G132
DONALD J. CRAM,NORMAN L. ALLINGER AND H. STEINBERG
this system they traveled equal distances. Angyal" further found identical Rf values in four other solvent systems. Dr. A . B. Foster tested a sample of the synthetic ether in his
[CONTRIBUTION FROM THE
Macro Rings.
Vol. 76
borate ionophoresis system,' and found that the compound paralleled natural bornesitol in its behavior. MADISON, WISCONSIS
DEPARTMENT O F CHEMISTRY O F THE
UNIVERSITY O F CALIFORXIA AT L O S ANGELES]
VII. The Spectral Consequences of Bringing Two Benzene Rings Face to Face1 BY DONALD J. CRAM,NORMAN L. ALLINGER~ AND H. STEINBERG RECEIVED MAY12, 1954
The preparation of the paracyclophane with n = m = 3, of I1 with n = 3, 77t = 4 and with n = m = 4, and of I11 with 12 are reported. The variations in spectral properties of these classes of compounds with their molecular geometries are discussed. The ultraviolet absorption spectra of cis- and trans-1,2-diphenylcyclopentaneare compared. IZ =
The previous papers in this series3 have reported obtainable member ( n = m = 2 ) 5 a contains nonthe preparation of the series of eight paracyclo- planar benzene rings,j the possibility is evident phanes I in which n = m and in which n = m - 1 that as the steric constraints are released by increasfor all values of n and m from n = m = 2 to n = ing the length of the methylene bridges the arom = 6, with exception of the compound in which matic nuclei might become planar before the irn = nz = 3 . The less symmetrical cycles (I) with electrons of each ring cease to affect each other's n = 2 , m = 4, withn = 3 andm = 6, andwithn = spectral characteristics. In compounds of class I1 4, m = 6 have also been r e p ~ r t e d as , ~ well as com- and I11 the possibility exists that bent benzene rings pounds of class 111 with m = gdaand 10.3e,4b might be produced without the complicating feature of a second aromatic nucleus being in the vicinity of the first. Finally, in compounds of class IV, it was hoped that two benzene rings could be brought face to face without any accompanying complications due to distortion of the aromatic I I1 I11 rings from planar configurations. These two groups of compounds (I and 111) offer Results unique opportunities for the study of certain kinds The synthesis of the paracyclophane with m = n of transannular electronic and steric effects on physical and chemical properties. Spectral abnor- = 3 was carried out by the sequence formulated. malities, particularly in the ultraviolet region, have The mixture of the four cis-trans isomers of diester been observed to characterize the smaller paracy- IX obtained by the reduction of the aromatic diesclophanes I,3aand have been attributed to one or ter VI11 was not separated into its components, both of the following causes: ( I ) .rr-electron inter- but was used directly in the acyloin condensation. actions between the two benzene rings, ( 2 ) distor- The impure acyloin (probably a mixture of position tion of the benzene rings from their normal planar isomers) was submitted directly to a modified configurations. The present investigation reports Clemmensen reduction, the saturated hydrocarbon an attempt to separate and identify these effects XI being obtained in a 1% over-all yield from diesthrough a comparison of the spectral properties of ter IX. This yield is indicative of the unfavorable three types of compounds : the paracyclophanes (I) steric situation found for the ring-closing step and themselves; compounds belonging to classes I1 and compares with the yields for analogous sequences 111; compounds possessing the geometry of 117. in the preparation of the two next larger cycles as follows: compound I with m = 3, n = 4, yield (5%; I with m = n = 4, yield 22%. Dehydrogenation of the saturated hydrocarbon furnished the aromatic cycle, XII. Several other potential syntheses of XI1 were H also investigated. Reduction of unsymmetrical diI \ester VI11 with lithium aluminum hydride followed In compounds ol class I, although the sriiallest by treatment of the resulting diol with hydrogen bromide furnished the dibromide XIII. This (1) This work was supported in part by t h e Ofice of S a r a 1 Kesubstance failed to cyclize when subjected to the search. conditions of the Wurtz reaction, as had the dibro( 2 ) Dow Predoctoral Fellow a t U.C.L.A., 1953-1984. (3) (a) D. J. C r a m a n d H . Steinberg, THISJOURNAL, 73, 5691 (1951); ~ an attempt to mide XIV p r e v i o ~ s l y . ~Similarly, (b) H. Steinberg and D. J. Cram, i b i d . , 74, 5388 (1952); (c) D. J. XI1 by an intermolecular Wurtz reaction of obtain Cram and S. L. Allinger, ibid., 76, 726 (1954); (d) N.L. Allinger and dibromide XV and p-dibromobenzene failed to give D. J. Cram, i b i d . , 76, 2362 (1964); (e) D. J. Cram and H. U. Daeniker, the desired cycle. It would appear that the successibid., 76, 2743 (1954); ( f ) J. Abell and D. J. Cram, i b i d . , 76, 4406 (1954). ful application of the Wurtz reaction to prepara(4) (a) M. F. Bartlett, S. K. Figdor a n d I;. Wiesner, Canadian J . tion of the paracyclophanes is limited to those comC h a i n . , 30, 291 (1952); (b) K. Wiesner, D. R.1. MacDonald. R. €3.
