N.M.R.SPECTRA OF NORBORNENES
Sept. 20, 1963 [CONTRIBUTIOX FROM THE
FRICKCHEMICAL LABORATORY, PRINCETON
UNIVERSITY, PRISCETON,
2709 N. J , ]
Electronegativity of Substituents and Nuclear Magnetic Resonance Spectra of Norbornene Derivatives BY PIERRELASZLO’ AND PAULVON RAGUBSCHLEYER~ RECEIVED MAY3, 1963 The magnitudes of coupling constants between protons on vicinal saturated carbon atoms vary significantly with changes in the electronegativity of adjacent substituents. This effect has been demonstrated in a number of 5-endo-2-norbornenyl derivatives ( I ) . The internal chemical shifts for the same protons also depend on the substituent electronegativities. Contrary t o the literature, vinyl and ethyl derivatives are found not t o be suitable models for the estimation of these coupling constant and internal chemical shift effects, respectively. Additional features of the nuclear magnetic resonance spectra of compounds I are analyzed.
Coupling constants J between protons on vicinal carbon atoms C 1 H 1 - C Z H 2 depend not only on the dihedral angle $ between the C I H I C z and C1C2H2 plane^,^ but also on the electronegativity of substituents X, CH-CHX.4-1z The large variation of J with dihedral angle, a cos2 function, gives t h e very familiar Karplus curve.3 The effect of substituent electronegativity on J, although first noted many years ago,4 is much less generally appreciated. For simple aliphatic molecules this electronegativity variation is small,4but for vinyl compounds the effect can be of comparable magnitude to that of a change in g e ~ m e t r y . ~ For - ~ more complicated aliphatic molecules structural distortions due to the bulk of substituents tend t o obscure the electronegativity This is especially true for cyclopropanes, which, because of the quasi-double bond character of the carbon-carbon linkages, might be expected to be relatively sensitive to the electronegativity effect. Although significant variations in coupling constants of polysubstituted cyclopropanes have been found, no direct correlation with electronegativity is yet possible with the compounds ~ t u d i e dlZa .~, To investigate the effect of substituent electronegativities on coupling constants in more complicated aliphatic systems, rigid compounds of known and fixed geometry are needed. We report here on a series of endo-Diels-Alder adducts I of cyclopentadiene and various dienophiles. While this work was in progress, Williamson described the results of a similar study on the adducts I1 of hexachlorocyclopentadiene. l 2 The two investigations are complementary and both demon-
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c1
I
c1
I1
(1) On leave, Facult4 des Sciences, University of Paris, Paris, France. ( 2 ) Alfred P . Sloan Research Fellow. (3) M . Karplus, J . C h e m P h y s . , 30, 11 (1959); H . Conroy in R . A. Raphael, E C. Taylor, and H. Wynberg, E d . , “Advances in Organic Chemistry. Methods and Results,” Vol. 2 , Interscience Publishers, Inc., h-ew York, N . Y . , 1960, pp. 310-311. (4) R E Glick and A. A Bothner-By, J C h e m . P h y s . , 25, 362 (1956). ( 5 ) C N Banwell, N Sheppard. and J . J . Turner, Speclrochim. Acto, 16, 794 (19601, C . N . Banwell and N. Sheppard. Mol. P h y s , 3, 351 (1960). (6) J S Waugh and S.Castellano, J C h e m . P h y s . , 36, 1900 (1961). ( 7 ) J Feeney, A Ltdwith, and I,. H . Sutcliffe,J . C h e m . Soc., 2021 (1962). ( 8 ) T . Schaefer, C a n . J . C h e m . , 40, l ( 1 9 6 2 ) . (9) H M . Hutton and T. Schaefer, ibrd, 40, 875 (1962); 41, 684 (1963); J D. Graham and M. T. Rogers, J . A m . C h e m . Soc., 84, 2249 (1962) (10) C i Y Banwell and N . Sheppard, quoted by Hutton and Schaefer, preceding reference. (11) H S Gutowsky, G. G . Belford, and P . E . McMahon, J . C h e m . P h y s . , 56, 3383 (1962). (12) K . 1- Williamson, J . A m . C h e m . Soc., 85, 516 (1963). (12a) N O T E ADDED I N PROOF -Such correlations have now been reported: K . L. Williamson, C. A. Lanford, and C . R . Nicholson, Abstracts, 145th hTational Meeting of the American Chemical Society, New York, N. Y . , Sept , 1963
