2401
AEoelecrobtained by the linear SEE relationship. I n the meanwhile, the linear SEE relationship appears to provide an extremely valuable approximate relationship as discussed above.
Acknowledgment. We are pleased to acknowledge the helpful assistance of Professor Arnett, Dr. C. Douty, and Mr. T. Murty in obtaining preliminary results with their calorimeter.
Analysis of the Nuclear Magnetic Resonance Spectra of Some 2,6-Bridged Bicyclo [2.2.1]heptane Derivatives Kermit C. Ramey,'" David C. Lini,'" Robert M. Moriarty,lb H. Gopal,lband Harold G. Welshlb Contribution from the Research and Development Department of the ARCO Chemical Company, a Division of Atlantic Richjeld Company, Glenolden, Pennsylvania, and the Department of Chemistry, The Catholic University of America, Washington, D. C. Received December 15, 1966 Abstract : The 60- and 100-Mcisecnuclear magnetic resonance spectra of several bridged bicyclo[2.2.l]heptane derivatives have been analyzed in detail. These compounds possess the common structural feature of a 2,6 oxygenated bridge which may be either a lactone as in 5-exo-iodo-6-endo-hydroxybicyclo[2.2.1 ]heptane-2-endo-carboxylic acid lactone (1) and the 5-exo-bromo (2), 5-exo-acetoxy (3), 5-exo-tosyloxy(4), 2-exo-methyl-5-exo-iodo (9, 2-exomethyl-5-exo-bromo ( 6 ) derivatives; or the 2,6 bridge may be an oxido unit as in 4-exo-tosyloxy-6-oxatricyclo[3.2.1 13v6]nonane(7)and 4-exo-acetoxy-6-oxatricyclo[3.2.1.13~8]nonane (8). Chemical shifts for all the protons in these structures have been assigned, and the geminal and vicinal couplings measured. Aromatic solvent shifts observed for compounds 1,5, and 7 are discussed in terms of solvent-solute collisional complexes of defined stereochemistry. Long-range couplings for the proton pairs : 1-4,3-endo-7(a), 5-endo-7(b), 2-exo-6-exo,l-3-exo, 2-exo-4, and 6-exo4 in compound 1 were observed and most of these were confirmed using spin-decoupling techniques. For the C5endo proton doublet in the lactone derivatives it is noteworthy that the principal coupling is with the C7(b)proton; this amounts to about 2.5 cps while the Cj-endo-Cs-exo vicinal coupling is negligibly small (0.3 cps) and the CS endo-C4-vicinal ranges from 0.5 to 1.0 cps. The nmr spectra of the 5-keto and 7-keto derivatives in the 2,6 lactone series are discussed in relationship to the changes in chemical shifts relative to the precursor secondary alcohols. acid lactone (9) and The alcohol-ketone pairs are 5-exo,6-endo-dihydroxybicyclo[2.2.l]heptane-2-endo-carboxylic 5-keto,6-endo-hydroxybicyclo[2.2.1]heptane 2-endo-carboxylicacid lactone (10); 6-endo,7(b)-dihydroxybicyclo[2.2.1]heptane-2-endo carboxylic acid lactone (11) and 7-keto,6-endo-hydroxybicyclo[2.2.l]heptane-2-endo-carboxylic acid lactone (12). ,
T
he norbornyl system has served as a substrate for the generation and evaluation of numerous mechanistic hypotheses in modern organic chemistry. Mechanistic conclusions originating from studies in the norbornyl series frequently have been based upon the structures of rearranged products. Detailed nuclear magnetic resonance spectral analyses in this series obviously are of importance in facilitating the elucidation of rearrangement products. Moreover, due to the conformational rigidity of these systems, long-range
1
2
3
4
(1) (a) The ARCO Chemical Co., Glenolden, Pa.; (b) The Catholic University of America, Washington, D. C.
