-9-methyl-9-pivaloylfluorene (III). Method 1: pivaloylation of 9-methylfluorene. Based on the method reported by McCollum (1977), after earlier reports by Ullman & Wustemburger (1905) and Wanscheidt & Moldavski (1931), treatment of 9-fluorenone with CH3MgI followed by aqueous NH4Cl provided 9-methyl-9-fluorenol, which was reduced to 9-methylfluorene with H3PO2/I2 in HOAc. In the manner reported for the preparation of 9-pivaloylfluorene from fluorene (Meyers et al., 1991), 9-lithiated 9-methylfluorene treated with pivaloyl chloride at 213 K readily produced (III). Flash chromatography provided colorless crystals [71% yield, m.p. 431–431.5 K (uncorrected); same m.p. after heating to 458 K and allowing to recrystallize from the melt], shown by X-ray analysis to be exclusively the sp rotamer, which was likewise the case for solutions of (III) as determined by 1H NMR at room temperature. IR (concentrated CDCl3): δ 1680 cm−1 (sharp, strong; C═O); 1H NMR (Varian VXR 300; CDCl3): δ 1.54 (s, 3H, 9-CH3), 0.67 [s, 9 H, C(CH3)3]; 13C NMR (Varian VXR 300, 75 MHz for 13C; CDCl3), δ: 46.42 [C(CH3)3], 28.26 [C(CH3)3], 25.73 (9-CH3). Method 2: methylation of 9-pivaloylfluorene. To a stirred solution of ap-9-pivaloylfluorene (Meyers et al., 1991) in tetrahydrofuran under argon and maintained at 213 K, nBuLi (Aldrich, in hexanes) was injected. The deep red stirred solution was then maintained at 273 K and methyl iodide was injected. The solution became slightly yellow within 20 min. This mixture was stirred at room temperature for an additional 30 min. Thin-layer chromatography indicated the presence mainly of (III). The reaction was quenched with aqueous NH4Cl, the mixture extracted with ether, and the extract dried and rotary evaporated. Column chromatography (silica gel, 230–400 mesh; hexanes/ether 10:1) provided colorless crystals identified by 1H NMR as (III) [93% yield, m.p. 430.5–431.5 K (corrected)].
The 140 Friedel pairs measured were merged in the final cycles of refinement because the f'' terms in the scattering-factor expression are insignificant for this structure. All H atoms are riding.
Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1996); cell refinement: MSC/AFC Diffractometer Control Software; data reduction: PROCESS in TEXSAN (Molecular Structure Corporation, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: TEXSAN and SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP (Johnson, 1965); software used to prepare material for publication: TEXSAN, SHELXL97 and PLATON (Spek, 2000).
C19H20O | Dx = 1.144 Mg m−3 |
Mr = 264.35 | Melting point = 430.5–431.5 K |
Orthorhombic, Cmc21 | Mo Kα radiation, λ = 0.71069 Å |
Hall symbol: C 2c -2 | Cell parameters from 24 reflections |
a = 13.908 (2) Å | θ = 10.0–10.5° |
b = 15.3513 (18) Å | µ = 0.07 mm−1 |
c = 7.1860 (6) Å | T = 296 K |
V = 1534.3 (3) Å3 | Fragment, colorless |
Z = 4 | 0.51 × 0.49 × 0.43 mm |
F(000) = 568 |
Rigaku AFC-5S diffractometer | Rint = 0.023 |
Radiation source: sealed tube | θmax = 27.5°, θmin = 2.0° |
Graphite monochromator | h = 0→18 |
ω (rate 6° min–1) scans | k = 0→19 |
1137 measured reflections | l = −1→9 |
997 independent reflections | 3 standard reflections every 150 reflections |
