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Electronic effects of substituents on the barrier to rotation in secondary amides
303 Journal of Molecular Q Elsevier Scientific
Structure.
I5 (1973)
Publishing
Company,
303-306
Amsterdam
- Printed
ELECTRONIC EFFECTS OF SUBSTITUENTS ROTATION IN SECONDARY AMIDES
K.
GURUDATH
Department
(Received
RAO
AND
of Chemistry,
6 September
in The Netherlands
ON THE BARRIER
TO
C. N. R. RAti
Indian Institute of Technolqgy,
Kanpur-16
(India)
1972)
Extensive spectroscopic studies have been reported in the literature’12 regarding the structural and environmental factors that affect the barrier to rotation about the central C-N bond in secondary amides (I). Thus, when R is a bulky alkyl substituent or a phenyl group, the cis isomer is favoured; the cis
“\
Ho
H/N-c\R. (I)
isomer also predominates in secondary thioamides compared to the corresponding amides. Recently, molecular orbital calculations have been carried out on secondary amides and realistic estimates of the barrier heights, E, , have been obtained2-4. It was our interest to examine the electronic effects of substituents on the barrier heights and c&tram equilibiia in secondary amides. For this purpose, we have carried out CND0/2 calculations’ on substituted formanilides (II), and also determined the cis-tram isomer ratios in these derivatives from infrared spectroscopy. ‘=d-‘,
\ /H JN-=xo
The results of our CND0/2 calculations are summarised in Table 1 where we have listed the energy difference between the cis- and irans-isomers, barrier heights, CND0/2 charges on important atoms and the calculated dipole moments. The &-isomer is more stable than the trans-isomer in all the formaniiide deriv* To whom all correspondence
should be addressed.
304 TABLE CNDOj2
R
1 RESULTS
ON
FORMANlLtDES
Al?
(Ii)=
CNDO
Gb
{kcal mole-
’ ) (kcal mole- ’ )
c
pNHZ
0.3
12.4
watts eis
p-CWf
0.2
12.3
H
0.7
11.3
tram cis tram
p-CN
I.4
cis
9.1
rrutts CiS
p-NO=
0.7
8.3
fJ2ZllS
cis
charges
N
-a170 -0.178 -0.181 -0.189 i-O.348 -0.179 +0.342 -0.187 +0.346 -0.163 to.347 -0.162 i-0.346 -0.162 i-O.346 -0.161
+0.355 +0.350 +0.34s +0.343
P (0)
0
N
-0.352 -0.347 -0.338 -0.331 -0.329 -0.323 -0.321 -0.327 -0.314 -0.316
to.088 to.110 to.090 +o.i13 i-o.094 +0.117 +0.096 +0.115 -tarox i-o.121
3.5 5.0 4.3 2.2 3.6 3.4 6.0 1.2 8.0 3.1
a Structural parameters from related molecules were taken from the literature. a AE is the energy difference between cis- and rrans-isomers; E, values are with respect to the cisisomer.
