MAGNETIC RESONANCE IN CHEMISTRY, VOL. 24, 607-611 (1986)

'J(CC) and "J(CC) Coupling Constants in Monosubstituted Anilines. Additivity of Substituent Effects on Carbon-Carbon Couplings

Peter Sandor" and Lajos Radics NMR Laboratory, Central Research Institute of Chemistry, P.O. Box 17, H-1525 Budapest, Hungary

Isotropic carbon-carbon couplig constants over one and three chemical bonds have been measured in ortho-, meta- and para-substituted anilines, C,€€,NH,X (X = F, Cl, Br, I, OCH,, NH,, NO,, CN, COOCH,, CH,), in natural abundance. The experimental data are compared with J(CC) values calculated by assuming additivity of snbstituent coupling increments. Additivity conditions are discussed. Non-additivities are interpreted, on a qualitative level, in terms of mesomeric and inductive interactions between the substituents.

INTRODUCTION

During the last few years considerable interest has been focused on carbon-carbon spin-spin coupling constants, J(CC).l" The effects of substituents on the magnitude of J(CC) have been extensively studied for a large variety of chemical environments, ranging from alkanes to condensed aromatics.%' Most previous studies, however, were restricted to mono- substituted compounds, and coupling data on di- and poly-substituted molecules are still very In di- and poly-substituted systems, substituent-induced changes in the magnitude of J(CC) may be expected to obey some additivity rules, much in the same way for J(HH)+" and J(FF)" couplings. Since substituent-induced effects on scalar couplings usually become severely attenuated across formal single bonds, additivity rules are more conveniently studied in unsaturated molecules and aromatics. In order to explore the existence and nature of the additivity of substituent coupling increments in carbon-carbon couplings, we have measured (at the natural abundance level) one- and three-bond l3C-I3C coupling constants in a series of ortho-, meta- and paru-disubstituted benzenes. The experimental J(CC) data and their analysis in terms of additivity are the subject of this paper.

For practical considerations, one of the substituent groups, NIT2, remained unchanged throughout this study while the other substituent was varied over a wide range: C6H4(NHz)X, X = F , C1, Br, I, OCH,, NITz, NOz, CN, COOCH,, CH,. The choice of the amino group as the common substituent provided, on the one hand, the convenience of dealing, in most cases, with weakly coupled AB systems, a condition necessary for the accuracy of measurements and, on the other, offered the opportunity to study the effects

* Author to whom correspondence should be addressed.

0749-1.581/86/070607-05$05 .00 0 1986 by John Wiley & Sons, Ltd.

of both inductive and mesomeric interactions on J(CC).

EXPERIMENTAL

All compounds used were commercial products or were prepared by standard procedures. Purification of the compounds was achieved by distillation or repeated crystallization. Owing to the strict require- ments with regard to sensitivity, low concentrations or indifferent solvents could not be used. Samples (20-50% w/v) were usually prepared with hexadeu- terioacetone. In some instances (X = OCH, and NHz), deuterioacetonitrile or deuterionitromethane was used as the solvent in order to avoid the formation of Schiff's bases with acetone.

Natural abundance proton decoupled 13C spectra were recorded at 300K on a disk-augmented Varian XL-100115 NMR system (2.5.16 MHz) using 12 mm 0.d. sample tubes. Spectral widths were chosen so as to allow the time domain signals to be sampled into a 32 K (32-bit word-length) data table using acquisition times of 8-16 s (digital resolution 0.06-0.125 Hz per data point). Throughout the experiments, the line width of the mono-labelled isotopomers at the height of the 13C satellites was less than 5 Hz. The spectra of methyl meta-aminobenzoate and ortho- and para- fluoroanilines, giving strongly coupled and overlap- ping subspectra, were run at 50.3 MHz using a Bruker WP-200/SY spectrometer. The same, higher fre- quency instrument was employed to obtain the values of 'J(CC) and 3J(CC) in toluene and methyl benzoate, molecules for which the available literature data6 were less accurate. Apart from a few exceptions (indicated in Tables 2-4), J(CC) values were measured for both (A and B) carbon signals. The difference between the two independent readings was generally less than 0.1 Hz and always remained below 0.2 Hz. Therefore,

