32 1
Conformational Analysis of 1,4-Cyclohexadienes. Carbon-Proton Spin-Spin Coupling Constants in Dihydroaromatic Carboxylic Acids James L. Marshall,*Ia Larry G . Faehl,Ia Cat0 R. McDaniel, Jr.,la and N. Dale Ledfordlb Contribution from the Departments of Chemistry, North Texas State University, Denton, Texas 76203, and the University of South Alabama, Mobile, Alabama 36688. Received May 27, 1976
Abstract: [2,3,4,5,6-?H~; carboxyl- 13C]-1,4-dihydrobenzoic acid (2), [2,3,4,5,6,7,8-2H7; carboxyl- I3C]-1,4-dihydro-1 naphthoic acid (3), and [carboxyl-‘3C]-9,10-dihydro-9-anthroic acid (4) were synthesized and studied by proton N M R spectroscopy to obtain all homoallylic carbon-proton couplings. The ratio J C H / J H His 0.6, as previously observed for other systems. It is observed that these carbon-proton couplings are just as sensitive as proton-proton couplings to the degree of pucker(for ing of the dihydro ring. A comparison of theoretical and empirical J values is consistent, whereby the ratio Jeq.cq/Jdx.eq J H H ) decreases, and the ratio JAx.Ax/Jdx.eq (for J C H )increases throughout the series 2-4, showing the extent of puckering of 3 is intermediate between that of 2 and that of 4. Thus are generated three categories-‘‘flat”, “flattened boat”, and “true boat”-to compare the geometries of 2-4. Considerations of other types of coupling constants (vicinal and allylic) in the series 2-4 and in other compounds supports the designation of these separate categories.
It is clear that 1,4-dihydrobenzene and its monosubstituted derivatives are ~ l a n a r , and ~ - ~that 9, lo-dihydroanthracenes are puckered with 9-substituents axiaL6 However, some uncertainty shrouds the N M R conformational analysis of dihydroaromatic compounds in general, because the ratio of the two proton-proton homoallylic coupling constants 5Jc,s/ 5Jrrans (see 1) remains close to 1 throughout a series of com-
I c
pound^.^^^ Indeed, it has been proposed that this ratio of homoallylic coupling constants should not be used as a conformational tool.2b This hypothesis is somewhat surprising, because a strong geometrical dependence of 5 J H H had been previously shown (in single-path olefins).8 In the present account it is shown that part of the past difficulty in utilizing the ratio 5Jcls/5Jtrans arises from the fact that in proton-proton couplings this ratio uses axial-equatorial and equatorial-equatorial couplings. Perhaps better suited to the conformational analysis are axial-axial and axial-equatorial couplings, whose 5Jcls/5Jtrans ratio does not remain so close to 1. Since the substituent is axial, one must then use couplings to the substituent. In the present study the substituent was labeled with I3C,and proton couplings to this labeled carbon were used. It is then realized, from data generated from this anq from past studies, that there is a consistent trend in the ratio ‘Jcls/’Jtrans (both 5Jax.ax/5Jdx.eq and 5Jeq.eq/5Jdx.eq) throughout a series of dihydroaromatic compounds. Revults and Preliminary Remarks Spectral Analysis. The compounds synthesized in this study are 2-4. Compounds 2 and 3 were deuterated as shown so as
-2a
b
Y=D
3a
X=H, Y=D
X=D, V=H
L
X=O, Y-H
X-H.
