Muhammad I. Javed and Matthias Brewer*
The University of Vermont, Department of Chemistry, 82 University Place, Burlington, Vermont 05405
This is a nice and convenient method of preparing diazo compound from hydrazones. Dehydrogenation of hydrazones is usually achieved by toxic heavy-metal salts such as HgO or Pb(OAc)4. Now it can be achieved under mild condition using Et3N and DMSO activated with oxalyl chloride (to give Me2SCl2). Ammonium chloride salt by-product could be removed by filtration and solution of diazo compounds could be used directly in next reaction.
The University of Vermont, Department of Chemistry, 82 University Place, Burlington, Vermont 05405
This is a nice and convenient method of preparing diazo compound from hydrazones. Dehydrogenation of hydrazones is usually achieved by toxic heavy-metal salts such as HgO or Pb(OAc)4. Now it can be achieved under mild condition using Et3N and DMSO activated with oxalyl chloride (to give Me2SCl2). Ammonium chloride salt by-product could be removed by filtration and solution of diazo compounds could be used directly in next reaction.
Initial study using hydrazone 1 in CH2Cl2 was found to give desired diazo derivative. But in trapping the diazo compound with benzoic acid, significant amount of 4 was formed. This was due to higher solubility of ammonium chloride salt in CH2Cl2 which reacted with the diazo intermediate. The solvent was switched to THF and this gave better yield of trapping product 3 because ammonium chloride was less solution in THF and it was crashed out more completely.
Therefore, several hydrazones were studied. Yields of diazo products were determined by amount of nitrogen gas evolution (in THF) and by trapping with benzoic acid (in CH2Cl2) as seen in Tables 1 and 2, respectively.
In both experiments, the trend is clear that aryl hydrazones gave better and more consistent yields of diazo compounds than aliphatic hydrazones. In some cases, when the resulting diazo products were stable, they were separated by filtration and could be directly characterized, e.g. diazo 6.
For hydrazone 7, in addition to ester 11, trapping of the corresponding diazo compound also gave by-product ester 12 formed from participation of THF in trapping the stable benzylic cation species 9. This problem could be circumvented by using a mixture of 9:1 Et2O:CH2Cl2, which allowed most, if not all, ammonium chloride salt to be removed while limiting solvent participation as seen in Table 2 that yield of 11 (as well as 13) was vastly improved.
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