+
Ingraham and R . B. Kelly, C a i z n d i m l J . Rcseavrh, B28, 331 (1950).
(2) (L)C I I3rov.n J Chtin Soc 3265 (I'J5d), (I>) 327'1 (1053)
PREPARATION AND SPECTRA OF PARACYCLOPHANES
Dec. 5, 1954
4
1, Morpholine, S 3, 2 , KOH, H i 0
6133
CH36Hz
/,O ,=-COOCH, Hz
6 (CH2)3
1, Na 2, AcOH
i ( D C H 2 C O O C H 3
X
mixture of isomers
Pt
IX
La-CH2COOCH* VI11
Other approaches to the synthesis of cycle XI1 were equally discouraging. The pyrolysis of XVI13deither as the free diPd(C) acid6 or as the thorium salt' gave no isol(CH213 (CHZ)~ able cyclic product. The nitrile XVIII 250-320 under the conditions of a Ziegler cyclization8 also failed to yield the desired mateXI XI1 rial, as did the dibromide XVI in an c H 2 c H 2 B r attempted dialkylation of malonic ester. , - - - o C H 2 B r The synthesis of the cycle, p-dodecamethylene(CH2)3 (CH2)3 benzene (XXIV), is outlined in the formulation. Numerous attempts were made to ring-close diester X X through the acyloin reaction, but only starting XI11 XI\material and small amounts of polymer were obpounds typified by XVI in which both bromide tained. In a further attempt to obtain 111 (m = 8 ) , the aromatic ring of diester X X was reduced to functions are of the benzyl type.3a the corresponding cyclohexane derivative X X V , ~ 0 - c H z - A both isomers being isolated, one in a pure state, the other slightly contaminated with its isomer. (CH2)3 (CH2)3 (CHzIa The isomer obtained as the minor component was 1 assigned the trans structure by analogy with the 8r Br results obtained in the reduction of p - ~ y l e n e .This ~ XVI, A = Br XV provisional assignment is in accord with von AuXVII, A = COOH XVIII, A = CN wers' rulelo that trans isomers of this type possess the lower index of refracti0n.l' Since both of these CaH6-( CH2)3-COOC2H5 diesters resisted the acyloin ring-closure reaction 1, Succinic anhyd. I (the bulk of the starting material was recovered unAlC13 changed), the cis-diester X X V was converted to the fCH2)3COOH (CH2)3COOCHa corresponding acid chloride 1, SH2NH2, KOH 1,LiAIHa XXVI. The acyloin reaction ----+ has been carried out success+ 2' HBr fully on acid chlorides in sim2, CH30Hz (CHz)3COOCH3 ple cases,12 but not in cycliza-C-( CHz)2COOH tion reactions. When applied II 0 to the diacid chloride XXVI, XIX xx the acyloin reaction produced (CHdKH2Br (CHz)5COOCHa polymer, no starting material 1, NaCH(COOC2H5)~ 1, Na being recovered. An attempt t o prepare cycle I11 with m = 8 2, OHby a ring-closing Wurtz reac3, A, -COz (CH2)3CH2Br 4, CHI OH^' ( CH2)bCOOCH3 (6) M. Szwarc, J. Chem. Phys., 16, 128
r("t-?
