Exceptionally Active Catalysts for the Formation of Carbamates from Alcohols and Isocyanates: Molybdenum(VI) Dichloride Dioxide and Its DMF Complex

Abstract

Small amounts of MoO₂Cl₂ or MoO₂Cl₂(DMF)₂ catalyze carbamate formation from an alcohol and isocyanates: 0.1 mol% of the respective additive allow primary, secondary or tertiary alcohols to add to aliphatic or aromatic isocyanates of varied steric hindrance within 20 minutes at room temperature. Typically the corresponding carbamate resulted in 100% yield. Only particularly hindered substrates required 1.0 mol% of the catalyst while as little as 0.01% sufficed for the phenylcarbamoylation of menthol. Catalytic amounts of DMAP accelerate carbamate formation from certain alcohols and isocyanates, too.

Key words

addition - carbamoylation - DMAP - heterocumulene - urethane formation

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• 54a

  Carbamates 12, 15, 36, and 37 are new compounds.⁵⁵ Other carbamates were described (inter alia) in the following references:

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42

Cf. ref. 41a.

53

Typical Procedure for the Carbamoylation of Alcohols with Isocyanates: At 22 ˚C neat benzyl isocyanate (31; 0.52 mL, 0.56 g, 1.1 equiv) was added to MoO₂Cl₂(DMF)₂ (1.3 mg, 3.8 µmol, 0.1 mol%) and 1.0 M (-)-menthol (16; 594 mg, 3.80 mmol) in CH₂Cl₂ (3.8 mL). The mixture was stirred at r.t. for 20 min. CH₂Cl₂ (10 mL) and H₂O (7 mL) were added. The aqueous phase was extracted with CH₂Cl₂ (2 × 12 mL). The combined organic phases were dried over Na₂SO₄. Removal of the solvent under reduced pressure gave N-benzyl-O-[(1R,2S,5R)-2-isopropyl-5-methylcyclo-hexyl] carbamate (35; 1.10 g, 100%) as a white solid (mp 92 ˚C). It was a pure compound as evidenced by ^¹H NMR spectroscopy and combustion analysis.

55

All new compounds (12, 15, 36, and 37) gave satisfactory ^¹H NMR and ^¹³C NMR spectra and correct combustion analyses for C, H, and N (±0.30%). Analytical details will be published in a full paper in due course.