Steric influences on the stability and synthetic accessibility of tetraalkylcyclopentadienes

Journal of Molecular Structure 478 (1999) 163–168

Steric influences on the stability and synthetic accessibility of tetraalkylcyclopentadienes Jason S. Overby, Erik D. Brady, Stephanie C. Slate, Timothy P. Hanusa* Department of Chemistry, Vanderbilt University, Nashville, TN 37235, USA Received 15 June 1998; accepted 13 October 1998

Abstract The phase transfer-catalyzed addition of cyclohexyl bromide to cyclopentadiene under basic conditions yields 2,3,5,5 0 tetracyclohexylcyclopentadiene as the sole tetra-substituted isomer. An X-ray crystal structure reveals that some angle distortion exists around the quaternary carbon, including a compressed C5 ring angle (99.4(5)⬚). The reason for the production of only the 2,3,5,5 0 isomer from the seven possible for tetracyclohexylcyclopentadiene was probed with molecular mechanical and semiempirical MO calculations. The 2,3,5,5 0 form was determined to be the lowest in strain energy, although not in the heat of formation. Close intramolecular H…H contacts are present in the cyclohexyl-substituted cyclopentadienes that are absent in isopropyl analogs. 䉷 1999 Elsevier Science B.V. All rights reserved. Keywords: Crystal structures; Cyclopentadienes; Steric Effects

1. Introduction Highly alkylated cyclopentadienes find uses ranging from industrial lubricants [1] to sterically bulky ligands for transition metal complexes and catalysts [2]. The phase transfer-catalyzed addition of alkyl groups to cyclopentadiene under basic conditions is a practical route to these hydrocarbons [3], although isomeric mixtures of variously substituted cyclopentadienes C5RnH6⫺n are usually formed. For example, Dehmlow and Bollmann [4] found that the phase-transfer catalyzed synthesis of tetraisopropylcyclopentadiene led to the formation of three different isomers (1,2,3,4; 1,3,5,5 0 ; 2,3,5,5 0 ) in addition to one isomer of pentaisopropylcyclopentadiene (1,2,3,5,5 0 ). For some * Corresponding author. Tel.: 001 615 3224667; Fax: 001 615 3224936; e-mail: [email protected]

applications, only certain cyclopentadiene isomers are useful (e.g., only a C5R4H2 isomer with an allylic hydrogen can serve as a precursor to the [C5R4H] ⫺ anion), and there is understandable interest in optimizing the yield of cyclopentadiene isomers with a given pattern of substitution. If kinetic factors were a principle determinant of the isomer distribution in phase-transfer addition reactions, the structural features of the intended substituents could alter the substitution patterns. We examined this possibility by the synthesis of cyclohexyl-substituted cyclopentadienes. Although the cyclohexyl group is larger and has more conformational degrees of freedom than does the isopropyl moiety, both are secondary alkyl groups and their steric properties are often similar. For example, the comparable cone angles of P(i-Pr)3 (160⬚) and P(cC6H11)3 (170⬚) [5] suggest that the additional atoms

0022-2860/99/$ - see front matter 䉷 1999 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(98)00762-5

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Table 1 Crystal data and summary of data collection for 2,3,5,5 0 -C5(Cy)4H2 Chemical formula Formula weight Color of crystal Crystal dimensions, mm Crystal system Space group Cell dimensions (at 20⬚C, 25 reflctns) ˚ a, A ˚ b, A ˚ c, A ˚3 V, A Z D (calcd), g/cm 3 ˚ Wavelength (Cu Ka), A Absorption coefficient, mm ⫺1 Type of scan Scan width Limits of data collectn Correction for decay Total no. of reflections collected No. with I ⬎ 3.0s (I) Variables refined R(F) Rw(F) Goodness of fit Max D /s in final cycle Max/min peak (final difference ˚ 3) map) (e ⫺/A

C29H46 394.68 colorless 0.43 × 0.58 × 0.70 Orthorhombic Pbcn (#60) 11.949 (3) 18.530 (6) 11.539 (4) 2555 (1) 4 1.026 1.54178 3.87 v –2u (1.89 ⫹ 0.30 tan u ) 6⬚ ⱕ 2u ⱕ 120.2⬚ None 2192 960 132 0.069 0.091 3.21 0.00 0.21/–0.17

of the cyclohexyl group only modestly increase the effective bulk of the ligand. Tricyclohexylcyclopentadiene has been generated with the phase-transfer approach [6], and we expected that an extension of the reaction to form the more highly substituted tetracyclohexylcyclopentadiene (C5(Cy)4H2) derivatives would give an isomer distribution that could be compared with that for tetraisopropylcyclopentadiene (HCp 4i). As described later, the outcome of attempts to prepare C5(Cy)4H2 was different from that for the HCp 4i reactions. Several varieties of tetraalkylcyclopentadienes were then examined with molecular mechanics calculations to determine whether estimates of the steric strain in these molecules could provide an insight into their relative synthetic accessibility.

