| Abstract
| - 5-Azido-3-benzyl-4-formyl-1-phenylpyrazoles 1a−c extrude dinitrogen upon heating in toluene togive the corresponding nitrenes, which immediately rearrange via a new ring-opening ring-closurereaction to produce an equimolar mixture of 4-cyano-2-phenyl-3-phenylazofurans 2a−c and 3-benzyl-4-cyano-1-phenylpyrazoles 3a−c. The formation of the 4-cyano-2-phenyl-3-phenylazofurans 2a−cis the first example in the pyrazole series of a nitrene rearrangement, in which the parentheterocyclic system of the product differs from that of the starting material. The isolation ofequimolar amounts of the two products points to the fact that their formation occurs by twomechanistically interconnected pathways, between which the exchange of a redox equivalent takesplace. Evidence for the existence of two mechanistically interlinked pathways is presented, andthe insight into the stoechiometry of the reaction is taken advantage of to optimize the reactionwith respect to either of the two products 2 or 3. Thus, it is demonstrated how one can bias the twopathways using external reagents, thereby changing the product distribution ratio 2:3 from 1:1 inthe unbiased case, to 1:4 in one direction, and to better than 20:1 in the other direction.
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