¹³C n.m.r. spectra of 2‐substituted pyrimidines

Abstract

Abstract--'T n.m.r. chemical shifts and carbon-proton coupling constants of 2-substituted pyrimidines are reported. The carbon chemical shifts are correlated with 7i-electron densities. Substituents which cause deshielding at the directly bound carbon (e.g. NH,, OCH, and F) exert a more powerful effect in the benzene series than in the pyridine or pyrimidine series. The carbon-proton coupling constants do not correlate with the electronegativity of the substituents. Carbon-proton coupling constants and protonproton coupling constants over the same number of bonds do not obey the Karabatsos relationship. The changes in the carbonproton coupling constants in 2(ZH)-pyrimidinone and 2(2H)-pyrimidinethione which accompany anion and cation formation are reported.

Carbon-13 NMR determination of the tautomeric and ionization states of folate in its complexes with Lactobacillus casei dihydrofolate reductase

“…The shift changes on replacing a OMe with a OH group in monosubstituted benzenes are the following: for the directly substituted carbon, -4.5 ppm; ortho, 1.7 ppm; meta, 0.4 ppm (Levy et al, 1980;Ewing, 1979). However, the inductive effect at the substituted carbon is expected to be smaller in a 2-substituted pyridine or a 4-substituted pyrimidine due to competition for electronic charge between the ring nitrogen and the electron-withdrawing substituent (Craik, 1983;Turner & Cheeseman, 1976;Cheeseman et al, 1979). For example, an NH2 -* OMe substitution in a monosubstituted benzene causes a substituent shift at Cl of 13.4 ppm (Levy et al, 1980;Craik, 1983) compared with a corresponding substituent shift of only 4.4 ppm in a 2-substituted pyridine (Stothers et al, 1972) and 4.4 ppm in 4-substituted 2-amino-6-methylpteridine (unpublished results).…”

Density Functional Theory/GIAO Studies of the 13C, 15N, and 1H NMR Chemical Shifts in Aminopyrimidines and Aminobenzenes: Relationships to Electron Densities and Amine Group Orientations

“…The 1 H, 13 C, and 15 N NMR chemical shifts of nitrogen heterocyclic compounds have been the subject of a number of experimental and theoretical studies. − There have also been a number of NMR studies of chemical shifts in aminobenzenes , and aminopyrimidines. − An important aspect of magnetic shielding in amine-substituted aromatic compounds, which appears not to have been addressed in previous NMR studies, is the dependence of NMR chemical shifts on amine group orientations. Experimental and theoretical studies of amine substituted aromatic compounds show that the amine group hydrogens are out of the plane by an amount which depends on a balance between π-electron delocalization across the C−N bond and the tendency of the amine group to form strongly directed sp 3 -hybridized orbitals. − There are relatively few structural studies among these compounds because of spectral complexity arising from multiple, large-amplitude vibrations associated with low NH 2 torsion and inversion barriers.…”

“…Since these represent large fractions of the total shift ranges for these two nuclei, the changes in electronic structures should also be large. Indeed, one of the earliest NMR correlations was that between 1 H and 13 C NMR chemical shifts and π-electron densities of aromatic and heteroaromatic systems. 1,2 This was a puzzle in the early studies of aromatic molecules because changes in the diamagnetic term are small, while changes in the paramagnetic term should be in the opposite direction.…”

“…The theoretical basis for the correlations was addressed by several groups. [3][4][5][6] The 1 H, 13 C, and 15 N NMR chemical shifts of nitrogen heterocyclic compounds have been the subject of a number of experimental and theoretical studies. [7][8][9] There have also been a number of NMR studies of chemical shifts in aminobenzenes 10,11 and aminopyrimidines.…”

Regioselectivity in the amination of methylsulfanyl-substituted azines with O-mesitylenesulfonylhydroxylamine

“…In the 1 H NMR spectra of III, IV, Va, Vb, VIa, and VIb, signals from protons in the aromatic ring and CH 3 group were displaced downfield relative to the corresponding signals of the initial azines, obviously due to the presence of positive charge. In the 13 C NMR spectra of Va, Vb, VIa, and VIb, carbon atoms in the α-position with respect to the nitrogen resonated in a weaker field than those located in the β-position, which is typical of pyrimidine derivatives [11]. The MeS carbon signal in the spectra of III, Va, and Vb is displaced downfield by 17-19 ppm relative to the corresponding signals of their precursors I and II due to the presence of positive charge on the sulfur atom.…”