691 reflections with I > 2σ(I) | intensity decay: 0.7% |
Refinement on F2 | Hydrogen site location: inferred from neighbouring sites |
Least-squares matrix: full | H-atom parameters constrained |
R[F2 > 2σ(F2)] = 0.043 | w = 1/[σ2(Fo2) + (0.0884P)2 + 0.0776P] where P = (Fo2 + 2Fc2)/3 |
wR(F2) = 0.138 | (Δ/σ)max = 0.001 |
S = 1.02 | Δρmax = 0.26 e Å−3 |
997 reflections | Δρmin = −0.13 e Å−3 |
102 parameters | Extinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
1 restraint | Extinction coefficient: 0.025 (4) |
Primary atom site location: structure-invariant direct methods | Absolute structure: Absolute structure could not be determined |
Secondary atom site location: difference Fourier map | Absolute structure parameter: 0 (10) |
C19H20O | V = 1534.3 (3) Å3 |
Mr = 264.35 | Z = 4 |
Orthorhombic, Cmc21 | Mo Kα radiation |
a = 13.908 (2) Å | µ = 0.07 mm−1 |
b = 15.3513 (18) Å | T = 296 K |
c = 7.1860 (6) Å | 0.51 × 0.49 × 0.43 mm |
Rigaku AFC-5S diffractometer | Rint = 0.023 |
1137 measured reflections | 3 standard reflections every 150 reflections |
997 independent reflections | intensity decay: 0.7% |
691 reflections with I > 2σ(I) |
R[F2 > 2σ(F2)] = 0.043 | H-atom parameters constrained |
wR(F2) = 0.138 | Δρmax = 0.26 e Å−3 |
S = 1.02 | Δρmin = −0.13 e Å−3 |
997 reflections | Absolute structure: Absolute structure could not be determined |
102 parameters | Absolute structure parameter: 0 (10) |
1 restraint |
x | y | z | Uiso*/Ueq | ||
O1 | 1.0000 | 0.8478 (2) | 0.6436 (5) | 0.0886 (13) | |
C1 | 0.8178 (2) | 0.67810 (19) | 0.4404 (5) | 0.0664 (9) | |
C2 | 0.7542 (2) | 0.6318 (3) | 0.3297 (8) | 0.0826 (11) | |
C3 | 0.7858 (3) | 0.5753 (2) | 0.1967 (6) | 0.0781 (11) | |
C4 | 0.8814 (3) | 0.56247 (18) | 0.1667 (5) | 0.0649 (8) | |
C4a | 0.94769 (19) | 0.60772 (14) | 0.2745 (3) | 0.0467 (6) | |
C9 | 1.0000 | 0.7067 (2) | 0.5121 (5) | 0.0440 (8) | |
C9a | 0.91558 (19) | 0.66596 (15) | 0.4107 (4) | 0.0463 (6) | |
C10 | 1.0000 | 0.8074 (2) | 0.5015 (5) | 0.0480 (9) | |
C11 | 1.0000 | 0.8595 (2) | 0.3169 (6) | 0.0479 (8) | |
C12 | 1.0000 | 0.8083 (3) | 0.1383 (7) | 0.0763 (14) | |
C13 | 0.9101 (4) | 0.9174 (3) | 0.3204 (7) | 0.1076 (15) | |
C14 | 1.0000 | 0.6788 (3) | 0.7193 (6) | 0.0649 (11) | |
H1 | 0.7957 | 0.7161 | 0.5316 | 0.080* | |
H2 | 0.6884 | 0.6394 | 0.3466 | 0.099* | |
H3 | 0.7412 | 0.5450 | 0.1251 | 0.094* | |
H4 | 0.9021 | 0.5239 | 0.0753 | 0.078* | |
H12a | 1.0000 | 0.8476 | 0.0313 | 0.114* | |
H12b | 0.9441 | 0.7716 | 0.1308 | 0.114* | |
H13a | 0.9060 | 0.9496 | 0.2061 | 0.161* | |
H13b | 0.9142 | 0.9572 | 0.4231 | 0.161* | |
H13c | 0.8539 | 0.8816 | 0.3341 | 0.161* | |
H14a | 1.0000 | 0.6164 | 0.7241 | 0.097* | |
H14b | 0.9414 | 0.6976 | 0.7742 | 0.097* |
U11 | U22 | U33 | U12 | U13 | U23 | |
O1 | 0.177 (4) | 0.0474 (15) | 0.0419 (16) | 0.000 | 0.000 | −0.0118 (14) |
C1 | 0.0621 (16) | 0.0650 (16) | 0.072 (2) | −0.0028 (14) | 0.0138 (16) | 0.0021 (18) |
C2 | 0.0560 (15) | 0.086 (2) | 0.106 (3) | −0.0162 (16) | 0.001 (2) | 0.016 (3) |
C3 | 0.074 (2) | 0.074 (2) | 0.086 (3) | −0.0242 (16) | −0.019 (2) | 0.006 (2) |
C4 | 0.093 (2) | 0.0470 (13) | 0.0546 (17) | −0.0134 (13) | −0.0135 (17) | −0.0016 (13) |
