Q4(b). 'Etard reaction, Clemmensen reduction, Michael addition

Introduction. (i) is a controlled oxidation of a benzylic CH₃ to CHO, (ii) the reduction of an aryl ketone C=O to CH₂ with an ester present, (iii) the 1,4-addition of a ketone enolate (or enamine) to methyl vinyl ketone.

(i) Toluene to benzaldehyde: 'Etard reaction

Scheme: The 'Etard complex is hydrolysed to the aldehyde; over-oxidation is avoided.

Reagent: chromyl chloride, CrO₂Cl₂, in CS₂ (or CCl₄), then water. Chromyl chloride abstracts benzylic H and forms a brown insoluble 'Etard complex, C₆H₅CH(OCrOHCl₂)₂; hydrolysis gives benzaldehyde. Because the complex precipitates, the aldehyde is not oxidised further to benzoic acid. (Alternative: CrO₃ in acetic anhydride gives benzylidene diacetate, which is hydrolysed to benzaldehyde.)

(ii) Aryl ketone to CH₂: Clemmensen reduction

Scheme: Only the ketone C=O is reduced; the aromatic ring and the ester survive.

Reagent: zinc amalgam and concentrated HCl (Clemmensen reduction). Electrons transfer from the zinc surface to the protonated carbonyl (zinc carbenoid/organozinc intermediates, not the alcohol), and the C=O becomes CH₂. Aryl ketones are reduced especially well. The ethyl ester largely survives the acidic medium (any acid formed can be re-esterified); the Wolff–Kishner reduction, NH₂NH₂/KOH, would hydrolyse the ester completely. Hydrogenolysis with H₂, Pd/C in acid is an alternative for benzylic ketones.

(iii) Cyclohexanone + MVK: Michael addition

Scheme: Stork enamine version of the Michael reaction.

Name: Michael addition (conjugate, 1,4-addition) of the ketone's α-carbon to the β-carbon of methyl vinyl ketone. Reagent: a base such as NaOEt in EtOH or catalytic KOH (enolate as Michael donor), or better the Stork enamine method: pyrrolidine forms the enamine, which adds to MVK, and aqueous acid hydrolyses the iminium back to the ketone. Mild conditions stop the reaction at the 1,5-diketone; with strong base and heat this diketone would go on to an intramolecular aldol (Robinson annulation) to give octalone.

Conclusion. (i) 'Etard reaction, CrO₂Cl₂/CS₂ then H₂O; (ii) Clemmensen reduction, Zn(Hg)/conc. HCl; (iii) Michael addition, base (NaOEt) or pyrrolidine enamine (Stork) followed by H₃O⁺.

Application. The ketone of (ii) is the Friedel–Crafts succinoylation product of anisole; Clemmensen reduction and ring closure is the Haworth route to tetralones and naphthalenes, and the Michael product of (iii) is the Robinson-annulation precursor of octalone, the classic ring-building step of steroid synthesis.