couplings which are often of a n unexpectedly large magnitude may be detected. This paper presents detailed analyses of the spectra of compounds 1-8. The compounds included in this study are of current and particular interest because of uncertainties of interpretatior?" and previous erroneous assignments.2b
Chemical Shifts I n a preliminary communication of a portion of this work,3 we pointed out that the chemical shifts of the C1- and Cz-exo protons in compound 1 were anomalous in the sense that the C1 proton appeared at a lower field position relative t o the C2-exo proton which is attached to the carbon atom bearing the carbonyl group of the lactone. This assignment was required to explain the magnitude of the couplings associated with the C8-exo proton. A n earlier interpretation of the spectrum of 2 used the reverse assignment of the chemical shift of the C1- and Cz-exo protons.2b This incorrect assignment was used recently by Jensen and Miller2" who corrected stereochemical assignments of Traylor and Factor for the structure of the oxymercuration product derived from 5-norbornene-2-endo-car(2) (a) F. R. Jensen and J. J. Miller, Tetrnhedron Letters, 40, 4861 (1966); (b) E. Crundwell and W. Templeton, J . Chem. SOC.,1400 (1964). (3) R.M. Moriarty, H. Gopal, H. G. Walsh, K. C. Ramey, and D. C. Lini, Tetrahedron Letters, 38, 4555 (1966).
Ramey, et a[. 1 Nmr Analysis of Bicyclo[2.2.l]heptane Derivatives
2402
A
5
8
6
9
B
I
ccc
-
C+H2
C
Figure 1. 100-Mc/sec nmr spectra of: A, compound 2 ; B, compound 6, and C , compound 8, in solution in CDQ.
boxylic acid.4a,b The incorrect chemical shift assignments used by Jensen and Miller for the CI- and CY exo protons, however, d o not influence the validity of their conclusions with respect to the configuration of the C5-endo proton in compounds 1 and 2 which they studied. Figure 1A-C presents the IOO-Mc/sec nmr spectra of compounds 2, 6, and 8, respectively (Table I presents chemical shifts). For lactones 2 and 6 one may immediately discern a similarity; namely, three resonances equivalent to one proton each appear at downfield positions and these resonances are clearly (4) (a) T. G. Traylor and A. Factor, Abstracts, 147th National Meeting of the American Chemical Society, Philadelphia, Pa., April 1964, p 36N. (b) Professor Traylor informed us of his revision of the stereochemistry originally proposed by him and Factor& for the oxymercuration product of 5-norbornenc-2-e~zdo-carboxyl~c acid on Nov 20, 1964, in a seminar in this department,
Journul of the American Chemical Society
89.10
separated from the complex upfield absorption. The absorptions around T 5.1 and 6.1 are logically assigned to the C6-exo and C5-endo protons, respectively. The resonance at T 6.75 in 2 might be due t o the C?-exo proton but this assignment leads t o a number of unreasonable interactions. For example, spin-decoupling experiments revealed that the proton in 2 at T 6.75 was coupled with the CG-exoproton to the extent of 5 cps. This would be an unprecedentedly large longrange coupling for such protons. This coupling was clearly better accommodated by a vicinal rather than a 1,3 arrangement of these interacting protons. The spectrum of 6 clarifies this point in that a methyl group occupies the site in this derivative of the C2-exo proton present in 1 and 2, yet a resonance still occurs at T 7.20. Accepting this alternative assignment, i.i.., that C1 proton gives rise t o the absorption at 7 7.2, appearance of this proton in 6 as principally a doublet is due t o the absence of a proton at C? for coupling. Furthermore, any reasonable analysis of spectra in this series, as will be shown below, demands these relative assignments of the C1- and CS-exoprotons. The reason for this unexpected reversal i n the chemical shifts of the C1- and C2-exo protons is based upon a model in which the C1 proton is axial with respect t o the lactone ring and is located in the region of maximum deshielding resulting from the ring current associated with the lactone carbonyl group. This interpretation is also consistent with the results obtained for compounds 5 and 6 . The upfield shifts of about 0.4 ppm for the C1 proton in compounds 5 and 6 is probably due t o the anisotropy of the carbon-carbon u bond of the methyl group a t C,.j Accordingly, the Cs-exo proton in 1 should also experience extra shielding, and reference t o the data in Table I bears out this prediction. The differerence in magnitude of the upfield shift is associated with the dihedral angle between the C2-CH3 bond and the C1-H, C3-exo-H, and C3-eudo-H bonds. The maximum upfield shift is experienced by the Cs-exo proton, A6 C,-ex-o(l)-C:i-cso(5) = $0.54. The dihedral angle here is close t o 0". The C?-CH;I and C1-H dihedral angle is about 40"and A6 Cl(l)-C1(5) = $ 0.44. The dihedral angle for the C2-CH3-C3-erdo-H is about 120" and A6 C3-erzdo(l)--C3-end(5) = -0.42. Thus, this proton suflers a large deshielding effect due t o the anisotropy of the C-CH:] bond. The effect upon the C6-exo and CiLb)protons is very small. Replacement of the carbonyl group of the lactone by a methylene group, as in compounds 7 and 8, causes a further upfield shift of the C1 protons relative to the position in the lactones. The spectrum of 8, Figure l C , exhibits a typical AB part of an ABX pattern for protons 8(a) and 8(b). Inspection of models reveals that only one of the CS methylene protons has the correct geometry for spinspin coupling with the C2-exoproton. I n most cases the assignment of resonances corresponding t o the C3-exo, C3-endo,and 7(a), 7(b) proton pairs was straightforward. The expected larger value of J3.exo-3.erido over J7(a)-i(b) as found by Laszlo and Schleyer was of diagnostic value heresG Differentiation of the chemical shifts of the individual protons of these (5) J. W. ApSinion, W. G. Craig, P. V. Demarco, D. W. Mnthicson, L. Saundcrs, and W. B. Whalley, Chern. Cornntiri~.,359 (1966). (6) P. Laszlo and P. von R. Schleyer, J . A m . C h e m . S O C . ,86, 1171 ( 1964).
