1 T
SNR SPECTRA OF SUBSTITUTED CYCLOPROPANES
Substituent Effects.
V1I.l
279 1
Carbon-13 Nuclear Magnetic
Resonance of Substituted Cyclopropanes
by Paul H. Weiner and Edmund R. Malinowski Department of Chemistry and Chemical Engineering, Steaens Institute of Technology, Hohoken, S e w Jersey 07050 (Received J a n u a r y 9, 1967)
Carbon-13 chemical shifts and directly bonded carbon-hydrogen coupling constants are reported for a series of cyclopropyl compounds. The chemical shifts of the a-carbon atoms shorn a linear correlation with similarly substituted methanes. Carbon-13 shifts of methanes, benzenes, vinyl groups, methyl ketones, and cyclopropanes are compared by a method of least squares. In most of these cases, the correlation of carbon-13 shifts between different systems seems to be a function of more than two factors.
Introduction Chemical shifts and coupling constants, observed in nmr spectra, are strongly dependent upon the various substituents of the molecule. Some coupling constants are directly whereas others are pairwise a d d i t i ~ e with ~ - ~ respect to the substituent groups. For example, directly bonded C-F, Si-H, and Sn-H and nondirectly bonded Sn-H coupling constants are pairmise additive.' In contrast, the carbon-13 shifts of unsubstituted hydrocarbons are directly additive and one can assign constitutive parameters.* Shifts of the monosubstituted hydrocarbons deviate from the simple constitutive rule.9 The linear alkanes also conform to a simple additivity rule.1° Cyclopropyl derivatives are an excellent subject for investigation because the ring is rigid and free from complications which might arise from carbon-carbon bond rotations. Unfortunately the proton spectra of cyclopropanes, in particular monosubstituted cyclopropanes, are complicated and in many instances computer techniques for the analysis of the spectra are required." Even after such analyses have been completed, it is difficult to assign the resulting shifts and couplings to the proper nuclei. In general, carbon13 spectra are easier to analyze although more difficult to obtain. For these reasons we have chosen to investigate the effects of substituents on the carbon-13 spectra of cyclopropanes. The state of knowledge of carbon-13 spectra can be found in a recent review by Stothers.I2 The unusually
large shielding of the carbon-13 nuclei of cyclopropane has been explained by Burke and Lauterbur13 as the result of a diamagnetic ring-current effect which originates from the peculiar electronic distribution of the three-membered ring. llaciel and Savitsky14 measured the carbon-13 shifts of cyclopropyl methyl ketone and cyclopropyl cyanide. Their results clearly show that the shifts of the ring carbons are affected by the presence of the substituent. (1) Part V I : T. Vladimiroff and E. R. Malinowski, J . Chem. P h y s . , 46, 1830 (1967).
(2) (a) E. R. Malinowski, J . Am. Chem. SOC.,83, 4479 (1961); (h) E. R. Malinowski, L. 2. Pollara, and J. P. Larm:mn. ibid., 84, 2649 (1962). (3) K. Tori and T. Nakagawa, J . P h y s . Chem., 68, 3163 (1964). (4) A . W.Douglas, J . Chem. Phys., 40, 2413 (1964). (5) E. R. Malinowski and T. Vladimiroff, J . - I m . Chem. Soc., 86, 3575 (1964). (6) T. Vlndimiroff and E. R. Malinowski, J . Chem. P h y s . , 42, 440 (1965). (7) E. R. Malinowski, T. Vladimiroff, and R. F. Tavares, J . P h y s . Chem., 70, 2046 (1966). (8) G. B. Savitsky and K. Kamikawa, ihid., 68, 1956 (1964). (9) G. B. Savitsky, R. M . Pearson, and K. Namikawa, ibid., 69, 1425 (1965). (10) E. G. Paul and D. M. Grant, J . Am. Chem. Soc., 85, 1701 (1963). (11) K. B. K i b e r g and B. J. Nist, ihid., 85, 2788 (1963). (12) J. B. Stothers, Quart. Rev. (London), 19, 144 (1965). (13) J. J. Burke and 1'. C. Lauterbur, J . Am. Chem. Soc., 86, 1870 (1964). (14) G. E. Maciel and G. B. Savitsky, J . P h y s . Chem., 69, 3925 (1965).
Volume 7 1 , iyumber 9
August 1967
2792
PAUL
H. M'EINER
AND
~~~
EDYUND R. J'IALINOWSKI
~
Table I : 13C' Chemical Shifts (ppm Relative to W H J ) and JCH Coupling Constants (cps) for Ring Carbons in Substituted Cyclopropanes Compound no.
