Organometallics 1995,14,418-422
418
Polysilylation of Naphthalene Revisited: Synthesis and Structural Study of 1,2,3,5,6,7-Hexakis(trimethylsilyl)1,2,3,5,6,7-hexahydronaphthalenef Micheline Grignon-Dubois" Laboratoire de chimie organique et organomktallique, Universitk Bordeaux I, 351 Cours de la Libtration, 33405 Talence-Cedex, France
Michel Laguerre and Michel Saux Laboratoire de chimie analytique, Facultk de Pharmacie, Universitt Bordeaux II, 3 Place de la Victoire, 33076 Bordeaux, France Received July 26, 1994@ Revisiting t h e reductive polysilylation of naphthalene using Me3SiCUMgElMPT reagent, we have shown t h a t t h e reaction was not correctly interpreted. The hexasilylated product 2 previously described h a s been reformulated as 6 on the basis of NMR d a t a and X-ray study. In addition, a molecular mechanics study of both 2 and 6 allowed understanding of the polysilylation process.
Introduction Reductive silylation of naphthalene 1 with an excess of Me3SiCVMg/HMPT reagent has been previously reported to give a hexasilylated derivative in 20-30% yie1d.l The s-cis dienic structure 2 (Figure 1)has been ascribed to this compound on the basis of the W spectra and mechanistic considerations. We have recently shown that silylation of quinoline2and isoquinoline3also lead to hexasilylated derivatives, but in these cases we obtained the s-trans dienic structures 3-6 (Figure 1) as demonstrated by NMR and X-ray studies. These results and the fact that the physical chemistry data reported for 2 seemed to us more in accordance with a s-trans dienic skeleton prompted us to reinvestigate this reaction. We now present evidence for the reformulation of 2 and a rationale for the silylation process based on structural parameters and molecular mechanics calculations (MM3).4
Results and Discussion The silylation has been conducted as reported in ref 1, excepting in the MesSiCl quantity used (7 equiv versus naphthalene instead of 6 equiv), and the temperature and reaction time (85 "C for 24 h instead of 100 "C during 72-96 h). These modifications allowed us to increase the yield from 20-30% to 42%. Our sample shows the same spectroscopic and physicochemical data as previously reported1 (see Experimental
* To whom correspondence should be addressed (tel. 56846286; fax 56846646). + Dedicated to Norbert DufTaut, a pioneer in organosilicon chemistry, who initiated French research in this field. Abstract published in Advance ACS Abstracts, December 1,1994. (1)(a)Dunogues, J.; Calas, R.; Biran, C.; Duffaut, N. J . Orgunomet. Chem. 1966, 30, 943. (b) Laguerre, M.; Dunogues, J.; Calas, R. Tetrahedron Lett. 1981,22,1227. (2) Grignon-Dubois,M.; Fialeix, M.; Rezzonico, B. Can. J . Chem. 1990,68,2153. (3)Grignon-Dubois, M.;Fialeix, M.; Leger, J.-M. Can. J . Chem. 1993,71,754. (4) Allinger, N. L.; Yuh, Y. H.; Lii, J. H. J . Am. Chem. SOC.1989, 111, 8551. @
Section). In particular, it exhibits the same 116 "C melting point (EtOH) and a 273 nm Amax in W. We assigned it the s-trans dienic structure 6 (Figure 11, which is in better agreement with the NMR data and the W spectra. These conclusions were supported by a single-crystal X-ray analysis. An ORTEP drawing of 6 is shown in Figure 2. Bond lengths, valence and dihedral angles, and Si..*Si distances are given in Tables 11-IV (supplementary material) and Figure 3. In the solid state, 6 exhibits a nonplanar dienic linkage with a dihedral angle of 156.3' leading to a curved shape of the carbon frame. The molecule is symmetric about a CZ axis orthogonal to the plane of the skeleton and passing through the middle of the C4a-C8a bond. This symmetry is well demonstrated by the superimposition of the two cyclohexene moieties, which leads to a rms of 0.02 (methyl groups excluded). Four silicons are positioned on the ex0 side (Si1 and Si5 in the axial position and Si3 and Si7 in the equatorial) and two on the endo side (Si2 and Si6 in axial). They can be classified in two equivalent groups of three, which are positioned alternatively up and down on each sixmembered ring (Figure 3). Their location determined an almost equilateral triangle, due to the similarity of the Si- .Si distance values (average 4.426 The 5.19 A distance between Si1 and Si5 shows that they are only slightly interacting despite their cis-diaxial relationship, which is certainly responsible for the curved shape of the skeleton. Indeed, only one H.--H and one C-.*C nonbonding distance for these silyl groups are shorter than the sum of the van der Waals radii5" (respectively 1.92 and 3.77 A long). This is the same for the vicinal silicon groups Si6 and Si7, for which the shortest 3.98 Cam 42 distance is very close to the 4 A normal minimal value.6a Only Si2 and Si3 led to a noticeable steric hindrance with a 3.76 A measured C* *C distance. An
A
A).