2. Experimental 2.1. General considerations Proton and carbon ( 13C) NMR spectra were obtained on a Bruker NR-300 spectrometer at 300 and 75.5 MHz, respectively, and were referenced to the residual 1H resonances of CDCl3 (d 7.24 and 77.0) and cross-referenced to tetramethylsilane. Infrared data were obtained on an ATI Mattson Genesis Series FT-IR spectrometer. GC/MS data were obtained with a HP 5890 Series II Gas Chromatograph and a HP 5971 Series Mass Selective Detector. Elemental analysis was performed by Desert Analytics Microanalytical Laboratory, Tucson, AZ. 2.2. Materials Adogen䉸 464, cyclohexyl bromide (Aldrich) and KOH (Fisher) were commercial samples and used without further purification. CDCl3 was stored over molecular sieves. 2.3. Synthesis of 2,3,5,5 0 tetracyclohexylcyclopentadiene, (2,3,5,5 0 -C5(Cy)4H2) In a procedure similar to that used for tricyclohexylcyclopentadiene [6], a 3-l three-neck Morton flask was fitted with a condenser, mechanical stirrer, heating mantle, thermometer and an inlet adapter. To this were added a supersaturated aqueous solution of KOH (approximately 750 g KOH in 1.5 l total volume, ⬃ 9 M) and Adogen䉸 464 (23 ml). Freshly cracked cyclopentadiene (41 ml) and cyclohexyl bromide (308 ml) were added and stirring was started. The mixture turned brown and became warm (50⬚C). The vigorously stirred mixture was maintained at 60⬚C overnight, after which it was extracted with hexanes (3 × 100 ml). The hexanes were then removed by rotary evaporation to leave an orangebrown viscous oil. Analysis with GC/MS revealed that several tricyclohexylcyclopentadienes and a single tetrasubstituted isomer were formed in the reaction in a 32 : 68 ratio. The oil was transferred to a silica gel chromatography column and eluted with hexanes. A yellow band was collected from which the hexane was removed under vacuum. The remaining solvent was removed under high vacuum, leaving a viscous orange

J.S. Overby et al. / Journal of Molecular Structure 478 (1999) 163–168 Table 2 Atomic fractional coordinates and isotropic thermal parameters for P P 2, 3,5,5 0 -C5(Cy)4H2 (Biso ˆ …8p2 =3† 3iˆ1 3jˆ1 Uij a*i a*j ai ·aj ) Atom

x/a

y/b

C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) C(15) H(1) H(2) H(3) H(4) H(5) H(6) H(7) H(8) H(9) H(10) H(11) H(12) H(13) H(14) H(15) H(16) H(17) H(18) H(19) H(20) H(21) H(22) H(23)

0 0.0103(5) 0.0063(4) 0.1084(4) 0.1237(5) 0.2271(6) 0.3310(6) 0.3172(5) 0.2142(5) 0.0070(5) ⫺ 0.1100(5) ⫺ 0.1102(6) ⫺ 0.0486(7) 0.0683(7) 0.0691(6) 0.0188 0.1008 0.059 0.1329 0.2163 0.2363 0.3931 0.344 0.3088 0.3819 0.2251 0.2056 0.0439 ⫺ 0.1483 ⫺ 0.1476 ⫺ 0.1851 ⫺ 0.074 ⫺ 0.0869 ⫺ 0.0462 0.1045 0.1076 0.1445 0.0345

0.0535(4) 0.1061(3) 0.1733(2) 0.0073(3) ⫺ 0.0476(3) ⫺ 0.0957(3) ⫺ 0.0496(4) 0.0047(3) 0.0520(3) 0.2413(2) 0.2671(3) 0.3383(3) 0.3256(4) 0.3006(4) 0.2321(3) 0.0922 ⫺ 0.0194 ⫺ 0.0773 ⫺ 0.0228 ⫺ 0.1228 ⫺ 0.1277 ⫺ 0.0798 ⫺ 0.0248 ⫺ 0.0201 0.0345 0.079 0.084 0.278 0.2309 0.2754 0.3522 0.3754 0.2898 0.3693 0.2913 0.3374 0.2194 0.1945