Received 15 August 1985 Accepted (revised) 9 October 1985

608 P. SANDOR AND L. RADICS

the accuracy of the experimental couplings is believed to be better than f 0 . l H z . No attempt was made to determine the relative signs of the couplings, as all 'J(CC) and 3J(CC) values may be safely assumed to be positive in these molecules. ' J One-bond 13C--12C -+13C-'3C isotope shifts, as obtained from the analyses of the AB resonance patterns, ranged from -0.01 to -0.03 ppm, resulting in higher nuclear shieldings. Isotope shifts over three chemical bonds in no case exceeded the experimental errors.

3t-Bond orders and theoretical 13C--13C coupling constants were calculated by the SCPT INDO method developed by Blizzard and Santry,13 taking into account for the latter all the perturbations associated with Fermi contact, orbital-dipole and spin-dipole mechanisms. Spin-spin interactions were treated with standard INDO MO parameteri~ation.'~ One-centre integrals Scz(0) and (r-3)c had values of 3.7174 and 2.5505 a.u. , respectively. Calculations were carried out using standard geometries.16

Tablel. Experimental 'J(CC) and 3J(CC) couplings (Hz) in monosubstituted benzenes' and in the unsub- stituted benzene molecule

Substituent

F CI Br I OCH, NHZ NO, CN COOCH,c CH,d H H H

'J(12) ' J (23 ) 'J(34) = 'J (16 ) = 'J (56 ) = 'J (45 )

70.80 56.65 56.20 65.15 55.75 56.10 63.65 54.85 56.10 60.95 54.45 56.10 67.05 57.75 56.15 61.15 58.65 56.00 67.40 56.10 55.30 60.05 56.45 55.10 58.60 56.10 55.20 57.00 56.40 55.95

57.0 55.3 f 0 . 5 55.95 f 0.04

3J(25) J(14) =3J(36) Ref

10.45 6.70 6 10.65 7.80 6 10.65 8.15 6 10.60 8.70 6 9.20 7.10 6 9.25 7.90 b 9.70 7.40 6 0.95 9.05 6 9.30 8.80 This work 9.55 9.00 This work

18 10.08 f 0.1 19 10.01 f 0.03 6

-

a Data for the substituted derivatives are rounded to f0.05 Hz. bAveraged values from Refs 6 and 17. .

'J(C-1,CO) = 74.90 Hz and 3J(C-3,C0) = 4.60 Hz. 'J(C-I,CH,) = 44.25 Hz and ,J(C-3,CH3) = 3.80 Hz.

RESULTS AND DISCUSSION

Experimental carbon-carbon coupling constants ob- tained for rnetu-, para- and ortho-substituted anilines are given in Tables 2, 3 and 4, respectively. In order to quantify the additivity of substituent effects for a given J(CC) , the experimental values were compared with Jcalc, the hypothetical coupling constants calcu- lated assuming additivity of the substituent coupling increments. The pertinent differences, A = Jexp - Jcaic, are also reported in Tables 2-4. In view of the common NH, substituent, values of Jcalc may be expressed simply as

where n is the number of chemical bonds separating the interacting nuclei i and j (n = 1, 3), k is the position of the X substituent (NH, is assumed to be at C-1) [k = 2 (ortho), 3 (rnetu), 4 Cpuru)] and "J(ij)""z and "J(CC)" represent the value of the given coupling constant in aniline and unsubstituted benzene, respectively. While the 'J(CC) and 3J(CC) data for monosubstituted benzenes were available (or redeter- mined) with an accuracy of f0 .1Hz (see Table l ) , literature values for 'J(CC)" (Table 1) show an inconsistency that greatly exceeds the uncertainties of the other two terms in the above expression. In order to minimize systematic errors in the values of Jcalc, the experimental nJ(CC)H couplings in Eqn (1) were replaced by what may be termed as their 'reference' values, 'J(CC)rH and 3J(CC)rH. Following literature analogies,10~20 we assumed that, in rnetu-substituted anilines, the interactions between substituent groups were negligibly small and, hence, for a sufficiently large set of data, the average value of the differences Jexp - Jcalc should be zero. Least-squares minimization performed on the 60 '.Iexp and 30 3Jexp couplings measured for the rnetu-substituted anilines gave lJ(CC)rH = 55.50 f 0.15 and 3J(CC)p = 10.15 f 0.05 Hz (95% confidence limit), in remarkable agreement with the 55.3 f 0.5 and 10.08 f 0.1 Hz,