to remove interfering couplings from the olefinic protons (the proton N M R of 2 and 3 with no deuteration exhibited patterns too complex to analyze). It was apparent from the proton N M R spectra of 2 and 3 that a roughly 50:50 product distribution resulted in both cases. Table I gives the proton N M R parameters obtained from 2-4. Figure 1 shows, as an example of the N M R analysis conducted herein, the deuterium-decoupled proton spectrum of 2. With a mixture of 2a and 2b, two AB portions of an ABX (proton-proton-carbon) system should appear, giving rise to two overlapping spectra of eight lines each. The chemical shifts of the methylene proton in 2a and 2b were slightly different (by 0.0 15 ppm), and accordingly two closely overlapping four-line methylene patterns could be recognized (Figure lb). Thus, the two 5 J H H sphttings and the two ~ J Csplittings H could be discerned. The chemical shifts of the methine proton in 2a and 2b were virtually identical, and the two overlapping four-line methine patterns were essentially superimposed (Figure 1a). From the methine pattern the 2 J splitting ~ ~could be discerned. The total spectrum of 2 was analyzed as a “six”-spin system to give the data presented in Table I. The deuterium-decoupled proton N M R spectrum of 3 was analyzed in an identical fashion. The spectrum of 3 differed from that of 2 in that the chemical shifts of the methylene protons in 3a and 3b were substantially different (0.1 5 ppm), and consequently the two four-line methylene patterns were separate. The analysis of 4, with no deuteriums, was a more straightforward aromatic proton-decoupled analysis of the ABC portion of an ABCX pattern. The 5 J H H values for 2 taken from Table I should be considered to be more accurate than those obtained in a previous study9 in which the two splittings were not specifically resolved, and these values appearing in Table I should replace the previous values. Spectral Assignments. The uncertainty that existed in the data for 2 and 3 was the correct assignment of the two AB patterns; e.g., in 2 it was certain that the isotopic isomer with 5 J C H = 4.65 also had ’JHH = 9.19 Hz, but from spectral analysis itself, it could not be ascertained whether these two values belonged to the trans-JcH and c1s-J”” (2a), or to the cis-JcH and trans-JHH (2b). The assignments that were made for 2 were based on two considerations: (1) for 1,4-dihydrobenzene itself,’O 5Jc,shas been shown to be larger than 5Jtrans:
Marshall, Faehl, McDaniel, Ledford
/
Conformational Analysis of 1,4-Cyclohexadienes
322 Table I. Carbon-Proton and Proton-Proton Coupling Constants for Dihydroaromatic Carboxylic Acids 2-4
2 3 4
9.19 3.84 trans-JcH, just as would be anticipated (an axial-axial coupling should be large).8 The spectral assignments for 4 follow directly from previous work6 that has established chemical shifts for the 9-substituted-9,lO-dihydroaromatic system. Justification for Using JCHValues: the J C H I J H HRatio. In the present study it was assumed that homoallylic JCH values correlate with homoallylic JHH values. The basis for this assumption is a previous study12 in which it was shown that for labeled carboxylic acids, JCH correlates with J H H for 2-, 3-, 4-, and 5-bonded couplings in olefins, aromatics, acetylenes, and aliphatics (correlation coefficient = 0.98). For a given type of coupling it was shown12 that JCH/JHH is about 0.6 (the slope of the plot was 0.62). Since compound 2 is flat, then the geometry of cis-’JCH should approximate that of Cis-’JHH, and likewise the geometries of trans-’JcH and t r a n ~ - should ~ J ~ ~ be similar. Compound 2 thus offers a system suitable to test this correlation for homoallylic couplings. Indeed, from Table I these ratios are calculated to be: cis-JcH/cis-JHH = 5.751 9.19 = 0.63, and tranS-JcH/tranS-JHH = 4.6517.56 = 0.62. This excellent agreement between CiS-JCHICiS-JHH, transJcH/tranS-Jb{H, and the previously observed ratio of 0.62 is no doubt to some degree fortuitous, for identical geometries are not to be expected for a JCH geometry and its respective JHH geometry, but it is nevertheless clear that homoallylic J C H values in the conformational analysis of 2-4 can be related to the geometrically equivalent JHH values. Thus, cis-trans ratios Journal of the American Chemical Society
for JCH should be just as meaningful as cis-trans ratios for JHH. Indeed, for flat 2, it is calculated from Table I that C i S - J c H / trans-JcH = 1.24, and that cis-JHH/trans-JHH = 1.22; thus, it is clear that the cis-Jltrans-J ratio for this flat system is about 1.21, for both proton-proton and carbon-proton systems. Previously observed Cis-JHHItrans-JHH ratios for flat systems have been 1.20 for dihydrobenzene,1° 1.2 for 1,4-dihydrobenzyl alcohol,2aand 1.1 for 3-fluoro-l,4-dihydrobenzylalcohol.2a A further check is possible on the suitability of using ’JCH values in the conformational analysis of 2-4. From the I3Csatellite proton spectrum of dihydroanthracene itself, an axial-axial proton-proton homoallylic coupling constant of 4.8 Hz has been determined.’ The axial-axial carbon-proton homoallylic value of 3.2 H z taken from Table I for 4 thus generates a ratio Of J C N I J H H = 3.214.8 = 0.67, again close to the previously observedI2 value of 0.62.