˚ 2) Biso (A

Z/c 1/4 0.1510(4) 0.1863(4) 0.2623(5) 0.1656(6) 0.1869(7) 0.1992(7) 0.2954(6) 0.2746(5) 0.1128(4) 0.0876(5) 0.0157(6) ⫺ 0.0962(6) ⫺ 0.0732(6) ⫺ 0.0007(6) 0.0723 0.3321 0.1616 0.0942 0.2559 0.1233 0.2161 0.1285 0.367 0.2986 0.2056 0.3382 0.1559 0.0451 0.1589 ⫺ 0.0003 0.0584 ⫺ 0.1399 ⫺ 0.1391 ⫺ 0.145 ⫺ 0.0328 0.0155 ⫺ 0.0438

4.7(4) 4.6(2) 4.3(2) 4.9(3) 6.7(3) 8.2(4) 8.6(4) 7.0(4) 6.0(3) 4.3(2) 6.5(3) 7.8(4) 8.1(4) 8.7(4) 7.3(4) 5.6 5.8 8.1 8.1 9.8 9.8 10.3 10.3 8.4 8.4 7.2 7.2 5.2 7.7 7.7 9.4 9.4 9.8 9.8 10.4 10.4 8.7 8.7

Table 3 ˚ ) and angles (deg) for 2,3,5,5 0 -C5(Cy)4H2 Selected bond lengths (A Atom

Distance

Atom

C(1)–C(2) C(1)–C(4) C(2)–C(3) C(3)–C(3)

1.506(6) 1.559(6) 1.311(6) 1.479(9)

C(2)–C(1)–C(2) C(4)–C(1)–C(4) C(1)–C(2)–C(3) C(2)–C(3)–C(3)

0

Angle 0 0 0

99.4(5) 113.4(6) 112.1(4) 108.2(3)

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oil. Upon standing, colorless crystals of tetracyclohexylcyclopentadiene formed. Washing the crystals with small amounts of cold hexanes removed any remaining traces of the trisubstituted isomers, although the high solubility of 2,3,5,5 0 -C5(Cy)4H2 in hexanes reduced the yield to only 6.1%, m.p. 94⬚C– 97⬚C. Anal calcd. for C29H46: C, 88.25; H, 11.75. Found: C, 88.33; H, 11.66. 1H NMR (CDCl3); d 5.62 (s, 2 H, C5 ring H); 3.48 (q, Cy ring H); 2.17 (s, Cy ring H); 1.77–1.58 (br m, Cy ring H); 1.30– 1.07 (br m, Cy ring H). 13C NMR (CDCl3): d 151.0 (CyC); 132.7 (CyC); 62.4 (C5 ring C); 39.1 (Cy); 36.7 (Cy); 34.2 (Cy); 27.9 (Cy); 27.2 (Cy); 27.0 (Cy); 26.5 (Cy). Principal IR bands (KBr, cm ⫺1): 2925 (vs), 2853 (s), 1708 (w), 1447 (m), 1346 (w), 1260 (w), 1163 (w), 1085 (w), 1038 (w), 985 (w). MS (EI) m/z: 394 (M ⫹), 311 (M ⫹ – Cy). 2.4. X-ray crystallography of 2,3,5,5 0 tetracyclohexylcyclopentadiene A suitable crystal of 2,3,5,5 0 -C5(Cy)4H2 was located and mounted on the end of a glass fiber. All measurements were performed on a Rigaku AFC6S diffractometer with graphite monochromated Cu-Ka ˚ ) radiation. Relevant crystal and data (l ˆ 1.54178 A collection parameters for the present study are given in Table 1. Cell constants and orientation matrices for data collection were obtained from systematic searches of limited hemispheres of reciprocal space; sets of diffraction maxima were located whose setting angles were refined by least squares. The space group Pbcn was determined from consideration of cell parameters, statistical analysis of intensity distributions and systematic absences. Subsequent solution and refinement of the structure confirmed the choice. Data collection was performed using continuous v –2u scans with stationary backgrounds (peak:background counting time ˆ 2 : 1). Data were reduced to a unique set of intensities and associated sigma values in the usual manner. The structure was solved by direct methods (SHELXS-86, DIRDIF) and Fourier techniques which yielded the positions of all nonhydrogen atoms. The hydrogen atoms were placed in idealized positions based on packing considerations ˚ . The positions were fixed for and d(C–H) ˆ 0.95 A the final cycles of refinement. A final difference map

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Fig. 1. ORTEP diagram of the non-hydrogen atoms of 2,3,5,5 0 -C5(Cy)4H2, giving the numbering scheme used in the text. Thermal ellipsoids are shown at the 30% probability level.