'1
L-QA
La-cH2CH2Br L-a-CI12CH2Br
Zc-
XXII
XXI
C=O
I
XXIII
Zn
XXIV
(1948). (7) L. Ruzicka, W. Brugger, M. Pfeiffer. H. Schinz and M. Stoll, Helu. Chim. Acta, 9, 499 (1926). (8) K. Ziegler, H. Eberle and H. Ohlinger, Ann., 604, 94 (1933). (9) 0 . Miller, Bull. SOC. chim. Bclg., 44, 513 (1935). (10) K. von Auwers, Ann., 410, 84 (1920). (11) NOdifficulty should be experienced here in applying von Auwers' rule since in $-disubstituted cyclohexane derivatives, the trans isomer is the more thermodynamically stable [N. L. Allinger, Expcrientia, 10, 328 (1954).] (12) A. Basse and H. Klinger, Bey., S1, 1217 (1898).
6134
DONALD J. CRAM,NORMAN L. ALLINGER AND H. STEINBERG
tion with dibromide X X I as starting material also failed. -(
CH2)sCOOCHa
XXV c i s and trans
/i'CH*):j-COCl
vL( m
CH2)r-COCl XXVI
The two half-reduced paracyclophanes (11) with = 3, n = 4 and m = n = 4 were prepared by the
Vol. 76
hydrogenation (glacial acetic acid with a platinuni catalyst) of one of the two benzene rings of I with m = 3 , n = 4 and m = n = 4, respectively. Discussion Figure 1 records the ultraviolet absorption spectra of the paracyclophanes I in which n and m are varied stepwise one carbon a t a time from m = n = 2 to m = 5 , n = 6. The most striking feature characterizing the relationships between these spectra is the discontinuity in the progression of the curves from normal (as compared to open-chain models) to abnormal as the values of rn and n become small. Thus I with m = n = 4 differs only slightly in its spectral properties from I with m = n = 5 but differs markedly from I with m = 3, n = 4. This discontinuity is apparent in both the re215 mp) and that of low gion of high intensity (A 270 mp), the series of intensity absorption (A bands in both places moving toward longer wave lengths and lower intensities as the values of m and n become smaller than 4. A similar discontinuity is found in the spectra of the more unsymmetrical paracyclophanes (see Fig. 2) in which n 5 m - 2 , again the requirement for a normal spectrum being that both m and n be equal to or greater than 4. As the bands move toward longer wave lengths, they decrease in intensity, and the E'S of even the larger cycles (m 2 4, n 2 4) are all about 10% below those of the open-chain models.
--
5
4
0 m
-
3
2
I 10
240
x rmu)
I 280
I
320
Fig. 1.-Ultraviolet absorption spectra of the more symmetrical paracyclophanes in 95% ethanol (Cary spectrophotometer, model PMS). The curves, with the exception of the open-chain model a t the bottom, have been displaced upward on the ordinate axis by 0.5 log E unit increments from the curve immediately below.
Fig 2.--Ultraviolet absorption spectra of the relatively unsymmetrical paracyclophanes in 95% ethanol (Cary spectrophotometer, model PMS). The curves, with the exception of the open-chain model a t the bottom, have been displaced upward on the ordinate axis by 0.5 log e unit increments from the curve immediately below
Distortion of the Aromatic Rings from Planar Configurations.-The establishment of the fact that the aromatic nuclei in the cycle I with m = n = 2 are distorted from the normal planar configuration5 suggests that the same thing might he true of some of the other smaller members of the homologous series. In structure A (the profile of
PREPARATION AND SPECTRA OF PARACYCLOPHANES
Dec. 5, 1954
6135
the paracyclophane in which m = n = 2 ) 5it is evident that strong repulsive forces are acting between the two benzene rings. 10.13 8.
-T - 4-
1.558.
k1.486
These data fix a lower limit of 3.09 A. for the distance that two benzene rings can be from one another (face to face) without strong repulsive forces coming into play. Table I reports the results of calculations of the maximum inter-benzoid distances for the paracyclophanes, the assumptions being made that: (a) the aromatic rings are planar; (b) the bond distances are normal and the bond angles are as normal as possible; (c) the two rings are as directly over one another as possible. These calculations indicate that in those cycles with n = 3 or less and m = 4 or less, the interbenzenoid distances a t either one or both ends of the molecule must be less than 3.09 8. The tentative conclusion is evident that the benzene rings are non-planar in the cycles where m = n = 3 or smaller, and that other bond angles are distorted, the distortion being greater the smaller the values of m amd n.13 Furthermore, the cycles with both m and n equal to 4 or more are probably planar, the geometry of the compounds having m = 3 and n = 3 to 6 being ambiguous.