A
-
(5) (a) Bock, H.; Ruppert, K.; Nather, C.; Havlas, 2.;Herrmann,H.F.; h a d , C.; Gtjbel, I.; John, A.; Meuret, J.; Nick, S.; Rauschenbach, A,; Seitz, W.; Vaupel, T.; Solouki,B.Angew. Chem.,Int. Ed. Engl. 1992, 31,550 and references therein. (b)Allen, F. H.; Kennard, 0.;Watson, D. G.; Brammer, L.; Orpeen, A. G.; Taylor, R. J . Chem. Soc., Perkin Trans. 2 1987,51.
0276-733319512314-0418$09.00/0 0 1995 American Chemical Society
Polysilylation of Naphthalene Revisited
Organometallics, Vol.14,No.1, 1995 419
Si*
Si* Si*
Si*
Si*
Si*
3
Si* 4
Si*
Si* I
5
Si*
Si*
6
Si*
I
8
Si* = SiMe3
Figure 1. s-cis and s-trans dienic linkages obtained (or postulated for 2) from naphthalene, quinoline, and isoquinoline. a s-cis and s-trans structure as in 2 and 6 (Figure 1).In particular, comparison of the W Am, values, 237 nm (14600)for 7,263nm (4830)for 8,and 273 nm (13500) for the hexasilylated compound, had led the previous authors1 to assign it the s-cis structure as in 8. In fact, the important magnitude difference of the E values C23 seems more in accordance with a s-trans structure, and the silicon group, are not without influence on the Am, value. In order to demonstrate the effect of a trimethylsilyl group in allylic position, we have measured the Amax in 3-(trimethylsilyl)cyclopentene, for which we found 208 nm. Comparison of this value to the 173 nm calculated value in cyclopentene (see Experimental Section) clearly shows the bathochromic effect of an Figure 2. ORTEP drawing of compound 6 showing the allylic trimethylsilyl group linked to a cyclene. It has atom-numbering scheme. (Hydrogen atoms have been not been possible to calculate the Am, values in 2 and omitted for clarity.) 6, the silicon atom not being parametrized. In order to obtain a better evaluation of the contribution of the additional packing analysis of the crystal lattice reveals silicon groups in 6, we have calculated the A, of its very weak van der Waals contacts, which is in actwisted carbon framework. The 236 nm calculated cordance with the observed low stability of the crystal value, which can be compared to the measured values under X-ray (see Experimental Section). Steric hinreported in l i t e r a t ~ r e ~for 9 ~7 (242 nm, e = 17 400, or drance leads to opening of the C-C-Si valence angles 237 nm, E = 14 600),shows that the slight torsion of and lengthening of all the Si-Cdg bonds (average 1.902 the dienic linkage (156.3") has only a weak effect on the A), a s depicted in Figure 3. As a consequence, some Am, . From these results, absorption could be estimated important closings of the Si-C-H angles are observed. around 390 nm for 2 and 360 nm for 6 (four allylic silyl The C-C bonds in allylic positions only slightly deviate groups in each molecule). In fact, the X-ray structure from the 1.506 A normal value, whereas those in of 6 shows that only the two C1-Si and C5-Si bonds homoallylic positions are considerably longer (1.560 are in a perfect perpendicular orientation versus the It is worth noting that these last values are related to double bond, which leads to a maximum stabilizing n the shorter Si- .Si distances a s we previously observed i n t e r a ~ t i o n . Considering ~ this structural feature, the