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2.3 Proposed Mechanism:
Regarding the mechanism of the transformation of 2-hydroxy-2,3-dihydro-1Himidazo[1,2-a]pyrimidinium
salts into 2-amino-1H-imidazoles, we presume that the
reaction proceeds via an unusual Dimroth-type rearrangement 14-16 (Scheme-4).
Scheme-4: Proposed Mechanism for the Dimroth-type Rearrangement of 2-
Hydroxy-2,3-dihydro-1H-imidazo[1,2-a]pyrimidinium Salts III-12
N
N N R1 Br OH
R 2 R3
N N
R 2
NH 2
R1 OH
R 3
N2H4 fast
HN NH R 1
HN
R 2
H 2NHN
O
R 3
N
N
N R1 OH
R 2 R3
R 1HN
N
N
H
H 2NHN
R 2
OH
R 3
N
N
N R1 OH
R 2 R3
1 A B
- H 2O
slow
R 1HN
C D E 2
In the first step, the 2-hydroxy-2,3-dihydroimidazo[1,2-a]pyrimidinium salt (1)
undergoes cleavage of the pyrimidine ring, resulting in the generation of pyrazole and
2-amino-5-hydroxyimidazolidine (C), which is in equilibrium with the open form (D).
This can cyclize again leading to the isomeric 2-amino-5(4)-hydroxyimidazolidines
(E). Both isomers (C) and (E) were detected by mass-spectrometry. Final
dehydratation upon microwave irradiation, results in the rearranged 2-amino-1Himidazoles
2. This dehydratation step, as judged by mass-spectrometry, was found to
be the slowest step of the transformation. While some 2-amino-5(4)hydroxyimidazolidines
(E) lost water spontaneously at room temperature, other
required higher temperature upon microwave irradiation. Therefore all reactions were
run at 100 °C, preventing the sequence from stopping after the first step.
-
N
H
N
N
H
N
R 2
R 3