C O M M U N I C A T I O N S
compounds. Further investigations to expand the scope of this
reaction are currently under active investigation in our laboratories.
Acknowledgment. The authors thank Bogdan Belgorodsky, Dr.
Shlomit Gali, Dr. Dan Grinstein, and Sharon Gil-Chaimov for their
contributions, and Tel-Aviv University for its generous financial
support.
Figure 2. Proposed mechanism for the catalytic oxidation of the hydrazo
compounds using H2O2 and TiCl3/HBr catalyst.
Supporting Information Available: Synthetic procedures and
characterization of compounds 2 and 3. This material is available free
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The mechanistic studies of the discovered process were begun
with the evaluation of the HBr-HOBr cycle, starting with validation
of the hypobromous acid formation. On the basis of the olefin
hydrobromination reaction reported by Saint and others,12 we used
trans-methylstyrene as a HOBr-trapping substrate (Figure 3). The
reaction was performed with catalytic and stoichiometric amounts
(as compared to trans-methylstyrene starting material) of HBr
reagent. In each case, a corresponding amount of hydrobromination
product was observed, clearly indicating the formation of HOBr
species in our process.
We have performed stoichiometric oxidations of hydrazo starting
materials (entries 1-3, Table 1) by using an in situ generated
hypobromous acid from N-bromoacetamide and HOCl.13a These
oxidation reactions produced the same yields of the azo products
as our catalytic process, providing a strong support to our claim
that HOBr is indeed the oxidant of the hydrazo compound in the
reaction. It should be mentioned that no azo products were detected
when we attempted to react stoichiometric amounts of hydrazo
derivatives with HOCl or directly with H2O2.
Additional studies included evaluation of other metal-containing
compounds as potential cocatalysts in our system. Specifically, VO-
13
(acac)2 and CuI,14 both capable of single-electron redox trans-
formation, were used instead of TiCl3; however, no hydrazo
oxidation was observed with these compounds.
In summary, a novel method for preparing azo compounds via
catalytic oxidation of corresponding hydrazo precursors has been
discovered. This oxidation process was highly efficient and selective
and represents a valuable addition to the chemistry of azo
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Soc., Perkin Trans. 1 1988, 8, 2297-2303.
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(14) Goodgame, D. M. L.; Goodgame, M.; Rayner, G. W. Nature 1969, 222,
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