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Hindered rotation in amidines
Journal of Molecular Structure, 218 (1990) 165-167 Elsevier Science Publishers B.V., Amsterdam - Printed
HINDERED
165 in The Netherlands
ROTATION IN AMIDINES
I. WAWER Department in Chemistry, The University, 02 093 Warsaw, Poland
SUMMARY The 15N chemical shifts and frequencies of dCcN bond from Raman spectra of N'N' -dimethyl-N2-substituted smidines were related to the rotational barriers around C-N' bond obtained from "C
NMR.
INTRODUCTION Numerous amidines have biological and pharmaceutical activity. The configuration and molecular dynamics of the compounds with amidine group N=C-N are, therefore recently widely investigated. Amidines contain two different nitrogen atoms: amine (N') and imine (N2) type, however, because of conjugation between the N2 =C ;,PH3 ACH I R
3
double bond and the lone electron pair at N' both carbon-nitrogen bonds exhibit a partial double bond character. The rotation around C-N' is hindered and the activation para13 meters of the rotation were estimated from C d-NMR line shape analysis (refs. l-3). The rotational barriers (AGf)
were deter-
mined by the electronic and steric factors introduced by the substituents R and R' and reflected the degree of the double bond character of the C-N' bond. The aim of this study was to relate the obtained rotational barriers with the other parameters which give the information about the nitrogen lone pair delocalization and the bond orders within the amidine group. EXPERIMENTAL N'N'-dimethyl-$-substituted amiytnes were synthesized and purified as described in ref. 4. The N NMR spectra were recorded
002%2860/90/$03.50
0 1990 Elsevier
Science
Publishers
B.V.
166
on the Bruker AM500 spectrometer at 50.7 MHz using CH3N02 as sn internal reference and the INEPT technique (optimized for 2J (15N-C,-H) of about 2.5 Hz for shortening the accumulation time. 1500 - 3500 scans were accumulated for 1 M amidine solution in CDC13. The Raman spectra were recorded on a CARY 82 spectrometer with Ar' laser from Spectra Physics; pure amidines were placed in a capillary. RESULTS AND DISCUSSION Earlier studies suggested that 15N NMR chemical shifts can be used as a probe of the nitrogen lone pair delocalization in conjugated systems and significant correlations between 6'5 N and nG* were found (ref. 5). The 15 N chemical shifts of N'N'-dimethylN2-substituted amidines indicate (Table 1) that some correlation between $I5 N and A G* exists in formamidines with different substituent at phenyl ring i.e. in a series where steric factor within the range of amidine group does not change. The upfield shift of N2 nitrogen signal with increasing rotational barrier can be observed whereas the changes of chemical shift of amino nitrogen N' occur in an opposite direction. The downfield shift of N' may be explained by an increase in conjugation of the lone pair with nelectrons of the N2-C bond. This delocalization of electron pair does not appear a dominant factor in acetamidines which are sterically crowded (ref. 3). The changes of the bond orders in the amidine moiety should be reflected in shifting of the N2=C and C-N' bands in Raman spectra of N2 -substituted amidines. Amidines exhibit strong Raman band in the range 1600-1660 cm" which was attributed to the N2=C stretching vibration (ref. 6) and the bands in the range 1200-1400 cm" expected for C-N' vibrations; ?
however is not characteristic (superposition with other viC-N" bration similarly as in amides). The polar form contribution to the structure of amidine is getting more significant with the increasing rotational barrier and the shortening of C-N' bond accompanied by a lengthening of the N2=C bond can be expected. The above correlation in the carbon-nitrogen bonds length in amidines was confirmed by the X-ray data (ref. 7) and (assuming that decreasing frequency of Raman band can be attributed to the bond lengthening) it is observed also in Raman spectra of the studied compounds (Table 1).
167
-
The author thanks Professor J. Oszczapowicz for supplying the
amidines.
TABLE 1 The 15N chemical shifts and Raman frequencies of \)C,N2 of formamidines (R=H) and acetamidines (R=CH3)
R' ~~ PBr - C6H4 %H5 pCH3 - C6H4 C6H4- CH2
R
’ 5Nbpm> N'
N2
fl C=N2 -1
(cm >
dG+ ( kJ
mol-')
H
- 157.0
- 299.7
1630
64.8a
H"
- 153.5 - 151.1
- 301.7 - 302.0
1637 1640
63.3a 60.3a
H
- 163.6
- 311.9
1647 1655
52.4b 50.6b
1655 1615
50.4b 53.0c
n - C4Hg
H
n-C6H13 'gH5
H cH3
n-C6H13
CH3
- 141.8
- 306.0
1627
<40b
a from (ref. I), b from (ref. 2), ' from (ref. 3). REFERENCES 1 2 3 4
I. Wawer, Pal. J. Chem., 62 233 (1988). I. Wawer, Magn. Reson. Chem., 27 557 (1989). I. Wawer, ibid., 26 601 (1988)r J. Oszczapowicz, E. Raczyliska, Pal. J. Chem., 57 419 (1983) and J. Chem. Sot., Perkin Trans., 2, 1963 (198 ). 5 G.J. Martin, M.L. Martin, J-P. Guesnard, 1 $N NPlR Spectroscopy, Springer Verlag, Berlin, Heidelberg, New York, 1988 pp. 101-104. 6 D.N. Shigorin, Ya.K. Syrkin, Izvest. Akad. Nauk SSSR, Ser. Fiz. 2, 225 (1945) end Zhur. Fiz. Khim. 23, 241 (1949). 7 R. Anulewicz, T.M. Krygowski, B. Pniewska, J. Crystall. Spectr. Res., I7, 661 (1987).
