J . Phys. Chem. 1991, 95, 20-25
20
ARTICLES Conformational Behavior of 1-Tetralone, 1-Benzosuberone, Dibenzosuberone, and Some Structurally Related Aryl Ketones from NMR Spectroscopy Pauline Jint and Timothy A. Wildman* Department of Chemistry, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4MI (Receioed: December 14, 1989: In Final Form: June 5, 1990)
The conformational preferences and behavior of the following ketones have been studied by 'HNMR spectroscopy and by semiempirical molecular orbital calculations using the AMI method: I-tetralone, 1-benzosuberone, dibenzosuberone, dibenzosuberenone, acetophenone, 2-methylacetophenone, benzophenone, and 2-methylbenzophenone. Where possible, benzylic coupling constants have been interpreted in terms of averages over torsions in a double-well potential to obtain minimum energy conformations and torsional barrier estimates. Calculations of classical averages for a hindered, rigid rotator demonstrate that the long-range couplings are insensitive to barrier heights if the potential energy minima are closely spaced. In such cases, the couplings are indicative of the separation of the torsional minima rather than the barrier height. While the optimum geometries obtained with AM1 agree with the those deduced from the NMR parameters, AM1 appears to underestimate barrier heights. In the absence of useful couplings, the chemical shifts for the benzophenones have been interpreted as averages over internal rotation. Both the shifts and the computations are consistent with the view that the phenyl groups are twisted by about 30' with respect to the carbonyl plane in the preferred conformation.
Introduction As part of a study of the reketonization kinetics of various o-methylaryl ketones, we have been interested in the conformational behavior of a variety of structurally related aryl ketones, both in the ground-state keto form and in the less stable (and relatively uncharacterized) enol form. In the reketonization process, the conformational behavior of the reactant enol and the product ketone appears to control the relative rates of hydrogen transfer for this seriest4 In the present work, the conformational preferences of several ground-state ketones have been investigated by ' H N M R spectroscopy. The compounds studied were acetophenone ( I ) , 2-methylacetophenone (2), l-tetralone (3), l benzosuberone (4), dibenzosuberone (9, dibenzosuberenone (6), benzophenone (7).and 2-methylbenzophenone (8).
is found to be proportional to the expectation value of sin2 4, where 4 is the dihedral angle between a benzylic CH bond and the plane of the aromatic ring:
6J,(CH2,H) = 6J90"(sin2 4)
(1)
where angular brackets denote a canonical average. A similar relationship appears to govern 4J,(CH2,H), although this coupling has an additional ~ - c o n t r i b u t i o n . ~If- ~the ~ ' ~potential governing rotation (or torsion) about the exocyclic bond C 1Ccu is separable and its form can be deduced, the barrier to conformational interconversion may be estimated by comparison of computed values of (sin2 4) as a function of barrier height with the experimental values of (sin2 4) from eq 1. Through analogy with eq 1, the couplings to the meta ring protons are given by the formula
0
I,X=H 2, X = CHR
3
4
5, X = CHzCHz
6, X = CH=CH
The conformational behavior of each molecule has been investigated, usually by examination of long-range spinspin coupling constants5-8over six formal bonds between the benzylic protons and the protons of the aromatic rings, 6J,(CH2,H). Barriers to interconversion have been estimated where possible by the J method.6 This technique utilizes the fact that such couplings are transmitted by a u-a polarization mechanism and hence have magnitudes proportional to the overlap between the CH bond and the a-system. Both experimentally and theoretically, 6Jp(CH,,H) Present address: Department of Chemistry, University of California, Berkeley, CA 94720.
