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1H N.M.R. spectra of some 5-substituted-3-isopropyl-6-methyl uracils
Journal of Molecular Structure, 174 (1988)281-284 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
281
'H N.M.R. SPECTRA OF SOME 5-SUBSTITUTED-3-ISOPROPYL-6-METHYL URACILS
B.Z.JOVANOVIC',t.D.TADIC', M.D,MUSKATIROVIC1,M.B.PESIE* and S.I.BOGDANOVIC2 1 2
Department of Organic Chemistry, Faculty of Technology and Metallurgy, University of Belgrade, P.O. Box 494, YU-11001 Belgrade, Yugoslavia Faculty of Agriculture, Department of Biochemistry,Universityof Belgrade, P.O. Box 127, YU-11081 Belgrade, Yugoslavia
ABSTRACT The IH n,m.r. spectra of some 5-substituted-3-isopropyl-6-methyl uracils were determined in deuterated dimethyl sulfoxide (d6-DMSO), The substituents are H, Cl, Br, I, NO , NH , N(CH3)*, C H , C H -NO -p and C6H4-NH2-p. It has been shown that relat?ve chemical s ift6(&s) gf4N 2-H proton of some of the examined compounds correlated linearly with Hammett u ('konstant for the substituents in position 5 of uracil ring, The correlation wvth the u constant was interpreted as providing evidence that the inductive effect of th!!substituent on chemical shift is dominating one. INTRODUCTION As it is known some 5-halo-3-alkyl-6-methyluracils are used as selective herbicides. The herbicidal activity of these compounds depends on their structure, the chemical changes during a process of their herbicidal activity and the products formed by those changes (ref. 1). In our previous investigation (ref.2-3) concerning the mechanism of thermal degradation of some 5-substituted3-alkyl-6-methyluracils, it has been shown, that by heating these compounds dealkylation of N-alkyl group takes place with the formation of the corresponding olefine and 5-substituted-6-methyluracil. The assumed mechanism includes a cyclic transition state in which heterolytic cleavageof C,-N(3) bond, as well as of the Cs-H bond takes place with the formation of the C,-CB double bond, similarly to B-syn-eliminationi.e., pyrolytic Ei elimination of esters, xanthates and amino-oxides via a six-membered cyclic transition state (ref. 4). On the basis of the obtained results it was concluded that the heterolytic cleavage of C,-N(3) bond is more advanced in transition state than heterolytic cleavage of CB-H bond. The degree of dealkylation of N-alkyl group in position 3 of uracil ring besides its structure depends on nature of the substituent in position 5 and can be attributed to its inductive effect.
0022-2860/88/$03.50 0 1988 Elsevier Science Publishers B.V.
282
The general formula of these compounds is
Where S is H, Cl, Br, I, N02, NH2, N(CH3)2, C6H5, C6H4-NO*-p and C6H4-NH2-p. In this work 'H n.m.r. spectra of above compounds are discussed in view of their structure and the effect of the substituents in position 5 on chemical shift of N(,)-H proton. EXPERIMENTAL All uracil derivatives were synthesized following the procedure already described in the literature
and identified by microanalysis, MS and 'H n.m.r
data.(ref.2-3) The 'H n.m.r. spectra of 5-substituted-3-isopropyl-6-methyl Uracils were recorded in deuterated dimethyl sulfoxide (d6-DMSO) with tetramethylsilan (TMS) as internal reference standard using Varian FT-80 A spectrophotometer. In all cases the 'H n.m.r. spectra gave satisfactory signal integration. The chemical shifts of aromatic protons of 5-(p-nitrophenyl)and 5-(p-aminophenyl)-3-isopropyl-6-Imethyl uraciland corresponding coupling constants (AA'BB' system) were consistent with the expected structures. All spectra of uracil derivatives were determined using approximatly 0.3M solution at 37+-2OC. RESULTS AND DISCUSSION The obtained results emphasize the conclusion that all uracil derivatives exist in 2,4-dioxo form. (structure I). The values of the chemical shifts of N(,)-H proton of 5-substituted-3-isopropyl-6-methyl uracils are presented in Table 1 (6 in ppm), as well as the relative chemical shifts of N(,)-H proton (As)
(relative to 3-isopropyl-6-,methyl uracilwhich has H atom in position 5 of
the ring). In the same Table are given the corresponding Hammett am constants , which as it is known represent the approximate measure of the inductive effect of the substituent.(ref.5)
283
The data from Table -H proton depends
1 confirm
the assumption
on the nature
N(1) ring. The electron-withdrawing
substituents
down field chemical
shifts
value).