respectively, reported recently by Diehl et ~ 2 1 . ' ~ [For theoretical considerations justifying the definition of "J(CC),H, see below.] Summation of the probable errors of the individual terms used in the calculations of individual differences Jexp - JCdc gives 20.45 and f0.35Hz as the accuracy limits for the additivity criteria of one- and three-bond coupling constants, respectively.

rnetu-substituted anilines

From inspection of the data in Table 2, it can be seen that substitution-induced increments in 3Jexp fully obey the criteria of additivity and, despite substantial variations with the nature of the substituents, the differences A remain well below the estimated f0.35Hz limits of accuracy. In contrast, at least one third of the 60 'Jexp values in Table 2 show significant deviations from additivity. Closer examination of the pertinent A values reveals that non-additivity typically occurs with couplings 'J(23) and 'J(16) and, interestingly, the respective A values, apart from a few exceptions, are of the same magnitude but opposite in sign. The constancy of /A1 over a wide range of substituents suggests that inductive and/or mesomeric interactions cannot be responsible for the observed non-additivities. On the other hand, non-additivities of the substituent effects may be expected to occur if the benzene ring distortions caused by the common amino group in the rnetu-substituted derivatives differ from those in aniline.21 In view of the lack of appropriate experimental geometries for rnetu-substituted anilines, no direct test of this hypothesis seems to be feasible. SCPT-INDO calculations made on artificially dis- torted benzene rings,22 however, revealed that even moderate changes in the individual C-C-C bond angles that accompany substitutions (0.5-lo) are sufficient to give rise to the experimentally observed differences in 'J(CC) values, while three-bond

?I(CC) AND 3J(CC) COUPLINGS IN ANILINES 609

Table 2. Experimental one- and three-bond carbon-carbon couplings (Hz) and calculated non-additivities A (in parentheses) in meta-substituted anilines

' J ( i j ) 3J(ij)

X

Fa

CI

Br

I

OCH,

NH2

NO2

CNC

COOCH3d

CH,*

i j=12 23

62.60 74.80 (+0.50) (+0.85) 61.00 69.30

(-0.40) (+1.00) 6 0 ~ 5 67.95

(i.0.15) (1-1.15) 59.75 65.05

(-0.35) (+0.95) 63.90 70.90

(+0.50) (10.70) 64.70 64.70

(+0.40) (+0.40) 62.30 71.60b

(+0.55) (+1.05) 62.35 63.85

(+0.25) (f0.65) 62.50 61.85b

(+0.75) (f0.10) 61.65 60.30

(-0.40) (+0.15)

34 45

71.10 57.10 (-0.20) (-0.05) 65.50 55.90

(-0.15) (-0.35) 63.90 55.30

(-0.25) (-0.05) 61,45 54.65 (0.00) (-0.30) 66.95 57.90

(-0.60) (-0.35) 60.95 58.80

(-0.70) (-0.35) 67.80b 56.60

(-0.10) (0.00) 60.15 56.80

(-0.40) (-0.15) 59.20 56.60

(+0.10) (0.00) 57.20 56.70

(-0.30) (-0.20)

56 16

59.80 61.30 (+0.45) (-0.55) 59.45 61.25

(+0.20) (-0.50) 59.60 61.10

(+0.35) (-0.65) 59.55 61.25

(-0.30) (-0.50) 59.20 61.10

(-0.10) (-0.70) 58.80 60.95

(-0.35) (-0.70) 59.20 60.35

(1-0.75) (-0.60) 58.80 60.20

(+0.55) (-0.55) 58.70 60.50

(10.35) (-0.35) 58.95 61.25

(-0.15) (-0.35)