atives; the energy difference between the two isomers calculated by us for formanilide agrees excellently with the experimentai value of Suzuki et aL6. The calculated dipole moment of formanilide also agrees well with the experimental value’ of 3.4 D. The barrier height calculated by us for formanilide is lower than the experimental value (17.7 kcal mole- ‘) of Carters in CDC13 solution. This is probably due to the effect of the solvent medium. In addition to providing a polar medium, chloroform can hydrogen bond to the carbonyl group; both these factors are likely to increase the barrier height’* ‘. We see from Table 1 that the barrier height si~i~cantly decreases with increase in the electron-withdrawing power of the para-substituent. Thus, E, is about 12 kcal mole-’ when the pam-substituent is an amino group compared to 8 kcal mole-x in the p-nitro derivative. The barrier heights from NMR spectroscopy reported by Carter’ for a few substituted formanilides are in accord with the trend in substituent effects reported here, In para-substituted N,N-dimethylbenzamides (ill), however, eiectron-withdrawing groups increase the barrier height to rotation about the C-N bond g_ This is understandable, since the subRC4-b
CM3
\ 0
/
//C-N\CH 3
tm stituents on the phenyl groups in (II) and {III) would affect the C-N bond order (and hence the ba~lerh~lght) difEerently_In (III), onlyelectron-donating groups on the benzene ring would have resonance interaction (complementary substitutions
305 with theamidegroup in theparaposition while in (II),eiectron-withdrawing groups would be complementary to the amino groups; this results in opposite trends in substituent effects on the C-N bond order in the two amides. On the basis of the above findings we would expect a decrease in the proportion of the c&isomer in formanilideswith increase in the electron-withdrawing power of the substituent, since the &-isomer is thermodynamicahy more stable in these derivatives. This is exactly what we observe experimentally as can be seen from the data on isomer ratios obtained by infrared spectroscopy (Table 2). In Table 2, we have listed the Hammett G constants of the para-substituents” along TABLE cis-
2
AND
ffYZnS-ISOMER
POPULATIONS
IN j7Wff-SUBSTITUTED
FOXMANIL~DES
AND THIOACETANILIDES
Hanmett a constant
0/OTrans isomer Fornranilides (II)
Titioacetanilides (IV) ( If’-CHJ )”
P-N(CHs)z
- 0.600
P-CH3
-0*1?0
54 56 57 58
46 56 58 64
p-Br
f-o.232
p-0CH3
-0.268
0.0
H
+0.680” + I .27c
~C02C2Hs p-NO2
66 68 71
72 846
3 Solvent chloroform (25 “C). The isomer ratios were determined from intensities of the characteristic N-H bands: relative intensities of Amide (I) and Amide (II) bands also gave simifar results. b Solvent chloroform (30 “C); from ref. I 1. c This o constant is applicable to aniline derivatives. d Temperature: 50 “C.
with the per cent trans-isomer in formanilides to illustrate this point. The recent thioformanilides (IV, R’=H) data of Walter and Kubersky’ ‘9 * ’ on para-substituted and thioacetanilides (IV, R’=CH3) also show the same trend as the formanilides. RC6H4 \
/R
,/"-K* (lx)
The barrier heights in (IV) would be expeLted to decrease with increase in the electron-withdrawing power of R just as in formanilides (XI), The present results clearly point out the importance of the eIectronic effects of substituents on the barrier heights to rotation in secondary amides. REFERENCES 1 W. E. STEWART AND T. H. SIDDALL III, Chem. Rev., 70 (1970) 517. 2 C.N.R.RAo,K.G. RAO,A.GOELAND D. BALASUBRAMA~IAN,~ Ckerrr.Suc.(A),(I971)3077. 3 A. S. N. MURTHY, I(.G. RAOANDC, N. R. RAO,J. Amer. Ckrent.Sue., 92 (1970) 3544.
306 4 3. F. YAN, F* A. MOMAN,
R. HOFFMANN AND H_ A. SCHERAGA,_f_Whys, Chem-, 74 (1970) 420. 5a J. A, POPLE, D. P. SANTRY AND G. A. SEGAL, J. Chem, Phys., 43 (1965) S129. .5b J. A. POPLE AND G. A. Z&GAL, J. Gem. Phys., S136; 44 (1966) 3289. 6 L. SUZUKI, M. TSLJBOI,T. SNIMANOUCHI AND S. MKZUSHIMA,Spectruchim. Actu, 16 (1%) 471. 7 1. SUZUKI, Nippon .Kagaku Zus~hi, 80 (1959) 353. 8 R. E. CARTER, Acta Chem. Scar& 22 (1968) 2643. 9 L. M. JACKMAN, T. E. KAVANAGW AND R. C. HAV~ON, Org. Magn. Res., I (1969) 109. 10 J. HI=, Physic& Organic Chmktry, McGraw-Hi11 Book Co., New York, 2nd editon, 1962, p. 87. 1I W.. WAX.IXR AND Ht. P. KUIIE~KY, ~~~ct~~ci%nz.Acfa, 26A (IPXJ) 1153. 12 W. WALTER AND H. P, KUBERSKY, J. &@ot. structure, %I (1972) 207.