0022-365419112095-0020$02.50/0
where the first and second terms correspond to the u-T and u mechanisms, r e s p e ~ t i v e l y 5Jw" . ~ ~ ~is~0.34 ~ (fO.O1) Hz and 5J180" is 0.32 (fO.O1) Hz," where the errors have been estimated by the present authors. In this report, information from spectral parameters has been compared with inferences from semiempirical molecular orbital ( 1 ) Haag, R.; Wirz, J.; Wagner, P. Helv. Chim. Acta 1977, 60, 2595. (2) Scaiano, J. C. Chem. Phys. Lett. 1980, 73, 319. (3) Scaiano, J. C. Tetrahedron 1982, 38, 819. (4) Grellmann, K.-H.; Weller, H.; Tauer, E. Chem. Phys. Lett. 1983, 95,
195. Baron, U.; Bartelt, G.;Eychmiiller, A.; Grellmann, K.-H.; Schmitt, U.; Tauer, E.; Weller, H. J. Phorochem. 1985, 28, 187. (5) Wasylishen, R.; Schaefer, T. Can. J. Chem. 1972, 50, 1852. (6) Parr, W. J. E.; Schaefer, T. Arc. Chem. Res. 1980, 13, 400. (7) Schaefer, T.; Sebastian, R.; Wildman, T. A,; Dettman, H. Can. J . Chem. 1982, 60, 2274. (8) Schaefer, T.; Wildman, T. A,; Sebastian, R. Can. J. Chem. 1982,60, 1924. (9) Schaefer, T.; Sebastian, R.; Penner, G. H. Can. J. Chem. 1985, 63, 2597. (IO) Barfield, M.; Fallick, C. J.; Hata, K.; Sternhell, S.; Westerman, P. W. J. Am. Chem. SOC.1983, 105, 2178. ( 1 1 ) Schaefer, T.; Laatikainen, R. Can. J. Chem. 1983, 61, 2785.
0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 1, 1991 21
Conformational Behavior of Some Aryl Ketones
TABLE I: Long-Range Coupling Constants for 2,2-Dideuterio-l-tetralone(3), I-Benzosuberone (4), Dibenzosuberone (S), and Dibenzosuberenone (6) in Acetone-d, at 303 K O tetralone benzosu berone dibenzosuberone di benzosu berenone 4Jo(CH2.S) 5J,(CH2,6) 6Jp(CH237) 'J,,,(CH2?8)
-0.910 (3) 0.308 (2) -0.670 (3) 0.464(3)
4J.,(CH2,6) 'J,(CH2,7) 6Jp(CH2.8) $J,(CH2,9)
-0.677 (8) 0.264(7) -0.407 (6) 0.323(8)
4J,(4,10) = 4J0(4,10') 5J,(3.10) = 'J,(3,10') 6Jp(2,10) = 6Jp(2,10') 5J,(l,10) = 5J,(1,10')
-0.829 (5) 0.20' -0.546 (4) O.2Ob
14Jo(4,10)l 'J,(3,10) 16Jp(2,10)l 'J,(I ,IO)
IO
0
'
0. I
- - 7- _
0.65
5-
1
1
'
3
--__ \
5 34 23 16
+m,m
em 22-30 60
> I 7 and 90°, highly unfavorable energetically because the methyl substituent is in proximity to the second phenyl group in such conformations. The left-hand side should be similar to that in Figure 3, and the shifts should thus be comparable to those calculated for benzophenone. However, the unique ortho proton and the two meta protons of the methylated ring remain distinct. In support of this approximation, the observed shifts of the para protons in benzophenone and 2-methylbenzophenone are 0.045 and 0.039 ppm, respectively, and the meta proton shift of 0.042 ppm is close to the average of the H3 and H5 shifts for 2-methylbenzophenone, which is 0.043 ppm. The observed shift differences for 2-methylbenzophenone are -0.477 (-0.440), 0.003 (-0.010), 0.039 (0.080), and 0.083 (0.127) ppm, which compare well with the values calculated for the AMI surface (in parentheses). On the other hand, the -0.599 ppm shift calculated for H, of the (3Oo,3O0)conformer is rather larger than that measured. The shifts for the unsubstituted ring resemble those for benzophenone itself.
