J. Am. Chem. SOC.1981, 103, 5363-5372 that for CI-CI so that our measurements with their large uncertainties provide no experimental evidence on the point. Some supporting evidence is available in the results of X-ray diffraction 2-dione,I5a molecule investigation of [4.4.2]propella-3,8-diene-l1,1 with a CBD subunit, and trans-3,4-di-tert-butylcyclobutane1,2-dione” The first of these has distances of 1.551 (3) A, 1.534 (3) A, and 1.572 (3) A for the bonds corresponding to C&, C1-C4, and C -C4 in CBD; the second has distances of 1.560 (3) A, 1.527 (2) and 1.560 (3) A. Thus the CI-C1 type distance is in each case longer than normal and the CI-C4 type smaller than its neighbors. The C=O bond length in CBD (rg = 1.194 (2) A) is about 0.02 A shorter than those found in aliphatic ketones. We attribute the shortening to diminished nonbond repulsions involving the C=O and adjacent bonds, and the oxygen atom and geminal carbons. Since the proximity of the opposing bonds and atoms depends most importantly on the CCC bond angle at the base of the carbonyl group, the C = O bond length should decrease as this angle decreases. This is observed to be the case: In CBD and
A,
(15) Nahlovsky, Z.; Nahlovsky, B.; Strand, T. G. Actu Chem. S c u d . 1970, 24. 2617.
5363
in the similar propelladienedione and tert-butyl compounds mentioned above the C=O bond lengths are respectively 1.194 (2) A, 1.192 (2) A, and 1.191 (2) A, (C=O base angles -90’); and in 2-cyclopentane-1 ,4-dione,I8 and p - b e n z o q ~ i n o n e(~ ~ C 4 base angles respectively 108.2 (4)’ and 118.1 (3)’) the C=O bond lengths are 1.208 (2) 8, and 1.225 (2) A. The measured amplitudes of vibration are generally in good agreement with those calculated from our assumed force field. The agreement is poorest for the ring bonds. However, we attribute no special significance to this fact because the values of these amplitudes are highly correlated with the values of the distances which bear large uncertainties.
Acknowledgment. We are grateful to Professor J. M. Conia for providing us with the sample of CBD, to Lise Hedberg for advice with the force field calculations, and to the National Science Foundation which supported this work through Grant No. C H E 74-13527 and No. C H E 78-04258. Supplementary Material Available: Tables of total intensities, final backgrounds, average molecular intensities, symmetry coordinates, force constants, and calculated frequencies (1 1 pages). Ordering information is given on any current masthead page. (18) Hagen, K.; Hedberg, K. J. Mol. Struct., 1978, 44, 195. (19) Hagen, K.; Hedberg, K. J . Chem. Phys. 1973, 59, 158.
Conformational Studies of Dihydrotetraphenylmethanes. 1. X-ray Crystallographic and Solution IH NMR Studies of trans- 1,4-Dihydr0-4-tritylbiphenyland Its 4’-Bromo Derivative: Boat-Boat Inversion in a Congested Cyclohexa- 1,4-diene Anthony K. Cheetham,*+Martin C. Grossel,*t and John M. Newsamt Contribution from the Chemical Crystallography Laboratory, 9 Parks Road, Oxford, OX1 3PD, England, and the Department of Chemistry, Bedford College University of London, Regent’s Park, London, NWI 4NS, Enland. Received January 9, 1981