C4a | 0.0662 (14) | 0.0354 (10) | 0.0386 (11) | −0.0064 (10) | −0.0024 (12) | 0.0028 (10) |
C9 | 0.056 (2) | 0.0421 (15) | 0.0335 (16) | 0.000 | 0.000 | −0.0003 (15) |
C9a | 0.0600 (14) | 0.0397 (10) | 0.0392 (11) | −0.0039 (10) | 0.0031 (13) | 0.0023 (10) |
C10 | 0.065 (2) | 0.0432 (17) | 0.0353 (16) | 0.000 | 0.000 | −0.0030 (15) |
C11 | 0.066 (2) | 0.0384 (15) | 0.0397 (17) | 0.000 | 0.000 | 0.0026 (15) |
C12 | 0.134 (4) | 0.057 (2) | 0.0380 (19) | 0.000 | 0.000 | 0.005 (2) |
C13 | 0.122 (3) | 0.122 (3) | 0.079 (3) | 0.057 (3) | 0.013 (3) | 0.018 (3) |
C14 | 0.105 (3) | 0.052 (2) | 0.0375 (19) | 0.000 | 0.000 | 0.0038 (17) |
O1—C10 | 1.195 (5) | C4a—C4ai | 1.455 (5) |
C1—C2 | 1.386 (5) | C9—C9a | 1.517 (3) |
C1—C9a | 1.389 (4) | C9—C10 | 1.547 (5) |
C2—C3 | 1.363 (6) | C9—C14 | 1.549 (5) |
C3—C4 | 1.362 (5) | C10—C11 | 1.549 (5) |
C4—C4a | 1.390 (4) | C11—C12 | 1.506 (6) |
C4a—C9a | 1.399 (4) | C11—C13 | 1.534 (4) |
C10—C9—C14 | 108.9 (3) | C9a—C4a—C4 | 119.8 (3) |
C9—C10—C11 | 123.9 (3) | C4—C4a—C4ai | 131.53 (17) |
C9a—C9—C14 | 110.34 (19) | C9a—C9—C9ai | 101.4 (3) |
C9a—C9—C10 | 112.85 (19) | C4a—C9a—C1 | 120.4 (3) |
O1—C10—C9 | 118.5 (3) | C4a—C9a—C9 | 110.7 (2) |
O1—C10—C11 | 117.6 (3) | C1—C9a—C9 | 129.0 (3) |
C2—C1—C9a | 117.9 (3) | C12—C11—C13 | 108.4 (3) |
C3—C2—C1 | 121.5 (3) | C13i—C11—C13 | 109.2 (5) |
C4—C3—C2 | 121.2 (3) | C12—C11—C10 | 117.4 (3) |
C3—C4—C4a | 119.2 (3) | C13—C11—C10 | 106.6 (3) |
C9a—C4a—C4ai | 108.62 (15) | ||
C10—C9—C9a—C1 | −58.9 (4) | C4ai—C4a—C9a—C9 | −1.2 (2) |
C14—C9—C9a—C1 | 63.2 (4) | C4—C4a—C9a—C9 | 179.3 (2) |
C10—C9—C9a—C4a | 122.8 (2) | C2—C1—C9a—C4a | −0.8 (4) |
C14—C9—C9a—C4a | −115.2 (2) | C2—C1—C9a—C9 | −179.0 (3) |
C9a—C1—C2—C3 | 0.6 (5) | C9ai—C9—C9a—C4a | 1.8 (3) |
C1—C2—C3—C4 | −0.3 (6) | C9ai—C9—C9a—C1 | −179.9 (2) |
C2—C3—C4—C4a | 0.2 (5) | C9a—C9—C10—O1 | 122.9 (2) |
C3—C4—C4a—C9a | −0.4 (4) | C9a—C9—C10—C11 | −57.1 (2) |
C3—C4—C4a—C4ai | −179.8 (2) | O1—C10—C11—C13 | −58.2 (3) |
C4ai—C4a—C9a—C1 | −179.7 (2) | C9—C10—C11—C13 | 121.8 (3) |
C4—C4a—C9a—C1 | 0.8 (4) |
Symmetry code: (i) −x+2, y, z. |
Experimental details
Crystal data | |
Chemical formula | C19H20O |
Mr | 264.35 |
Crystal system, space group | Orthorhombic, Cmc21 |
Temperature (K) | 296 |
a, b, c (Å) | 13.908 (2), 15.3513 (18), 7.1860 (6) |
V (Å3) | 1534.3 (3) |
Z | 4 |
Radiation type | Mo Kα |
µ (mm−1) | 0.07 |
Crystal size (mm) | 0.51 × 0.49 × 0.43 |
Data collection | |
Diffractometer | Rigaku AFC-5S diffractometer |
Absorption correction | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 1137, 997, 691 |
Rint | 0.023 |
(sin θ/λ)max (Å−1) | 0.650 |
Refinement | |
R[F2 > 2σ(F2)], wR(F2), S | 0.043, 0.138, 1.02 |
No. of reflections | 997 |
No. of parameters | 102 |
No. of restraints | 1 |
H-atom treatment | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.26, −0.13 |
Absolute structure | Absolute structure could not be determined |
Absolute structure parameter | 0 (10) |
Computer programs: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1996), MSC/AFC Diffractometer Control Software, PROCESS in TEXSAN (Molecular Structure Corporation, 1997), SHELXS97 (Sheldrick, 1990), TEXSAN and SHELXL97 (Sheldrick, 1997), ORTEP (Johnson, 1965), TEXSAN, SHELXL97 and PLATON (Spek, 2000).