electron-donating
Conversely,
a negative
to correlate
ituents with substituent
where A&stands
N(,)-H
proton,
to 5-isopropyl-6-methyl
deshild
uracil
substituents
the relative
constant
for relative
tant and where
TABLE
relative
in position
shift of 5 of uracil
giving
the
(positive
cause an upfield
A6
shift, with
A6 value.
The attempt
according
that the chemical
of the substituents
(a,) using Hammett
chemical
shift
p is the slope of obtained
to JaffC
chemical
can be considered
shifts for different
subst-
equation
(in ppm); urn-for substituent
cons-
plot (in ppm/a) gave the result which
as fair
(r=0.938,
n=8).(ref.6)
1. 'H n.m.r. chemical shifts of N&1)-H proton of 5-substituted-3-isopropyl-6-me h 1 uracils
Substituent in position
Relative chemical shift (Ab,ppm)
N(l)-H 5
(b,ppm)
"m
0
H
10.85
Cl
11.32
+0.47
0.37
Br
11.35
to.50
0.39 0.35
0
I
11.30
+0.45
NO2
11.50
to.65
0.71
NH2
10.25
-0.60
-0.16
N(CH3)2
10.60
-0.25
-0.21
'gH5 C6H4-NO*-p
10.95
to.10
0.06
11.25
to.40
C6H4-NH2-p
10.00
-0.85
The plot of relative corresponding
Hammett
The correlation possible
to assign
chemical
urn constants obtained
shifts of N is given
-H proton (1) in Fig.1.
from the data given
proportionality
constant
in Table
(A6,ppm)
vs. the
1, have made
it
(slope of the plot) with the value
p= 1.27. The exclusion cantly
of the point for NH2 substituent
the value of the proportionality
improves
the correlation
satisfactory
(r= 0.976,
The correlation ect of the substituent
coefficient
constant
does not affect
signifi-
p= 1.05, but considerably
that it classifies
the correlation
as
n = 7).
of A6 with urn constants on chemical
indicated
that the electronic
shifts of N (,)-H proton
is predominantly
eff-
284 inductive.
possible
is emphasized
This conclusion
NO2 substituent
to assume
by the fact that the value of A6 for
with a, and not with 0; constant
is correlated
the direct conjugation
though
it is
of the lone pair of electrons
on N
atom and NO2 substituent.
(1)
Ah,PPrn
I
-0.5-
I -0.2
I 0.2
I 0
Fig. 1. The plot of relative
chemical
-3-isopropyl-6-methyl
uracils
Almost
the same proportionality
could be calculated the correlation molecule
I
I
0.6
0.8
shifts of N(,)-H vs. Hammett-o, constant
(P
q
proton
of the average
5 of uracils
a,
in 5-substituted-
constants 1.14, r = 0.972,
from the data given in the literature
of 5-substituted
in position
t 0.4
n = 6)
(ref. 7) concerning
-H and N(3) -H chemical shifts in the (1) and Hammett om constant for the substituent
value N
uracils ring.
REFERENCES 1 2 3 4 5 6 7
R.Wegler, Chemie der Phlanzenschutz-und Schxdlings-bekampfungsmittel., Band II, Springer-Verlag, Berlin-Heidelberg-New York, (1970) pp.356. P.D.Tadic, M.B.PeSic and S.K.Ries, J.Agr.Food them., 19 (1971) pp.46. B.f.JovanoviC, f.D.TadiC, M.D.MuSkatiroviC and M.B.PeSi&, J.Serb.Chem.Soc., 51 (1986) pp.389. C.H.De Puy and R.W.King, Chem.Rev., 60 (1960) pp.431. J.Shorter, Correlation Analysis in Organic Chemistry: An Introduction to Linear Free-energy Relationships, Oxford University Press, (1979) pp 9. H.H.JaffP, Chem.Rev., 53 (1953) pp.191. J.P.Kokko, L.Mandell and J.H.Goldstein, J.Amer.Chem.Soc., 84 (1961) pp.1042.