Abstract: Single-crystal X-ray diffraction measurements on 1,4-dihydro-4-tritylbiphenyland on two crystal modifications of its 4’-bromo derivative reveal that the three molecular structures are remarkably similar, the only variation being the sense of puckering of the cyclohexa-1,4-dienering. Crystals of 1,4-dihydro-4-trit lbiphenyl are orthorhombic, space group Pnam, Z = 4, lattice parameters a = 10.083 (l), b = 12.128 (2), c = 18.247 (5) 1201 independent reflections gave a final R of has space group Pnam, Z = 4, a = 13.012 (2), b 0.050. The orthorhombic form of 4’-bromo-l,4-dihydro-4-tritylbiphenyl = 17.264 (3), c = 10.431 (1) A; 894 independent reflections gave a final R of 0.046. In contrast, the monoclinic form has space group Pc, Z = 2, a = 9.01 1 (l), b = 16.187 (3), c = 8.463 (2) A, p = 108.98 (1)’; 1205 independent reflections gave a final R of 0.041, Comparison of the three structures suggests that the cyclohexa-1,4-diene ring in 1,4-dihydrc-4-tritylbiphenyls exists in either a shallow potential well with a single minimum corresponding to an essentially planar cyclohexadiene geometry or a double-minimum potential well in which there is rapid boat inversion. There is no evidence for the highly puckered boat conformation previously suggested. A reassessment of the ‘H NMR data for these compounds also supports this view. The close similarity of the molecules analyzed provides a rare opportunity for the evaluation of the relative importance of intramolecular interactions and intermolecular or “packing” forces on the solid-state geometries of these molecules. It is concluded that while the steric requirements of the dihydrotetraphenylmethane molecule prescribe an essentiallyplanar cyclohexa- 1,4-diene geometry, the exact conformation of the ring is controlled by crystal packing requirements.
1;
Introduction L ~ IH NMR~coupling constants ~ provide a convenient ~ ~ probe into the solution conformations of organic molecules.’ In principle, for a derivative of cyclohexa- 1+diene (1),2 an analysis of vicinal couplings could indicate the overall ring conformation. Chemical Crystallography Laboratory. t Bedford College University of London.
However, such couplings are generally small when saturated carbon~ centers are ~ involved,~ and the effect of large variations in ring geometry may be reflected by only slight changes in the magnitudes of such short-range interaction^.^ Allylic couplings: (1) Sternhell, S. Q. Reu. Chem. SOC.1969, 23, 236. (2) These will be numbered as 1,4-dihydrobenzenes for convenience. (3) Macial, G. E.; McIver, J. W., Jr.; Ostlund, N. S.;Pople, J. A. J . Am. Chem. SOC.1970, 92,4491.
0002-7863/81/1503-5363$01.25/0 0 1981 American Chemical Society
5364 J . Am. Chem. SOC.,Vol. 103, No. 18, 1981 over four bonds, are potentially more useful. For cyclohexadiene derivatives there is good agreement between observed and calculated coupling constants so that measured values may be interpreted in terms of molecular g e ~ m e t r y . ~ -However, ~ these interactions are relatively insensitive to the extent of ring puckering when this becomes large,5 and, more seriously, the small values of such coupling constants limit the precision with which they can be determined experimentally. Considerable emphasis has therefore been placed on the interpretation of homoallylic couplings.* Homoallylic coupling occurs over five bonds and thus encompasses the entire cyclohexadiene ring framework. It attains a maximum value when the bonds to the coupled protons are nearly parallel.e11 Fortunately, this maximum is apparently of the order of 12 which is substantially larger than the maximum allylic coupling (ca. 3 Hz) and can be measured experimentally with good precision. Further, this coupling varies relatively uniformly with the extent of ring puckering which can be defined by the angle We may distinguish four coupling pathways [see 11, though for a symmetrically puckered ring the two trans
