Friday, September 18, 2009

Reactions Using Iodine Azide without Explosions
How do you carry out organic synthesis when the reagent has a tendency to blow up?  Brandt and Wirth constructed a microreactor to do the job.  In this case, one of the main benefits of a microreactor is that very small amounts of material are needed.  Once the reactants were mixed, they passed through a capillary with a volume of 0.196 mL where they were heated to promote the reaction.

The reaction of interest was between iodine azide and various derivatives of benzaldehyde.  Under these conditions, the weak iodine - nitrogen bond breaks and the reaction involves a free radical mechanism. The aldehyde Hydrogen gets abstracted and the resulting radical reacts with iodine azide to give an acyl azide and more Iodine atoms to continue the radical chain reaction.

The resulting Acyl Azide then undergoes a Curtius Rearrangement when heated.  N2 leaves, the benzene ring migrates from the carbonyl carbon to the first nitrogen, and a lone pair on the first nitrogen forms a pi-bond with the carbonyl carbon (replacing the bond from the benzene ring.) This produces an Isocyanate, which is quite reactive towards nucleophiles.  With excess azide ions present, the isocyanate reacts with a second azide ion (via an ionic mechanism this time) to produce the final product - a carbamoyl azide.

They don't report any explosions, but otherwise seem to have had disappointing results.  The reaction did work, but it was rather slow and gave low yields.  The yield did improve if they first distilled the benzaldehyde before the reaction, but it was still low.

Brandt, J., & Wirth, T. (2009). Controlling hazardous chemicals in microreactors: Synthesis with iodine azide Beilstein Journal of Organic Chemistry, 5 DOI: 10.3762/bjoc.5.30

No comments: