Science of Synthesis

Tidwell, T. T., Science of Synthesis, (2006) 23, 583.

The original dehalogenation method of Staudinger, as modified by McCarney and Ward,[‌32‌] provides a useful and efficient method for the formation of volatile ketenes. Dehalogenation of 2-bromoacyl halides can be used to prepare methylketene (‌2‌),[‌8‌,‌32‌] ethylketene (‌3‌),[‌8‌,‌11‌] isopropylketene,[‌11‌] and tert-butylketene.[‌11‌,‌72‌] Zinc activated by ultrasound[‌73‌] or simple heating[‌74‌] is useful in ketene preparation. The dehalogenation of 2-bromopropanoyl bromide by zinc, with distillation of the volatile ketene and the solvent at low pressure, is used to prepare a clean solution of methylketene that, on treatment with a catalytic amount of a chiral tertiary amine, gives the dimer ‌27‌.[‌48‌] Reduction of ‌27‌ with lithium aluminum hydride gives 1-hydroxy-2-methylpentan-3-one (‌86‌) with a high enantiomeric excess (‌Scheme 26‌). The dimer ‌27‌ and derivatives, including ‌86‌, are used in the preparation of a variety of other products.[‌48‌,‌49‌,‌75‌–‌78‌] Methylketene (‌2‌) for stereoselective dimerization is normally prepared by the pyrolysis of propanoic anhydride (see Section ‌23.14.1.1.2‌)[‌37‌] or by dehydrochlorination of propanoyl chloride with in situ dimerization (see Section ‌23.14.1.2.6‌).[‌47‌]

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References

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• 8.Meeeee-Meee, (8888) 8/8, 88.
• 8.Meeeee-Meee, (8888) M 88-8, 8888.