pseudo-axial-pseudo-equatorial couplings are identical. For a planar cyclohexa-l,4-diene, 5Jcisis thought to be ca. 9.2-9.6 H Z , ~ * ' ~ - " ,rising ' ~ - ' ~to a maximum of ca. 12 H z for CY = ca. 145°.8,9,"J2 5Juans,however, is significantly smaller and values in the range 7.1-8.04 H z have been reported for supposedly planar derivatives." In addition 5Jhar.falls with increasing ring puckering, having a value of 4.3 H z when CY = ca. 145O.I2 Since 5Jcisand SJtrans vary in different senses with CY,the ratio of the two can provide a more accurate indicator of the ring conformation than the actual magnitudes of the couplings themselves?JlJ5 Calculated homoallylic coupling constants as yet show only a qualitative agreement with e~periment.~.l~.",'~,'~ We have determined the exact molecular geometries of systems in which homoallylic coupling may be accurately measured. This has allowed us to establish a direct correlation between coupling constant values and dihydroaromatic ring conformations in rigid structures. However, we have also found that solid-state conformations of flexible, 1,4-dihydrobenzoic acids are closely related to those (4) Barfield, M.; Dean, A. M.; Fallick, C. J.; Spear, R. J.; Sternhell, S.; Westerman, P. W. J. Am. Chem. SOC.1975, 97, 1482. ( 5 ) Grassel, M. C.; Hayward, R. C. J. Chem. SOC.,Perkins Trans. 2 1976, 851. (6) Cheetham, A. K.; Newsam, J. M.; Grossel, M. C.; Hayward, R. C. J . Chem. SOC.,Perkin Trans. 2 1977, 1153. (7) Grossel, M. C.; Cheetham, A. K.; James, D.; Newsam, J. M. J . Chem. Soc., Perkin Trans. 2 1980, 471. ( 8 ) Rabideau, P. W. Acc. Chem. Res. 1978, Zl, 141. (9) Grossel, M. C.; Perkins, M. J. J . Chem. SOC.,Perkin Trans. 2 1975, 1544. (10) Marshall, J. L.; Faehl, L. G.; McDaniel, C. R., Jr.; Ledford, N. D.; J . Am. Chem. SOC.1977, 99, 321. (1 1) Grossel, M. C. Tetrahedron Lett. 1980, 2Z, 1075. (12) Rabideau, P. W.; Paschal, J. W.; Patterson, L. E. J. Am. Chem. Soc. 1975,97, 5700. (13) The angle a is that between the planes defined by the double bonds of the dihydrobenzene ring, Le., between the planes C(l)-C(2)4(3)-C(4) and C(4)4(5)-C(6)-C(l) in 4 (see ref 9). (14) Grossel, M. C.; Perkins, M. J. Nouu. J . Chim. 1979, 3, 285. (IS) Rabideau, P. W.; Paschal, J. W.; Marshall, J. L.; J . Chem. SOC.. Perkin Trans. 2.
(16) Barfield, M.; Sternhell, S. J . Am. Chem. SOC.1972, 94, 1905.
Cheetham et al.
Figure 1. Factors controlling the shape of the cyclohexa-1,4-diene potential energy well (after Laane and Lord"): (i) torsional strain, (ii) angle strain, (iii) overall effect.
adopted in solution7 despite possible perturbations arising from packing effects in the crystal. One effect which may complicate the interpretation of long time-scale spectroscopic data (e.g., 'H NMR spectra) is that due to thermal ~ibration.9,'~ The shape of the potential well occupied by the cyclohexa-l,4-diene ring is governed mainly by two factors, namely, torsional strain about the C(l)-C(2), etc., bonds and angle strain (see Figure l).I7 The former favors a planar structure while the latter is minimized in a boat conformation having a = ca. 150". The combination of these two effects apparently produces a shallow single-minimum potential energy well in which the lower vibrational levels may be readily populated thermally. Indeed far-IR measurements on cyclohexa-lQ-diene itself17suggest that at room temperature in the gas phase there is a significant population of energy levels corresponding to large amplitude boat boat vibrations about a planar energy-minimum geometry. A similar situation is believed to exist in s ~ l u t i o n ,for ' ~ the magnitude of 'Jtrans about 1,4-dihydrohexadeuteriobenzenevaries with temperature in a manner which