HINDERED
165 in The Netherlands
ROTATION IN AMIDINES
I. WAWER Department in Chemistry, The University, 02 093 Warsaw, Poland
SUMMARY The 15N chemical shifts and frequencies of dCcN bond from Raman spectra of N'N' -dimethyl-N2-substituted smidines were related to the rotational barriers around C-N' bond obtained from "C
NMR.
INTRODUCTION Numerous amidines have biological and pharmaceutical activity. The configuration and molecular dynamics of the compounds with amidine group N=C-N are, therefore recently widely investigated. Amidines contain two different nitrogen atoms: amine (N') and imine (N2) type, however, because of conjugation between the N2 =C ;,PH3 ACH I R
3
double bond and the lone electron pair at N' both carbon-nitrogen bonds exhibit a partial double bond character. The rotation around C-N' is hindered and the activation para13 meters of the rotation were estimated from C d-NMR line shape analysis (refs. l-3). The rotational barriers (AGf)
were deter-
mined by the electronic and steric factors introduced by the substituents R and R' and reflected the degree of the double bond character of the C-N' bond. The aim of this study was to relate the obtained rotational barriers with the other parameters which give the information about the nitrogen lone pair delocalization and the bond orders within the amidine group. EXPERIMENTAL N'N'-dimethyl-$-substituted amiytnes were synthesized and purified as described in ref. 4. The N NMR spectra were recorded
002%2860/90/$03.50
0 1990 Elsevier
Science
Publishers
B.V.
166
on the Bruker AM500 spectrometer at 50.7 MHz using CH3N02 as sn internal reference and the INEPT technique (optimized for 2J (15N-C,-H) of about 2.5 Hz for shortening the accumulation time. 1500 - 3500 scans were accumulated for 1 M amidine solution in CDC13. The Raman spectra were recorded on a CARY 82 spectrometer with Ar' laser from Spectra Physics; pure amidines were placed in a capillary. RESULTS AND DISCUSSION Earlier studies suggested that 15N NMR chemical shifts can be used as a probe of the nitrogen lone pair delocalization in conjugated systems and significant correlations between 6'5 N and nG* were found (ref. 5). The 15 N chemical shifts of N'N'-dimethylN2-substituted amidines indicate (Table 1) that some correlation between $I5 N and A G* exists in formamidines with different substituent at phenyl ring i.e. in a series where steric factor within the range of amidine group does not change. The upfield shift of N2 nitrogen signal with increasing rotational barrier can be observed whereas the changes of chemical shift of amino nitrogen N' occur in an opposite direction. The downfield shift of N' may be explained by an increase in conjugation of the lone pair with nelectrons of the N2-C bond. This delocalization of electron pair does not appear a dominant factor in acetamidines which are sterically crowded (ref. 3). The changes of the bond orders in the amidine moiety should be reflected in shifting of the N2=C and C-N' bands in Raman spectra of N2 -substituted amidines. Amidines exhibit strong Raman band in the range 1600-1660 cm" which was attributed to the N2=C stretching vibration (ref. 6) and the bands in the range 1200-1400 cm" expected for C-N' vibrations; ?
however is not characteristic (superposition with other viC-N" bration similarly as in amides). The polar form contribution to the structure of amidine is getting more significant with the increasing rotational barrier and the shortening of C-N' bond accompanied by a lengthening of the N2=C bond can be expected. The above correlation in the carbon-nitrogen bonds length in amidines was confirmed by the X-ray data (ref. 7) and (assuming that decreasing frequency of Raman band can be attributed to the bond lengthening) it is observed also in Raman spectra of the studied compounds (Table 1).
167
-
The author thanks Professor J. Oszczapowicz for supplying the
amidines.
TABLE 1 The 15N chemical shifts and Raman frequencies of \)C,N2 of formamidines (R=H) and acetamidines (R=CH3)
R' ~~ PBr - C6H4 %H5 pCH3 - C6H4 C6H4- CH2
R
’ 5Nbpm> N'
N2
fl C=N2 -1
(cm >
dG+ ( kJ
mol-')
H
- 157.0
- 299.7
1630
64.8a
H"
- 153.5 - 151.1
- 301.7 - 302.0
1637 1640
63.3a 60.3a
H
- 163.6
- 311.9
1647 1655
52.4b 50.6b
1655 1615
50.4b 53.0c
n - C4Hg
H
n-C6H13 'gH5
H cH3
n-C6H13
CH3
- 141.8
- 306.0
1627
<40b
a from (ref. I), b from (ref. 2), ' from (ref. 3). REFERENCES 1 2 3 4
I. Wawer, Pal. J. Chem., 62 233 (1988). I. Wawer, Magn. Reson. Chem., 27 557 (1989). I. Wawer, ibid., 26 601 (1988)r J. Oszczapowicz, E. Raczyliska, Pal. J. Chem., 57 419 (1983) and J. Chem. Sot., Perkin Trans., 2, 1963 (198 ). 5 G.J. Martin, M.L. Martin, J-P. Guesnard, 1 $N NPlR Spectroscopy, Springer Verlag, Berlin, Heidelberg, New York, 1988 pp. 101-104. 6 D.N. Shigorin, Ya.K. Syrkin, Izvest. Akad. Nauk SSSR, Ser. Fiz. 2, 225 (1945) end Zhur. Fiz. Khim. 23, 241 (1949). 7 R. Anulewicz, T.M. Krygowski, B. Pniewska, J. Crystall. Spectr. Res., I7, 661 (1987).