.._..' , I
.
I
....
t H i(CHpl,&H.J,-CH,
I
TABLE I CALCULATED' MOLECULAR GEOMETRY(I~EALIZED) OF PARACYCLOPHANES AND SHIFTS IN ABSORPTION BANDSOF THE ULTRAVIOLET SPECTRA FROM NORMALITY~
0 D
Dist.,
A.
Xmax of cycles
Cmpd. c-1 c-4 m n to c - 1 ' to c - 4 ' 214 2 2 1.54 1.54 . 1.80 2.26 2 3 3 2.12 3.16 2 4 2 3 3 2.52 2.52 -Zd 3 . 4 1 3 4 2.84 1 5.27 3 6 3.50 3 4 4 3.73 3.73 3 4 5 .. 2 4 6' 1
.
..
..
- Xmax open-chain models
Bands a t Amax for model, mpc 219 223 259 265 267 6 21d 21 , .. 20d 7 10 7d lod 6d 6 -Id 23d 2 4 od 9d - 2 1 .. 2d 4d 3 1 3 d 1 1 1 1 2 1 1 1 2 1 1 1 1
..
. .. .. ,. ..
..
273 2gd 26 9 21
7 3 1 1 1
Based on bond distances and where possible on bond angles taken from ref. 14. The spectra of the bis-(p-alkylphenyl)-alkanes appear to be identical to one another.& cModel is bis-(p-ethylpheny1)-butane. dShoulders (estimated). e Spectra of larger paracyclophanes are identical with I, wz = 4,~t= 6. a
Crystallographic data on a number of aromatic compounds in which the aromatic nuclei occupy parallel planes indic?te an interplanar distance of approximately 3.40 A. for quite a variety of sub-
-*
-
(13) The geometry of compound I with m = 3 is being examined through X-ray crystallographic analysis in these laboratories b y Dr. K. N. Trueblood.
3 I 240
I 280
Xlmrl,
I
I
I 280
240 AlrnPl.
Figs. 3 and 4.-Ultraviolet absorption spectra of cycles I11 (Fig. 3) and I1 (Fig. 4) in 95y0ethanol (Cary recording spectrophotometer, model P M S ) . The curves, with the exception of the open-chain model compound at the bottom, have been displaced upward on the ordinate axis by 0.5 log e unit increments from the curve immediately below. (14) J. M. Robertson, "Organic Crystals and Molecules," Curnell University Press, Ithaca, N. Y . , 1953, pp. 157, 206, 27, 270, 274. (15) (a) H. Ott, Ann. Physik, 85, 81 (1928); (b) J. M. Robertson, J . Chcm. Soc., 1222 (1951); (c) 615 (1935), a n d 1195 (1936); (d) J. M. Robertson and I. Woodward, ibid., 219 (1937); (e) J. M . Robertson and J. G. White, i b i d . , 607 (1945).