Compound structure
______ 4, ppm _______ carbon
0 carbon
1
16.7
16.7
2 3
34.8 16.6 {16.2
4
----
JCH,cps----b carbon
Ref
162
162
b
29.7 (27.2 (26.2
184 179
167 I69
a a
...
...
C
34.3
28.0
157
153
a
5
43.6
27.0
173
161
a
6
76.6
42.0
...
170
a
7
30.6
2.5.6
165
167
a
8
39.2
23.2
160
161
a
9
36.8
31.1
173
173
a
10
41.2
29.7
...
' Measured in this laboratory. J . Phys. Chem., 69, 3925 (1965).
a
carbon
* J. Burke and P. C. Lauterbur, J . -4.m. Chem. Soc., 86, 1870 (1964).
Experimental Section All measurements ivere made on neat liquids with a Varian DP-60 spectrometer operating at 15.0% Xc/sec and at a temperature of approximately 27". The carbon-13 resonance was observed in the rapid passage dispersion mode. Samples were placed in a nonspinning 15-mm o.d. thin-wall tube. For calibration purpows, a sample of enriched methyl iodide contained in a sealed 5-mm tube was placed inside the 15mm tube containing the sample. Spectra were calibrated by the usual side-band technique with a HewlettPaclrard wide-range oscillator, Jlodel 200CDR. The wide-range oscillator frequency was counted accurately with a Hewlett-Paclrard frequency counter, ;\Iode1 X2B. Because high radiofrequency power and rapid sweep rates are required for recording natural abundance carbon-13 fpectra, distortions of the signals result. In determining chemical shifts, errors caused by this distortion can be minimized or eliminated by taking the mean of an upfield and downfield pass. In Table I shifts of compounds 2-6 were determined from graphs in the usual manner. Because the methyl iodide signal overlaps the signal of the sample, comThe Journal of Physical Chemistry
a
C
G. Maciel and G. B. Savitsky,
pounds 7-9 were calibrated in the following manner. Spectra were recorded with and xithout the methyl iodide reference sample. From these two sets of data, it was possible to determine all chemical shifts and coupling constants. Shifts were measured with an accuracy of approximately hO.2 ppm and couplings with an accuracy of approximately f2 cps. Positive shifts are low-field shifts and negative shifts represent upfield shifts with respect to methyl iodide. S o bulk-susceptibility corrections were made. Samples obtained from commercial sources, containing carbon13 in natural abundance were used without further purification or dilution.
Spectral Assignments Assignments were made 011 the basis of peak intensities and of splitting patterns due to proton coupling. Owing to "magnetisation transfer," the second peak of a carbon-13 doublet to be scanned is of lower intensity than the first. Triplets and quartets are similarly skewed. This phenomenon is a great aid in making spectral assignments, especially when the peaks of different carbon nuclei overlap. An example of such overlap is shown in Figure 1 for
13C
2793
NMRSPECTRA OF SUB~TITUTED CYCLOPROPANES
i l
Table 11: *SCChemical Shifts (ppm Relative to W H J ) and JOE Coupling Constants (cps) for Carbon Atom of the Substituents Compound
F'3CSN
142.9
[t-WOOH
202.0
D"" I)-(XQ
:
Figure 1. The 15.085 Mc/sec carbon-13 spectra of cyclopropyl carboxylic acid.
60.5
127
161.4 147.9
155
97.6
146
H
Pi
OH
cyclopropyl carboxylic acid where the methine doublet is hidden under the methylene triplet. Lines 1 4 are the quartet signals of methyl iodide, the external standard. By comparing the upfield and downfield passes, in light of "magnetization transfer," the following assignments were made. Lines 5, 6, and 8 represent the methylene triplet and lines 7 and 9 represent the methine doublet. Line 10 is the resonance of the ca,rboxyl carbon. Carbon-13 shifts and directly bonded carbonhydrogen coupling constants for the ring carbons are shown in Table I. The designation a refers to the carbon bearing the substituent and p refers to the unsubstituted carbons of the ring. As shown in the table, the values for cyclopropyl cyanide compare favorably with that reported in the literature. Shifts and coupliqgs of the carbon atoms of the substituents are tabulated in Table 11.