suggests that the population of the higher vibrational levels is reduced at lower temperatures, resulting in a more nearly planar time-averaged geometry.I4 The introduction of substituents will distort this picture and may lead to a steepening of the potential well, or a shift in the energy minimum to a puckered conformation (as, for example, in 2,6-dimethyl1,4-dihydrobenzoic acid7), or give rise to a double minimum potential energy s ~ r f a c e . ' ~ *This ' ~ latter situation is found for 1,4-dihydronaphthalenes and 9,1O-dihydroanthra~enes.~*~~~**~ However, substituents may also directly affect the magnitude of the observed homoallylic coupling through electronic interact i ~ n s . ~ * ' JC-1 ~ J ~substitution exerts little influence on the homoallylic co~plings,69~ but substituents on the vinylic carbon atoms may modify the n-electron density by mesomeric or inductive effect^.^.^^ Such influences have served merely to confuse the experimental interpretation^.^,^-'^ A large single substituent can also greatly modify the shape of the cyclohexa-1,4-diene potential energy well. For example, the bulky trityl group could act as a conformational lock, and it has been suggestedz4 that with this (17) Laane, R.; Lord, R. C. J. Mol. Specfrosc. 1971, 39, 340. (18) Ermer, 0. Strucf. Bonding (Berlin) 1976, 27, 196. (19) Allinger, N. L.; Sprague, J. T. J . Am. Chem. SOC.1972, 94, 5734. (20) Rabideau, P. W.; Burkholder, E. G.; Yates, M. J.; Paschal, J. W. J. Am. Chem. Soe. 1977, 99, 3596. (21) Marshall, J. L.; Folsom, T. K. J. Org. Chem. 1971, 36,2011. Marshall, J. L.; Ihrig, A. M.; Jenkins, P. N. J . Org. Chem. 1972, 37, 1863. (22) Marshall, J. L.; Faehl, L. G.; Ihrig, A. M.; Barfield, M. J. Am. Chem. SOC.1976,98, 3406. (23) Heilbronner, E.; Brogli, F.; Vogel, E. J . Electron. Specfrosc.Relat. Phenom. 1976, 9, 227. (24) Durham, L, J.; Studebaker, J.; Perkins, M. J. J . Chem. SOC.,Chem. Commun. 1965,456.
J. Am. Chem. SOC.,Vol. 103, No. 18, 1981 5365
Structural Studies of trans-1,4-Dihydro-4-tritylbiphenyls group located at C- 1 , a cyclohexadiene ring would be forced into a highly puckered boat geometry with the trityl group adopting a pseudo-equatorial position. 'H N M R data on several 1,4-dihydro-4-tritylbiphenyls were considered to support this view.z For example, the measured cis-homoallylic coupling for 2a was re-
Table I. Experimental Details for the Three X-ray Crystallographic Analyses parent
crvstal size
ported to be ca. 11 Hz, a value which was thought to be consistent with a highly folded ring geometry.25 In a more recent analysis of the spectrum of 2a, however, a slightly smaller value of 10.5 H z was found.26 When this is compared with the corresponding couplings in hexadeuterio- 1 ,Cdihydrobenzene (Jcis= 9.6 H Z ) ~ ' and 1,4-dihydropentadeuteriobenzoicacid (Jh = 9.2 Hz),'O both of which are believed to be essentially planar, doubt is cast on the original suggestion that the dihydroaromatic ring is highly puckered in the dihydrotritylbiphenyls. Further, the spectra of the cis isomers 2a and 3a show a remarkably high-field two-proton aromatic absorption (at ca. r 3.8) assigned" to the ortho protons of the isolated phenyl substituent, the presence of which cannot be convincingly explained through conformations in which the substituents are placed in a pseudo-equatorial environment. In view of these ambiguities we have reexamined in detail the solution IH N M R spectra of the cis and trans isomers of 1,4dihydro-4-tritylbiphenyl (2) and their 4'-bromo derivatives (3). W e have also completed X-ray crystallographic studies on each of these