Q3(a). Acetylacetone with MeMgI; conversion to hexane-2,4-dione

Reading of the question: “2,4-pentadione” is pentane-2,4-dione (acetylacetone) and “2,4-hexadione” is hexane-2,4-dione.

Introduction. The CH₂ between the two carbonyl groups of acetylacetone is strongly acidic, because its anion is shared by both C=O groups. A Grignard reagent is a much stronger base than it is a nucleophile towards such a ketone.

(i) One equivalent of MeMgI

Scheme: The Grignard reagent is protonated; no C=O addition takes place.

1. CH₃⁻ of MeMgI removes the active methylene H; methane is evolved (one mole per mole of dione; this is the Zerewitinoff estimation of active H).
2. The product is the magnesium enolate CH₃C(OMgI)=CHCOCH₃, held as a six-membered chelate. It is delocalised and no longer electrophilic, so no tertiary alcohol forms; on acid work-up acetylacetone is recovered.

(ii) To hexane-2,4-dione: alkylation of the dianion

Scheme: The second, more basic carbanion is formed at a terminal CH₃ and is alkylated there.

1. The first equivalent of base removes the central H; the second removes a terminal CH₃ hydrogen, giving the dianion.
2. The terminal carbanion is the less stabilised, more basic and more nucleophilic site, so one equivalent of CH₃I alkylates only there (γ-alkylation, Hauser).
3. Acid work-up: CH₃COCH₂COCH₂CH₃, hexane-2,4-dione. (Monoalkylation of the ordinary anion with CH₃I would put the methyl on C-3 instead: 3-methylpentane-2,4-dione.)

Conclusion. With one equivalent of MeMgI acetylacetone only loses its active H: CH₄ and a chelated magnesium enolate form. Hexane-2,4-dione is made by forming the dianion (2 NaNH₂/NH₃ or 2 LDA) and methylating the terminal carbon with CH₃I.

Application. Dianion alkylation of β-dicarbonyl compounds (Hauser, Weiler) is a standard way to lengthen the chain at the γ-carbon, used in the synthesis of polyketide fragments.