Discussion R'la~iel'~investigated the carbon-13 shifts of the carbon atoms bearing the substituents in vinyl compounds. He found that a plot of the carbon-13 shifts of vinyl compounds DS. the analogously substituted phenyls is roughly linear, with a slope close to unity. He reasons that such a relationship is t o be expected because (a) the carbon atoms of both systems are sp2 hybridized, (b) each carbon contributes a p orbital to a R bond, and (c) the substituent bears the same geometrical relationship in both systems. Correlations such as this illustrate the systematic behavior of substituents on carbon shifts of various parent systems. Such studies provide an important guide for theoretical investigation. For this reason we have plotted in Figure 2 the carbon-13 shifts of the (Y carbon of monosubstituted cyclopropanes against those of
H
D-
164.4
147.1
158
&H 218.0 156.9 147.1
169
similarly substituted methanes. As seen in the figure, the experimental points vary linearly. The point corresponding to the disubstituted chlorine derivatives also falls close to the line. This is not too surprising since the chlorine atoms occupy similar positions in both molecules. The least-squares slope is not 1.09 as in Maciel's plot, but 0.82. The fact that the slope is not unity tempts one to conclude that this is the result of a difference in hybridization of the carbon atoms (sp3 for methanes and sp2 for cyclopropanes). However, one must be cautious in using such correlations to assign degrees of hybridization since many other factors can contribute. The linear behavior of the carbon-13 shifts discussed here is evidence that the substituent preserves much of its character and affects the shifts of carbon atoms of different hybridization somewhat in the same manner. The scatter shown in the graph is real (;.e., greater than experimental error) and could result from the fact that (15) G. E. Maciel, J. Phys. Chem., 69, 1947 (1965).
Volume 71,Number 9 Auguat 1987
PAULH. WEINERAND EDMUND R. MALINOWSKI
2794
Table I11 : a-Carbon-13 Chemical Shifts in Similarly Substituted Compounds (Relative to W H J ) 0 X
H F c1 Br I COzCzHs CHzBr CHzCl CHzOCzHs 0
C8HsX"
CHaX
CsHaX
16.7
17.2' 94.7' 44.2' 28.7'
148. Od 183.1' 154.4b 142.6' 115.7' 151.6' 153. Ob
34.7
O.Od
39.7' 37.2'
I/
CCHI 0
41.3'
34.3 16.6
36.8 39.2 (76. 6)h
44.0'
50.9' 79. 0' 25.2' 76.6' 66.8' 48.9' 69. Od 19.0' 38.8' 39.2' 24, 0' 38.2'
CH-CHX'
I1
CHaCX
218. Ok 189.9' 188.gb 177. 3b 212.1b
158. 8b
145.4 134.9 105.7 149.1 152.5 153.0 155.1
157.3b
157.8
217.8'
159. 1' 170.4' 178.2' 157.1' 167.6' 170.6' 157.0' 174.9' 161.4'
149.3 168.5 172.2
158.0'
219.9'
231. 1' 190. 0' 225.3' 188.7" 197.Ok 216.2'
160.3'
44.8" 45.1' (74.od)h
* This laboratory. * G. E. Maciel, J. Phys. Chem., 69, 1947 (1965). Data taken from compilations of E. R. Malinowski, T. Vladimiroff, and R. Taveres, ibid., 70, 2046 (1966). P. C. Lauterbur, J. Chem. Phys., 26, 217 (1957). e G. E. Maciel and J. J. Natterstad, H. Spiesecke and W. G. Schneider, ibid., 35,722 (1961). 'G. B. Savitsky and K. Namikawa, J. Phys. Chem., ibid., 42, 2427 (1965). 67, 2430 (1963). I, 1,l-Dichlorocyclopropane and methylene chloride. J. W. Emsley, J. Feeney, and L. H. Sutcliffe, "High ResoluSee footnote i, tion NMR Spectroscopy," Vol. 2, Pergamon Press, Inc., New York, N. Y., 1966, p 1030. See footnote i, p 997. p 1009. J. B. Stothers and P. C. Lauterbur, Can. J. Chem., 42, 1563 (1964). " G. E. Maciel, J . Chem. Phys., 42, 2746 (1965). " K. Frei and H. S. Bernstein, ibid., 38, 1215 (1963). ' G. Maciel and G. B. Savitsky, J. Phys. Chem., 69,3925 (1965).