14

5.55 (-0.25)

6.85 (-0.05)

7.20 (-0.05)

7.90 (10.10)

6.20 (0.00) 7.10

(+0.10) 6.50b (0.00) 8.10b

(-0.05) 8.00

(+0.10) 8.10 (0.00)

25

4.50 (+0.05)

5.65 (+0.10)

5.90 (0.00) 6.50

(f0.05) 4.95

(iO.10) 5.80

(t0.15) 5.05

(-0.10) 6.70

(-0.10) 6.60

(+0.05) 6.65

(-0.10)

36

7.95 (-0.25)

8.10 (-0.30)

8.10 (-0.30)

8.10 (-0.25)

6.80 (-0.15)

7.10 (+0.10)

7.50b (+0.05)

8.50b

7.30 (+0.25)

7.30 (0.00)

(-0.20)

a 'J(F, C-3) = 240.90, 'J(F,C-2) = 24.60, 'J(F,C-4) = 21.45, 3J(F,C-l) = 11.05, ,J(F,C-5) = 10.20 and 4J(F,C-6) = 2.30 Hz. Signs were not determined.

Only lines around one of the carbon signals were observed. 'J(C-3,CN) = 79.20 Hz. 'J(C-3,CO) = 73.80 Hz.

" 'J(C-3,CH,) = 43.70 Hz.

carbon-carbon couplings remain unaltered on such bond-angle distortions.

These calculations have shown, further, that the sum of the six 'J(CC) couplings in benzene also remains unchanged with C-C-C bond angle variations, a result that provides justification for the procedure adopted for deriving the reference values J(CC),H and 3J(CC)rH.