Structure.
I5 (1973)
Publishing
Company,
303-306
Amsterdam
- Printed
ELECTRONIC EFFECTS OF SUBSTITUENTS ROTATION IN SECONDARY AMIDES
K.
GURUDATH
Department
(Received
RAO
AND
of Chemistry,
6 September
in The Netherlands
ON THE BARRIER
TO
C. N. R. RAti
Indian Institute of Technolqgy,
Kanpur-16
(India)
1972)
Extensive spectroscopic studies have been reported in the literature’12 regarding the structural and environmental factors that affect the barrier to rotation about the central C-N bond in secondary amides (I). Thus, when R is a bulky alkyl substituent or a phenyl group, the cis isomer is favoured; the cis
“\
Ho
H/N-c\R. (I)
isomer also predominates in secondary thioamides compared to the corresponding amides. Recently, molecular orbital calculations have been carried out on secondary amides and realistic estimates of the barrier heights, E, , have been obtained2-4. It was our interest to examine the electronic effects of substituents on the barrier heights and c&tram equilibiia in secondary amides. For this purpose, we have carried out CND0/2 calculations’ on substituted formanilides (II), and also determined the cis-tram isomer ratios in these derivatives from infrared spectroscopy. ‘=d-‘,
\ /H JN-=xo
The results of our CND0/2 calculations are summarised in Table 1 where we have listed the energy difference between the cis- and irans-isomers, barrier heights, CND0/2 charges on important atoms and the calculated dipole moments. The &-isomer is more stable than the trans-isomer in all the formaniiide deriv* To whom all correspondence
should be addressed.
304 TABLE CNDOj2
R
1 RESULTS
ON
FORMANlLtDES
Al?
(Ii)=
CNDO
Gb
{kcal mole-
’ ) (kcal mole- ’ )
c
pNHZ
0.3
12.4
watts eis
p-CWf
0.2
12.3
H
0.7
11.3
tram cis tram
p-CN
I.4
cis
9.1
rrutts CiS
p-NO=
0.7
8.3
fJ2ZllS
cis
charges
N
-a170 -0.178 -0.181 -0.189 i-O.348 -0.179 +0.342 -0.187 +0.346 -0.163 to.347 -0.162 i-0.346 -0.162 i-O.346 -0.161
+0.355 +0.350 +0.34s +0.343
P (0)
0
N
-0.352 -0.347 -0.338 -0.331 -0.329 -0.323 -0.321 -0.327 -0.314 -0.316
to.088 to.110 to.090 +o.i13 i-o.094 +0.117 +0.096 +0.115 -tarox i-o.121
3.5 5.0 4.3 2.2 3.6 3.4 6.0 1.2 8.0 3.1
a Structural parameters from related molecules were taken from the literature. a AE is the energy difference between cis- and rrans-isomers; E, values are with respect to the cisisomer.