6136
DONALD J. CRAM,NORMAN L. ALLINGER AND H. STEINBERG
els of I11 with m = 9 suggest that either the aromatic ring is non-planar, or the bond angles in the methylene bridge are modified, or the two effects combine to relieve the steric repulsions inherent in the system. The fact that I11 with m = 8 could not be made suggests that I11 with m = 9 is a t least as strained as the paracyclophanes with m = n = 3 or m = 3, n = 4, since the same kinds of ringclosing reactions on the same kinds of starting materials were utilized.I6 Similar trends are evident in the ultraviolet spectra (see Fig. 4) of the paracyclophanes in which one benzene ring has been reduced to a cyclohexane ring (II),I7 A cyclohexane is somewhat thicker than a benzene ring, but it is impossible to say which system (I or I1 with m and n the same) is the most constrained since the bond orientations in I1 are more adaptable than in I . Judging by the spectra, the benzene rings in I1 with m = 3, n = 4 and in I11 with m = 10 are about comparably constrained. The conclusion that can be drawn from these spectral comparisons is that the warping of the aromatic rings in the paracyclophanes is at least partially responsible for their abnormal electronic spectral properties. Transannular Electronic Effects in the Smaller Paracyc1ophanes.-It was suggested previously3a that the abnormal ultraviolet absorption spectra of the smaller paracyclophanes might be due, a t least partially, to transannular resonance effects associated with either the ground or excited states of the two aromatic systems. Ingrahaml* carried out both valence bond (V.B.) and molecular orbital (L.C.X.O.) calculations in an attempt to determine whether or not there is any stabilization of the molecule due to 2pu-type bonds between the two benzene rings of I, m = n = 2 . Keglecting effects due to the two bridging chains and non-planarity of the benzene rings, the valence bond method predicted 4 kcal. more resonance energy for I ( m = n = 2) than for two infinitely separated benzene rings. However, 16 kcal. was required to bring two benzene rings face to face and 3 A. apart from infinite separation) due to repulsive forces. On the other hand, the L.C.A.O. method due to its neglect of repulsive forces predicted the same energy for the two benzene rings face to face as completely separated, unless one or both aromatic rings were deficient one electron. The valence bond calculation also indicated a slight shift of the 260 mp benzene band toward shorter wave lengths. In connection with the question of the nature of the electronic forces operating between the two benzene rings in the smaller paracyclophanes, the geometries and spectral properties of a number of molecular complexes are of interest. In most of the complexes that involve aromatic nuclei, one component of the complex carries electron-withdrawing and the other electron-releasing groups, (16) I n t h e preparation of I with m = n = 3 a n d m = 3 , 71 = 4, the fully reduced cis-diester (such a s I X ) underwent ring closure. I n t h e attempted preparation of I11 with m = 8, t h e fully reduced cis-diester XXV failed t o ring close in even trace amounts. Compound I11 with in = 9 was prepared by a sequence similar t o t h a t employed for I11 with m = 12. T h e former synthesis will he reported in a future paper in this series. (17) Attempts t o prepare I 1 with m = II = 2 failed. (18) I,. L . Ingraham, J . Chem. P h y s . , 18, 988 (1950).
Vol. 76
and the forces binding the two components of molecular complexes in general are considered to be polar in character (acid-base interactions in the Lewis sense).lg In a number of cases the crystal structures of complexes involving two aromatic components have been determined, 2oa,e and frequently the molecules are stacked in columns, with the two components forming alternate layers parallel to each other, the aromatic rings not having coaxial orientation. In these crystals, the distances betweeq the parallel aromatic nuclei vary from 3.2 to 3.5 A. In p-nitroaniline where the nitro group of one ring overlies part of the ring of a second molecule (the rings occupy parallel planes) the line closest intermolecular 0 . . C approach is 2.66 A., a distance much shorter than that found in ordinary crystals.20b-d Some of these molecular complexes persist a t least partially in ~ o l u t i o n 1 g b ~ ~ ~ and absorb light a t longer wave lengths and lower intensities than do the separate components of the complex. Finally, Nakamoto measured the dichroisms of the n-electron absorption bands in the ultraviolet spectra of the crystals of a number of common aromatic compounds,22aas well as of the crystals of four molecular complexes of the type discussed above.22b In the former case, the light absorbed with its electric vector vibrating parallel to the plane of the aromatic nuclei was always of longer wave length ( E being higher) than the light with its vector vibrating perpendicular to the same plane. I n the latter case the situation was reversed. These effects were as follows. I n the simple crystals the overlap of n-electron clouds in the direction perpendicular to tee benzene rings is small (the rings are 3.4-3.5 A. apart), and accordingly the density distribution of a-electrons is larger and the electrons are more mobile in the direction parallel than in the direction perpendicular to the rings. I n the crystals of complex, the overlap of n-electron clouds in the direction perpendicular to the benzene rings is larger (the rings are 3.16-3.26 8. apart), and thus the density distribution of n-electrons becomes larger and the electrons become more mobile in the direction perpendicular than in the direction parallel to the rings. The distance between the benzene rings in the smaller paracyclophanes is either smaller or comparable to that found in the above molecular compounds. Furthermore, the general progression of bands in the ultraviolet spectra of the paracyclophanes toward longer wave lengths as the two benzene rings get closer together might possibly be partially associated with transannular resonance effects, the excited states being more resonance stabilized than the ground states, as is common in
.