was featureless. Fractional coordinates and isotropic thermal parameters for all atoms are listed in Table 2; selected bond distances and angles are listed in Table 3. 2.5. Computational details All calculations were performed using the MacSpartan Plus package (Wavefunction, Inc., Irvine, CA, USA) (SYBYL for molecular mechanics, PM3 for semiempirical MO). The X-ray coordinates of both 1,2,3,4-tetraisopropylcyclopentadiene [7] and 2,3,5,5 0 -tetracyclohexylcyclopentadiene were used as starting geometries for optimization. Each structure was appropriately modified to generate the remaining isomers. Each compound was optimized until a minimum structure was found. 3. Results and discussion 3.1. Synthesis of 2,3,5,5 0 tetracyclohexylcyclopentadiene When C5(i-Pr)3H3 (HCp 3i) and C5(i-Pr)4H2 (HCp 4i) are synthesized from C5H6 and i-PrBr, 50% KOH is typically employed as the base. We have found that increasing the concentration of KOH to generate a supersaturated solution (ca. 9 M) favors the formation of the tetraisopropyl-substituted ring over the

triisopropyl product [8]. We extended this synthetic variation to the production of tetracyclohexylcyclopentadiene. Reaction of five equivalents of cyclohexyl bromide with freshly cracked cyclopentadiene and a supersaturated solution of KOH led to the formation of tetraand tricyclohexylcyclopentadiene in a 68 : 32 ratio as determined by GC/MS. The tetrasubstituted product could by separated by careful column chromatography on silica gel. Initially isolated as a viscous orange oil, 2,3,5,5 0 -C5(Cy)4H2 forms colorless crystals in the oil on standing. Washing the crystals with small amounts of cold hexanes removes any remaining traces of C5(Cy)3H3. The NMR data of this material are consistent with either the 2,3,5,5 0 or 1,4,5,5 0 isomer, and an X-ray crystallographic study was performed to determine the substitution pattern. 3.2. Solid state structure of 2,3,5,5 0 tetracyclohexylcyclopentadiene The cyclopentadiene derivative crystallizes as large blocks in the orthorhombic space group Pbcn with a crystallographically imposed C2 axis through the quaternary carbon C(1), rendering only half of the molecule unique. An ORTEP view of the molecule showing the numbering scheme used in the tables is shown in Fig. 1. The bond distances and angles in the C5 ring should

J.S. Overby et al. / Journal of Molecular Structure 478 (1999) 163–168

Fig. 2. Strain energy (SYBYL) and heat of formation (PM3) for tetracyclohexylcyclopentadienes as a function of ring position.

not be affected by the hyperconjugation between the methylene carbon and p -systems that influences the geometry of cyclopentadiene itself [9]. There is some evidence for this in the slightly greater alteration in the bond lengths in 2,3,5,5 0 -C5(Cy)4H2 than in C5H6 ˚ )/CH–CH2 (1.498(2) (e.g., CH–C(Cy)2 (1.506(6) A ˚ ), CHyC(Cy) (1.311(6) A ˚ )/CHyCH (1.344(1) A ˚ ), A ˚ )/CH–CH (1.460(2) and C(Cy)– C(Cy) (1.479(9) A ˚ ), although the differences are not always statistiA cally significant. More obvious is the compression of the CH–C(Cy)–CH angle (C(2)–C(1)–C(2 0 )) to 99.4(5)⬚, compared to the analogous CH–CH2 –CH angle in C5H6 of 102.7⬚. This is accompanied by an enlargement of the C(4)–C(1)–C(4 0 ) angle to 113.4(6)⬚, which reflects a slight rehybridization of the quaternary carbon atom, probably driven by the closeness of the gem-Cy groups (e.g., C(5)…C(5 0 ) ˆ ˚ ). 3.54 A 3.3. Molecular mechanics study of tetrasubstituted cyclopentadienes As noted earlier, when isopropyl bromide is employed in the phase-transfer catalyzed synthesis of substituted cyclopentadienes, three HCp 4i isomers and a single pentasubstituted isomer are isolated. This contrasts strongly with the fact that only 2,3,5,5 0 C5(Cy)4H2 is isolated from the analogous reaction with cyclohexyl bromide. The latter result, however,

