Synthetic pathways to remove amine group

Assistant

The conversion of methyl(1-phenylethyl)amine (1) to acetophenone (2) or its oxime (3) is formally an oxidative removal of a secondary amine group and its replacement by a carbonyl or oximino function. The benzylic carbon in 1 is already at the correct oxidation level to become a ketone – only the C–N bond must be replaced by a C=O (or C=NOH) bond. This can be accomplished through intermediates that allow hydrolytic displacement of nitrogen.

Below are several reliable synthetic pathways, with particular attention to laboratory practicality.

1. Via imine formation and hydrolysis / transimination

The most straightforward approach is to oxidise the secondary amine to the corresponding N-methylimine (4), then either hydrolyse it to acetophenone or trap it with hydroxylamine to give the oxime directly.

a) Oxidation to the N-methylimine

A variety of mild oxidants convert 1 into Ph–C(CH₃)=NCH₃ (4).

• Bleach method (NaOCl / base)

Treatment of 1 with aqueous sodium hypochlorite at 0 °C gives the N-chloroamine, which upon addition of a base (e.g. DBU or NaOH) undergoes dehydrochlorination to the imine 4.

• Swern‑type oxidation

Although classical Swern conditions (DMSO, (COCl)₂, then Et₃N) are mostly used for alcohols, related protocols employing N–tert-butylbenzenesulfinimidoyl chloride can oxidise secondary amines to imines in good yields.

• Catalytic aerobic oxidation

CuCl (or CuI) with TEMPO under an O₂ atmosphere, or RuCl₃/O₂ systems, cleanly dehydrogenate benzylic secondary amines to imines.

b) Hydrolysis to acetophenone

Imine 4 is easily hydrolysed by stirring with dilute aqueous acid (e.g. 1 M HCl) at room temperature:

After neutralisation, extraction gives acetophenone in high yield.

c) Direct conversion to acetophenone oxime

Alternatively, 4 can be treated directly with hydroxylamine hydrochloride and sodium acetate in aqueous methanol. A transimination reaction occurs, liberating methylamine and precipitating acetophenone oxime:

This one‑pot sequence (oxidation → oxime formation) is particularly convenient for preparing the oxime without isolating the ketone.

2. Via nitrone formation and hydrolysis

Secondary amines can be oxidised to nitrones, which hydrolyse to ketones and N-alkylhydroxylamines.

Oxidation of 1 with dimethyldioxirane (DMDO) in acetone at −20 °C, or with H₂O₂ catalysed by Na₂WO₄, affords the nitrone 5:

Subsequent acidic hydrolysis (e.g. dilute H₂SO₄) cleaves the nitrone:

Acetophenone is obtained in good overall yield. The nitrone can also be intercepted with hydroxylamine to give the oxime, though the imine route is more direct.

3. Oxidative deamination with *N*-halosuccinimides

N-Bromosuccinimide (NBS) under photochemical or radical initiation converts 1 to an α‑bromoamine that readily eliminates HBr to form the imine 4. The imine is then processed as above. This method avoids aqueous bleach and can be useful for sensitive substrates.

4. Metal‑catalysed acceptorless dehydrogenation

Modern catalytic methods using ruthenium or iridium pincer complexes can dehydrogenate secondary amines directly to imines, releasing H₂ gas. For example, Shvo’s catalyst or Milstein‑type complexes operate at 100–150 °C in toluene. The resulting imine 4 is hydrolysed to acetophenone. This green approach generates no stoichiometric by‑products other than water.

Summary of the most practical laboratory route

Acetophenone:

1. Dissolve methyl(1-phenylethyl)amine in CH₂Cl₂, add aqueous NaOCl at 0 °C, stir briefly.
2. Add DBU (or 2 M NaOH), warm to room temperature, extract the imine.
3. Stir the organic layer with 1 M HCl for 30 min. Separate, dry, and evaporate to obtain acetophenone.

Acetophenone oxime:

After step 2, replace the HCl hydrolysis with a mixture of NH₂OH·HCl and NaOAc in MeOH/H₂O. The oxime precipitates and can be filtered off.

These transformations remove the –NHCH₃ group cleanly and provide high yields of the desired carbonyl or oximino product, illustrating general strategies for the oxidative deamination of benzylic amines.

Assistant

Yes, the chlorination of (1R)-N-methyl-1-phenylethanamine with TCCA (trichloroisocyanuric acid) proceeds with complete retention of configuration at the benzylic carbon, yielding (1R)-N-chloro-N-methyl-1-phenylethanamine.