(19) (a) R. S. Mulliken, THIS JOURNAL, 79, 600 (1950), and 74, 811 (1952); (b) J. Landauer and H. McConnell, ibid., 74, 1221 (1952); (c) W. Brackman, Rec. Lvav. chim., 68, 147 (1949); M. J. S. Dewar, J . Cizem. Soc., 406 (1946); (d) J. Weiss, i b i d . . 245 (1942). (20) (a) H. hl. Powell, G. Huse and P. W. Cooke. ibid , 1 5 3 , 433 (1943); (b) S . C. Abrahams and J. hf. Robertson, Acta C r y s t , 1, 252 (1948); (c) P. J. A . McKeown, A. R. Ubbelohde and I. IVoodward, ibid , 4, 391 (1951); (d) S . C. Abrahams, THIS J O U R K A L , 74, 2692 (19.52); (e) K. Osaki and M. AIatsuda, Acta C r y g l , in press. (21) I,. Michaelis and S . Granick, THISJ O U R N A L , 66, 1023 (1944). (22) (a) IC. S a k a m o t o , ibid., 74, 390, 392 (1952); ( b ) 74, 1739 (1932 j.
PREPARATION AND SPECTRA OF PARACYCLOPHANES
Dec. 5, 1954
?
organic molecules. Although repulsive forces between the two rings undoubtedly predominate, the type of structures indicated in B might stabilize the cycles somewhat as has been indicated in the valence bond calculation of Ingraham.I8 If this
6137 I
I
-.. .,
4
'.I
type of stabilization exists, it would be different from that found in the molecular complexes in two respects: the attractive forces would be far less polar in character; the rings have coaxial orientation. I n an attempt to determine the consequences of bringing two benzene rings face to face without the complication of deforming the aromatic rings, the ultraviolet absorption spectra of cis- and tans-l,2diphenyl~yclopentane~~ were compared, as well as those of XXVII and XXIX (Fig. 5). A very slight trend is evident for the absorption bands to move
3
.L .. .,
i b 0
-
2
I
XXVII
XXVIII
Trans
XXIX
Isomer
- -- ----
!
!
! toward longer wave lengths and lower intensities I as one passes from the structures in which the two phenyls are more trans (trans-l,2-diphenylcyclo0 24 0 280 pentane or compound XXIX) to those in which the (m Y). two phenyls are more cis (cis-l,2-diphenylpentane Fig. 5.-Ultraviolet absorption spectra of cis and trans or compound XXVII). Although this trend is isomers of disubstituted five-membered ring compounds in qualitatively similar t o that observed for the paracyclophanes as rn and n become smaller, quantita- 95% ethanol (Cary recording spectrophotometer, model tively there is no comparison. Furthermore, the PMS). The lower pair of curves are displaced downward differences might be due to other than transannular on the ordinate axis by 1.0 log E unit. effects. If the two phenyl groups in cis-1,2-di- XXVIII), Kdc/Kmeso being equal to 20.27 Since phenylcyclopentane and in XXVII are completely the equilibrium constants between diastereomerieclipsed, the benzene rings possess a geometry cally related substances such as meso- and dl-1,2relative to each other comparable to that of the dihydroxydibenzyl are undoubtedly close to unity,28 paracyclophane with m = 2, n = 4. There is the AFO for cis-acetonide + trans-acetonide considerable doubt, however, that two phenyl amounts to about -1.8 kcal. The question arises groups cis to one another on a five-membered ring as to just how deformed the bond angles in a moleare eclipsed completely. Aston, et u Z . , ~ ~concluded cule such as cis-1,2-diphenylcyclopentaneor XXVII from thermal data that cyclopentane was not have to become before the 1-carbons of each planar, and Pitzer, et u L . , ~calculated ~ from spectral phenyl are separated by 3.4 A. This condition is and thermal data that the puckered was more sta- met if a is expanded from its normal 109 to 115" ble than the planar configuration. Donahue and (the analogous bond angle in the paracyclophane TruebloodZ6have determined the crystal structure with m = n = 2 is distorted similarly), and 0 (see of hydroxyproline and found that the carbon carry- diagram) is adjusted to 75" by distorting the fiveing the hydroxyl group is 0.4 A. out of the plane of membered rings from the planar configuration. the other four-ring atoms. Similarly, if a! is 120°, 8 must be 56" if ths 1-carbons That considerable repulsive forces operate tend- of each phenyl are separated by 3.4 A. On the ing to prevent two phenyls from becoming eclipsed other hand, to get the 1-carbons of the two phenyls is clear from the values of the equilibrium constants I obtained between meso- and dl-l,2-dihydroxydibenzyl and their respective acetonides (XXVII and (23) Samples of these compounds were very generously supplied by Dr. H. A. Weidlich [see Be?., 71B,1601 (1938)l. 66, (24) J. G. Aston, H. L. Fink and S. C. Schumann, THISJOURNAL, 341 (1943). (25) (a) K . S. Pitzer, Science, 101, 672 (1945); (b) J. E. Kilpatrick, K. S. Pitzer and R . Spitzer, T H ~JOURNAL, S 69, 2483 (1947). (26) J . Donohue and K. N. Trueblood, Acta. C r y s t . , 6 , 419 (1952).