281
'H N.M.R. SPECTRA OF SOME 5-SUBSTITUTED-3-ISOPROPYL-6-METHYL URACILS
B.Z.JOVANOVIC',t.D.TADIC', M.D,MUSKATIROVIC1,M.B.PESIE* and S.I.BOGDANOVIC2 1 2
Department of Organic Chemistry, Faculty of Technology and Metallurgy, University of Belgrade, P.O. Box 494, YU-11001 Belgrade, Yugoslavia Faculty of Agriculture, Department of Biochemistry,Universityof Belgrade, P.O. Box 127, YU-11081 Belgrade, Yugoslavia
ABSTRACT The IH n,m.r. spectra of some 5-substituted-3-isopropyl-6-methyl uracils were determined in deuterated dimethyl sulfoxide (d6-DMSO), The substituents are H, Cl, Br, I, NO , NH , N(CH3)*, C H , C H -NO -p and C6H4-NH2-p. It has been shown that relat?ve chemical s ift6(&s) gf4N 2-H proton of some of the examined compounds correlated linearly with Hammett u ('konstant for the substituents in position 5 of uracil ring, The correlation wvth the u constant was interpreted as providing evidence that the inductive effect of th!!substituent on chemical shift is dominating one. INTRODUCTION As it is known some 5-halo-3-alkyl-6-methyluracils are used as selective herbicides. The herbicidal activity of these compounds depends on their structure, the chemical changes during a process of their herbicidal activity and the products formed by those changes (ref. 1). In our previous investigation (ref.2-3) concerning the mechanism of thermal degradation of some 5-substituted3-alkyl-6-methyluracils, it has been shown, that by heating these compounds dealkylation of N-alkyl group takes place with the formation of the corresponding olefine and 5-substituted-6-methyluracil. The assumed mechanism includes a cyclic transition state in which heterolytic cleavageof C,-N(3) bond, as well as of the Cs-H bond takes place with the formation of the C,-CB double bond, similarly to B-syn-eliminationi.e., pyrolytic Ei elimination of esters, xanthates and amino-oxides via a six-membered cyclic transition state (ref. 4). On the basis of the obtained results it was concluded that the heterolytic cleavage of C,-N(3) bond is more advanced in transition state than heterolytic cleavage of CB-H bond. The degree of dealkylation of N-alkyl group in position 3 of uracil ring besides its structure depends on nature of the substituent in position 5 and can be attributed to its inductive effect.
0022-2860/88/$03.50 0 1988 Elsevier Science Publishers B.V.
282
The general formula of these compounds is
Where S is H, Cl, Br, I, N02, NH2, N(CH3)2, C6H5, C6H4-NO*-p and C6H4-NH2-p. In this work 'H n.m.r. spectra of above compounds are discussed in view of their structure and the effect of the substituents in position 5 on chemical shift of N(,)-H proton. EXPERIMENTAL All uracil derivatives were synthesized following the procedure already described in the literature
and identified by microanalysis, MS and 'H n.m.r
data.(ref.2-3) The 'H n.m.r. spectra of 5-substituted-3-isopropyl-6-methyl Uracils were recorded in deuterated dimethyl sulfoxide (d6-DMSO) with tetramethylsilan (TMS) as internal reference standard using Varian FT-80 A spectrophotometer. In all cases the 'H n.m.r. spectra gave satisfactory signal integration. The chemical shifts of aromatic protons of 5-(p-nitrophenyl)and 5-(p-aminophenyl)-3-isopropyl-6-Imethyl uraciland corresponding coupling constants (AA'BB' system) were consistent with the expected structures. All spectra of uracil derivatives were determined using approximatly 0.3M solution at 37+-2OC. RESULTS AND DISCUSSION The obtained results emphasize the conclusion that all uracil derivatives exist in 2,4-dioxo form. (structure I). The values of the chemical shifts of N(,)-H proton of 5-substituted-3-isopropyl-6-methyl uracils are presented in Table 1 (6 in ppm), as well as the relative chemical shifts of N(,)-H proton (As)
(relative to 3-isopropyl-6-,methyl uracilwhich has H atom in position 5 of
the ring). In the same Table are given the corresponding Hammett am constants , which as it is known represent the approximate measure of the inductive effect of the substituent.(ref.5)
283
The data from Table -H proton depends
1 confirm
the assumption
on the nature
N(1) ring. The electron-withdrawing
substituents
down field chemical
shifts
value).
electron-donating
Conversely,
a negative
to correlate
ituents with substituent
where A&stands
N(,)-H
proton,
to 5-isopropyl-6-methyl
deshild
uracil
substituents
the relative
constant
for relative
tant and where
TABLE
relative
in position
shift of 5 of uracil
giving
the
(positive
cause an upfield
A6
shift, with
A6 value.