with gem-disilyl derivatives,6 which also shows the 273 nm Am, value appears in good accordance with both influence of steric hindrance on bond lengths. the dienic twisting and a double bathochromic effect of Spectroscopic data of 6 lead to the following comthe silicon. Concerning the 'H NMR data, it is worth ments. lH NMR data and UV spectra had been previously reported for 1,2,3,5,6,7(7) and 1,2,3,4,6,7- noting that both relative chemical shifts and 3J coupling hexahydronaphthalene (8),7which respectively present (6) Laguerre, M.; Grignon-Dubois, M. J.Mol. Struct. 1994,319, 167. (7)Marvel, E. N.; Kaple, G.; Delphey, J.; Platt, N.; Polston, N.; Tashiro, J. Tetrahedron 1973,29, 3797.
(8)Hiickel, W.; Krausa, W. J . Liebigs Ann. 1962, 654, 49. (9) (a) Weidner,U.; Schweig, A. Angew. Chem.,Znt. Ed. Engl. 1972, 11, 146. (b) Unwalla, R. J.; Profeta, S., Jr.; Cartledge, F. K. J . Org. Chem. 1988,53, 5658.
Grignon-Duboiset al.
420 Organometallics, Vol. 14,No. 1, 1995
SI
SI
l1
SI
SI
4 'I
2 6
SI
Figure 3. Shortest Si-.*Sinonbonding distances (A) and important valence bond angles (deg) and bond distances (A). constants for 6 are in good agreement with those reported for 4 and 7 but not 8.2,1 In order to have a better idea of the conformational behavior of 6 without the crystal packing effect, we performed a molecular mechanics study. In particular, it appears interesting t o examine angle valence deformations and conformational preferences due to steric hindrance among the six silyl groups. The MM3 force field4 was chosen as giving the best results after comparison between crystallographic and computational structures built ex nihilo for several polysilylated compounds. Indeed, to check the accuracy of this force field toward highly distorted polysilylated derivatives, we have calculated the 1,2,3,4,5,6-hexakis(trimethylsilyl)benzene molecule, whose X-ray structure has been described by Sakurai et al.1° It is worth noting that even in this extreme case, we found a rms of 0.2 A if one considers the superimposition of all the silicons and aromatic carbons. The optimized structure obtained for 6 has been compared to its X-ray structure. Superimposition of all the heavy atoms led to a rms value of 0.066 A, showing that the conformation in the crystal is very close to the lowest energy one. This is well demonstrated by the dienic twisting angles, which are respectively 23.7 and 22.8". In order to systematically explore the conformational space of 2 and 6, we performed both a stochastic dynamics simulationll and a Monte Carlo conformational search12starting from the X-ray structure for 6 and a local minimum for 2. The comparison of the calculated energy values shows that 6 (97.6 kJ/mol) is more stable than 2 (121.5 kJ/mol). This difference is essentially due to the greater van der Waals contribution in 2. Monte Carlo calculations (10)Sakurai, H.; Ebata, K.; Kabuto, C.; Sekiguchi,A. J . A m . Chem. SOC.1990,112,1799. (11)van Gunsteren, W.F.; Berendsen, H. J. C. Mol. Simul. 1988, 1, 173. (12)(a) Chang, G.; Guida, W. C.; Still, W. C. J . Am. Chem. Soc. 1989, 111, 4379. (b) Saunders, M.;Houk, K. N.; Wu, Y.-D.; Still, W. C.; Lipton, M.; Chang, G.; Guida, W. C. J . Am. Chem. SOC.1990,1121419.