\
l
(27) P. H. Hermans, 2 . Physik. Chem., 118, 337 (1924). (28) F. A. Abd Elhafez and D . J. Cram, THIS JOURNAL, 1 6 , 339 (1953).
613s
DONALD J. CRAM,NORMAN L. ALLINGERAND H. STEINBERG
VOl. 76
3.0 A. apart, if a is 115" then 0 must be 37". The actual dimensions in cis-1,2-diphenylcyclopentane probably lie somewhere nearer the last values, which would place the 1-carbons of each benzene ring farther apart than the distance calculated for the 1-carbons (2.84 A.) of the paracyclophane with m = 3, n = 4, whose ultraviolet absorption spectrum is still far from normal. The above considerations suggest that both the lack of planarity of the aromatic rings and transannular resonance effects operate in the smaller paracyclophanes to give them their abnormal ultraviolet absorption spectra. Transannular activating and deactivating influences and directive effects of functions substituted for hydrogen in the smaller paracyclophanes are currently under investigation. The Infrared Spectra of the Paracyc1ophanes.The principal bands found in the infrared spectra of the paracyclophanes I have been recorded.29 In I with n = m = 2, strong bands appeared a t 6.30, 10.75 and 11.20 p which do not appear in the open-chain model compounds.3a Similar bands, though of lesser intensity, were found also in the compounds with m = 2 , n = 3 or 4.3a The bands near 6.3 p in these smaller cycles might possibly be due to an anomalous >C=C< stretching. The cycles with m > n > 3 show no unusual bands below 9 p which are consistent throughout the series, the structural feature responsible for nearly every band in this region being known. Some of the bands above 9.0 p are of diagnostic value for the cycles. Experimental
tracted with ether. The ether extracts were washed with water and dilute sodium bicarbonate and were dried. After removal of the ether, the product was distilled t o give a yellow oil, b.p. 160-200° (1 mm.). The distillate was heated under reflux with 2 g. of Raney nickel in 150 ml. of methanol for 1 hour. The nickel was removed by filtration, and the solvent was evaporated. The residue was distilled through a 2-foot column of the Podbielniak type, and after a 2-g. forerun, the desired ester was collected as a colorless liquid, b.p. 24-243' (4 mm.), n% 1.5509, wt. 30.1 g. (52%). Anal. Calcd. for CY~H2204: C , 73.60; H , 6.80. Found: C, 73.40; H, 6.79. p-Carboxy-p '-carboxymethyl- 1,3-diphenylpropane .--Saponification of the ester VI11 gave the acid, yield 90%. Crystallization of this material from aqueous acetic acid gave a crystalline powder, m.p. 215-217". Anal. Calcd. for ClSH1804:C, 72.47; H, 6.08. Found: C, 72.27; H, 6.23. p-Carbomethoxy-p '-carbomethoxymethyl-1,3-dicyclohexylpropane (M).-The aromatic ester 17111, 27.4 g., was hydrogenated with 2 g. of platinum oxide in 75 ml. of pure acetic acid, the theoretical amount of hydrogen being taken up in 14 hours. The catalyst was removed from the solution bv filtration, and. after evaDoration of the solvent. the product was distilled, b.p. 216-219' (4 mm.), Zz5D 1.4779, wt. 27.1 g. (9
