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Methyl­ation of 9-li­thia­ted ap-9-pivaloyl­fluorene, (I), as well as pivaloyl­ation of 9-li­thia­ted 9-methyl­fluorene provided rotationally stable sp-9-methyl-9-pivaloyl­fluorene, (III), C19H20O, which lies about a crystallographic mirror plane. Fluorene (I) exists exclusively in the ap configuration in solution (NMR) as well as in the crystalline state, reflecting the unfavorable interaction between the tert-butyl and fluorene-ring π electrons in the sp configuration. The existence of (III) exclusively in the sp configuration indicates that, in this case, the interaction between the tert-butyl group and the fluorene-ring π electrons provides relatively more thermodynamic stability than the steric interaction between the tert-butyl and 9-methyl groups (ap configuration).

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S010827010001235X/gd1104sup1.cif
Contains datablocks global, III

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S010827010001235X/gd1104IIIsup2.hkl
Contains datablock III

CCDC reference: 156166

Comment top

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. CO···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.

Experimental top

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; CO); 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)].

Refinement top

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.

Computing details top

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).

Figures top
[Figure 1] Fig. 1. The molecular structure and atom numbering scheme for (III) with displacement ellipsoids at the 30% probablilty level. Atoms C9, C14, C10, O1, C11 and C12 lie on a plane of symmetry. [Symmetry code: (i) 2 − x, y, z.]
sp-9-methyl-9-pivaloylfluorene top
Crystal data top
C19H20ODx = 1.144 Mg m3
Mr = 264.35Melting point = 430.5–431.5 K
Orthorhombic, Cmc21Mo Kα radiation, λ = 0.71069 Å
Hall symbol: C 2c -2Cell parameters from 24 reflections
a = 13.908 (2) Åθ = 10.0–10.5°
b = 15.3513 (18) ŵ = 0.07 mm1
c = 7.1860 (6) ÅT = 296 K
V = 1534.3 (3) Å3Fragment, colorless
Z = 40.51 × 0.49 × 0.43 mm
F(000) = 568
Data collection top
Rigaku AFC-5S
diffractometer
Rint = 0.023
Radiation source: sealed tubeθmax = 27.5°, θmin = 2.0°
Graphite monochromatorh = 018
ω (rate 6° min–1) scansk = 019
1137 measured reflectionsl = 19
997 independent reflections3 standard reflections every 150 reflections
691 reflections with I > 2σ(I) intensity decay: 0.7%
Refinement top
Refinement on F2Hydrogen site location: inferred from neighbouring sites
Least-squares matrix: fullH-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 = 1.02Δρmax = 0.26 e Å3
997 reflectionsΔρmin = 0.13 e Å3
102 parametersExtinction correction: SHELXL97, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
1 restraintExtinction coefficient: 0.025 (4)
Primary atom site location: structure-invariant direct methodsAbsolute structure: Absolute structure could not be determined
Secondary atom site location: difference Fourier mapAbsolute structure parameter: 0 (10)
Crystal data top
C19H20OV = 1534.3 (3) Å3
Mr = 264.35Z = 4
Orthorhombic, Cmc21Mo Kα radiation
a = 13.908 (2) ŵ = 0.07 mm1
b = 15.3513 (18) ÅT = 296 K
c = 7.1860 (6) Å0.51 × 0.49 × 0.43 mm
Data collection top
Rigaku AFC-5S
diffractometer
Rint = 0.023
1137 measured reflections3 standard reflections every 150 reflections
997 independent reflections intensity decay: 0.7%
691 reflections with I > 2σ(I)
Refinement top
R[F2 > 2σ(F2)] = 0.043H-atom parameters constrained
wR(F2) = 0.138Δρmax = 0.26 e Å3
S = 1.02Δρmin = 0.13 e Å3
997 reflectionsAbsolute structure: Absolute structure could not be determined
102 parametersAbsolute structure parameter: 0 (10)
1 restraint
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
O11.00000.8478 (2)0.6436 (5)0.0886 (13)
C10.8178 (2)0.67810 (19)0.4404 (5)0.0664 (9)
C20.7542 (2)0.6318 (3)0.3297 (8)0.0826 (11)
C30.7858 (3)0.5753 (2)0.1967 (6)0.0781 (11)
C40.8814 (3)0.56247 (18)0.1667 (5)0.0649 (8)
C4a0.94769 (19)0.60772 (14)0.2745 (3)0.0467 (6)
C91.00000.7067 (2)0.5121 (5)0.0440 (8)
C9a0.91558 (19)0.66596 (15)0.4107 (4)0.0463 (6)
C101.00000.8074 (2)0.5015 (5)0.0480 (9)
C111.00000.8595 (2)0.3169 (6)0.0479 (8)
C121.00000.8083 (3)0.1383 (7)0.0763 (14)
C130.9101 (4)0.9174 (3)0.3204 (7)0.1076 (15)
C141.00000.6788 (3)0.7193 (6)0.0649 (11)
H10.79570.71610.53160.080*
H20.68840.63940.34660.099*
H30.74120.54500.12510.094*
H40.90210.52390.07530.078*
H12a1.00000.84760.03130.114*
H12b0.94410.77160.13080.114*
H13a0.90600.94960.20610.161*
H13b0.91420.95720.42310.161*
H13c0.85390.88160.33410.161*
H14a1.00000.61640.72410.097*
H14b0.94140.69760.77420.097*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
O10.177 (4)0.0474 (15)0.0419 (16)0.0000.0000.0118 (14)
C10.0621 (16)0.0650 (16)0.072 (2)0.0028 (14)0.0138 (16)0.0021 (18)
C20.0560 (15)0.086 (2)0.106 (3)0.0162 (16)0.001 (2)0.016 (3)
C30.074 (2)0.074 (2)0.086 (3)0.0242 (16)0.019 (2)0.006 (2)
C40.093 (2)0.0470 (13)0.0546 (17)0.0134 (13)0.0135 (17)0.0016 (13)
C4a0.0662 (14)0.0354 (10)0.0386 (11)0.0064 (10)0.0024 (12)0.0028 (10)
C90.056 (2)0.0421 (15)0.0335 (16)0.0000.0000.0003 (15)
C9a0.0600 (14)0.0397 (10)0.0392 (11)0.0039 (10)0.0031 (13)0.0023 (10)
C100.065 (2)0.0432 (17)0.0353 (16)0.0000.0000.0030 (15)
C110.066 (2)0.0384 (15)0.0397 (17)0.0000.0000.0026 (15)
C120.134 (4)0.057 (2)0.0380 (19)0.0000.0000.005 (2)
C130.122 (3)0.122 (3)0.079 (3)0.057 (3)0.013 (3)0.018 (3)
C140.105 (3)0.052 (2)0.0375 (19)0.0000.0000.0038 (17)
Geometric parameters (Å, º) top
O1—C101.195 (5)C4a—C4ai1.455 (5)
C1—C21.386 (5)C9—C9a1.517 (3)
C1—C9a1.389 (4)C9—C101.547 (5)
C2—C31.363 (6)C9—C141.549 (5)
C3—C41.362 (5)C10—C111.549 (5)
C4—C4a1.390 (4)C11—C121.506 (6)
C4a—C9a1.399 (4)C11—C131.534 (4)
C10—C9—C14108.9 (3)C9a—C4a—C4119.8 (3)
C9—C10—C11123.9 (3)C4—C4a—C4ai131.53 (17)
C9a—C9—C14110.34 (19)C9a—C9—C9ai101.4 (3)
C9a—C9—C10112.85 (19)C4a—C9a—C1120.4 (3)
O1—C10—C9118.5 (3)C4a—C9a—C9110.7 (2)
O1—C10—C11117.6 (3)C1—C9a—C9129.0 (3)
C2—C1—C9a117.9 (3)C12—C11—C13108.4 (3)
C3—C2—C1121.5 (3)C13i—C11—C13109.2 (5)
C4—C3—C2121.2 (3)C12—C11—C10117.4 (3)
C3—C4—C4a119.2 (3)C13—C11—C10106.6 (3)
C9a—C4a—C4ai108.62 (15)
C10—C9—C9a—C158.9 (4)C4ai—C4a—C9a—C91.2 (2)
C14—C9—C9a—C163.2 (4)C4—C4a—C9a—C9179.3 (2)
C10—C9—C9a—C4a122.8 (2)C2—C1—C9a—C4a0.8 (4)
C14—C9—C9a—C4a115.2 (2)C2—C1—C9a—C9179.0 (3)
C9a—C1—C2—C30.6 (5)C9ai—C9—C9a—C4a1.8 (3)
C1—C2—C3—C40.3 (6)C9ai—C9—C9a—C1179.9 (2)
C2—C3—C4—C4a0.2 (5)C9a—C9—C10—O1122.9 (2)
C3—C4—C4a—C9a0.4 (4)C9a—C9—C10—C1157.1 (2)
C3—C4—C4a—C4ai179.8 (2)O1—C10—C11—C1358.2 (3)
C4ai—C4a—C9a—C1179.7 (2)C9—C10—C11—C13121.8 (3)
C4—C4a—C9a—C10.8 (4)
Symmetry code: (i) x+2, y, z.