performed on 7 and 8 have confirmed that the s-trans structure (10.43 kJ/mol) is more stable than the s-cis isomer (42.11 kJ/mol). Both 2 and 6 present four silyl groups in allylic positions. Comparison of the lowestenergy conformers of 6 showed that two C1-Si and C5Si bonds are in a perfect perpendicular orientation versus the double bond, which leads to a maximum stabilizing ?G i n t e r a ~ t i o n .In ~ contrast, this arrangement is no longer possible for 2, due to the presence of four successive silicon groups on the same cycle. In this case, only the C1-Si bond can adopt a similar, but less favorable (84"), orientation. From a mechanistic point of view, this conformational study of hexasilylated regioisomers shows that the double-1,2,3-substitutedarrangement, which avoids severe spatial hindrance, is thermodynamically more favorable than the 1,2,3,4,6,7-one. Indeed, the former geometry allows arrangement of all the bulky silyl groups alternatively above and below the ring plane with only trans axial-axial or axial-equatorial interaction. This is not possible when four silyl groups are borne by the same ring, which leads to overcrowding due to a vicinal trans-diequatorial relationship. This conclusion is exemplified by the results we previously obtained with quinoline2and i~oquinoline,~ demonstrating that reductive silylation of two-fused six-membered aromatic rings follows the same general route described in Figure 4. Actually, the 1,2,6,7-tetrasilylintermediate had been isolated in the case of naphthalene.lb Concerning the stereochemistry of the three disilylation steps, examination of the X-ray structure of 6 shows that the two 1,2-additions are trans, whereas the last 1,6addition is cis. In the case of quinoline, we came to the same conclusion on the basis of lH NMR analysis.2 To fully rationalize these results, a study associating MM3 calculations and X-ray and NMR data is now in progress for a series of polysilylated derivatives.
Polysilylation of Naphthalene Revisited
Organometallics, Vol. 14,No. 1, 1995 421
SiMe, ~ Me$
X=H Y =Y H= N X=NY=H
Me3%
'T
~ M~~S~&SW e ~ Me3Si
i
Me3Si
'%SiMe3
,
rrY**y,SiMe3 SiMe3
SiMe3
Figure 4. General pathway of the reductive hexasilylation of naphthalene and its heterocyclic analogues. Experimental Section Melting points were determined on a Metler capillary apparatus and are uncorrected. IR spectra were obtained on a Perkin-Elmer 457 spectrophotometer. UV spectra were recorded on a Varian DMS 90 (double beam) apparatus. A base line correction program was performed before each run. The lH and 13CNMR spectra were recorded on a Bruker AM 250 spectrometer (CDCl3solutions,TMS as internal standard), and 29SiN M R spectra, on a Bruker AC 200. Mass spectra were measured on an AEI MS 12 spectrometer. Elemental analyses were performed by service central d'analyse du CNRS (F-69390 Vernaison, France). Materials. Unless specifiedotherwise, reagent-grade chemicals were used as received. HMF'A (Aldrich) was degassed before use by an ultrasonic cleaning bath. Trimethylchlorosilane was distilled from magnesium powder prior to use. Reductive silylation was carried out under an argon atmosphere by employing vacuum line techniques. Silylation of Naphthalene. To a vigorously