Imine-Enamine Tautomerization

[ieva-jonusaite]

Requirement

The system shall identify a substructure pattern H-N-C=C, where:

• N is a nitrogen atom bearing a mobile hydrogen
• The first C is a carbon atom connected to the nitrogen
• The second C is a carbon atom double-bonded to the first carbon

The system should allow for the bidirectional interconversion to the tautomeric form N=C-C-H, which involves the migration of the mobile hydrogen from nitrogen to the terminal carbon and a corresponding shift of the double bond from C=C to N=C.

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Background

The hydrogen migration is facilitated by the electron-donating properties of nitrogen, which can stabilize both the imine and enamine forms through different electronic arrangements.

• In the enamine form, the nitrogen lone pair (an available pair of electrons not used in bonding) can participate in conjugation (the sharing of electrons across alternating single and double bonds) with the C=C double bond, providing additional stabilization.
• Conversely, the imine form benefits from the stability of the C=N double bond.

Rule No: PT_03_00

Rule No: PT_04_00

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Exceptions: Blocked Tautomerization due to Steric Hindrance

Background

The Imine-Enamine tautomerization requires a specific section of the molecule to temporarily adopt a flat (planar) geometry in the region around the nitrogen and the adjacent carbon atoms to allow for the necessary electronic and bond rearrangement.
However, if large, physically obstructive groups are attached to the carbon atoms of the double bond, they can create a "crowded" three-dimensional environment. This crowding, known as steric hindrance, can physically block the molecule from achieving the required flat shape, thus preventing the tautomeric shift from occurring.

Requirement

The system shall identify substructures that fit the H-N-C=C pattern but where both carbon atoms adjacent to the imine group (=NH) bear non-hydrogen and/or aromatic substituents.
Upon identification of such a structure, the system must prevent the bidirectional interconversion to the N=C-C-H tautomeric form.

Examples

1. Carbon Connected to Non-Hydrogen Substituents:
   These are groups where the carbon atom is bonded to atoms or groups other than hydrogen, such as:
   • Alkyl groups: methyl (–CH₃), ethyl (–C₂H₅), isopropyl, tert-butyl, and higher homologues
   • Halogens: fluorine (–F), chlorine (–Cl), bromine (–Br), iodine (–I)
   • Hydroxyl group: (–OH)
   • Thiol group: (–SH)
   • Phosphanyl group: (–PH₂)

2. Aromatic Rings:
   These include:
   • Benzene ring (phenyl group)
   • Naphthyl group (fused benzene rings)
   • Furan ring (aromatic heterocycle)