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parallels the outcome of attempts to form poly(trimethylsilyl)cyclopentadienes, for which 2,3,5,5 0 C5(SiMe3)4H2 is the only tetrasubstituted isomer observed [10]. This suggests that the range of available isomers is critically sensitive to the steric demands of the substituents, as greater substitution on the sp 2 hybridized carbon would be expected to raise the total energy of the isomer. This was supported by PM3 calculations on all seven possible tetracyclohexylcyclopentadienes; for example, the 2,3,5,5 0 and 1,2,3,4-C5(Cy)4H2 isomers have heats of formation of –63.5 and –72.3 kcal mol -1, respectively (Fig. 2). As there is no evidence for the latter isomer in the reaction products, the product mixture evidently reflects kinetic control of the isomer distribution. In order to determine whether the structural influence of the substituents on the product distribution could be interpreted with the use of molecular mechanics calculations, we studied the steric strain energies of variously substituted tetralkylcyclopentadienes. As illustrated in Fig. 2, the C5(Cy)4H2 structure with the least steric strain is isomer a (2,3,5,5 0 7.2 kcal mol ⫺1), which is also the isomer that is found experimentally. The next most stable isomer b (1,3,5,5 0 ) has more than twice the strain energy (15.4 kcal mol ⫺1), and the other possible isomers have 20 kcal mol ⫺1 strain energy or more. The 1,2,3,4-isomer g, which would be useful in ligand synthesis, was found to have the highest strain energy of all (28.0 kcal mol ⫺1). An inspection of the minimized structures (SYBYL or PM3) suggests that the origin of the increased strain evidently lies in Cy…Cy 0 contacts and close H…H 0 distances. In b, for example, CH…CH 0 contacts between cyclohexyl rings on the carbon atoms at the ˚ (PM3); H…H 0 1 and 5 positions are as close as 3.29 A separations between the same rings are as small as ˚ . Such contacts, of course, are missing in a. 1.75 A Interestingly, parallel SYBYL calculations on the HCp 4i isomers indicate that the difference in energy between the least and most strained isomers (a and f, respectively) is only 12.1 kcal mol ⫺1. Two of the HCp 4i isomers prepared by Dehmlow and Bollman [4] correspond to the two lowest in energy (a and b), but the third isolated isomer g differs from that of the third lowest in energy (c) only in the position of the double bonds. Thus whether the isolated isomers are the initially generated products, or

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whether they arise by rearrangement from other forms (e.g., g from c or d), steric congestion in HCp 4i will not favor its 2,3,5,5 0 isomer to the extent that it does in C5(Cy)4H2. The reason that the C5(Cy)4H2 isomers display a larger range of strain energies than do the HCp 4i analogues is not simply a function of the closest contacts in the molecules. The shortest separations themselves are not always greatly different between the isopropyl and cyclohexyl derivatives. In the b isomer of HCp 4i, for example, CH…CH 0 contacts between isopropyl groups on the carbon atoms at the 1 and 5 positions are calculated to be as close as ˚ ; H…H 0 separations between the same groups 3.29 A ˚ . These values are similar to are as small as 1.73 A those in the 2,3,5,5 0 -C5(Cy)4H2 analog. However, the cyclohexyl-substituted rings have additional contacts that are absent in the HCp 4i isomers. For instance, in ˚ b-C5(Cy)4H2 there are two contacts at less than 1.9 A between hydrogen atoms on the g -CH2 units of the cyclohexyl ring on the C5-ring carbon 1 and the hydrogen atom on the C5-ring carbon 2. These and other long distance, but still energetically significant, contacts do not exist in the isopropyl-substituted isomers; their collective effect is to magnify the energetic differences between the C5(Cy)4H2 isomers more than in the HCp 4i compounds. It should be stressed that the selectivity introduced by steric factors in phase-transfer addition reactions does not mean that highly substituted cyclopentadienes cannot be formed by alternate synthetic means. For example, phase transfer-catalyzed reactions employing t-BuBr with cyclopentadiene never produce any C5(t-Bu)4H2; only isomers of the trisubstituted product are recovered [3]. However, 1,2,4,5-C5(t-Bu)4H2 has been prepared by reducing 1,2,3,4-tetra-t-Bu-cyclopentenone to the alcohol followed by dehydration [11]. The starting cyclopentenone is not produced by sequential addition reactions.

4. Conclusions Under phase transfer-catalyzed conditions that generate three different isomers of tetraisopropylcyclopentadiene, only the 2,3,5,5 0 isomer of tetracyclohexylcyclopentadiene can be isolated. Molecular mechanics calculations suggest not only that this isomer has the lowest steric strain, but also that other possibilities are substantially higher in energy. The results indicate that phase-transfer addition to cyclopentadiene is strongly influenced by the steric properties of the substituents, and that the product mixture reflects kinetic control of the isomer distribution.

Acknowledgements Acknowledgment is made to the National Science Foundation for support of this research.

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