stirred suspension of magnesium (1.59 g; 6.6 x mol) in HMPA (65 mL) and freshly distilled trimethylchlorosilane (21.4 g; 198 mmol), a solution of naphthalene (3.84 g; 30 mmol) dissolved in HMF'A (10 mL) was added dropwise within 0.5 h. During addition, the temperature was kept at 85 "C. The mixture was then stirred for 24 h at this temperature and then cooled to room temperature. Addition of 50 mL of cyclohexane led to precipitation of the MgCld2HMPT complex, which was filtered off. f i r evaporation of the solvent, the crude product was distilled under reduced pressure, providing 1,4-bis(trimethylsilyl)-1,4-dihydronaphthaleneas a colorless liquid (bp 82 "C (0.2 mmHg), 2.3 g, 28%). The remaining yellow viscous oil (10.4 g) was crystallized from ethanol to give 6 as yellowish crystals (7.2 g; 42%). N M R Data for l,l-Bis(trimethylsilyl)-l,4-dihydronaphthalene. IH (CDC13, 250 MHZ): 0.0 (s, 18 H), 3.0 (d, 2 H, J 1.6 Hz),5.6 (d, 2 H, J 1.6 Hz), 6.9-7.0 (m, 4 H). (CDCl3, 62.9 MHz): SiMe3, -2.04; CH, 34.04, 123.6, 124.8, 128.0; C, 135.3. 29Si(CDC13, 39.73 MHz): 3.54. Structural Data for Compound 6. Mp 116 "C (capillary), 1it.lmp 116 "C. U V (C6H12): 1,273 nm (13 500). lH (CDCl3, 250 MHz): -0.03 ( 8 , 9 H), 0.06 (s,9 H),0.12 ( 8 , 9 H), 1.27 (d, 1H, J 0.8 Hz), 1.85 (broad s, 2 H), 5.24 (d, 1H, 2.7 Hz). I3C (CDCl3, 62.9 MHz): SiMe3, -2.4, -0.6, -0.4; CH, 15.7, 25.7, 30.1,121.15; C, 129.2. 29Si(CDC13,39.73MHz): 3.4; 4.15; 4.9. Anal. Calcd for CzsH,jzSi,j: C, 59.28; H, 11.02; Si, 29.70. Found: C, 59.02; H, 11.23; Si, 29.21. X-ray Crystallography of Compound 6. A prismatic single crystal of C2sH6zSie ( f k= 567.32) having approximate dimensions of 0.25 x 0.25 x 0.30 mm was obtained at room temperature from ethanol solution and mounted on a glass fiber in a random orientation. The intensity data collection was performed at 25 "C on an Enraf-Nonius CAD-4 diffractometer. The data were measured with the d 2 8 scan technique and a variable scanning rate, using graphite-monochromated Cu Ka radiation. A total of 3967 reflections were collected, of
Table 1. Positional and Thermal Parameters and Their Estimated Standard Deviations atom
X
Y
Z
B (AZ)"
Si1 Si2 Si3 Si4 Si5 Si6 c1 c2 c3 c4 c5 C6 c7 C8 c9 c10 c11 c12 C13 C14 C15 C16 C17 C18 C19 c20 c 21 c22 C23 C24 C25 C26 C27 C28
0.2425(2) 0.4839(2) 0.3496(3) 0.0725(2) 0.2541(2) 0.1485(2) 0.2706(5) 0.2564(5) 0.2198(6) 0.1905(5) 0.1834(5) 0.2467(5) 0.2860(6) 0.3575(6) 0.3758(6) 0.3127(5) 0.3006(9) 0.1951(8) 0.1602(8) 0.247(1) 0.371l(8) 0.425(2) 0.0301(7) 0.0104(8) 0.0687(8) 0.3604(7) 0.2585(9) 0.21 19(8) 0.08 l(2) 0.0859(9) 0.210(1) 0.5127(7) 0.4836(7) 0.5618(8)
0.0683(5) -0.0064(5) -0.3565(6) -0.1299(5) -0.361(5) 0.3012(5) 0.033(1) 0.147(1) 0.154(1) 0.017(1) -0.098( 1) -0.088(1) -0.196(1) -0.208( 1) -0.074( 1) 0.038(1) 0.062(2) 0.230(2) -0.048(2) -0.382(2) -0.499(2) -0.354(2) 0.002(2) -0.155(3) -0.280(2) -0.07 l(2) 0.088(2) -0.186(2) 0.292(2) 0.339(2) 0.453(2) 0.1 16(2) 0.071(2) -0.140(2)
0.8714(1) 0.8549(2) 0.8598(2) 0.667 l(1) 0.58OO(1) 0.6343(2) 0.7579(4) 0.7263(4) 0.6588(4) 0.6284(4) 0.6690(4) 0.7296(4) 0.7561(4) 0.8 138(4) 0.8474(4) 0.8247(4) 0.9485(6) 0.8536(6) 0.8606(6) 0.8667(6) 0.8231(6) 0.9335(7) 0.7003(6) 0.5929(6) 0.7061(8) 0.6274(5) 0.5274(5) 0.5380(5) 0.5623(9) 0.6790(9) 0.6402(7) 0.9 140(6) 0.7860(6) 0.876l(7)