materials. Preliminary reports of the results obtained for the cis and trans isomers of the parent hydrocarbon (2) have interesting feature of the already a ~ p e a r e d . ~A~particularly ,~~ crystal structure of the trans system (2a) is that, contrary to previous suggestions, the trityl substituent occupies a pseudo-axial position. For both isomers (2) the cyclohexadiene ring is almost planar (ack= ca. 175°,28atrm = ca. 1 7 2 O *'). This is in marked contrast to the preliminary results of an earlier crystallographic study of the trans-4'-bromo derivatives (3b) in which the trityl substituent was found to occupy the pseudo-equatorial position of a more highly puckered cyclohexadiene ring (a = 165°).25,30 In order to obtain more accurate structural information, we have reexamined the structure of this material. Further, as 4'bromo-trans-l,4-dihydro-4-tritylbiphenyl(3b) occurs in two distinct crystalline forms, it presents an unusual opportunity for examining the same molecular system in differing crystal environments. In this paper we present the results of solution 'H N M R studies and single-crystal X-ray measurements on both modifications of 3b together with a detailed discussion of the results from the parent hydrocarbon (2b). Details of our work on the cis isomers 2a and 3a will follow.31
Experimental Section The three sets of intensity data were collected on an Enraf-Nonius CAD-4F diffractometer3*and, to optimize clarity, the basic experimental details for each analysis are tabulated in Table I. Salient features of the analyses are described below. All crystallographic calculations were performed on the Oxford University 1906A computer, using the CRYSTALS suite of programs." Atomic scattering factors were taken from Cromer and Mann34 and S ~ a g n a(H~ only). ~ In the final cycles of (25) Atkinson, D. J.; Perkins, M. J. Tetrahedron Lett. 1969, 2335. (26) Benati, L.;Tiecco, M.; Tundo, A.; Taddei, F. J . Chem. SOC.B 1970, 1443. (27) Garbisch, E. W., Jr.; Griffith, M. J. J. Am. Chem. SOC.1968, 90, 3590; 1970, 92, 1107. (28) Grossel, M. C.; Cheetham, A. K.; Hope, D. A. 0.; Lam, K. P.; Perkins, M. J. Tetrahedron Let?. 1979, 1351. (29) Grossel, M. C.; Cheetham, A. K.; Newsam, J. M. Tetrahedron Lett. 1978. 5229 -. . -, __ - . (30) Atkinson, D. J. Ph.D. Thesis, University of London, 1969. (31) Cheetham,.A. K.; Grossel, M. C.; Hope, D. A. O., in preparation. (32) Enraf-Nonius CAD-4F Diffractometer User Handbook, Enraf-Nonius, Delft, Holland, 1977. (33) Carruthers, J. R.CRYSTALS User Manual, 1975, Oxford University Computing Laboratory.
OTBR
MTBR
no no hOl, h = 2n h01, I = 2n Okl;k + I = Okl, k + I = 2n 2n 0.3 mm cube 0.25 mm cube 0.4 X 0.2 X 0.1 mm
photographic survey yes reflection conditions h01, h = 2n
unit cell definitiona no. of reflections 6 range data collection max e geometry mode scan type, w - n e , n= no. of measd reflections data reduction corrections
22 25 15 < e < 18" 16 < e
22
< 18" 15 < e < 19"
25" bisecting
25" bisecting
1.67
1.33
26" FLAT (see text) 1.33
3072
3247
2924
Lorentz Lorentz polarization polarization rejection criterion I G 3oIb I G 3oI no. of independent 1201 894 structure factor moduli structure refinement least-squares full full matrix hydrogen XYZ xyz, U[iso] refinement no. of variables 200 220 66 65 no. of H constraints R 0.0496 0.0458 Rw 0.0575 0.0543
Lorentz polarization I < 301 1205
blockdiagonal xyz
365 96 0.0413 0.0443
a Data relating to the reflections used in the least-squares optimization of the unit cell parameters and orientation matrix, 01 refers to the standard deviation derived from the counting statistics.