O1—C10 | 1.195 (5) | ||
C10—C9—C14 | 108.9 (3) | C9a—C9—C10 | 112.85 (19) |
C9—C10—C11 | 123.9 (3) | O1—C10—C9 | 118.5 (3) |
C9a—C9—C14 | 110.34 (19) | O1—C10—C11 | 117.6 (3) |
C10—C9—C9a—C1 | −58.9 (4) | C10—C9—C9a—C4a | 122.8 (2) |
C14—C9—C9a—C1 | 63.2 (4) | C14—C9—C9a—C4a | −115.2 (2) |
We reported previously that the ap rotamer of 9-pivaloylfluorene, (I), is the exclusive conformation in solution (NMR), as well as in the crystalline state (Meyers et al., 1991). Shortly afterwards, we reported the surprising observation that the related 9-hydroxy-9-pivaloylfluorene, (II), exists solely in the sp conformation, both in solution and as crystals (Meyers et al., 1992). The exclusive ap conformation of (I) reflects its thermodynamic stability relative to the sp conformation, in which there is unfavorable interaction between the tert-butyl group and fluorene-ring π electrons. While this same unfavorable interaction would be realised in the sp conformation of (II), it was suggested that, in this case, intramolecular hydrogen bonding, viz. C═O···HO, in this conformation lowered the energy sufficiently to make the sp rotamer thermodynamically preferred. Replacement of the 9-OH group by CH3 would not afford such hydrogen bonding. However, it was not known a priori whether the steric interaction between the tert-butyl and the fluorene-ring π electrons (sp rotamer) or between the tert-butyl and 9-CH3 groups (ap rotamer) would impart the lesser thermodynamic stability.
9-Methyl-9-pivaloylfluorene, (III), was prepared by two quite different routes (see Scheme), namely methylation of (I) and pivaloylation of 9-methylfluorene. The crystalline products were identical, melting sharply without decomposition. 1H NMR at room temperature exhibited a singlet for the pivaloyl protons at δ 0.67, indicating their strong shielding by the fluorene ring, characteristic of the sp rotamer of 9-methyl-9-pivaloylfluorene in solution (Meyers et al., 1992), and the absence of the signal near δ 1.25 which is exhibited by the deshielded pivaloyl protons of related ap counterparts (Robinson et al., 1994; Meyers et al., 1991).
This study firmly establishes the sp conformation of crystalline (III) (Fig. 1). Selected geometric parameters of (III) are shown in Table 1. They are very similar to the parameters of the corresponding bonds of (II) (Meyers et al., 1992), for the most part differing by no more than about 1° or 0.01 Å. Those that differ to a larger extent are in the direction which could be associated with intramolecular hydrogen bonding in (II), exemplified by the following comparisons of corresponding angles and torsion angles of (II) and (III), respectively: O1—C10—C9 115.4 (4) and 118.5 (3)°; C10—C9—C9a—C4a 125.6 (4) and 122.8 (2)°; C10—C9—C9a—C1 − 53.5 (5) and −58.9 (4)°; and C14—C9—C9a—C1 67.8 (5) and 63.2 (4)°.
It is concluded that the sp conformation of (III) is not associated with intramolecular hydrogen bonding or crystal-packing forces, this being the exclusive rotamer also in solution, and is therefore the sterically induced thermodynamically favored rotamer. The fact that both (II) and (III) exist solely as their sp rotamers while the 9-H parent compound (I) exists exclusively as its ap rotamer might not have been predicted from our recent observations of the related but more sterically restricted 9-H, –OH and –CH3 9-(o-tert-butylphenyl)fluorenes (Hou et al., 1999; Meyers et al., 1999; Robinson et al., 1998), in which intramolecular hydrogen bonding is not a possibility. In the latter series, both the 9-H and 9-OH fluorenes exist exclusively in the same singular conformation (crystalline form and solution), while the 9-CH3 fluorene exists in the opposite conformation, showing that an H atom and an OH group have similar steric influence, while a CH3 group imparts a substantially greater effect. With this in mind, the fact that (II) and (III), but not (I), both have sp configured further supports the probability that this conformation for (II) is promoted by intramolecular hydrogen bonding, but for (III), is induced by steric factors.