H, H, H, H,' H,'
-0.216 -0.002 0.048 0.075 0.156
-0.224 -0.008 0.032 0.052 0.097
-0.011 0.028 0.033 0.045 0.090
-0.094 0.059 0.080
-0.155 0.057 0.081
0.113 0.080 0.076
" I n ppm. Positive values denote deshielding. bThese sums exclude the contribution from carbonyl anisotropy, which should be similar to that for acetophenone. CFrom the tensor components relative to T M S in ref 48, namely, uII = 234 ppm along the CH bond, u22 = 146 ppm in the ring plane, and q3= 9 ppm normal to the ring plane.
barrier of 1 1 . 1 kJ mo1-I while the t ~ r s i o n aand l ~ ~microwaveSZ ~~~ spectra suggest values between 19.3 and 20.5 kJ mol-'. Notice that the magnitude of the rotational barrier in benzaldehyde is not to be compared with the low barrier on the benzophenone surface, but more appropriately with the diagonal path. The computed molecular geometries were used to calculate ring-current-induced shifts from eq 7. These shifts were used in turn, along with the relative energies, to determine average shifts for each proton from JJuH($I?$2)
(Ah) =
exp[-@V($I?$2)I d#l
JJexPI-@V(#17$2)1
+
d$' d$z
Summary and Conclusions d#Z
(8)
+
where uH is the isotropic value ( u I I H uZ2" u33H)/3. The integrals were approximated by numerical quadrature to a precision of 0.1%. Intermediate values of the energy or the chemical shifts were obtained by bilinear interpolation between grid points. Contributions from the ring current, the shielding anisotropy of the carbon atoms of the distant phenyl group, and the carbonyl shielding anisotropy are given in Table IV. These numbers represent averages over the surface in Figure 3. Also included in the table are the average shifts (H, + H,')/2 and (H, + H,')/2 calculated for the AMI surface, for the completely rigid (3Oo,3O0) conformer and for completely free internal rotation. It seems likely that the contribution from the carbonyl anisotropy will be similar in acetophenone and benzophenone; therefore, it has been excluded from these averages shifts. The average shifts are thus equated with the shift differences for benzophenone relative to acetophenone. Within this model, both the ring current and the anisotropy of the carbon shielding tensors make significant contributions to the net shift^.^^,^^ For benzophenone, the average shifts for the rigid (3Oo,3O0) conformer are in closest agreement with the observed shift differences for the ortho and meta protons in Table 111. However, the values from the AMI surface, for which the barrier on the diagonal is high but the barrier for the minimum energy path is relatively low, are not inconsistent with the measured shifts. A high barrier to internal rotation is not necessarily required to explain these shifts. On the other hand, free rotation of both phenyl groups results in larger shieldings for the ortho and meta protons than for the para proton, which is qualitatively incorrect. The fact that all of the models overestimate the deshielding of the para proton suggests that further refinements are required (52) Kakar, R. K.; Rinehart, E. A.; Quade, C. R.; Kojima, T. J . Cfiem. Pfiys. 1970, 52, 3803.
The value 6J90* = -1.04 f 0.03 Hz leads to an internally consistent interpretation of the spectra and conformational behavior of the ketones studied. Model calculations with symmetric double-well torsional potentials indicate that 6Jp(CH2,H)and 6J,(CH,H) are relatively insensitive to the height of the barrier unless the potential minima are widely separated. However, these couplings are sensitive to the separation of the minima. On the basis of benzylic couplings, the alicyclic rings of 1tetralone and 1-benzosuberone are rapidly inverting between two boatlike conformers. In dibenzosuberone, the second aromatic ring decreases the separation of the minima and lowers the barrier by decreasing the flexibility of the central ring. In dibenzosuberenone, the central ring is even more highly constrained. The AM 1 calculations appear to provide a reasonable description of these torsional potentials and of those for acetophenone and 2methylacetophenone, but barrier heights are underestimated somewhat. The IH chemical shifts of benzophenone relative to acetophenone and of 2-methylbenzophenone relative to 2-methylacetophenone have been interpreted as averages over the hindered internal rotation. Comparison with calculated shifts suggests that the phenyl groups are twisted by about 30' with respect to the carbonyl group. This is consistent with results from crystallography. The AMI barrier to internal rotation is about 3 kJ mol-', which is probably an underestimate of the true height. Acknowledgment. This work was supported by a grant from the Natural Sciences and Engineering Research Council of Canada. We are grateful to Dr. Donald Hughes for assistance in obtaining the spectra and to Professors Michael McGlinchey and David Santry for useful comments on chemical shifts. Supplementary Material Available: Tables SI-SI11 listing the results of high-resolution analyses for compounds 1-6 and Figures S 1 4 3 showing the AM1 torsional potentials for compounds 3-5 (9 pages). Ordering information is given on any current masthead page.