para-substituted anilines

From the inspection of the A values reported in Table 3, it can be concluded that non-additivities of 'J(CC) couplings in this group of derivatives are due mainly to mesomeric interactions between the substituents. This can be seen from the following. (a) Deviations from additivity are the largest in molecules which contain either strong n-donating (NH2, OCH,, F) or n-withdrawing (NOz, CN, COOCHJ groups as the X substituent. For these compounds, each one-bond coupling between ring carbon atoms exhibits marked non-additivity . (b) n-Donating and n-withdrawing substituents act on the magnitude of individual 'J ( i j ) couplings in an opposite sense: for molecules with a n-donating X substituent one finds A(12) > 0, A(23) < 0 and A(34) > 0, whereas in molecules with a n-withdrawing X substituent the signs of the respective A values are inverted. These variations in the sign of the A values are readily interpreted within the framework of INDO MO approximations. Our calculations showed that in para-substituted anilines with strongly n-donating substituents (X = NH2, OCH,, F) mesomeric interactions cause the calculated

~~~

Table 3. Experimental one- and three-bond carbon-carbon

X

Fa

CI

Br

I

OCH,

NHZ

NO'

CN"

couplings (Hz) and calculated non-additivities A (in parentheses) in para-substituted anilines

' J f i j ) 3J(i j )

i j=12 23 34 25 16 56 45 14 36

62.20 59.30b 72.20 9.35 4.90 (+0.35) (-0.50) (+0.90) (-0.20) (+0.45) 61.85

(+0.10) 61.55

(-0.10) 61.40

(-0.35) 62.50

(+ 0.70)

59.00 (+O. 10) 58.15

(+0.15) 57.80

(+0.20) 60.10

(-0.80) 62.50

59.40 60.70

59.70 60.60

d

(+0.85) -

(-1.55) (+1.45)

(-1.05) (+1.00)

66.40 (+0.75)

64.85 (+0.70) 61.70

(+0.25) 68.30

(+0.75) 62.50

9.60 (-0.1 5)

9.70 (-0.05)

9.50' (-0.20)

8.30 (0.00) d

t0.85) - 67.30 8.50' -0.60) (-0.30) 60.20 9.85 -0.35) (-0.20)

5.80 t0.25)

6.10 t0.20)

6.60 t0.15)

5.55 t0.70)

d

- 4.70

6.30

6.20

-0.45)

-0.50) 59.70 60.05 58.85 8.25

cOOcH' (-1.15) (+0.80) (-0.25) (-0.15) (-0.35) 61.65 59.15 57.60 8.75 7.05 CH3g (+0.05) (-0.40) (+0.10) (+0.10) (+0.30)

a 'J(F,C-4) = 232.80, 'J(F,C-3) = 'J(F,C-5) = 22.15, ,J(F,C-2) = 3J(F,C-6) = 7.50 and 4J(F,C-l) = 1.95 Hz. Signs were not determined. bOnly inner lines of the two AB subspectra were observed. Accuracy: f0.5 Hz. ' Only lines around one of the carbon signals were observed.

Not detectable because of magnetic equivalence. " 'J(C4,CN) = 83.95 Hz, 3J(C-2,CN) = 5.60 Hz. 'J(C-4,CO) = 78.35 Hz, 3J(C-2,CO) = 4.80 Hz. ' 'J(C-4,CH3) = 46.00 Hz, 3J(C-2,CH3) = 4.00 Hz.

610 P. SANDOR AND L. RADICS

n-bond orders of the C-1-C-2 and C-3-4-4 bonds to increase, and that of the C-2-C-3 bond to decrease, compared with the values in the monosubstituted derivatives. An opposite change in the respective n-bond orders obtains when substituent X has a strong n-electron-withdrawing property (X = NO2, COOCH,, CN). It should be mentioned, however, that no acceptable correlation could be found between A values and calculated non-additivities of n-bond orders. (c) For each particular ' J ( i j ) coupling, the A values in Table 3 follow the same trend as do the pertinent aRo((;Y) constant^.^, However, no linear correlation exists between the two quantities.

Vicinal 3J(25) [ = ,J(36)] couplings also seem to be influenced by the n-framework. The sign of the A values is positive when the X substituent is a n-donor and negative when it is a n-acceptor group. The overall n-electron density of the benzene ring, as calculated here by the INDO MO method, follows the same trend, i.e. it increases for two n-donors and decreases for one n-donor plus one n-acceptor substituent. These two observations are in agreement with earlier contentions claiming that three-bond carbon-carbon couplings depend on the sum of the n-bond orders.24 The good additivity observed for vicinal couplings involving substituted carbon atoms, ,J(14), however, suggests that transmission of substituent effects across three aromatic bonds is a complex phenomenon and its clarification needs further theoretical investigation.

ortho-substituted anilines

