3,4 and 7 membered heterocycles

HETEROCYCLES OF 3, 4 AND 7 MEMBERS

Details: German Fernandez; HETEROCYCLES OF 3, 4 AND 7 MEMBERS; Views: 7585

The properties of three-membered rings are related to strain, due to the low bond angles. Angular strain gives rise to opening reactions to form acyclic products.

In this section we will study the following heterocycles: oxirane, thiirane, 2H-azirine, aziridine, dioxirane, oxaziridine, 3H-diazirine and diaziridine.

oxirane

Boiling point: 10.5ºC
Ring stress: 114 KJ/mol
dipole moment: 1.88 D
Characteristics: Colorless liquid, soluble in water, very poisonous in the gas phase.

Tyrian

Boiling point: 55ºC
Ring stress: 83 KJ/mol
dipole moment: 1.66 D
Characteristics: Colorless liquid, partially soluble in water.

2h-azirine

Boiling point:
Ring stress: 170 KJ/mol.
dipole moment:
Characteristics: Very unstable substance, with high reactivity in the C=N bond

aziridine

Boiling point: 57ºC
Ring stress: 113 KJ/mol.
dipole moment: 1.89 D
Characteristics: Colorless liquid, soluble in water, poisonous, with an ammonia-like odor.

Details: German Fernandez; HETEROCYCLES OF 3, 4 AND 7 MEMBERS; Views: 11797

Physical properties: It has an annular stress of 114 KJ/mol. The dipole moment is 1.88 D
Spectroscopic data: Spectrum UV: λ=171nm. NMR: δH= 2.54, δC= 39.7

physical-properties

Details: German Fernandez; HETEROCYCLES OF 3, 4 AND 7 MEMBERS; Views: 26857

Intramolecular cyclization (Williamson intramolecular): Halogenated alcohols in position 2 cyclize in basic media to form oxiranes.

williamson synthesis of oxirane

Mechanism:

Williamson synthesis mechanism

Read more: Oxirane synthesis methods

Details: German Fernandez; HETEROCYCLES OF 3, 4 AND 7 MEMBERS; Views: 27443

Oxiranes act as Lewis bases, protonating on oxygen in acidic media. The second type of reactivity is related to ring strain that favors opening reactions.

Isomerization to carbonyl compounds: Oxiranes in the presence of Lewis acids isomerize to form aldehydes or ketones. Thus, oxirane in the presence of BF ₃ turns into acetaldehyde

oxirane-reactivity

{white-box} Example - Propose a mechanism for the following transformation:

Solution - The BF₃ protonates the oxygen of the oxirane, converting it into a good leaving group and favoring the opening of the cycle. A rearrangement to a hydroxycarbocation will give the expected product.