refinement, truncated three-term Chebyshev polynomials were used as the weighting schemes>6 and, in all three cases, there was no evidence for secondary extinction and corrections for anomalous dispersion and absorption were unnecessary. (a) Parent Hydrocarbon: tra~-1,4-Dihydro-Qtritylbiphenyl. Crystal data: C31H26M = 398, orthorhombic, space grou Pnam, a = 10.083 ( l ) , b = 12.128(2),c= 18.247(5)A,U=2231.41,'Z=4,dO= 1.17, d, = 1.18 g c m 3 , Mo Ka radiation, X = 0.71069 A, ~r.= 0.7 cm-'. The parent hydrocarbon (2b) was prepared as previously described3' and recrystallized from light petroleum (bp 60-80 "C) in the form of rhombs. The systematic absences (Table I) are consistent with twoo space group, Pnu2, and Pnum. For structure solution, in the expectation that intramolecular symmetry would be absent, the noncentrosymmetric ~ partially successful in that apPnaZl was assumed and M U L T A N ~was proximate coordinates for nearly half the nonhydrogen atoms were extracted from one of the resulting phase sets. The remainder of the molecular skeleton was obtained by a combination of least-squares and Fourier calculations. After subsequent refinement, all the hydrogen atoms were located in a difference Fourier synthesis and their coordinates were included as variables in refinement, although subject to a series of Waser-type constraints on bond lengths and angle^.'^,.'^ While refinement was possible, with only limited instability, in Pna2, it became clear that the structure possesses an intramolecular mirror plane. Indeed, Hamilton's significance test" shows that the noncentrosymmetric P n ~ 2 ~ is invalid even at the a = 0.5 level. The centrosymmetric Pnam was
~
(34) Cromer, D. T.; Mann, J. B. Los Alamas Scientific Laboratory Report, LA-3816, 1967. (35) Spagna, R.,personal communication. (36) Carruthers, J. R.; Watkin, D. J. Acta Crysrallogr., Sect. A 1979, 35, 698. (37) Hey, D. H.; Perkins, M. J.; Williams, G. H. J . Chem. SOC.1965, 110. (38) Germain, G.; Main, P.; Woolfson, M. M. Acta Crystallogr., Sect. A 1971, 27, 368. (39) Waser, J. Acta Crystallogr., Sect. A 1963, 16, 1091. (40) Hamilton, W. C. Acta Crystallogr., Sect. A 1965, 18, 502.
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J . Am. Chem. SOC.,Vol. 103, No. 18, 1981
therefore taken to be the correct choice of space group. The final residuals were R = 0.0496 and R, = 0.0575. (b) Orthorhombic Form of 4’-Bromo- ~-1,4-dihydro-4-tritylbiphenyl (OTBR). Crystal data: C31H25Br,M = 447.4, orthorhombic, space group Pnum, u = 13.012 (2), b = 17.264 (3), c = 10.431 (1) A, U = 2343.2 A’, d, = 1.35 g cm-3 for Z = 4; Mo K a radiation, X = 0.71069 A, p = 18.8 cm-I. A crystalline sample of the 4’-bromo derivative (3b) was kindly supplied by K. P. Lam and M. J. Perkins. This sample had been prepared as previously described” and was recrystallized from light petroleum (bp 60-80 “C). An optical examination of the sample revealed the occurrence both of rhombs and of clusters of elongated plates, but no needles were apparent. A well-formed rhomb was selected and, as the unit cell derived on the diffractometer was in good agreement with that reported by Atkin~on,’~ we proceeded to collect intensity data (Table I). The position of the bromine atom was deduced from a Patterson synthesis and, using this for a phasing model, all of the anticipated carbon atoms were located in a lFol Fourier synthesis. The systematic absences (Table I) imply that this system suffers from the same space group ambiguity as the parent hydrocarbon, although, in this case (presumably because of the heavy weighting by the bromine), refinements in Pna2, were unstable. In the subsequent analysis we found no evidence in favor of the noncentrosymmetric symmetry and, therefore, in common with the parent, Pnam was taken to be the correct space group. After refinement, all the expected hydrogen atoms were located in a difference Fourier synthesis. Their coordinates and isotropic thermal parameters (U[iso]) were refined