strate the dependence of J on electronegativity. We here extend and modify some of the conclusions of the earlier investigation. Results Analysis of N.m.r. Spectra.-Desired features of the spectra of 5-endo-substituted 2-norbornenes (I) were analyzed by first-order theory. Typical spectra, t h a t of the chloro compound (I, R = C1) and of the cyano compound (I, R = CN) are given in Fig. 1. The spectrum of the nitro compound (I, R = NOz) has been published as part of a study of norbornene derivat i v e ~ . ’ The ~ olefinic hydrogens H b and Hc and the broad bridgehead proton peaks H a and H d can readily be discerned in the spectra. One of these resonances comes a t a position very close to that of the bridgehead protons of norbornene (I, R = H) and is assigned Ha on this basis. The other bridgehead resonance, H d , is shifted downfield. These assignments are reinforced by a quantitative assessment of the electronegativity effect of the substituent R discussed below. Both exo protons a t C-5 and C-6 (He and Ht) are deshielded-they lie approximately in the plane of the magnetically anisotropic double bond13-and the resonances of both can be distinguished from others (H, and the C-7 methylene group) in the molecules. He is further deshielded by the substituent R. The observed chemical shifts are summarized in Table I . First-order analysis yielded the values of the nonzero coupling constants describing the spin-spin interactions of hydrogens H a , H e , Hr, H,, and H d ; namely, J e f , Jfg, J d e , J e g , and J a t . With one exception, the interaction of H e with H d and with H, gave rise to a triplet. (The exception (I, R = CH2=CHSOZ-) gave four lines for H e , a pattern typical of the X part of an ABX spectrum.) The triplet usually observed for proton He-an apparent AzX spectrum-is due to the fortuitous near identity of J e g and J d e . I t is not a “deceptively simple ~ p e c t r u m ” ~the ~ ; requirements 0 and for the latterI4are not met in this system: J d g the chemical shift difference between protons H, and H d is relatively large, e . g . , a t least 30 C . P . S . for I , R = C1 (Fig. 1). Actually, ]de and J,, are slightly different, but the resolution available was insufficient t o distinguish them; Table I1 lists only their average, I l z ( J d e Jeg). For each substituent R in Tables I and 11, electronegativities ER” are given. In some cases the internal chemical shifts between Hb and Hc were sufficient (-10-15 c.P.s.) to permit observation of a typical eight-line multiplet (Fig. 1) in the olefinic region; it consisted of an AB spectrum, each line of which was again split into two by coupling to the vicinal bridgehead proton, H b by Ha and Hc by Hd. The results are included in Tables I and 11. I t
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(13) R R Fraser, Con J . C h e m , 40, 78 (1962). (14) R . J . Abraham and H . J . Bernstein, ibid , 39, 216 (1961). (15) Taken from J . R . Cavanaugh and B . P . Dailey, J . C h e m . P h y s . , 34, 1099 (19611, where available. or calculated by t h e same procedure from d a t a measured here or taken from t h e literature.
PIERRELASZLO AND PAULVON RAGUSSCHLEYER
2710
I. R
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CHEMICAL SHIFTS OF
ER
I/.(&
Vol. 85
TABLE I 5-endo-SUBSTITVTED NORBORNENE DERIVATIVES, C.P.S.
+ €I.)Hd
OOCCHa 3.72 366 NO%* 3.70 368 CI 3.25 370 Br 2.96 369 SO>CH=CHz 2.94 365 CsHs 2.75 357 COCHI 2.70 351 CHO 2.69 364 COOH 2.60 362 COOCIHs 2.55 360 CN 2.49 375 H 2.1 357.5 Ref. 13. Estimated approximately.
COUPLING CONSTANTS I N
H.
E.
187 212 185.5 186 193 179sh 187 192 191 189 192 171
170 180 173.5 169 179 173.5 169 . 174.5
... 171 179 171
310 296 268 254 211 199 172 (1713)~ 179 171 167 (96)'
€ I . Ht
HI
125.5 (128)b
135 135 124 128
- Hd 123
He
184.5 (168)* 123 119 87 71
...
...
112 123 110 127 (9Wb
(Wb 56 61 42 0
84
72.5 68 18 20 - 15 ( - 16)b - 12 - 18 -25 ( - 73)B
TABLE I1 5-endo-SUBSTITUTED NORBORNENE DERIVATIVES, C.P.S.
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ER Jer 'VUeg J.d Jf. J.5 EJda Jer Jeg Jb. (Jab. J o d 3.72 7.9 3.25 .. 3.5 14.4 5.4 2.9.3 . O 3.70 7.8 3.9 .. .. 15.6 ... 3.25 8.2 3.5 12.0 3.75 15.2 2.8,3.0 Br 2.96 8.2 3.45 12.5 4.0 15.1 5.3 3 .O, 3.0 SOCH=CH, 2.94 8.7 3.85C .. 3.3 16.4 ... ... 2.75 8.9 3.7 10.4 3.7 16.2 ... ... CaHa 2.70 ... . .d .. .. .. ... COCHI CHO 2.69 8.7 .. 12.0 3.5 .. 5.6 2.4.2.5 COOH 2.60 8.5 4.0' 11.6 3.4 16.5 5.7 2.5.2.6 COOCsHs 2.55 8.9 4.1d .. 3.5 17.1 5.7 2.5.2.6 CN 2.49 9.2 3.6 11.5 3.5 16.4 5.7 2.9.2.5 Hb 2.1 ... .. .. .. .. 5.8 2.95.2.95 Estimated from the published spectrum, ref. 13. Values determined by analysis of the "C-H patterns. ref. 19; Pauling electronegativity used. Individual values observable here: 3.0 and 4.7 C.P.S. For R = COR', M. Green and E. A. C. Lueken (Helv, Chim.Acta, 45, 1870 (1962)) indicate values of 3.5-4.0 c.p.s. for I,..
I, R
=
OOCCHI NO** CI
was not possible to distinguish between HL, and H, and only the position of the center of gravity of the olefinic multiplet is reported. This position is structure de-
Discussion Chemical Shifts.-.The differences in chemical shifts between exo protonsat c.5 and c.6 (H. and H,) gave a good correlation with substituent electronegativities (En), independent of anisotropy effects16 (Fig. 2).