9.7(1) 9.6(1) 10.4(2) 9.6(1) 8.4(1) 10.0(2) 6.4(4) 6.2(4) 6.4(4) 6.7(4) 6.8(4) 6.2(4) 6.5(4) 7.7(4) 7.3(4) 7.0(4) 19.3(7) 12.6(6) 14.4(6) 19.1(8) 11.3(5) 22(1) 13.4(6) 2%1) 15.1(6) 12.0(5) 12.3(6) 12.2(5) 34U) 18.2(8) 16.7(7) 11.8(6) 15.5(7) 15.9(7)
LI B values for anisotropicallyrefined atoms are given in the form of the isotropic equivalent displacement parameter defined as (4/3)[a2B(1,1) f b2B(2,2) czB(3,3) ab(cos y)B(1,2) ac(cos ,8)B(1,3) f bc(cos a)B(2,3)1.
+
+
+
which 3922 were unique and not systematically absent; 2123 were used for calculation (F L 3a(F)). The intensities were corrected for Lorentz and polarization effects but not for absorption. As for many polysilylated derivatives, the crystals proved to be unstable under X-ray radiation. In the first experiment, after a total exposure time of 75 h, the total loss in intensity was 41.1%. A second crystal was chosen and measured but did not result in improved data. This fact explains the large B factors for some methyl groups ((211, C14, C16, C18, C23). The structure was solved by direct methods using the MULTAN 80 program13for electron density synthesis. Blockdiagonal matrix least-squares refinements were performed for a scale factor and positional and anisotropic thermal parameters of the non-hydrogen atoms. The hydrogen atoms were included in the calculations and refined with isotropic thermal parameters. The function minimized was Xw(lFoI - IFc1)2,and
Grignon-Dubois et al.
422 Organometallics, Vol. 14, No. 1, 1995
in the program: X-ray structures were minimized to a final rms gradient 50.005 kJl(mo1.A) via the truncated Newton conjugate gradient (TNCG) method (1000 cycles). Stochastic Dynamics Simulation. This variant of molecular dynamic (forces from the force field are increased by frictional and random forces, which simulate some of the properties of a solvent medium) is implemented in Macro2. Model.ll The MM3* (91) force field was chosen. The kinetic X-ray Crystal Structure Data for 6: Monoclinic, space energy was increased from 300 t o 900 K with a bath constant oup P21/c, a = 17.077(4) A, b = 10.102(3)A, c = 23.832(7) 0.2 ps. A total time of run of 50 ps (time step 1 fs) was B = 109.05(2)",V = 3886(1) A3, 2 = 4, D, = 0.970 g ~ m - ~ , of chosen, one conformer was sampled each 1 ps and minimized p = 21.3 cm-I, no. of variables = 555, R = 0.081, R, = 0.072, with the truncated Newton conju ate gradient method (TNCG, and S = 1.17. 1000 cycles, rms ~0.005 kJ/(moE I), and all the unique (heavy Electronic Spectral Calculations. The UV spectra were atoms only) conformers within a 50 kJ energy range were calculated according the ZINDO/S method of M. Zerner, as reported and classed by ascending energy. The run used the implemented in HyperChem Version 3 for Windows (Autodesk, minimized X-ray conformation as starting geometry for 6 and Inc.). Configuration interaction was first calculated with the a local minimum for 2. Hamiltonian AM1, RHF spin pairing, total charge = 0, and Monte Carlo-Style Conformational Search. This prospin multiplicity = 1 for lowest state. Then the UV spectral gram has been used as implemented in MacroModel.12 The calculations were performed using the following overlap automatic setup has been selected, Le., single bonds variable, weighting factors: 1.267 for u-d7 and 0.585 for J C - J C . ~ ~ The chiral center set, flexible ring opened, and 1000 steps made accuracy of the calculations was checked versus ,A of per input structure in a 50 kJ energy range. Each conformer cyclohexene (found, 182 nm; calc, 174.4 nm) and 1,2-dimethwas fully minimized (TNCG, 1000 cycles, rms, ~ 0 . 