On treatment with liquid ammonia at -33°C, the quaternary pyrimidinium salts, i.e.
1-methylpyrimidinium methyl sulfate, 1,2-dimethylpyrimidinium iodide, 1,4,6-trimethyl-pyrimidinium iodide and 1,2,4,6-tetramethylpyrimidinium iodide demethylate yielding pyrimidine. 2-methyl-, 4,6-dimethyl- and 2,4.6-trimethylpyrimidine, respectively. It was observed that under these conditions 1-methyl-[1,3- 15
N]-pyriniidiniuni methyl sulfate yields [1- 15
N]-pyrimidine. By measuring the PMR spectra of above- mentioned pyrimidinium salts in liquid ammonia it is shown that these salts undergo covalent amination on the 1,6-azomethin bond. These results indicate that the demethylation reaction occurs via
an Addition-NucleophileRing-Opening-Ring Closure mechanism.
On treatment with active methylene compounds in basic media the quaternary pyrimidinium salts, i.e.
methyl 1-methylpyrimidinium sulfate, 1-methyl-4-phenylpyrimidinium iodide and 1-methyl-5- phenylpyrimidinium iodide are converted into pyridine derivatives. The mechanism of the reaction is discussed.
On treatment of the quaternary pyrimidinium salts i.e.
1-methyl-4-phenylpyrimidinium iodide and 1-methyl-5-phenylpyrimidinium iodide with cyanamide, O
-methylisouronium chloride or bis[S-methylisothiouronium] sulfate in basic media, 2-amino-4-phenylpyrimidine and 2-amino-5-phenylpyrimidine are formed respectively. A ring transformation is involved in which the two-atom fragment N(1)-C(2) of the pyrimidine ring is replaced by an N-C fragment of the reagent. On reacting 1-methylpyrimidinium iodide with benzamidinium chloride or pivalamidinium chloride in a solution of sodium ethoxide in ethanol, 2-phenylpyrimidine and 2- tert
-butylpyrimidine are formed respectively.
It is proved by 15
N-labelling that this nucleophilic substitution occurs via
a ring transformation in which the N(1)- C(2)-N(3) fragment of the pyrimidine is replaced by the N-C-N fragment of the amidine. These reactions are new examples of a nucleophilic substitution occurring according to an S N
Reaction of 4-alkoxy- or 4,6-dialkoxypyrimidines with 1 equivalent of triethyloxonium tetrafluoroborate yields 4-alkoxy-N-ethyl or 4,6-dialkoxy-N-ethylpyrimidinium salts, respectively. With two or more equivalents of this reagent, rearrangement of N-ethyl-alkoxypyrimidinium salts into 1-ethyl-3-alkyl-1,4(3,4)-dihydro-4-oxopyrimidinium salts takes place. These rearrangements can also be performed by heating. The mechanism of these rearrangement reactions is discussed.
The crystal and molecular structures of two isomeric compounds, 1-ethyl-4,6-diethoxypyrimidinium tetrafluoroborate and 1,3-diethyl-1,4(3,4)-dihydro-6-ethoxy-4-oxopyrimidinium tetrafluoroborate, reaction products of 4,6-diethoxypyrimidine with Meerwein reagent [O(C 2
] , have been determined by means of X-ray diffraction.
1-Ethyl-4,6-diethoxypyrimidinium tetrafluoroborate is monoclinic a=10.794, b=13.361,c=10.892 Å, β
=112.6°, space group P2 1
/n, four molecules per unit cell.
1,3-Diethyl-1,4(3,4)-dihydro-6-ethoxy-4-oxopyrimidinium tetrafluoroborate is monoclinic, a=17.637, b=14.054, c=11.501 Å, β
=101.7°, space group C2/c, eight molecules per unit cell.
In both structures the fluoroborate ions are disordered. The bond distances in the π-electron systems are reasonably well described in terms of a small number of resonance structures.
Treatment of 1,3-diethyl-1,4(3,4)-dihydro-4-oxopyrimidinium tetrafluoroborate and its 2-phenyl, 6-phenyl, 6-methyl and 6-ethoxy derivatives with aqueous ammonia resulted in the formation of a mixture of open-chain compounds i.e. N
-ethyl-3-(ethylamino)acrylamides and N
-ethyl-3-[formyl(acetyl,benzoyl)ethylamino]-acrylamides. They are formed by cleavage of the pyrimidine ring between the N(1)-C(2) and N(3)-C(2) bond, respectively. In liquid ammonia the same ring cleavage generally occurs; however, in the case where a 6-ethoxy group is present, recyclisation can take place, leading to 6-(ethylamino)pyrimidine derivatives. This degenerate ring transformation has been observed also with the 2-methyl and 2-phenyl derivative of 1,3-diethyl-1,4(3,4)-dihydro-6-ethoxy-4-oxopyrimidinium tetrafluoroborate. Evidence is presented by means of 1
H-NMR and 13
C-NMR spectroscopy that all these reactions are iniated by attack of NH 3
at the C(2)-position. Some of the above-mentioned open-chain compounds underwent a ring closure to the initially used 1,3-diethyl-1,4(3,4)-dihydro-4-oxopyrimidinium tetrafluoroborates on treating them with hydrofluoroboric acid in absolute ethanol.
On treatment with liquid ammonia at -33° the quaternary pyrimidinium salts i.e. 4-ethoxy-1-ethyl- and 4,6-diethoxy-1-ethylpyrimidinium tetrafluoroborate undergo amino-de-ethoxylation, yielding 1,4-dihydro-1-ethyl-4-iminopyrimidine hydrogen tetrafluoroborate and a mixture of 1,4-dihydro-6-ethoxy-1-ethyl-4-imino- and 1,6-dihydro-4-ethoxy-1-ethyl-6-iminopyrimidine hydrogen tetrafluoroberate, respectively. 1
H-NMR and 13
C-NMR spectroscopic evidence is presented for the fact that compounds 1 and 3 easily give σ-adducts at position 2. Using 15
N-labelled ammonia it was shown that in these amino-de-ethoxylation reactions the substitution at C(4) or C(6) does not involve ring opening but probably occurs via an S N
) process. Reaction of 4-ethoxy-1-ethyl-2-phenyl-, 6-ethoxy-1-ethyl-4-phenyl-, 4,6-dimethoxy-1-ethyl-2-phenyl- and 4,6-dimethoxy-1-ethyl-2-methylpyrimidinium tetrafluoroborate with liquid ammonia gives besides the amino-de-ethoxylation product degenerate ring transformations leading to the N-deethylated products 14-16 and 4(6)-ethylaminopyrimidines 17-19. The salt 11 and 1,6-dihydro-1-ethyl-6-imino-4-phenylpyrimidine hydrogen tetrafluoroborate undergo, with potassium hydroxide, a Dimroth rearrangement to pyrimidines 20 and 17, respectively.
The mechanism of the conversion of pyrimidine into 5-ethyl-2-methylpyridine has been investigated. It has been proved, using the labelled compounds [1,3- 15
N]pyrimidine, [4,6- 14
C]pyrimidine and [5- 14
C]pyrimidine, that this reaction proceeds via
a mechanism, in which the pyrimidine ring is fragmentated into two molecules of HCN and one molecule of N
-methylacetaldimine. Four molecules of this imine undergo an aldol type condensation leading to 5-ethyl-2-methylpyridine.