The attempt
according
that the chemical
of the substituents
(a,) using Hammett
chemical
shift
p is the slope of obtained
to JaffC
chemical
can be considered
shifts for different
subst-
equation
(in ppm); urn-for substituent
cons-
plot (in ppm/a) gave the result which
as fair
(r=0.938,
n=8).(ref.6)
1. 'H n.m.r. chemical shifts of N&1)-H proton of 5-substituted-3-isopropyl-6-me h 1 uracils
Substituent in position
Relative chemical shift (Ab,ppm)
N(l)-H 5
(b,ppm)
"m
0
H
10.85
Cl
11.32
+0.47
0.37
Br
11.35
to.50
0.39 0.35
0
I
11.30
+0.45
NO2
11.50
to.65
0.71
NH2
10.25
-0.60
-0.16
N(CH3)2
10.60
-0.25
-0.21
'gH5 C6H4-NO*-p
10.95
to.10
0.06
11.25
to.40
C6H4-NH2-p
10.00
-0.85
The plot of relative corresponding
Hammett
The correlation possible
to assign
chemical
urn constants obtained
shifts of N is given
-H proton (1) in Fig.1.
from the data given
proportionality
constant
in Table
(A6,ppm)
vs. the
1, have made
it
(slope of the plot) with the value
p= 1.27. The exclusion cantly
of the point for NH2 substituent
the value of the proportionality
improves
the correlation
satisfactory
(r= 0.976,
The correlation ect of the substituent
coefficient
constant
does not affect
signifi-
p= 1.05, but considerably
that it classifies
the correlation
as
n = 7).
of A6 with urn constants on chemical
indicated
that the electronic
shifts of N (,)-H proton
is predominantly
eff-
284 inductive.
possible
is emphasized
This conclusion
NO2 substituent
to assume
by the fact that the value of A6 for
with a, and not with 0; constant
is correlated
the direct conjugation
though
it is
of the lone pair of electrons
on N
atom and NO2 substituent.
(1)
Ah,PPrn
I
-0.5-
I -0.2
I 0.2
I 0
Fig. 1. The plot of relative
chemical
-3-isopropyl-6-methyl
uracils
Almost
the same proportionality
could be calculated the correlation molecule
I
I
0.6
0.8
shifts of N(,)-H vs. Hammett-o, constant
(P
q
proton
of the average
5 of uracils
a,
in 5-substituted-
constants 1.14, r = 0.972,
from the data given in the literature
of 5-substituted
in position
t 0.4
n = 6)
(ref. 7) concerning
-H and N(3) -H chemical shifts in the (1) and Hammett om constant for the substituent
value N
uracils ring.
REFERENCES 1 2 3 4 5 6 7
R.Wegler, Chemie der Phlanzenschutz-und Schxdlings-bekampfungsmittel., Band II, Springer-Verlag, Berlin-Heidelberg-New York, (1970) pp.356. P.D.Tadic, M.B.PeSic and S.K.Ries, J.Agr.Food them., 19 (1971) pp.46. B.f.JovanoviC, f.D.TadiC, M.D.MuSkatiroviC and M.B.PeSi&, J.Serb.Chem.Soc., 51 (1986) pp.389. C.H.De Puy and R.W.King, Chem.Rev., 60 (1960) pp.431. J.Shorter, Correlation Analysis in Organic Chemistry: An Introduction to Linear Free-energy Relationships, Oxford University Press, (1979) pp 9. H.H.JaffP, Chem.Rev., 53 (1953) pp.191. J.P.Kokko, L.Mandell and J.H.Goldstein, J.Amer.Chem.Soc., 84 (1961) pp.1042.