subject to a series of Waser-type constraints on bond lengths and angles. In an additional constraint, the U[iso]’s for H ( l ) and H(4) were linked to the mean-square deviations of C( 1) and C(4), respectively, in the direction of the C-H bond. This was necessary because u[iso] for both these protons had become negative during earlier refinements. The final residuals were R = 0.0458 and R, = 0.0543. (c) Monoclinic Form of 4’-Bromo-frans-l,4-dihydro-4-tritylbiphenyl (MTBR). Crystal data: C31H25Br,M = 447.4, monoclinic, space group Pc, a = 9,011 (l), 6 = 16.187 (3), c = 8.463 (2) A, 6 = 108.98 ( l ) ” , U = 1167.4 A3,d, = 1.36 g cm-: for Z = 2, Mo Ka radiation, X = 0.71069 A, p = 18.8 cm-I. Crystals of the monoclinic form of the 4’-bromo derivative (3b) were taken from a second preparation similar to the above (Le., (b)). Crystallization from light petroleum (bp 60-80 “C) at ca. 25 “ C afforded crystals of two distinct forms, in approximately equal quantities. These consisted of rhombs and well-faceted plates. Again no needles were apparent, although some of the plates were elongated perpendicular to the crystallographic 6 axis. One of the plate-like crystals was selected for analysis on the diffractometer and the derived unit cell is in good agreement with that reported by A t k i n ~ o n(Table ~ ~ I). Because of the shape of the crystal, data collection was implemented using the FLAT mode.32 Each reflection was measured at the value of rotation about the scattering vector which resulted in minimum absorption, subject to some restraints on this rotation imposed by the geometry of the diffractometer hardware. The systematic absences (Table I) are consistent with two space groups, Pc and P2/c. The latter would imply that the molecule has a center of symmetry or an intramolecular twofold axis, neither of which is in accord with the molecular configuration. The noncentrosymmetric Pc was therefore assumed and confirmed by the subsequent analysis. For structure solution, coordinates for the bromine atom were taken from A t k i n ~ o n )and ~ the remainder of the molecular skeleton was elucidated by successive least-squares and lFol Fourier calculations. Pc is a polar space group, in which the position of the molecule may be defined arbitrarily along the x and z axes. To circumvent instabilities in the refinement, therefore, the sum of the shifts along the x and z axes were After further refinement of the atomic each constrained to be zero.33*39 coordinates and anisotropic thermal parameters, all the anticipated hydrogens were found in electron density difference syntheses. Their coordinates were refined, subject to Waser-type constraints as above, although their isotropic temperature factors were fixed at U[iso] = 0.05. The final residuals are R = 0.0413 and R, = 0.0443. Bond lengths and interatomic distances found in the three structures are listed inTable 11, while Table I11 compares interatomic angles. The numbering scheme employed is shown in Figure 2; OTBR and MTBR refer to the orthorhombic and monoclinic forms of 4’-bromo-1,4-dihydro-4-tritylbiphenyl (3b), respectively.
Results and Discussion X-ray Crystallographic Results. In t h e solid state, trans-l,4dihydro-4-tritylbiphenyl (2b) adopts a conformation in which the cyclohexadiene ring has a boat geometry (a- = +172.lo)l3 with the trityl substituent placed pseudo-axially (see Figure 3a).29 In contrast, both crystalline modifications of 4’-bromo-trans-l,4-
Cheetham et ai. Table II. Bond Lengths (A) and Interatomic Distances (A), with Estimated Standard Deviations in Parentheses c(l)-C(7) C(l)-C(2) C(1)-C(6) C(2)-C(3) C(5)-C(6) C(3)-C(4) C(4)-C(5) C(4)-C(11) CU)C(21) C(7)C(31) C(7)-C(4 1) C(7)-C(4) C ( 7 ) - W 1) C(4)-C(41) C( 11)-C(41) C(ll)C(12) C(ll)C(16) C(12)€(13) C(13)C(14) C(14)-C(15) C(15)-C(16) C (14)-Br C(2 1) C ( 2 2 ) C(31)-C(32) C(21)-C(26) C(31)C(36) C(22)C(23) C(32)-C(33) C(23)-C(24) C(33)4(34) C(24)C(25) C(34)-C(35) C(25)-C(26) C(35)C(36) C(41)-C(42) C(41)
