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Isoxazoles, pyrazoles, and isothiazoles
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This chapter investigates isoxazoles, pyrazoles, and isothiazoles. Isoxazole, pyrazole, and isothiazole are the parent structures of the 1,2-azole family of heterocycles, having a nitrogen atom plus one other heteroatom in a 1,2-relationship in a five-membered ring. The aromatic sextet is completed by delocalization of the lone pair from the second heteroatom. Consequently, as in pyridine, the nitrogen atoms of the 1,2-azoles have a lone pair available for protonation. However, the 1,2-azoles are significantly less basic than the 1,3-azoles because of the electron-withdrawing effect of the adjacent heteroatom. Isoxazole and isothiazole are essentially non-basic heterocycles, and even pyrazole is a much weaker base than the corresponding 1,3-azole imidazole. As with substituted imidazoles, substituted pyrazoles may exist as a mixture of tautomers. The chapter then looks at the synthesis, electrophilic substitution, and anion chemistry of isoxazoles, pyrazoles, and isothiazoles.
Title: Isoxazoles, pyrazoles, and isothiazoles
Description:
This chapter investigates isoxazoles, pyrazoles, and isothiazoles.
Isoxazole, pyrazole, and isothiazole are the parent structures of the 1,2-azole family of heterocycles, having a nitrogen atom plus one other heteroatom in a 1,2-relationship in a five-membered ring.
The aromatic sextet is completed by delocalization of the lone pair from the second heteroatom.
Consequently, as in pyridine, the nitrogen atoms of the 1,2-azoles have a lone pair available for protonation.
However, the 1,2-azoles are significantly less basic than the 1,3-azoles because of the electron-withdrawing effect of the adjacent heteroatom.
Isoxazole and isothiazole are essentially non-basic heterocycles, and even pyrazole is a much weaker base than the corresponding 1,3-azole imidazole.
As with substituted imidazoles, substituted pyrazoles may exist as a mixture of tautomers.
The chapter then looks at the synthesis, electrophilic substitution, and anion chemistry of isoxazoles, pyrazoles, and isothiazoles.
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