Relay Visible-Light Photoredox Catalysis: Synthesis of Pyrazole Derivatives via Formal [4 + 1] Annulation and Aromatization

Abstract

A relay visible-light photoredox catalysis strategy has been accomplished. Three successive photoredox cycles (one oxidative quenching cycle and two reductive quenching cycles) are engaged in a single reaction with one photocatalyst. This strategy enables formal [4 + 1] annulation of hydrazones with 2-bromo-1,3-dicarbonyl compounds, which functionalizes three C-H bonds of hydrazones. This method affords rapid access to a complex and biologically important pyrazole scaffold in a step-economical manner with high efficiency under mild conditions.

Photocascade Catalysis: A New Strategy for Cascade Reactions

“…The rational combination of photocatalytic SET oxidation and reduction could also provide a promising approach for developing new cascade reactions. In contrast to their previous study on the photocatalytic cascade alkylation of C(sp 2 )−H and C(sp 3 )−H bonds of aldehyde hydrazones with 2,2‐dibromo‐1,3‐dicarbonyls, the groups of Zhu and Yu jointly revealed that the reaction of aromatic‐aldehyde‐derived N ‐tetrahydroisoquinoline hydrazones 43 and diethyl 2‐bromo‐1,3‐dicarbonyl compounds 44 could undergo three successive photoredox cycles to give the corresponding complex and biologically important fused pyrazoles 45 in good yields (Scheme ) . Specifically, in this process, one SET reduction and two SET oxidations are situated together to sequentially activate the 2‐bromo‐1,3‐dicarbonyl compounds 44 and the key intermediates, hydrazone 43‐A and dihydropyrazole 43‐C .…”

“… Visible‐light‐induced photocatalytic cascade reaction of hydrazones and 2‐bromo‐1,3‐dicarbonyl compounds …”

Visible-Light Photocatalysis of Aldehyde and Carbonyl Functionalities, an Innovative Domain

“…Three successive photoredox cycles (one oxidative cycle and two reductive quenching cycles) were engaged in a reaction with one photocatalyst. Multiple quenching cycles could be demonstrated in a single reaction involving formal 4+1 annulation of hydrazone with 2-bromo diethylmalonate [23] (Figure 30).…”

Radical‐Mediated Formal C(sp2)–H Functionalization of Aldehyde‐Derived N,N‐Dialkylhydrazones

“…Later on, the same group also investigated the reactivity of the monohalogenated diethyl 2‐bromomalonate ( 41 ) under similar reaction conditions . Remarkably, reactions between 41 and a range of N ‐benzyl‐derived hydrazones in the presence of fac ‐[Ir(ppy) 2 dtbbpy](PF 6 ) (dtbbpy = 4,4′‐di‐ tert ‐butyl‐2,2′‐bipyridine) as photocatalyst also yielded nitrogen heterocyclic compounds, in the form of 4‐functionalized pyrazole derivatives 42 , one of the ester groups having been lost (Scheme a).…”