May IO, 1967
2403 Table I. Chemical Shifts for Some Bicyclo[2.2.l]heptane Derivatives
Chemical shifts, Compd 1 2 3
4 5 6 7%b @,b
1 6.78 6.75 6.80 6.87 7.22 7.20 7.48 7.38
6es 4.88 5.06 5.51 5.52 4.93 5.15 5.95 5.94
5 en 6.08 6.13 5.45 5.75 6.15 6.21 5.91
4 7.30 7.35 7.50 7.51 7.34 7.41 7.76 7.78
5.68
3e l 7.94 7.89 7.97 8.04 8.48 8.41 8.21 8.17
7
3 en 8.48 8.25 8.28 8.43 8.06 8.19 9.10 8.90
7
2e 9
7(a)
7(b)
7.46 7.48 7.45 7.60 8.83 (CHz) 8.84 (CHa) 7.76 7.70
7.68 7.72 8.01 8.02 7.64 7.71 8.15 8.10
8.18 8.26 8.38 8.43 8.12 8.24 8.53 8.48
Note a The chemical shifts for the protons in the 8 positions are 7 6.53,and 6.47for compound 7 and 6.19 and 6.34for compound 8. that the numbering of the oxido methylene group as Cs is not correct as far as nomenclature is concerned; Le., compounds 7 and 8 are tricyclo[3.2.1.13,8]nonanederivatives. However. we use the above incorrect nomenclature in order to focus on the relationship of these compounds t o their norbornyl analogs.
geminal pairs was accomplished by analysis of stereospecific long-range coupling using decoupling techniques; cide infra. The assignments of the Cz-exo, Ca-exo, and C 4 protons were implied from previous studies which showed that the spin-spin coupling of the Cz-exo and C3-exo protons should be relatively large, i.e., 7.5-11.5 cps, while the coupling of the C4 proton with the C3-exo, C3-endo, Cj-endo, C7(a),and C7(b) protons should be comparatively ~ m a l l e r . ~ ~ ~ ~ Further confirmation of the above chemical shift assignments derive from the nmr spectra of compounds 9,10, 11, and 12.
"5;' "104 RuOl
___f
0 9
10
In the case of alcohol 9 the Cj-endo proton at 60 Mc/sec appears as a singlet peak at T 6.30. The peak width at half-height is 2.8 cps. The CG-exo proton appears as a doublet at T 5.5. The principal coupling of the C5-endo proton is with the C7(b) proton, while the principal coupling, 5.0 cps, for the Ce-exo proton is with the C1 proton. Oxidation of 9 to 10 removes the Cj-etzdo proton absorption. The C6-exo proton is shifted by $0.55 ppm. Similarly, for 11, the C7(b) proton appears as a singlet with peak width at a halfheight of 4 cps. The C1 proton appears as a doublet of doublets with further small splitting (JG.eno-l 5 cps, J2.rxo-l 5 cps, J7(b)-1 1.5 cps, J1-4 1 cps). Oxidation of 11 to 12 removes the C7(,,)resonance and the C1 resonance is essentially unshifted. The small C7(b)-lcoupling also vanishes as expected.
-
-
-
N
(7) (a) I