Theoretical Considerations A previous theoretical study8 of (monoolefin) homoallylic coupling constants indicates a straightforward trend in which the calculated coupling constant increases as either, or (to a much greater degree) both, of the C-H bonds approach a parallel alignment with the atomic p orbitals of the a system. Subsequently, in an attempt to extend this trend to cyclohexadiene (dual path), the calculated coupling constants of the first study have been multiplied by a factor of 2.’ To ascertain if this latter approach might be too simplistic, SCF-INDOFPT calculation^^^ were done in the present study on 1,4cyclohexadiene itself a t various conformations. Table I1 gives the results. In Table I1 proton-proton coupling constants are given as the puckering angle cy varies (defined as the dihedral angle of the plane defined by the sp2 carbons and the plane defined by C(s)-C( I )-C(*l).The geometry of cyclohexadiene is taken from an electron diffraction study,I4 and for the various values of Q in Table I1 constant bond lengths and constant independent bond angles were retained. The maximum value of Q was taken a t 3 5 O , because a t this value the C-H bond is parallel to the olefinic p orbitals (within 0.1’). In order to determine the effect of a dual path vs. a single path, calculations for 2-butene were also done utilizing an identical geometry; these values also appear in Table 11. This calculated value for 2-butene is somewhat larger than that done by Barfield and Sternhell (compare 4.8 with 2.8); the cause for this difference most probably arises from the shorter bond lengths and larger internal angles of cyclohexadiene cited in the electron diffraction study14 (Barfield and Sternhell used “standard” bond lengths and bond angles).
/ 99:2 / January 19, 1977
323 Table 11. Calculated0 Proton-Proton Coupling Constants of 1,4-Cyclohexadiene at Various Puckering Angles
la
4a
le-&(2 Puckering angle, a,deg
0 10 20 30 35 35 b 2-Butene, O°C
3 Calculated J , Hz
l a , 4a (cis) l e , 4e (cis) l a , 4e (trans) 16.06 21.96 25.44 25.13 23.41 22.83 4.78
16.06 9.73 4.55 1.23 0.31 4.78
12.45 11.22 8.05 4.29 2.65 2.79 4.59
la, 2
le, 2
la, 3
le, 3
la, l e
4.10 3.20 2.86 3.16 3.53 3.58 3.85
4.10 5.28 6.41 7.14 7.26
-2.30 -3.26 -3.85 -4.02 -3.95 -4.01 -2.18
-2.30 -1.10 0.18 1.37 1.89
-13.85 -13.01 -10.85 -8.24 -7.04 -6.52 -9.44
3.85
-2.18
Energy, kcal/mol 0.0 0.67 3.08 8.08 11.89
Calculated by SCF-INDO-FPT;13 geometry taken from an electron diffraction s t ~ d y .All ’ ~ bond lengths and independent bond angles are not changed, except for the puckering angle cy, defined as the dihedral angle of the plane of C(2)-C(3)4(5)
