Versatile Pd‐Catalyzed CH Oxidative Cyclization of Homoallylhydrazones to Pyrazolines and Tetrahydropyridazines

Abstract

Pd‐catalyzed 5‐exo‐trig or 6‐endo‐trig CH oxidative cyclization of C‐homoallyl‐N‐sulfonylhydrazones to 5‐vinylpyrazolines or 6‐methylidene‐1,4,5,6‐tetrahydropyridazines, respectively, is shown to be controlled in a highly selective manner by the ionic character of the PdII catalytic center. This character is ultimately defined by the nature of the X ligand in the PdX2 salt serving as an in situ precursor of the active catalytic species involving BIPHIMIP {2‐(diphenylphosphino)‐1‐[2‐(diphenylphosphino)phenyl]‐1H‐imidazole} as an optimal spectator diphosphine ligand for both processes. Whereas acetates (X=OAc) favor the 6‐endo process, noncoordinating anions such as tosylates and triflates (X=OTs, OTf) favor the 5‐exo process.

Visible Light Mediated Photocatalytic N-Radical Cascade Reactivity of γ,δ-Unsaturated N-Arylsulfonylhydrazones: A General Approach to Structurally Diverse Tetrahydropyridazines

“…In this context, hydrazones represent an appealing alternative, [21][22][23][24] being easy to prepare and displaying a very peculiar reactivity. [25][26][27][28][29][30] Many synthetic approaches exploiting them have been reported, like Palladium 31,32 and Copper mediated cyclizations, 33,34 but especially, visible light-mediated processes. Photocatalytic transformations have, indeed, attracted huge interest in the last years, disclosing alternative routes to functionalized molecules under mild reaction conditions and with high functional group tolerance.…”

“…As a consequence, many efforts have been dedicated to new strategies for the construction of these nitrogen heterocycles: among those are [4 + 2] cycloaddition of azoalkenes, − 1,2-diaza-1,3-dienes, , or diazenes, and [3 + 3] formal cycloadditions exploiting cyclopropyl derivatives and hydrazones. , Nevertheless, the need for available starting materials and a good compatibility of reaction conditions with several functional groups has been the driving force for the use of other cyclization strategies. In this context, hydrazones represent an appealing alternative, − being easy to prepare and displaying a very peculiar reactivity. − Many synthetic approaches exploiting them have been reported, like palladium , and copper mediated cyclizations, , but especially visible light-mediated processes. Photocatalytic transformations have, indeed, attracted huge interest in the past years, disclosing alternative routes to functionalized molecules under mild reaction conditions and with high functional group tolerance. − In particular, visible light-mediated generation of N -centered radicals , has rapidly become one of the most efficient approaches in the synthesis of N -heterocycles (Scheme ).…”

Strategies and Methods for the Synthesis of 2‐Pyrazolines: Recent Developments (2012–2022)

“…Similar products, though not optically active and containing an exocyclic alkenyl group, can be obtained at moderate to good yields using 1,4‐benzoquinone as the oxidant and a diphosphine ligand, starting from γ , δ ‐unsaturated hydrazones 254 (Scheme 52). The authors observed that in the presence of para ‐toluene sulfonic acid (PTSA), the expected pyrazolines 255 were formed via a 5‐ exo‐trig process, whereas in the absence of PTSA, the six‐membered tetrahydropyridazines were formed as a result of 6‐ endo‐trig transformation [81] …”

“…The authors observed that in the presence of para-toluene sulfonic acid (PTSA), the expected pyrazolines 255 were formed via a 5-exo-trig process, whereas in the absence of PTSA, the six-membered tetrahydropyridazines were formed as a result of 6-endo-trig transformation. [81] Alternatively, as Wang and Han describe, an additional alkene can be used to insert an alkenyl group at C-5 of the pyrazoline ring. Specifically, β,γ-unsaturated hydrazones 256 can successfully react with bromoal-Scheme 50.…”