0 0 5kJ/ ylcyclohexene (found, 194 nm; calc, 186.8 nm). Under these (mol&). The least-used structures were used as starting conditions the following results were obtained cyclopentene, geometries only if their energies were within the energetic 172.99 nm; 7, 269.15, oscillator strength 0.36 (measured 263 window (50 kJ/mol of the lowest energy structure yet found). For 6, 47 conformers were found, but due to equivalencies nm, 4830);68,239.46, oscillator strength 1.048 (measured 237 of the methyl groups borne by silicon atoms, only 11 truly nm, 14 600,8or 242 nm, 17 400;7s-trans dienic framework in different classes of conformers were reported from 97.58 to the crystalline conformation of 6,235.87 nm, oscillator strength 118.3 kJ/mol, the first one being the X-ray structure. 0.888. For 2,44conformers were found, but as above, only 9 truly Molecular Modeling. Calculations were performed on a different classes were reported from 121.5 to 139.9 kJ/mol. SGI Indigo platform running Macromodel version 3.5 (ColumThe s-cis and s-trans nonsilylated dienes 7 and 8 were bia University, New York).l9 Conformational minima were treated as above, resulting in two conformers(10.43-11.04 kJ/ found using the modified MM3* (91)force field as implemented mol) with 7 and four conformers (42.11-55.62 kJ/mol) with 8. the weight w is defined as 4F3/u2(FJ2.The scattering factors used for non-hydrogenatoms were taken from ref 14, and those for hydrogen atoms, from Stewart et al.16 All calculationswere performed on a MicroVax computer using the MolEN program.16 Atomic parameters are given in Table 1. The molecule and its atom numbering scheme are shown in Figure
1
(13)Main, P.; Hull, S. E.;Lessigner, L.; Germain, G.; Declercq, J.P.; Woolfson, M. M. MULTAN 80: A System of Computer Programs for the Automatic Solution of Crystal Structures from X-Ray Diffraction Data. Univs of York, England, and Louvain, Belgium, 1978. (14)International Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV.(Present distributor: D. Reidel, Dordrecht, The Netherlands.) (15)Stewart, R. F.; Davidson, E. R.; Simpson, W. T. J. Chem. Phys. 1965,42, 3175. (16) MolEN is an interactive structure solution procedure (EnrafNonius, Delft, The Netherlands, 1990). (17) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1976, 42, 223. (18) Del Bene, J.; Jaffe, H. H. J . Chem. Phys. 1968,48, 1807.
Supplementary Material Available: Tables of bond distances, bond angles, torsion angles, hydrogen positional and thermal parameters, anisotropic thermal parameters, and least-squares planes (17 pages). Ordering informationis given on any current masthead page. OM9405955 (19) Macromodel (Mohamadi, F.;Richards, N. G. J.; Guida, W. C.; Liskamp, R.; Lipton, M.; Caufield, C.; Chang, G.;Hendrickson,T.; Still, W. C. J. Comput. Chem. 1990,11, 441).