atives; the energy difference between the two isomers calculated by us for formanilide agrees excellently with the experimentai value of Suzuki et aL6. The calculated dipole moment of formanilide also agrees well with the experimental value’ of 3.4 D. The barrier height calculated by us for formanilide is lower than the experimental value (17.7 kcal mole- ‘) of Carters in CDC13 solution. This is probably due to the effect of the solvent medium. In addition to providing a polar medium, chloroform can hydrogen bond to the carbonyl group; both these factors are likely to increase the barrier height’* ‘. We see from Table 1 that the barrier height si~i~cantly decreases with increase in the electron-withdrawing power of the para-substituent. Thus, E, is about 12 kcal mole-’ when the pam-substituent is an amino group compared to 8 kcal mole-x in the p-nitro derivative. The barrier heights from NMR spectroscopy reported by Carter’ for a few substituted formanilides are in accord with the trend in substituent effects reported here, In para-substituted N,N-dimethylbenzamides (ill), however, eiectron-withdrawing groups increase the barrier height to rotation about the C-N bond g_ This is understandable, since the subRC4-b
CM3
\ 0
/
//C-N\CH 3
tm stituents on the phenyl groups in (II) and {III) would affect the C-N bond order (and hence the ba~lerh~lght) difEerently_In (III), onlyelectron-donating groups on the benzene ring would have resonance interaction (complementary substitutions
305 with theamidegroup in theparaposition while in (II),eiectron-withdrawing groups would be complementary to the amino groups; this results in opposite trends in substituent effects on the C-N bond order in the two amides. On the basis of the above findings we would expect a decrease in the proportion of the c&isomer in formanilideswith increase in the electron-withdrawing power of the substituent, since the &-isomer is thermodynamicahy more stable in these derivatives. This is exactly what we observe experimentally as can be seen from the data on isomer ratios obtained by infrared spectroscopy (Table 2). In Table 2, we have listed the Hammett G constants of the para-substituents” along TABLE cis-
2
AND
ffYZnS-ISOMER
POPULATIONS
IN j7Wff-SUBSTITUTED
FOXMANIL~DES
AND THIOACETANILIDES
Hanmett a constant
0/OTrans isomer Fornranilides (II)
Titioacetanilides (IV) ( If’-CHJ )”
P-N(CHs)z
- 0.600
P-CH3
-0*1?0
54 56 57 58
46 56 58 64
p-Br
f-o.232
p-0CH3
-0.268
0.0
H
+0.680” + I .27c
~C02C2Hs p-NO2
66 68 71
72 846
3 Solvent chloroform (25 “C). The isomer ratios were determined from intensities of the characteristic N-H bands: relative intensities of Amide (I) and Amide (II) bands also gave simifar results. b Solvent chloroform (30 “C); from ref. I 1. c This o constant is applicable to aniline derivatives. d Temperature: 50 “C.
with the per cent trans-isomer in formanilides to illustrate this point. The recent thioformanilides (IV, R’=H) data of Walter and Kubersky’ ‘9 * ’ on para-substituted and thioacetanilides (IV, R’=CH3) also show the same trend as the formanilides. RC6H4 \
/R
,/"-K* (lx)
The barrier heights in (IV) would be expeLted to decrease with increase in the electron-withdrawing power of R just as in formanilides (XI), The present results clearly point out the importance of the eIectronic effects of substituents on the barrier heights to rotation in secondary amides. REFERENCES 1 W. E. STEWART AND T. H. SIDDALL III, Chem. Rev., 70 (1970) 517. 2 C.N.R.RAo,K.G. RAO,A.GOELAND D. BALASUBRAMA~IAN,~ Ckerrr.Suc.(A),(I971)3077. 3 A. S. N. MURTHY, I(.G. RAOANDC, N. R. RAO,J. Amer. Ckrent.Sue., 92 (1970) 3544.
306 4 3. F. YAN, F* A. MOMAN,
R. HOFFMANN AND H_ A. SCHERAGA,_f_Whys, Chem-, 74 (1970) 420. 5a J. A, POPLE, D. P. SANTRY AND G. A. SEGAL, J. Chem, Phys., 43 (1965) S129. .5b J. A. POPLE AND G. A. Z&GAL, J. Gem. Phys., S136; 44 (1966) 3289. 6 L. SUZUKI, M. TSLJBOI,T. SNIMANOUCHI AND S. MKZUSHIMA,Spectruchim. Actu, 16 (1%) 471. 7 1. SUZUKI, Nippon .Kagaku Zus~hi, 80 (1959) 353. 8 R. E. CARTER, Acta Chem. Scar& 22 (1968) 2643. 9 L. M. JACKMAN, T. E. KAVANAGW AND R. C. HAV~ON, Org. Magn. Res., I (1969) 109. 10 J. HI=, Physic& Organic Chmktry, McGraw-Hi11 Book Co., New York, 2nd editon, 1962, p. 87. 1I W.. WAX.IXR AND Ht. P. KUIIE~KY, ~~~ct~~ci%nz.Acfa, 26A (IPXJ) 1153. 12 W. WALTER AND H. P, KUBERSKY, J. &@ot. structure, %I (1972) 207.