Experimental details

Crystal data
Chemical formulaC19H20O
Mr264.35
Crystal system, space groupOrthorhombic, Cmc21
Temperature (K)296
a, b, c (Å)13.908 (2), 15.3513 (18), 7.1860 (6)
V3)1534.3 (3)
Z4
Radiation typeMo Kα
µ (mm1)0.07
Crystal size (mm)0.51 × 0.49 × 0.43
Data collection
DiffractometerRigaku AFC-5S
diffractometer
Absorption correction
No. of measured, independent and
observed [I > 2σ(I)] reflections
1137, 997, 691
Rint0.023
(sin θ/λ)max1)0.650
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.043, 0.138, 1.02
No. of reflections997
No. of parameters102
No. of restraints1
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.26, 0.13
Absolute structureAbsolute structure could not be determined
Absolute structure parameter0 (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).

Selected geometric parameters (Å, º) top
O1—C101.195 (5)
C10—C9—C14108.9 (3)C9a—C9—C10112.85 (19)
C9—C10—C11123.9 (3)O1—C10—C9118.5 (3)
C9a—C9—C14110.34 (19)O1—C10—C11117.6 (3)
C10—C9—C9a—C158.9 (4)C10—C9—C9a—C4a122.8 (2)
C14—C9—C9a—C163.2 (4)C14—C9—C9a—C4a115.2 (2)
 

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