Here is the reasoning:

1. Site of reaction – no involvement of the chiral centre

The transformation is a straightforward electrophilic chlorination at the nitrogen atom. The amine lone pair attacks an electrophilic chlorine atom from TCCA (or any Cl⁺ equivalent), forming an N–Cl bond. The mechanism does not break, form, or even temporarily labilise the C–N bond at the stereogenic centre. The benzylic carbon remains tetrahedral throughout and its connectivity (Ph, CH₃, H, NR₂) is unchanged.

2. Mechanism and stereochemical integrity

• Step 1: Nucleophilic attack of the amine nitrogen on Cl⁺ (delivered by TCCA), giving a quaternary ammonium intermediate, [Ph–CH(CH₃)–N⁺(CH₃)(H)(Cl)].
• Step 2: Deprotonation of the ammonium nitrogen by a base (e.g., excess starting amine or an added base) yields the neutral *N*-chloroamine.

Because the C–N bond is merely spectator in this process, the absolute configuration at the benzylic carbon is fully preserved. There is no mechanistic pathway for inversion or racemisation under these mild conditions.

3. Stability of the *N*-chloroamine enantiomer

Once formed, (1R)-N-chloro-N-methyl-1-phenylethanamine is a chiral molecule with the carbon as the sole stereogenic centre. N-Chloroamines are configurationally stable at carbon; they do not undergo spontaneous racemisation at room temperature. (The nitrogen itself inverts rapidly, but nitrogen inversion is irrelevant here because nitrogen is not a stereogenic centre—the chirality resides on carbon.)

4. Comparison with bleach

The same stereochemical outcome would be observed with NaOCl (bleach) – the nature of the chlorinating agent (TCCA, NaOCl, N-chlorosuccinimide, etc.) does not alter the fact that the reaction is an N-chlorination that leaves the benzylic carbon untouched. TCCA offers practical advantages (solid, easy to handle, non-aqueous conditions possible) but does not change the stereochemical course.

Important note for the next step

If you subsequently treat this chiral N-chloroamine with base to generate the imine (PhC(CH₃)=NCH₃), the benzylic carbon becomes sp²-hybridised and the stereocentre is lost. The imine, and ultimately acetophenone or its oxime, will be achiral. However, this has no bearing on the N-chlorination step itself—the chlorinated intermediate is formed enantiomerically pure.

Assistant

1. Which imine isomer forms preferentially?

Treating (1R)-N-chloro-N-methyl-1-phenylethanamine with one equivalent of NaOH does generate the imine PhC(CH₃)=NCH₃, which can exist as two geometric isomers:

• (E)-isomer: the phenyl group (priority substituent on C) and the N-methyl group are on opposite sides of the C=N bond.
• (Z)-isomer: phenyl and N-methyl are on the same side.

Stereoelectronic elimination

The reaction is an E2-type 1,2-elimination of HCl across the C–N bond. Base abstracts the benzylic proton while chloride leaves from nitrogen. For efficient orbital overlap, the H and Cl must be anti-periplanar.

In the N-chloroamine, the benzylic carbon has only one hydrogen (the original (R)-configuration places H, Ph, and CH₃ asymmetrically). The nitrogen is sp³ and can, in principle, invert, though N-chloroamines have a somewhat raised inversion barrier. Nevertheless, the molecule can sample the necessary conformers. The anti requirement dictates the relative geometry of the leaving groups, and that geometry translates into which face of the incipient C=N bond bears the N-methyl group.

Observed selectivity

Experimentally, base-promoted dehydrochlorination of N-chloro-α-methylbenzylamines generally affords the (E)-imine as the major or exclusive product. The (E) isomer is the thermodynamically more stable one (less steric repulsion between phenyl and N-methyl). Whether the reaction is under kinetic or thermodynamic control, the (E)-isomer dominates; any (Z)-isomer formed initially can isomerise under the basic conditions.

So, in practice, you obtain largely the (E)-N-methyl-1-phenylethanimine.

2. Does reduction of that imine give racemic amine?

Yes. The imine is achiral – it has a plane of symmetry (the C=N π-bond plane, ignoring the E/Z isomerism, which does not create a stereogenic centre). Reduction with an achiral reducing agent (e.g. NaBH₄, LiAlH₄, H₂/Pd) adds hydride to the prochiral carbon.

Because there is no chiral element in the substrate or the reagent to differentiate the two enantiotopic faces of the imine, the hydride attacks from either face with equal probability. The product is a 1:1 mixture of (R)- and (S)-N-methyl-1-phenylethanamine – a racemate. The geometry of the starting imine (E or Z) does not alter this outcome; it only affects the relative arrangement of substituents in the transition state, but not the facial selectivity at the carbon that becomes the new stereocentre.

Summary:

• Base treatment gives predominantly the (E)-imine.
• Achiral reduction of that imine yields racemic N-methyl-1-phenylethanamine.

To recover enantiomerically enriched amine, a chiral reducing agent or a chiral auxiliary on nitrogen would be required.