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Synfacts 2014; 10(12): 1258
DOI: 10.1055/s-0034-1379562
Synthesis of Heterocycles
© Georg Thieme Verlag Stuttgart · New York

A Photocatalytic Hydroamination Route to Dihydropyrazoles

Contributor(s):
Victor Snieckus
,
John I. Trujillo (Pfizer)
Hu X.-Q, Chen J.-R, * Wei Q, Liu F.-L, Deng Q.-H, Beauchemin AM, Xiao W.-J. * Central China Normal University, Wuhan and Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, P. R. of China; University of Ottawa, Canada
Photocatalytic Generation of N-Centered Hydrazonyl Radicals: A Strategy for Hydroamination of β,γ-Unsaturated Hydrazones.

Angew. Chem. Int. Ed. 2014;
53: 12163-12167
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Publication History

Publication Date:
18 November 2014 (online)

 
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Key words

photocatalysis - hydroamination - dihydropyrazoles - oxyamination

Significance

The synthesis of nitrogen-containing heterocycles from readily available precursors is of ongoing interest to the synthetic community. In particular, the dihydropyrazole ring system and its derivatives are found in compounds with varying types of biological activity (J. M. Alex, R. Kumar J. Enzyme Inhib. Med. Chem. 2014, 293, 427). A direct and atom-economical route to such systems is via a nitrogen-centered radical; however, until now this has been only accomplished by N–O bond cleavage to generate the N-centered radical. Of particular note are previous reports of a catalytic enantioselective iodoaminocyclization route (C. B. Tripathi, S. Mukherjee Org. Lett. 2014, 16, 3368) and stoichiometric TEMPO or azodicarboxylate routes to the ring system (X.-Y. Duan et al. J. Org. Chem. 2013, 78, 10692; M.-K. Zhu et al. Chem. Eur. J. 2013, 19, 5250).


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Comment

The current work demonstrates for the first time the direct catalytic transformation of an N–H bond of a hydrazone into an N-centered radical, which is then trapped by a pendant alkene. The radical is generated under mild photocatalytic conditions with [Ru(bpy)3]Cl2·6 H2O (2 mol%) as the catalyst to produce a variety of dihydropyrazoles B from readily accessible hydrazones in good yields. The investigators demonstrate the trapping of the carbon-centered radical with ­TEMPO or (PhTe)2 to give oxyamination and teluriumamination products in useful conversions. The utility and ease of set-up of photochemical reactions is becoming more appreciated by the synthetic community (Y. Su et al. Chem. Eur. J. 2014, 20, 10562).


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