In a manner similar to the case of the para-substituted derivatives, non-additivities of substituent effects in 'J(CC) couplings can be traced back mainly to mesomeric interactions between substituents. This follows from the observation that the sign of A values (above the error limit) is the same as that of the non-additivities of the pertinent n-bond orders. INDO MO calculations on a benzene ring with two n-donating substituents in the ortho-position revealed that n-bond orders increase for C-1-C-2, C-2-C-3, C-4-C-5 and C-1-C-5 bonds, whereas those for C-3-C-4 and C-5-C-6 decrease with respect to their value in the monosubstituted derivatives. An opposite variation in the n-bond orders obtains when one of the substituents is a n-withdrawing group, again in accordance with the signs of the respective A values.

A remarkable exception to these tendencies is represented by one-bond couplings between the substituted carbon atoms, 'J(12), displaying negative A values irrespective of the n-electronic character of the substituents. The negative sign suggests that, in these positions, mesomeric interactions may be overcompensated by either steric or inductive interactions between the substituents. Simple SCPT INDO model calculations, however, have shown that steric repulsion between ortho substituents should result in positive A values; hence, non-additivity of

I

Table 4. Experimental one- and three-bond carbon-carbon couplings (Hz) and calculated non-additivities A (in parentheses) in ortho-substituted anilines

' JO j ) 3J( i j )

X i j = 12

73.40 (-3.05)

69.40 (-1.40)

68.30 (-1.00)

66.05 (-0.55)

Fa

CI

Br

I

70.00 OCH3 (-2.70)

c

NHZ - 70.90b

NO2 (-2.15) 64.80

(-0.90) CNd

63.50 C00CH3e (-0.75)

61.10 (-1.55) CHB

23

74.75 t0.80) 68.80 t0.50) 67.20 t0.40) 64.20

(+0.10) 70.40

(+0.20) 64.50

(+0.20) 68.55b

(-2.00) 62.25

(-0.95) 59.90

(- 1.85) 60.60

(+0.45)

34 45

57.00 56.85

56.55 55.95

55.85 55.95

55.40 55.85

57.45 57.15

58.10

59.00 53.00

58.10 54.50

57.95 53.90

56.65 55.75

(-0.15) (+0.15)

(+0.30) (-0.65)

(+0.50) (-0.65)

(+0.45) (-0.75)

(-0.80) (+0.50)

(-1.05) -

(+2.40) (-2.80)

(+1.15) (-1.10)

(+1.35) (-1.80)

'(-0.25) (-0.70)

c

56

59.20

59.70 t0.45) 59.70 t0.45) 59.90

-0.15)

16

62.60 (+0.30)

61.50 (+0.10) 60.75

(+0.25) 60.25

14

6.1 Ob (+0.30)

7,25b (+0.35)

7.60b (+0.35)

8.00

36

5.70 t 1.25)

6.90 t 1.35)

7.20 t 1.30)

7.65 t0.65) (+0.15) (+0.20) (-0.20) (+1.20) 58.45 64.45 7.25 7.95 5.95 -0.85) (+1.05) (+1.05) (+1.00) (+1.10) 58.10 64.50 7.70 7.70 -1.05) (+0.20) (+0.70) (+0.70) -

c

60.70 58.40

59.35 60.65

59.90 59.40

59.00 62.20

t2.25) (-3.35)

t l .10) (-1.45)

t1.55) (-2.35)

-0.10) (+0.15)

a 'J(F,C-2) = 237.05, 'J(F,C-l) = 12.70, *J(F,C-3) = 18.45, 3J(F,C-4) = 6.75, 5) = 3.50 Hz. Signs were not determined.

Only lines around one of the carbon signals were observed. Not detectable because of magnetic equivalence. 'J(C-2,CN) = 80.45 Hz.

'J(C-2,CHa) = 44.25 Hz. e ~J(c-~,co) = 75.55 HZ.

25

9.10b t0.90)

8.60b t0.20)

8.55b t0.15)

8.15

6.00 7.30b 6.80

8.10b 8.60 7.60

7.90b 7.30 7.80 (0.00) (+0.25) (+I251 8.50 7.60 7.55

t0.40) (+0.30) (+0.80)

I(F,C-6) = 3.80 and 4J(F,C-

-0.50) (-0.15) (+1.65)

-0.05) (-0.10) (+0.80)

'J(CC) AND 3J(CC) COUPLINGS IN ANILINES 61 1

substituent coupling increments for one-bond coupling between substituted carbon atoms, A(12), may arise as the result of partial mutual compensation of the inductive effects by the substituents themselves. Confirmation of this assumption comes from our INDO MO calculations performed on o-fluoroaniline, which show that carbon atoms C-1 and C-2 are less positive in this molecule than expected on the basis of the calculated charge distributions in fluorobenzene and aniline. In addition, the theoretical A(12), obtained by using the SCPT INDO-based semiempiri- cal 'J 12 coupling constants ['J(12)gF&2-F = 72.25,

57.38Hz1, is also negative in sign, although its absolute value (1.03 Hz) is merely one third of the

1 J(12)ae:r IL )- - 61.74, 1J(12)&z0r = 68.92, 'J(CC):,,, =

experimentally observed difference. Inspection of the data in Table 4 shows that

non-additivities of coupling increments for vicinal interactions involving one of the substituted carbon atoms, A3J(14) and A3J(25), assume very similar values: they are nearly zero for n-acceptor (X = NO2, COOCH3, CN) and weakly interacting (X = C1, Br, I) substituents and positive for n-electron donor (X = NH2, OCH,, F, CH,) functions. Interestingly, pronounced deviations from additivity were found for three-bond couplings between proton-bearing carbon atoms C-3 and C-6. The pertinent non-additivities, A3J(36), are of positive sign and assume a value of about lHz, irrespective of the n-character of the second substituent.

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