'
the substituent does not occupy the exact same relationship (geometrically and electronically) in the different molecular systems. Other factors, such as specific solvents effects, etc., can also contribute to the scatter. For comparison of other molecular systems, we have also compiled in Table 111the carbon shifts of phenyls, vinyls, and methyl ketones. I n an attempt to evaluate such empirical correlations as described above on a more quantitative basis, we have calculated the slopes, intercepts, and root-mean-square deviations by a method of least squares. An IBM 1620 computer was used for calculations. The results are shown in Table The J o u r n a l of Physical Chemistry
IV. None of the root-mean-square deviations fall within the limits of experimental accuracy which is estimated to be approximately *0.2 ppm. M a ~ i e l ' s ~ ~ plot of the vinyls vs. benzenes gives the best correlation with a standard deviation *3.4 ppm. The cyclopropane-methane plot shows a slightly larger standard deviation, namely k4.4 ppm. Correlations of the carbonyl carbon shifts of methyl ketones are extremely poor, yielding standard deviations of 14.7, 16.8, and 18.0 ppm when plotted against ethylenes, benzenes, and methanes, respectively. In making the above twodimensional analysis, we are implicitly testing whether or not the carbon shifts can be reduced to a function
13C
NMRSPECTRA OF SUBSTITUTED CYCLOPROPANES
0'
10
20
30
40
50
60
70
80
90
Figure 2. The 1*C chemical shifts of the P carbon of substituted cyclopropanes v5. lac chemical shifts of similarly substituted methanes (relative to methyl iodide standard); ( 0 is the a,a-dichloro compounds).
Table IV : Least-Square Slopes, Intercepts, and Root-Mean-Square Standard Deviations of a-Carbon-13 Chemical Shifts for Similarly Substituted Compounds
2
U
axis
axis
slope
Intercept
Std dev, ppm
CC" CHaX CHaX CHaX CeHa
CHdHX CaHsX CH%=CHX CeHsX C3HsX 0
1.09 0.82 0.88 0.54 0.59
-19.6 2.78 112.9 134.7 -57.4
3.4 4.4 7.4 7.7 9.5
CHFCHX
CHaCX 0
0.58
116.8
14.7
CsHdl:
CHaCX 0
0.30
157.8
16.8
CHsX
CHiCX
207.5
18.0
II
II
-0.06
100
.
,
O'
10
20
.
30
, , ,
,
60
70
40 50 &Y,
bC13 W3N P P ~
II
2795
of two major factors. This approximation seems t o be adequate to first order for vinyl-benzene and methylcyclopropane. This is not always true, as can be seen from the plot of benzenes us. methanes in Figure 3. The large scatter of points gives evidence that more than two important factors are involved. One is now faced with the problem of determining exactly how many factors are involved. I n other words "n" axes are needed to represent the data adequately. For such
. .
"
80 90 100
(CH3X) PPm
Figure 3. The '*C chemical shifts of the (Y carbon of substituted benzenes us. similarly substituted methanes (relative to methyl iodide standard).
studies a method called factor analysis, developed by Malinow~ki,'~J~ seems applicable. This method helps one find the least number of axes or factors necessary to reproduce the data. We are currently studying this phase of the problem. Upon examining Table I we find no apparent relationship between the shifts of the LY and /3 carbons of cyclopropanes. Further examination of this table reveals that when the substituent causes an increase in JCH of the LY carbon it also causes a slight increase in JCHof the /3 carbon. I n the light of the Malinowski direct additivity relation' of JcH we conclude, from values listed in Table 11,that the cyclopropyl group has approximately the same effect as a methyl group on the JCH of substituted methanes. This is easily seen when we examine JCH values reported by Frei and Bernstein,'* namely, 127 and 142.5 cps for WHz(CH3) (C6Hs) and 13CH(OH)(CH3) (C6H6), respectively. When a cyclopropane ring is substituted for a methyl group in these compounds, JCH is found to be 127 and 146.0 cps, respectively (see Table 11).
Acknowledgment. The authors gratefully acknowledge the support of the U. S. Army Research Office (Durham), Contract No. DA-31-124-ARO-D-90. (16) E. R. Malinowski, Ph.D. Thesis, Stevens Institute of Teohnology, 1961 ; Dissertation Abstr., 23(8), Abstract 62-2027 (1963). (17) This method has been applied recently for the prediction of activity coefficients of nonelectrolytes determined from gas-liquid chromatography by P. T. Funke, E. R. Malinowski, D . E. Martire, and L. Z. Pollara, Separation Sci., 1, 661 (1966). (18) K. Frei and H. J. Bernstein, J. Chem. Phys., 38, 1216 (1963).
Volume 71, Number 9 August 1967
